Darshan Institute of Engineering & Technology · 2019. 11. 19. · specimen preparation for...

Darshan Institute of Engineering & Technology

Certificate

This is to certify that Mr./Ms.____________________________________

Enrollment No. ________________Branch_____________________________

Semester ____________ has satisfactory completed the course in the subject

__________________________________ in this institute.

Date of Submission: - __________________________

Staff in Charge Head of Department

DARSHAN INSTITUTE OF ENGINEERING AND TECHNOLOGY

MATERIAL SCIENCE AND METALLURGY (2131904)

B.E. III SEM. (MECHANICAL)

INDEX

Sr.

No. Description

Page

No.

Starting

Date

Ending

Date Sign Remark

1.

To get acquainted with the operation, construction, use and capabilities of a metallographic microscope and polishing machine and study procedure of specimen preparation for microscopic examination.

2.

To understand what is micro examination, importance of micro examination and study various ferrous and non-ferrous microstructures.

3.

To understand what is solid solutions,

space lattices, crystal structure and

bonding.

4.

To identify the different types of material available for design ,manufacturing and processing of various components based on structure-property-performance-processing relationships.

5. To understand the concept of iron carbon diagram and TTT curve.

6. To understand the procedure of Non Destructive tests.

7. To understand the concept of heat treatment and case hardening processes.

8. To understand the concept of powder metallurgy.

9.

To understand the concept of hardenability and obtain the hardness distribution curve with the help of Jomeny End quench test apparatus.

Metallurgical Microscope

1 Material science and Metallurgy (2131904)

Department of Mechanical Engineering


Date: ___ /___ /______ EXPERIMENT NO. 1

Objective

To get acquainted with the operation, construction, use and capabilities of a metallographic

microscope and polishing machine and study procedure of specimen preparation for

microscopic examination.

1.1 Definition

Metallography consists of the microscopic study of the structural characteristics of a metal or

an alloy. Metallography is the general study of metals and their behaviour, with particular

reference to their microstructure and macro-Structure.

Microstructure is the characteristic appearance and physical arrangement of a metal as

observed with a microscope.

Macrostructure is the appearance and physical arrangement as observed with the

naked eye or with a low power magnification.

Microstructure and Macrostructure of a metal or an alloy are closely interrelated with each

other and knowledge of both is essential for full understanding of any metal. Metallography

has wide scope and for the reason, a number of precise techniques (e.g. electron microscopy,

field ion microscopy, etc.) have been developed for the purpose.

1.2 Metallurgical Microscope

Metallurgical microscope is by far the most important tool of the metallurgist form both the

scientific and technical stand point.

Purpose

A metallurgical microscope helps determining:

Grain size and shape.

Size, shape and distribution of various phases and inclusion.

Mechanical and thermal treatments of the alloys.

A large range of metallurgical microscopes is available, for the above mentioned purposes,

all using the principal of examination by light reflected from the specimen surface (since

metal specimens are opaque). The typical metallurgical microscope is shown in Fig 1.2

Principle





As shown in Fig. 1.1, a horizontal beam of light from some light source is reflected, by

means of a plane-glass reflector, downward through the microscope objective onto the

surface of the specimen. Some of this incident light reflected from the specimen surface will

be magnified in passing through the lower lens system, the objective, and will continue

upward through the plane-glass reflector and be magnified again by the upper lens system,

the eyepiece. The initial magnifying power of the objective and the eyepiece is usually

engraved on the lens mount. When a particular combination of objective and eyepiece is used

at the proper tube length, the total magnification is equal to the product of the magnifications

of the objective and the eyepiece.

Fig. 1.1 The principle of Metallurgical microscope





Construction

A draw tube carrying eyepiece at its top end slides within the body tube of the microscope,

with the help of a rack anal pinion devices by rotating coarse and fine adjustment knobs.

Sliding of draw-tube within the body-tube varies the distance between the eyepiece and the

objective and thus helps focusing of the object. Fine adjustment facilitates final focusing of

the object.

Fig. 1.2 Metallurgical microscope

The objective, fitted at the down end of the body tube, resolves the structure of the metal

(specimen) whereas the eyepiece enlarges the image formed by the objective. Eyepieces are

made in a variety of powers, such as X5, X8, X10, etc., marked on the top of the eyepiece.

The source of light is inside the microscope tube itself. Light is diffused with the help of a

diffusing disc. The width of the light beam is controlled by the iris diaphragm. The incident

light strikes the plane glass reflector kept at 45° and is partially reflected down onto the

specimen. The rays of light get returned reflection from the (polished) specimen, pass

through the objective and glass reflector to form the final image which can be seen through

the eyepiece.

A photographic camera may be mounted above the eyepiece in order to record permanently

the metallographic structure of the alloy. The maximum magnification obtained with the

optical microscope is about 2000X.





1.3 Electron Microscope

At times it becomes necessary in metallurgical research to examine metal structures at very

high magnifications. Unfortunately the highest magnification possible with an ordinary

optical metallurgical microscope (Fig. 1.1) is in the region of X2000.For very high-power

microscopy (i.e., between X2000 and X200000) light rays are replaced by a beam of

electrons and this way developed an electron microscope.

Principle

Fig. 1.3 the Principle of an electron microscope

Most electron microscopy is carried out by using transmission-type instruments that produce

images of r transparent replicas of the etched specimen or of very thin metal obtained by

various techniques. It is necessary for electron microscope specimen to be transparent to the

electron beam. Replicas are produced in plastic or some other suitable material, which

reproduces faithfully the contours of the polished and etched specimen.

Thin foils of the metals which work as (electron microscope) specimen are of 100-2000. A°

thickness and are prepared by several available methods, one of which is Ion Bombardment

technique and another is Electro polishing method.

Construction and Operation





An electron microscope consists of an electron gun and condenser and projector lens.

Vacuum is necessary to allow passage of the electron beam. Electrons emitted by a hot

tungsten-filament cathode are accelerated, to form a high velocity beam, by the anode.

Depending upon the density and thickness of the replica (specimen) at each point, some of

the electrons are absorbed or scattered wi.ile the remainders pass through, i.e., transmit.

The magnetic field of the objective lens focuses and enlarges the electron beam that has

passed through the replica. Some of the electrons in this image are brought into a second

focus on a fluorescent screen by the projector lens.

1.4 Polishing Machine.

Fig.1.4 Schematic arrangement of the polishing machine

Fig. 1.4 shows the schematic arrangement of the specimen polishing machine available in the

laboratory. It is generally used after rough polishing of the specimen over fine emery papers.

Polishing is necessary to ensure a perfectly flat surface of the specimen, without any

scratches. A shining mirror like surface is required after polishing the specimen so as to get

the proper microstructure.

This machine consists of a cast iron disc mounted at the end of a vertical shaft. The shaft is

driven by an electric motor through pulleys and belt arrangement. The r.p.m. of the disc

available are 1400. A fine cloth is stretched over the top surface of the disc and is held in

position with help of a ring clamp. When in working, a fine abrasive powder suspended in

water is applied to the cloth on the revolving disc. The cloth is kept supplied with suspension

from a cylindrical container held above the disc. Alternatively, an abrasive paste may be





frequently applied to the cloth on the disc while supply of water may be regulated from the

cylindrical container to the disc. A P.V.C. tube brings liquid from the cylindrical container,

regulated by a cock, to the top surface of the disc. While polishing, the specimen is held

firmly in hand with a gentle pressure against the cloth on the revolving disc. This is

maintained so for a period as will permit mirror like finish on the surface of the specimen.

Thereafter, the specimen is washed with water and cleaned dry with alcohol. Care is taken

not to touch the polished surface with fingers, otherwise finger prints may be produced on

that surface requiring re polishing of the surface.

1.4 Preparation of Specimen

Specimen preparation or polishing is necessary to study its microstructure, because the

metallurgical microscope discussed earlier makes use of the principle of reflection of light

(from the specimen) to obtain the final image of the metal structure. Satisfactory

metallographic results can be obtained only, when the specimen has been carefully prepared.

Even the most costly microscope will not reveal the metal structure if the specimen has been

poorly prepared.

Procedure

The procedures for preparing the specimen both macro and micro-examination is the same,

except that in the latter the final surface finish is more important than in the former.

1. Selection of specimen. When investigating the properties of a metal or alloy, it is essential

that the specimen should be selected from that area (of the alloy plate or casting) which can

be taken as representative of the whole mass.

2. Cutting of the specimen. After selecting a particular area in the file mass, the specimen

may be removed with the help of a saw, a. panning tool, an abrasive wheel, etc.

3. Mounting the Specimen. If the specimen is too small to be held in hand for further

processing, it should be mounted in thermoplastic resin or some other low melting point

alloy.

4. Obtaining Flat Specimen Surface. It is first necessary to obtain a reasonably flat surface

on the specimen. This is achieved by using a fairly coarse file or machining or grinding, by

using a motor driven emery belt.

5. Intermediate and Fine Grinding. Intermediate and fine grinding is carried out using

emery papers of progressively finer grade.





The emery papers should be of very good quality in respect of uniformity of particle size.

Four grades of abrasives used are: 220 grit, 320 grit, 400 grit and 600 grit (from coarse to

fine); the 320 grit has particle sizes (of the silicon carbide) as about 33 microns and 600 grit

that of 17 microns (1 micron = 10-4

cm).

The specimen is first ground on 220 grit paper, so that scratches are produced roughly at right

angle to those initially existing on the specimen and produced through preliminary grinding

or coarse filing operation. Having removed the primary grinding marks, the specimen is wash

free of No. 220 grit. Grinding is then continued on the No. 320 paper, again turning the

specimen through 900 and polishing until the previous scratches marks are removed. The

process is repeated with the No. 400 and No. 600 papers. Grinding with the No. 200, No. 320,

etc., papers could be done in the following ways:

a) The specimen may be hand-rubbed against the abrasive paper which is laid over a flat

surface such as a piece of glass plate.

b) The abrasive paper may be mounted on the surface of a flat, horizontally rotating

wheel and the specimen held, in the hand, against it.

In either case, the surface of the abrasive paper (with a water proof base) shall be lubricated

with water so as to provide a flushing action to carry away the particles cut from the surface.

6. Rough Polishing. A very small quantity of diamond powder (particle size about 6

microns) carried in a paste that is oil-soluble is placed on the nylon cloth-covered surface of a

rotating polishing wheel. The lubricant used during the polishing operation is specially

prepared oil. The specimen is pressed against the cloth of the rotating wheel with

considerable pressure and is moved around the wheel in the direction opposite to rotation of

the wheel to ensure a more uniform polishing action.

7. Fine Polishing. The polishing compound used is alumina (Al2O3) power (with a particles

of 0.05 microns) placed on a cloth covered rotating wheel. Distilled water is used as a

lubricant. Fine polishing removes fine scratches and very thin distorted layer remaining from

the rough polishing stage.

8. Etching. Even after fine polishing, the granular structure in a specimen usually cannot be

seen under the microscope; because grain boundaries in a metal have a thickness of the order

of a few atom diameters at best, and resolving power of a microscope is much too low to

reveal their presence. In order to make the grain boundaries visible, after fine polishing the,

metal specimens are usually etched. Etching imparts unlike appearances to the metal

constituents and thus makes metal structure apparent under the microscope.





Before Etching, the polished specimen is thoroughly washed in running water. Then, the

etching is done either by

Immersing the polished surface (of the specimen) in the etching Reagent or by

Rubbing the polished surface gently with a cotton swab wetted with the Etching

Reagent.

After etching, the specimen is again washed thoroughly and dried.

Etching Reagents for Microscopic Examination

No Type of Etchant Composition Uses

1. Nital (i) Cone, Nitric acid

(ii) Absolute methyl alcohol

2 CC

98 CC

For etching steels, gray cast

iron &black heart malleable

2. Acid ammonium

persulphate

(i) Hydrochloric acid

(ii) Ammonium persulphate

(iii) Water

10 CC

10 gms

80 CC

For etching stainless steels.

3. Ammonia

hydrogen

peroxide

(i) Ammonium hydroxide

(0.880)

(ii) Hydrogen peroxide (3%

solution)

(iii) Water

50 CC

20-50 CC

50 CC

The best general etchant for

copper, brasses and bronzes.

4. Dilute hydro-

fluoric acid

(i) Hydrofluoric acid

(ii) Water

0.5 CC

99.5 CC

A good general etch-ant for

Al and its alloys

5. Keller’s reagent (i) Hydrofluoric acid

(ii) HCI

(iii) HNO3

(iv) Water

1 CC

1.5 CC

2.5 CC

95 CC

For (immersion) etching of

Duralumin type alloys

6. Mixed nitric and

acetic acids

(i) Nitric acid

(ii) Glacial acetic acid

50 CC

50 CC

A good general etch-ant for

Al and its alloys

References

1. Material Science and Metallurgy by O. P. Khanna

2. Introduction to Physical Metallurgy by Sidney H. Avner





Questionnaire

1. What is metallography?

2. Write a short note on metallurgical microscope.

3. Write a short note on electron microscope.

4. Draw a neat sketch of metallurgical microscope.

Microstructures





Objective

To understand what is micro examination, importance of micro examination and study

various ferrous and non-ferrous microstructures.

2.1 Macro Examination

Macroetching is the procedure in which a specimen is etched and evaluated macro

structurally at low magnifications. It is a frequently used technique for evaluating steel

products such as billets, bars, blooms, and forgings. There are several procedures for rating a

steel specimen by a graded series of photographs showing the incidence of certain conditions

and is applicable to carbon and low alloy steels. A number of different etching reagents may

be used depending upon the type of examination to be made. Steels react differently to

etching reagents because of variations in chemical composition, method of manufacturing.

Macro-Examinations are also performed on a polished and etched cross-section of a welded

material. During the examination, a number of features can be determined including weld run

sequence, important for weld procedure qualifications tests. As well as this, any defects on

the sample will be assessed for compliance with relevant specifications. Slag, porosity, lack

of weld penetration, lack of sidewall fusion and poor weld profile are among the features

observed in such examinations. It is normal to look for such defects either by standard visual

examination or at magnifications of up to 50X. It is also routine to photograph the section to

provide a permanent record. This is known as a photomacrograph.

2.2 Micro Examination

This is performed on samples either cut to size or mounted in a resin mold. The samples are

polished to a fine finish, normally one micron diamond paste, and usually etched in an

appropriate chemical solution prior to examination on a metallurgical microscope. Micro-

examination is performed for a number of purposes, the most obvious of which is to assess

the structure of the material. It is also common to examine for metallurgical anomalies such

as third phase precipitates, excessive grain growth, etc. Many routine tests such as phase

counting or grain size determinations are performed in conjunction with micro-examinations.

Microstructures




2.3 Study of Microstructures

Study of microstructure of the specimen is required to determine the metallurgical effects of

heat-treatment manufacturing processes (such as welding) etc. Trained metallographers are

able to evaluate the microscopical appearance of metals and to indicate the past history of the

metals so that the advisability of particular metallurgical method can be predicted. Many

photomicrographs and sketches of metallic structure included in the book, because all

metallurgical process have definite effects on the structure of the metals used and the

metallurgical nature of processes can be studied in terms of these metallographic effects.

Fig. shows microstructures of some important constituents.

Microstructures of Ferrous Metal

GREY CAST IRON S.G. IRON

WHITE CAST IRON MELLEABLE CAST IRON

Microstructures

3

Microstructures of Non Ferrous Metal

BRASS ALUMINUM

BRONZE GUNMETAL

References



Crystallography





Objective

To understand what is solid solutions, space lattices, crystal structure and bonding.

3.1 Solid Solutions

Metals generally form homogeneous liquid solutions in the liquid state. If, even after their

transformation to a solid crystalline state, the metals (alloy) retain their homogeneity and

consequently their solubility, a solid solution is said to have formed.

Thus solidified alloy, results in one kind of crystal in which both metals are present, but they

cannot be detected by the microscope, although the properties of the crystals are profoundly

altered.

Solid solutions form most readily when the solvent and solute atoms have similar sizes and

electron structure. For example, Brass is a solid solution of copper and zinc; a typical

composition is, copper 64% and Zn 36%. Copper atoms are solvent atoms and zinc atoms are

solute atoms.

Copper has an atomic radius of 1.278 Å, and Zinc has that of 1.332 Å.

Both Cu and Zn have 28 sub valence electrons.

They, individually form crystal structure of their own with a coordination number 12.

Copper and zinc form a substitutional solid solution.

A solid solution is simply a solution in the solid state and consists of two kinds of atoms

combined in one type of-space lattice. There is a homogeneous distribution of two or more

constituents in the solid state so as to form a single phase, i.e., the solid solution.

In a solid solution binary alloy system, the two metals (e.g., Cu and Ni) are completely

soluble in both the liquid and the solid states. A solid solution is the result of, metals

dissolving in each other's crystal lattice. Solid solutions are conductors, but not so good as the

pure metals on which they are based.

3.2 Types of solid solutions

Solid solutions occur in either of two distinct types, namely

Substitutional solid solution (1) Disordered (2) Ordered

Interstitial solid solution.

Crystallography




Substitutional solid solution

In substitutional solid solution, there is a direct substitution of one type of atom for another so

that solute atoms (Cu) enter the crystal to take positions normally occupied by solvent atoms

(e.g., nickel atoms); (Fig. 3.1).

In other words, in substitutional solid solution, the atoms of the solute substitute for atoms of

the solvent in the lattice structure of the solvent.

Fig. 3.1 Substitutional solid solution

Substitutional solid solution forms when the solute and solvent atoms possess equal or

approximately equal (within ± 7.5%) diameters; for example, atomic diameter of copper is

2.551 Å and that of nickel is 2.487 Å, and the two (i.e., Cu and Ni) form substitutional solid

solution. The great majority of the solid solutions are of substitutional type.

Disordered substitutional solid solution

In the formation of a substitutional solid solution the solute atoms do not occupy any specific

position but are distributed at random in the lattice structure of the solvent. This alloy is said

to be in a disordered condition. In the disordered condition, the concentration of solute atoms

can vary considerably throughout the lattice structure. (Fig. 3.2)

Fig. 3.2 Disodered substitutional solid solution

When a disordered substitutional solid solution crystallizes from the melt, there is a natural

tendency for the core of the dendrite to contain rather more atoms of the metal with higher

melting point, whilst the outer fringes of the crystal will contain correspondingly more atoms

of the metal of the lower melting point.

Crystallography




Ordered substitutional solid solution

The alloy in the disordered condition, if it is cooled slowly, under goes a rearrangement of the

atoms because of the diffusion that takes place during cooling. Diffusion tends to produce

uniform distribution of solute and solvent atoms. The solute atoms move into definite orderly

positions in the lattice (Fig. 3.3).

Fig. 3.3 Ordered substitutional solid solution

This structure is now known as ordered substitutional solid solution or super lattice.

Prolonged annealing tends to produce still more uniform and ordered solid solution. Cu-Zn,

Au-Cu, Cu2MnAI are some examples of ordered structures.

Interstitial solid solution

Interstitial solid solution forms when solute atoms are very small as compared to the solvent

atoms, they are unable to substitute solvent atoms (because of the large difference in

diameters of solvent and solute atoms) and can only fit into the interstices or spaces in the

crystal lattice of solvent atoms (Fig. 3.4).

Fig. 3.4 Interstitial solid solution

Those atoms which have atomic radii less than 1 angstrom (1 Å) are likely to form interstitial,

solid solutions. Such atoms are carbon (0.77 Å), nitrogen (0.71 Å), hydrogen (0.46 Å),

oxygen (0.6 Å), etc. Actually, atomic size is not the only factor that determines whether or

not an interstitial solid solution will form. Small interstitial solute atoms dissolve much more

readily in transition metals (such as Fe, Ni, Mn, Mo, Cr, W, etc.) than in other metals. Carbon

forms an interstitial solid solution with F.C.C. iron during the solidification of steel but it can

Crystallography




also be absorbed by solid iron provided the latter is heated to a temperature at which the

structure is F.C.C. This is the basis of carburizing steels. Nitrogen also dissolves interstitially

in solid steel, during the nitriding process.

3.3 Factors governing substitutional solubility

Several factors are now known, largely through the work of Hume-Rothery that controls the

range of solubility in alto systems. The different rules or factors are:

1. Crystal Structure Factor. The crystal lattice structure of the two (metal) elements should

same (i.e., both should be of b.c.c., f.c.c, or h.c.p. structure) for complete solubility, otherwise

the two solutions would not merge into each other. Also, for complete solid solubility the size

factor must usually be to than 8%.

2. Relative Size Factor. If two metals are to exhibit extensive solid solubility in each other it

is essential that their atomic diameters shall be fairly similar, since atoms is essential that

their atomic diameters shall be fairly similar, since atoms differering greatly in size cannot be

accommodated readily in the same structure (as a substitutional solid solution) without

producing excessive strain and corresponding instability. This is what is referred to when the

term size-factor is employed and extensive solid solubility is encountered only when the two

different atoms differ in size by less than 15%, called a favourable size factor (e.g., Cu-Ni).

If the relative size factor is between 8% and 15%, the alloy system usually shows a minimum

and if this factor is greater than 15%, substitutional solid solution formation is very limited.

3. Chemical-affinity Factor. The greater the chemical affinity of two metals, the more

restricted is their solid solubility. When their chemical affinity is great, two metals tend to

form an intermediate phase rather than a solid solution. Generally, the farther apart the

elements are in the periodic table, the greater is their chemical affinity.

4. Relative Valence (Valency) Factor. Consider two atoms, one with large valence electrons

and the other with small number of valence electrons. It has been found that the metal of high

valence can dissolve only a small amount of a lower valence metal, while the lower valence

metal may have good solubility for the higher valence metal.

For example, in the Al-Ni alloy system, both metals have f.c.c. structure. The relative size

factor is approximately 14%.However; Ni is lower in valence than Al and thus solid nickel

dissolves 5% aluminium, but the higher valence Al dissolves only 0.04% Ni.

3.4 Space lattice/crystal lattice

Crystallography




Lattice is the regular geometrical arrangement of points in crystal space. The atoms arrange

themselves in distinct pattern in space, called space lattice.Space lattice is the three

dimensional network of imaginary connecting the atoms (Fig. 3.5)

A Space lattice can be considered as an infinite array of points in space, so arranged that it

divides space into equal volumes with no sp ace excluded. An important characteristic of a

space lattice is that every point has identical surroundings. Space lattice or crystal lattice is

the arrangement of atoms in a crystal.

X-ray studies reveal that atoms in crystalline materials are arranged in a regular three

dimensional repeating pattern known as lattice structure. The lattice structure can be shown

by a network of lines, dividing the space into equal volumes. The points of intersection are

known as lattice points. Atoms are positioned about these points. As shown in Fig. 3.5 the

unit cell is the smallest portion of the lattice which when repeated in all directions gives rise

to lattice structure.

Fig. 3.5 Lattice structure, space lattice, lattice points and unit cell.

There are fourteen space lattices i.e., there are only 14 ways in which points can be arranged

in space so that each has identical surroundings. These 14 space lattices are known as Bravais

space lattices.

3.5 Unit cell and its lattice parameters

As already explained above, a space lattice can be considered as an infinite array of points in

space, so arranged that it divides space into equal volumes with no space excluded. Every

point, which is called a lattice point, has identical surroundings with every other point.

The smallest volume that contains the full pattern of repetition is called a unit cell. Identical

unit cells must completely fill the space when they are packed face to face, thus generating a

space lattice (Fig. 3.6). If a unit cell is so chosen that it contains lattice points only at its

Crystallography




corners, it is called a primitive unit cell or simple unit cell.

A primitive unit cell contains only one lattice point because each point at eight corners is

shared equally with eight adjacent unit cells. The edge length of the unit cell, called a lattice

constant or a lattice parameter, is a lattice translation in a given direction. In Fig. 3.6 simple

monoclinic, Triclinic, simple cubic are known as primitive cells. Unit cells for most crystal

structures are parallelepipeds or prisons having three sets of parallel faces.

Fig. 3.6 shows the unit cell geometry, which is, the shape of the appropriate unit cell

parallelepiped without regard to the atomic positions in the cell.

Within this framework, an x, y, z coordinate system is established with its origin at one of the

unit cell corners; each of the x, y, and z axis coincide with one of the three parallelepiped

edges that extend from this corner, as illustrated in Fig. 3.6

Fig 3.6 A unit cell with x, y and z coordinate axis, showing axial lengths (a, b, and c) and

interaxial angles (α, β and γ).

The unit cell geometry is completely defined in terms of six parameters: the three edge

lengths a, b and c and the three interaxial angles α, β and γ. These are indicated in Fig. 35.8

and are sometimes termed the Lattice Parameters of a crystal structure.

On this basis, there are found crystals having seven different possible combinations of a, b,

and c and α, β and γ, each of which represents a distinct crystal system.

3.6 Crystal systems

As explained above, crystal system is a scheme by which crystal structures are classified

according to unit cell geometry. This geometry is specified in terms of the relationships

between lengths a, b, and c and interaxial angles α, β and γ.

There are seven different crystal systems, namely

1. Cubic 4. Orthorhombics 2. Tetragonal

Crystallography




5. Rhombohedral 3. Hexagonal 6. Monoclinic

7. Triclinic.

The lattice parameter relationships and unit cell sketches for each crystal system are given in

Table 3.1 on front page.

Crystallography




Crystal system Axial Relationship Interaxial Angles Unit Cell Geometry

Cubic

Hexagonal

Tetragonal

Rhombohedral

Orthorhombic

Monoclinic

Triclinic

Table 3.1 Lattice Parameter Relationships and Figures Showing Unit Cell Geometries for the

Seven Crystal Systems

The cubic system, for which a = b = c and α = β = γ = 90°, has the greatest degree of

symmetry.Least symmetry is displayed by the triclinic system, since a ≠ b ≠ c and α ≠ β ≠ γ.

BCC and FCC structures belong to the cubic crystal system, whereas HCP falls within

hexagonal.

3.7 Crystal structure of metals

Introduction

A crystal is defined as an orderly array of atoms in space. For crystalline materials, crystal

structure shows the manner in which atoms or ions are arrayed in space. It is defined in terms

of the unit cell geometry and the atom positions within the unit cell.A regular and repetitious

pattern in which atoms of a crystalline material arrange themselves is known as the crystal

Crystallography




structure.

Crystal structure is defined as regular repetition of three dimensional patterns of atoms in

space. To some extent, the repeating pattern controls the external shape of crystal. Spare

lattice and Basis generate the crystal structure.

Space lattice + Basis Crystal structure

The Basis may consist-of one atom per lattice point. For example:

- FCC (Space lattice) + 1 Al atom at each lattice point (Basis) FCC crystal of alumn.

- BCC (Space lattice) + 1 Fe atom at each lattice point (Basis) BCC crystal of iron.

There are many different types of crystal structures, some of which are quite complicated.

Fortunately, most metals crystallize in one of the three relatively simple (space lattices)

structures, namely

1. The Body-centered cubic (BCC), (Cr, V, Mo, Na, Mn, α Fe, etc.)

2. The Face-centered cubic (FCC) (Al, Cu, Ag, Pb, γ Fe, etc.)

3. The close-packed hexagonal (CPH or HCP) (Mg, Zn, Cd, etc.)

Fig. 3.7 Crystal structures (Unit cells).

1. Body-centered cubic (B.C.C.) Structure [Fig 3.7(a)]

A B.C.C. unit cell has one atom in the centre of the cube and one atom each at all the

corners. Of course, the corner atom is shared by other adjoining body center cubes.

Thus the unit cell of BCC structure contains 8 atoms at the corner x ⅛ = 1 atom and 1

centre atom = 1 atom so Total = 2 atoms.

In BCC structure, the lattice constant a is related to the atomic radius r by: abcc =

4r/√3. Therefore the atomic packing factor is 0.68.

Crystallography




At room temperature, iron exhibits BCC structure. Other metals possessing this

structure are V, Mo, Ta, W, etc.

2. Face-centered Cubic (FCC) Structure [Fig. 3.7(b)]

A face-centered cube has an atom at each corner of the cube and in addition, one atom

at the intersection of the diagonals of each of the six faces of the cube.

Since each corner atom is shared by eight adjoining cubes and each face atom is

shared by only one adjacent cube, the unit cell contains: 8 atoms at the corners x ⅛ =

1 atom and 6 face-centered atoms x 1/2 = 3 atoms so Total = 4 atoms

This shows that FCC structure is more densely packed than the BCC structure.

Packing factor (volume of atoms/volume of cell) for FCC structure is 0.74 whereas

that of BCC structure was 0.68. This also proves that FCC structure is more densely

packed than the BCC structure.

The lattice constant à' of FCC structure is related to the atomic radius r by afcc = 4r/√2

The face-centered cube lattice, however, is unique in that it contains as many as four

planes of closest packing (111), each containing three close-packed directions <110>,

thus amounting to 12 physically distinct slip systems. No other structure possesses

such a large number of close packed planes and close-packed directions.

For this reason, metals possessing FCC structure can be plastically deformed at severe

rates. Metals possessing FCC structure are, Cu, Al, Pb, Ni, Co, etc.

3. Hexagonal Close Packed (HCP) Structure [Fig. 3.7(c) & (d)]

A hexagonal close packed structure has

- One atom at each corner of the hexagon,

- One atom at the centre of the two hexagonal faces (basal planes), and

- One atom at the centre of the line connecting the perpendiculars in case of three

rhombuses, namely DEFG, BDGH and BKED, which combine and form the HCP

structure.

Whereas, the unit cell of the cubic system (i.e. BCC or FCC) can be specified by a

single lattice parameter à' (Fig. 3.7 ) the hexagonal unit cell requires the width of the

hexagon à' and the distance between basal planes `c' for being specified. These

determine the axial ratio c/a, which is sometimes given.

The axial ratio of metals varies from 1.58 for beryllium to 1.88 for cadmium ('.' a =

2.9787 and c = 5.617). The atomic packing factor for HCP metal is equal to 0.74 (i.e.

Crystallography




same as for FCC metal). Metals such as zinc, cadmium, beryllium, magnesium etc.,

possess HCP structure.

3.8 Coordination number

Every atom in a crystal is surrounded by other atoms. By the term coordination number, we

mean the number of nearest atoms which are directly surrounding a given atom. The

coordination number may also be defined as the nearest neighbors to an atom in a crystal.

Fig. 3.8 Coordination number of carbon atom

Fig. 3.8 shows that the coordination number of carbon atom is four, because it has four

hydrogen atoms around it. When the coordination number is larger, the structure is more

closely packed. Coordination numbers for, a simple cubic, BCC, FCC & HCP are as below.

(1) Simple Cubic Structure

There is one atom at each of the (eight) corners of the cube. Any corner atom has four nearest

neighbour atoms in the same plane and two nearest neighbours (one exactly above and the

other exactly below) in a vertical plane.

Hence coordination number for simple cubic structure is 4 + 2 = 6. If 'a' is the side of the unit

cell, then the distance between the nearest neighbours will be equal to `a'.

(2) Body Centered Cubic (B.C.C.) Structure

In B.C.C. structure, there is one atom at each corner of the cube and one atom at the centre of

cube. For any corner atom of unit cell, the nearest atoms are the atoms which are at the

centres of unit cells. As such corner atom is surrounded by eight unit cells having body

centred atoms.

Crystallography




Fig.3.9 unit cell of BCC structure

Hence coordination number is 8. Similarly by considering the centred atom of each unit cell,

we can say that the coordination number is 8 because every centred atom is surrounded by

eight equidistant neighbours. The nearest distance between two atoms = √3a/2

(3) Face-centred Cubic (F.C.C.) Structure

In F.C.C. structure, there is one atom at each corner of the cube and one atom at the centre of

each face of the cube. For any corner atom of the unit cell, the nearest are the face centred

atoms. For any corner atom, there will be 4 face centred atoms of the surrounding unit cells in

its own plane, 4 face centred atoms below this plane and 4 face centred atoms above this

plane.

Fig. 3.10 unit cell of FCC structure

Hence the coordination number for this case is 4 + 4 + 4 = 12. The distance between two

nearest neighbours = a/√2

(4) Hexagonal Closed Packed (HCP) Structure

Crystallography




Fig.3.11 unit cell of HCP structure

The coordination number and the atomic packing factor for the HCP crystal structure are the

same as for FCC-12 and 0.74 respectively.

3.9 Bonding in solids

Introduction

All solids are composed of a very large number of atoms that are bonded together in some

manner. The cohesion between the atoms is dependent upon the character of the individual

elements. In solids, atoms occupy relatively fixed positions with regard to one another.

Atoms are held in the solid state by relatively strong interatomic forces which generally are

functions of temperature and pressure. These interatomic forces can be attractive or repulsive,

and the equilibrium spacing of atoms in a solid is obtained when the opposing forces are

balanced at a particular temperature and pressure. Solids offer resistance to an applied force;

this is because of the interatomic forces that hold atoms together.

When the applied force exceeds the interatomic forces (holding atoms together) the solids

tend to get deformed. The physical, chemical and electrical properties of different solids

depend upon the character, strength and directionality of the interatomic binding forces.

Binding Energy

The binding energy or bond energy may be defined as the energy required to return the atoms

to an infinite separation. It is the energy to break a bond or to separate the bonded atom. The

bond energy can also be expressed as the energy of formation of 1 mole of a substance from

its atoms or ions when brought together from an infinite distance to the equilibrium position.

The bond energy is equal but opposite in sign to the energy of dissociation of substance. The

bond energy can best be measured as the amount of energy required to break it that is the

Crystallography




amount of heat which must be supplied to vaporize the solid i.e., the state of infinite

separation of atom.

Sr. No. Materials Type of Bond Bond Energy (KJ/mol)

1. Iron Metallic 401.30

2. Sodium chloride Ionic 639.50

3. Silicon dioxide Covalent 1692.90

4. Nitrogen Intermolecular (van der Waals) 7.8

Table 3.2 Bond energy values for different materials and bond types.

The strength of the interatomic bond influences the melting and boiling points of the

substances. The melting and boiling points of the substance increase with the strength of the

bond.

3.10 Types of Bonds

The atomic bonds are of the following types:

Primary bonds

a) Ionic bond

b) Covalent bond

c) Metallic bond

Secondary bond

d) van der Waals bond (Molecular bond)

Primary Bond is Stronger and more stable than the secondary bond. An interatomic bond

whereas secondary bond is an intermolecular bond.

1. Weak attractive forces between polarized atoms.

2. Centres of + and of - electricity separated in each atom.

(a) Ionic Bond

- Ionic bond is also known as electrovalent or heteropolar bond.

- Ionic bond is probably the simplest example of interatomic bonding.

- Ionic bond is commonly found in inorganic chemistry.

- In the simplest case an ionic bond exists between a metallic atom (Sodium) and a non-

metallic atom (Chlorine).

- When atoms of sodium and chlorine are brought together, the valency electron of the sodium

atom is transferred to the chlorine atom, producing a positive sodium ion and a negative

Crystallography




chlorine ion, which are attracted to each other electro statically and form stable compound.

Fig. 3.12 The arrangements of atoms in ionic bond (Nacl)

- As shown above, the sodium atom has one electron (*) in its outer shell. It is released

(because it finds a lower energy level in chlorine atom) to join the seven electrons (*) in the

outer shell of the chlorine atom and thus NaCl, a stable compound forms.

- The ionic bond is no directional and pulls equally hard in all directions, because the

electrostatic attraction between the oppositely charge d ions acts in all directions.

- Ionic bonds result in the formation of a three-dimensional giant aggregate in which all units

are joined by strong ionic bonds, giving the solid.

(b) Covalent Bond

- Covalent bon d is also known as homopolar bond.

- The electronic structure of an atom is relatively stable if it has eight electrons in its outer

valence shell.

- Sometimes an atom may acquire these eight electrons by sharing electrons with an adjacent

similar atom.

- Thus, when electrons are shared (and not transferred) between atoms, it gives rise to a

covalent bond.

Crystallography




Fig. 3.10 The arrangements of atoms in covalent bond (CH4)

- An excellent example of covalent bonding is found in chlorine molecule. Here the outer shell

of each atom possesses seven electrons. Each chlorine atom would like to gain an electron,

and thus form a stable octet. This can be done by sharing of two electrons between pairs of

chlorine atoms thereby producing stable diatomic molecules. In other words, each atom

contributes one electron for the sharing process.

- Another example of covalent bonding is a molecule of methane, CH4 in which carbon atom

and hydrogen atoms share the electrons. The carbon atom has four electrons in its outer shell,

and these are joined by four more electrons, contributed singly by each of the four hydrogen

atoms.

- The number of covalent bonds formed by an element is equal to (8-N), where N is the

number of electrons outside the full shell. Covalent bond provides strong attractive forces

between atoms e.g., diamond, which is the hardest material found in nature and which is

entirely carbon has covalent bond.

(c) Metallic Bond

- Metallic bond applies in most pure metals and alloys.

- Metallic bonding may be regarded as a special form of covalent bonding.

- In covalent bonding particular atoms in the structure are linked together by particular pairs of

the valency electrons shared between them.

- Whereas, in metallic bonding, the valency electrons are not bound to any particular pairs of

atoms but move freely throughout the metal and form a negative electron cloud, which is

shared by the positive ions.

Crystallography




-

Fig. 3.11 The arrangements of atoms in metallic bond

- The positive metal ions tend to repel each other and take up positions according to some

geometrical pattern, but at the same time they are held together by their mutual attraction for

the electron cloud.

- Metallic bond is the characteristic of the elements having small numbers of valence electrons,

which are loosely held, so that they can easily be released to the common pool (cloud).

- Metallic bond is non-specific and non-directional, acting equally strong in all directions.

- This leads to highly co-ordinate close-packed structures, accounting for the unique plastic

properties of metals and for their ability to form alloys.

- The opaque lustre of metals is due to the reflection of light by free electrons.

(d) The Van der Waal Bond (or Intermolecular bond)

- Intermolecular or van der Waals forces are weak forces that account for mutual interaction

between molecules or inert atoms. Weak electrostatic attraction is due to unsymmetrical

electrical charges in electrically neutral (as a whole) atoms or molecules.

Crystallography




Fig. 3.12 The arrangements of atoms in molecular bond

- Intermolecular forces account for the properties of liquids and many molecular solids ranging

from small molecular weight up to polymers. Intermolecular bonds do not involve the

transfer or sharing of electrons between atoms.

There are three types of intermolecular bonds, namely:

1. Dispersion bonds,

2. Dipole bonds, and

3. Hydrogen bond.

Fig. 3.13 Intermolecular bonds

(i) Dispersion effect (electronic polarization) (ii) Dipole bond, and (iii) hydrogen bond

1. Dispersion bonds

- As electrons rotate around their nuclei, they tend to keep in phase; and since the electrons of

adjacent atoms in a molecule tend to repel each other, the result is that the molecule has a

small fluctuating net charge on each end. [As in Fig. 35.3 (i), the hydrogen molecule is

instantly charged negatively on the right end and positively on the left].

- The fluctuating charge on one molecule tends to interact with the fluctuating charge on a

neighbouring molecule, resulting in, a net attraction and thus the dispersion bond.

- Molecules of the inert gases, which consist of single atoms, are held together by dispersion

forces when the gases are solidified.

2. Dipole bonds

- Consider a molecule of hydrogen fluoride. There are two electrons that surround the positive

charges in the nucleus of the hydrogen atom and there are eight electrons surround the

nucleus of the fluorine atom i.e., electrons surrounded nucleus of fluorine more completely

than that of hydrogen.

- This creates an electrical imbalance. Consequently, the center of positive charge and the

center of negative charge do not coincide, rather, remain separated. This way produces an

Crystallography




electrical dipole.

- An electric dipole provides a mechanism for molecular bonding. The presence of a permanent

dipole moment increase the attraction forces between molecules and facilitates their closer

approach.

- A dipole bond is much weaker than the ionic bond but it is considerably stronger than the

dispersion bond.

3. Hydrogen bond

- Hydrogen bond is a special type of (strong) dipole bond that occurs between the molecules in

which one end is a hydrogen atom.

- The one electron belonging to the hydrogen atom is fairly loosely held, and if the adjacent

atom in the molecule is strongly electro-negative*, (*Electro negativity of an atom can be

defined as the power of attraction for the electron within a molecule). It may keep all the

electrons around itself, leaving the hydrogen atom in effect a positive ion.

- This tendency can produce a strong permanent dipole that can bond to other similar dipoles

with a force near that involved in the ionic bond.

- A good example of hydrogen bonding is water.Argon, calomel, Ice, paraffins and solid

carbon-di-oxide possess molecular bond.

References


Questionnaire

1. What do you understand by bonding in solids? 1 Name the various types. Explain any

one of them.

2. Define: (i) Crystal structure (ii) Space lattice (ill) Unit cell

3. Explain with neat sketches three main types of crystal structure of metals giving some

examples for each type.

4. Explain type of solid solutions.

5. Name and explain the factors governing solubility in substitutional solution.

Ferrous and Non Ferrous Material





Objective

To identify the different types of material available for design ,manufacturing and processing

of various components based on structure-property-performance-processing relationships.

4.1 Introduction

Ferrous materials contain iron, and the one element people use more than all other is iron.

Ferrous materials are the most important metals/alloys in the metallurgical and mechanical

industries because of their very extensive use. The widespread use of ferrous alloys is

accounted for by three factors:

1. Iron containing compounds exist in abundant quantities within the earth's crust;

2. Metallic iron and steel alloys may be produced using relatively economical extraction,

refining, alloying and fabrication techniques; and

3. Ferrous alloys are extremely versatile, in that they may be tailored to have a wide

range of mechanical and physical properties.

The principal disadvantage of many ferrous alloys is their susceptibility to corrosion.

4.2 Classification

4.3 Pig Iron

The name Pig Iron originated in the early days of iron ore reduction when the total output of

the blast furnace was sand cast into pigs - a mass of iron roughly resembling a reclining pig.

The oldest method of pig casting in sand beds has been largely superseded by pig casting

machines. Machines cast pigs are much cleaner than sand cast pigs and have no adhering





sand to contaminate the remelt process.

Pig (iron) for foundry use is roughly 50.8 x 22.8 x 10.1 cm (i.e., 20" x 9" x 4") in size.

Pig Iron, produced in a blast furnace, is the first product in the process of converting iron ore

into useful metal. The iron ore becomes pig iron when the impurities are burned out in a blast

furnace. Though still containing some impurities, pig iron has a high metal content.

Pig Iron, the product of blast furnace, has the following composition (approx.)

Carbon 3-4% Sulphur less than 1.0%

Manganese 0.1-1% Phosphorus 0.3-1.7%

Silicon 1-3% Iron Remainder

Pig Iron is the raw material for all iron and steel products. Pig iron is of great importance in

the foundry and in steel making processes.

1. Pig iron partly refined in a cupola produces various grades of Cast Iron.

2. By puddling or shotting processes, Wrought iron is produced from pig iron.

3. Steel is produced from pig iron by various steel making processes such as Bessemer, open-

hearth, oxygen, electric and spray steel making.

Pig iron is classified by chemical composition into three grades.

1. Basic pig iron

It is used for steel making and is low in silicon (1.5% max.) to prevent attack of the

refractory linings of refining furnaces and to control slag formation.

- Basic pig iron must be low in sulphur (0.04%) since sulphur is an active impurity in

steel and is not eliminated in the refining furnaces.

- Phosphorus normally is held to less than 1% and manganese to a range of 1 to 2%.

Carbon content varies from 3.5 to 4.4%.

2. Foundry pig iron

- It includes all the types that are used for the production of iron castings.

Foundry pig iron contains

Si 0.5 – 3.5 % S Up to 0.05%

C 3- 4.5 % P 0.035-0.9%

Mn 0.4 – 1.25 % Fe Remainder

3. Ferroalloys

- Ferroalloys are alloys of pig iron, each rich in one specific element. They are used as

additives, in iron and steel industries, to control or alter the properties of iron and

steel. Examples of ferroalloys are as follows:





1. Ferromanganese, which is pig iron that contains from 74 to 82% manganese.

2. Ferrosilicon, which is pig iron with 5 to 17% of silicon content.

4.4 Wrought iron

Wrought iron is a mechanical mixture of very pure iron and a silicate slag. Wrought iron is a

ferrous material, aggregated from a solidifying mass of pasty particles of highly refined

metallic iron with which a minutely and uniformly distributed quantity of slag is incorporated

without subsequent fusion.

Chemical composition of wrought iron would be:

C 0.02-0.03% P 0.05-0.25%

Si 0.02-0.10% Mn Nil-0.02%

Slag 0.05-.50% S 0.0084.02%

Properties

- Wrought iron is never cast. All shaping is accomplished by hammering, pressing,

forging, etc.

- Wrought iron is noted for its high ductility and for the ease with which it can be

forged and welded.

- The ultimate strength of wrought iron can be increased considerably by cold working

followed by a period of aging.

- Wrought iron possesses a high resistance towards corrosion.

- Wrought iron possesses the property of recovering rapidly from overstrain, which

enables it to accommodate sudden and excessive shocks without permanent injury. It

has a high resistance towards fatigue.

- The mechanical properties of wrought iron are largely those of pure iron. Because of

the nature of the slag distribution, however, tensile strength and ductility are greater in

the longitudinal direction or rolling direction than in the direction transverse to

rolling.

Fig. 4.1 shows the distribution of slag throughout the ferrite matrix in transverse section of

wrought iron.

- Structurally, wrought iron is a composite material; the base metal and the slag are in

physical association, in contrast to the chemical or alloy relationship that generally

exists between the constituents of other metals.

The form and distribution of (iron silicate) slag may be stringer-like, ribbon-like or platelets.





Fig. 4.1 Microstructure of wrought iron Slag (black) in ferrite matrix

- Tensile properties of wrought iron

Longitudinal Transverse

Tensile strength, kg/cm2 3380-3500 2530-2670

Elongation, % in 200 mm 18-25 2-5

Reduction in area, % 35-45 3-6

- Physical properties

Melting point ... 1510°C

Weight, kg/m3 ... 7680

Electrical resistance, 20°C, micro ohm/cm/sq. cm ... 11.97

Shear modulus, 26°C ... 0.83 x 106 kg/cm2

Poisson's ratio ... 0.30

Uses (Applications) of wrought iron

Wrought iron is available in the form of plates, sheets, bars, structurals, forging blooms and billets,

rivets, chains and a wide range of tubular products including pipe, tubing and casing, electrical

conduit, cold drawn tubing, nipples and welding fittings.

1. Building construction. Underground service lines and electrical conduit. Soil, waste, vent and

downspout piping.

2. Public works. Bridge railings, blast plates, drainage lines and troughs, sewer outfall lines,

weir plates, sludge tanks and lines.

3. Industrial. Condenser tubes, unfired heat exchangers, acid and alkali process lines, skimmer

bars, etc.

4. Rail road and marine. Diesel exhaust and air brake piping, ballast and brine protection plates,

hull and deck plating, tanker heating coils, etc.

5. Others. Gas collection hoods, coal handling equipment, cooling tower and spray pond piping.





Manufacture of wrought iron

Wrought iron is produced by the following processes:

1. Puddling process. 2. Aston's process.

4.5 Gray cast iron

Characteristics

- Gray Iron basically is an alloy of carbon and silicon with iron.

- It is readily cast into a desired shape in a sand mould.

- It contains 2.5-3.8% C, 1.1-2.8% Si, 0.4-1% Mn, 0.15% P and 0.10% S.

- It is marked by the presence of flakes of graphite in a matrix of ferrite (Fig. 4.2) pearlite or

austenite.





Fig. 4.2 Gray iron showing graphite flakes (black) and ferrite (white portion).

- Graphite flakes occupy about 10% of the metal volume.

- Length of flakes may vary from 0.05 mm to 0.1 mm.

- When fractured, a bar of Gray Cast Iron gives gray appearance.

- Gray Iron possesses lowest melting point of the ferrous alloys.

- Gray Cast Iron Processes high fluidity and hence it can be cast into complex shapes and thin

sections.

- It possesses machinability better than steel.

- It has high resistance to wear (including sliding wear).

- It possesses high vibration damping capacity.

- Gray iron has low ductility and low impact strength as compared with steel.

- Gray cast iron has a solidification range of 2400-2000°F.

- It has shrinkage of 1/8 inch/foot (1 mm/100 mm).

- It associates low cost combined with hardness and rigidity.

- Gray cast iron possesses high compressive strength.

- It has excellent casting qualities for producing simple and complex shapes.

Applications

- Machine tool structures (bed, frame and details).

- Gas or water pipes for underground purposes.

- Cylinder blocks and heads for I.C. Engines.

- Rolling mill and general machinery parts.

- Frames for electric motors.

- Manhole covers, Tunnel segment.





- Ingot moulds.

- Sanitary wares.

- Piston rings.

- Household appliances etc,

4.6 Malleable cast iron

Fig. 4.3 Ferritic malleable iron showing ferrite matrix and temper carbon (dark)

Characteristics

- Malleable CI is one which can be hammered and rolled to obtain different shapes.

- Malleable cast iron is obtained from hard and brittle white iron through a controlled

heat conversion process.

a) A ferritic malleable cast iron has Ferrite matrix.

b) A pearlitic malleable cast iron has Pearlite matrix.

c) An alloy malleable cast iron contains chromium and nickel and possesses high

strength and corrosion resistance.

- Malleable cast iron possesses high yield strength.

- It has high Young's modulus and low coefficient of thermal expansion.

- It possesses good wear resistance and vibration damping capacity.

- It can be used from - 60 to 1200°F.

- It has a solidification range of 2550--2065°F.

- It has shrinkage of 3/16 inch per foot (1.5 mm/100 mm).

- It has low to moderate cost.

- Malleable CI contains 2-3% C, 0.6-1.3% Si, 0.2-0.6% Mn, 0.15% P and 0.10% S.





Uses

- Automotive industry.

- Rail road.

- Agricultural implements.

- Electrical line hardware.

- Conveyor chain links.

- Gear case.

- Universal joint yoke.

- Rear axle banjo housing.

- Truck tandem axle assembly parts.

- Automotive crankshaft

- Crankshaft sprocket etc.

4.7 Nodular cast iron

Fig. 4.4 Nodular Cast Iron showing graphite nodules surrounded by ferrite (white)

Characteristics

- Unlike long flakes as in gray cast iron, graphite appears as rounded particles, or

nodules or spheroids in Nodular Cast Iron.

- The spheroidizing elements when added to melt eliminate sulphur and oxygen (from

the melt), which change solidification characteristics and possibly account for the

nodulization.

- Ductile cast iron possesses very good machinability.

- Soft annealed grades of Nodular cast iron can be turned at very high feeds and speeds.

- The properties of Nodular Cast Iron depend upon the metal composition and the

cooling rate.

- Nodular or Ductile Cast Iron contains 3.2-4.2% C, 1.1-3.5% Si, 0.3-0.8% Mn, 0.08%

P and 0.2% S.

- It possesses damping capacity intermediate between cast iron and steel.

- It possesses excellent castability and wear resistance.

Uses





- Paper industries machinery.

- Internal combustion engines.

- Power transmission equipment.

- Farm implements and tractors

- Earth moving machinery.

- Valves and fittings.

- Steel mill rolls and mill equipment,

pipes

- Pumps and compressors.

- Construction machinery.

4.8 White cast iron (mottled iron)

Fig. 4.5 White cast iron showing massive carbide areas (white) in fine pearlite (dark)

Characteristics

- White cast iron derives its name from the fact that its freshly broken surface shows a

bright white fracture.

- Unlike gray iron, white cast iron has almost all its carbon, chemically bonded with the

iron - as iron carbide, Fe3C. Iron carbide is a very hard and brittle constituent.

- Thus, white iron possesses excellent abrasive wear resistance.

- White iron under normal circumstances is brittle and not machinable.

- By using a fairly low silicon content, cast iron may be made to solidify as white iron.

- White iron castings can be made in sand moulds.

- White iron can also be made on the surface of a gray iron casting provided the

material is of special composition.

- If iron of proper composition is cooled rapidly, the free carbon will go in the

combined form and give rise to white iron casting.

- The white iron contains 1.8-3.6% C, 0.5-2.0% Si, 0.2-0.8% Mn, 0.18% P and 010% S

- The solidification range of white iron is 2550-2065°F.

- Shrinkage is 1/8 inch per foot (1 mm/100 mm).

- White iron (of a particular, chemical composition) is the first step in the production of





malleable iron castings.

Uses

- For producing malleable iron castings. For manufacturing those component parts

which require a hard and abrasion resistant material.

4.9 Plain carbon steel

Fig 4.6 Plain carbon steel (0.20% C) showing ferrite grains and grain boundaries

- Plain carbon steel is an alloy of iron and carbon and it is malleable.

- Carbon steels are different from (cast) iron as regards the percentage of carbon.

- It contain from 0.10 to 1.5% C whereas (cast) iron processes from 1.8 to 4.2% C.

- Carbon steels can be classified as Low carbon steel (or Mild steel), Medium carbon

steel and High carbon steel.

4.10 Mild steel

Mild steels or low carbon steels may be classified as follows:

(1) Dead mild steel - C 0.05 to 0.15%.

- It is used for making steel wire, sheets, rivets, screws, pipe, nail and chain.

- It has a tensile strength of 390 N/mm2 and a hardness of about 115 BHN.

(2) Mild steel containing - C 0.15 to 0.20%

- It has a tensile strength of 420 N/mm2 and hardness 125 BHN.

- It is used for making camshafts, sheets and strips for fan blades, welded tubing,

forgings, drag lines etc.

(3) Mild steel C 0.20 to 0.30%

- It has a tensile strength of 555 N/mm2 and a hardness of 140 BHN.

- It is used for making valves, gears, crankshafts, connecting rods, railway axles, fish





plates, small forgings etc.

4.11 Medium carbon steels

- Medium carbon steels contain carbon from 0.30 to 0.70%.

- Steels containing 0.35 to 0.45% carbons have a tensile strength of about 750 N/mm2.

They are used for making:

1. Connecting rods

2. Key stock

3. Wires and rods

4. Shift and brake levers

5. Spring clips

6. Axles

7. Gear shafts

8. Small and medium forgings, etc.

- Steels containing 0.45 to 0.55% carbon have a tensile strength of about 1000 N/mm2

and are used for making parts those are to be subjected to shock and heavy reversals

stress such as

1. Railway coach axles, Axles

2. Crank pins on heavy machines

3. Crankshafts, etc.

4. Spline shafts

- Steels containing 0.6 to 0.7% carbon have a tensile strength of 1230 N/mm2 and a

hardness of 400-450 BHN. Such steels are used for making

1. Drop forging dies

2. Set screws

3. Die blocks

4. Self tapping screws

5. Clutch discs

6. Valve springs

7. Plate punches

8. Cushion rings

9. Thrust washers etc.

4.12 High carbon steels

- High carbon steels contain carbon from 0.7 to 1.5%.

- Steels containing 0.7 to 0.8% carbon have a tensile strength of about 1400 N/mm2

and a hardness of 450-500 BHN. These steels are used for making:

Cold chisels

Drill bits

Wrenches

Jaws for vises

Wheels for railway

Wire for structural work

Shear blades

Automatic clutch discs

Hacksaws, etc.

- Steels containing 0.8 to 0.9% carbon have a tensile strength of about 660 N/mm2 and

a hardness of 500 to 600 BHN. Such steels arc used for making:





Rock drills

Railway rails

Clutch discs

Circular saws

Leaf springs

Machine chisels

Punch and dies

Music wires

- Steels containing 0.90 to 1.00% carbon (high carbon tool steels) have a tensile

strength of 580 N/mm2 and a hardness of 550-600 BHN. Such steels are used for

making:

Punch and dies

Seed discs

Springs

Pins

Shear blades

Keys etc.

- Steels containing 1.0 to 1.1% carbon are used for making:

Mandrels Machine tools Taps etc.


Taps Twist drills Thread dies,etc.


Files Reamers Cuting tool etc.

- SteeIs containing 1.3 to 1.5% carbon are used for making:

Wire drawing dies Metal cutting saw Tools for turning,

4.13 Alloy steels

- Steel is considered to be alloy steel when the maximum of the range given for the

content of alloying elements exceeds one or nu 11c of the following limits:

- Mn 1.65% - Si 0.60% - Cu 0.60%

- or in which a definite range or a definite maximum quantity of any of the following

elements is specified or required within the recongized field of constructional alloy

steels: Al, B, Cr, up to 3.99%, Co, Mo, Ni, Ti, W, V, or any other alloying element

added to obtain a desired alloying effect.

- Given below is the composition of a typical alloy steel

C 0.2-0.4%

Mn 0.5-1.0%

Si 0.3-0.6%

Ni 0.4-0.7%

Cr 0.4-0.6%

Mo 0.15-0.3%

Fe Balance

- Alloying elements alter the properties of steel (which is an alloy of iron and carbon)

and put it into a slightly different class from ordinary carbon steel.

- Advantages and Disadvantages of Alloy Steel.





- The important advantages and disadvantages in the choice of alloy steel from the

general point of view in relation to plain carbon steel are listed in the following

tabulation:

Advantages

(That May be Attained)

Disadvantages

(That May be Encountered)

- - Greater hardenability. - - Cost

- - Less distortion and cracking. - - Special handling

- - Greater stress relief at given hardness - - Tendency toward austenite retention

- - Higher elastic ratio and endurance

strength Less grain growth

- - Temper brittleness in certain grades

- - Lesser grain growths

- - Greater high temperature strength

- - Better machinability at high hardness

- - Greater ductility at high strength

Purpose of alloying

The purposes of alloying steels are:

1. Strengthening of the ferrite,

2. Improved corrosion resistance,

3. Better hardenability,

4. Grain size control,

5. Greater strength,

6. Improved machinability,

7. Improved temperature stability,

8. Improved ductility,

9. Improved toughness,

10. Better wear resistance,

11. Improved cutting ability,

12. Improved case hardening

properties

Effect of alloying elements

Carbon:

- Hardness

- Tensile strength

- Machinability

- Melting point.

Nickel:

- Increases toughness and resistance to impact

- Lessens distortion in quenching

- Lowers the critical temperatures of steel and widens the range of successful heat





treatment

- Strengthens steels

- Renders high-chromium iron alloys austenitic

- Does not unite with carbon.

Chromium:

- Joins with carbon to form chromium carbide, thus adds to depth hardenability with

improved resistance to abrasion and wear.

Silicon:

- Improves oxidation resistance

- Strengthens low alloy steels

- Acts as a deoxidizer.

Titanium:

- Prevents localized depletion of chromium in stainless steels during long heating

- Prevents formation of austenite in high chromium steels

- Reduces martensitic hardness and hardenability in medium chromium steels.

Molybdenum:

- Promotes hardenability of steel

- Makes steel fine grained

- Makes steel unusually tough at various hardness levels Counteracts tendency towards

temper brittleness

- Raises tensile and creep strength at high temperatures

- Enhances corrosion resistance in stainless steels

- Forms abrasion resisting particles.

Vanadium:

- Promotes fine grains in steel

- Increases hardenability (when dissolved)

- Imparts strength and toughness to heat-treated steel

- Causes marked secondary hardening

Tungsten:

- Increases hardness (and also red-hardness)

- Promotes fine grain

- Resists heat

- Promotes strength at elevated temperatures.





Manganese:

- Contributes markedly to strength and hardness (but to a lesser degree than carbon)

- Counteracts brittleness from sulphur

- Lowers both ductility and weldability if it is present in high percentage with high

carbon content in steel.

Copper: Copper (0.2 to 0.5%) added to steel

- Increases resistance to atmospheric corrosion

- Acts as a strengthening agent.

Aluminium:

- Acts as a deoxidizer

- Produces fine austenitic grain size

- If present in an amount of about 1%, it helps promoting nitriding.

Boron: Boron

- Increases hardenability or depth to which steel will harden when quenched.

Cobalt:

- Contributes to red-hardness by hardening ferrite

- Improves mechanical properties such as tensile strengths, fatigue strength and

hardness

- Refines the graphite and pearlite

- Is a mild stabilizer of carbides

- Improves heat resistance

- Retards the transformation of austenite and thus increases hardenability and freedom

from cracking and distortion.

Vanadium: Vanadium (0.15 to 0.5%)

- Is a powerful carbide former,

- Stabilizes cementite and improves the structure of the chill.

4.14 prominent alloy steels

Some of the popular alloy steels are:

A. Silicon steel

B. Silicon-Manganese steel

C. Nickel steel

D. Chrome-Nickel steel

E. Chrome-Vanadium steel

F. Molybdenum steel

G. Chrome-Molybdenum steel

H. Chrome steel

I. Manganese steel

J. Tungsten steel





K. Vanadium steel L. Cobalt steel.

(A) Silicon Steel

- Silicon steel contains C 0.10%, Mn 0.60% and Si 1.00%.

- Silicon imparts strength and fatigue resistance and improves electrical properties of

steel.

- Many bridges have been built of what is called Silicon Structural Steel. This is

stronger than carbon steel of equal ductility.

(B) Silicon-Manganese Steel

- Silicon-manganese steels contains C 0.40-0.55%, Si 0.04-1.8%, Mn 0.9-1.0%.

- Such steels are used for springs in the hardened and tempered condition (55 Si 2 Mn

90 steel), and for making punches and chisels.

(C) Nickel Steel

- Nickel steel contains C 0.35%, Ni 3.5%.

- Addition of nickel to structural steel results in an increase of strength, without a

proportionality great decrease of ductility.

- Nickel steels are used for storage cylinders for liquefied gases and for other low

temperature applications.

- Other uses of nickel steels are for heavy forgings, turbine blades, highly stressed

screws, bolts and nuts (40 Ni 3 steel).

(D) Chrome-Nickel Steel

- Chrome-nickel steel contains C 0.35%, Ni 1.25%, Cr 0.60%.

- Chrome-nickel steel will have, after heat treatment, almost the same strength and

ductility as 3.5% Nickel steel which has also been heat-treated, but it will not cost as

much.

- Chrome-nickel steels combine the effect of nickel (in increasing the toughness and

ductility) and chromium (in improving hardenability and wear resistance).

(E) Chrome-Vanadium Steel

- Chrome-vanadium steel contains C 0.26%, Cr 0.92%.

- Chromium and vanadium are added to low alloy steel to increase its hardenability and

to impart a grain structure that is finer than that of the standard chromium low-alloy

steels.

- Chrome-vanadium steel is used for making axles and shafts of automobiles, aero

planes and locomotives.





(F) Molybdenum Steel

- Molybdenum steel contains C 0.35%, Mo 0.76%.

- Molybdenum steel, when heat-treated, produces a structural steel which has increased

elastic limit without correspondingly decreased ductility. Molybdenum improves hot

hardness and strength of steel. Molybdenum steels are less affected by temper

brittleness.

- Molybdenum steels are used for making:

Aircraft landing gear

Coil and leaf springs

Pressure vessels

Transmission gear, etc.

(G) Chrome-Molybdenum Steel

- Chrome-molybdenum steel contains C 0.35%, Cr 1.06%, Mo 0.36%.

- Chrome-molybdenum steel has not as good a combination of strength and ductility as

nickel, chrome-nickel and chrome-vanadium steels, but it is quite easy to roll and

draw into tubes, to fabricate, and to weld, so that it is very popular for airplane

structural parts.

(H) Chrome Steel

- Chrome steel contains C 0.36%, Cr 0.57%.

- Chromium intensifies the effect of rapid cooling on steel. Therefore chromium is used

only in steels which are to be heat-treated.

- Chromium forms carbides and thus gives high hardness and good wear resistance. In

addition, chromium increases tensile strength and corrosion resistance of low alloy

steels.

- 8% of Cr steel use for Electrical purposes

- 15% of Cr steel use for Springs, ball and roller bearings.

(I) Manganese Steel

- Manganese low alloy steels contains Mn 1.6-1.9%, C 0.18-0.48%, Si 0.2-0.35%, S

and P < 0.040% each.

- Manganese increases hardness and tensile strength. A secondary effect is an increased

resistance to abrasion. The steel also withstands the shock test excellently.

- Manganese steels are used for making

Power shovel buckets

Grinding and crushing machinery

Railway tracks, etc.





(J) Tungsten Steel

- Tungsten low alloy steels are tool steels contains 2% tungsten, 1.70% chromium and

0.50% carbon.

- This is hard tough tool steel that is commonly used for making cutting tools.

- Tungsten forms carbides and prevents softening of the alloy at high temperatures.

- The tungsten steel may contain up to 1.5% tungsten. Tungsten steel is used for

making high speed cutting tools and permanent magnets.

(K) Vanadium Steel

- Vanadium is one of the most powerful scavengers that can be added to liquid steel for

the special purpose of removing oxygen. Vanadium has the effect of increasing the

strength and hardness of the metal. It produces a very small grain size.

- Except for castings, vanadium is seldom used as the sole alloying element in steel. It

is used together with chromium and other elements when high strength and anti-

fatigue properties are essential, as in springs, gears, shafts and heavy forgings.

(L) Cobalt Steel

- Cobalt tool steels are used where high frictional heats are developed. Cobalt imparts

additional red hardness to steel and cutting ability of tool is maintained at elevated

temperatures.

4.15 Tool steels

Tool and Die steels may be defined as special steels which have been developed to form, cut

or otherwise change the shape of a material into a finished or semi finished product.

Properties of tool steels

1. Slight change of form during hardening

2. Little risk of cracking during hardening

3. Good toughness

4. Good wear resistance

5. Very good machinability

6. A definite cooling rate during hardening

7. A definite hardening temperature

8. A good degree of through hardening

9. Resistance to decarburization

10. Resistance to softening on heating (red hardness).





Classification

The Joint Industry Conference (JIC), U.S.A. has classified tool steels as follows:

- T - W-High speed steel

- M - Mo-High speed steel

- D- High C, high Cr steel

- A - Air hardening steel

- O - Oil hardening steel

- W - Water hardening steel

- H - Hot work steel

- S - Shock resisting steel

As an example W8 means water hardening steel with 0.8% C.

4.16 stainless steels

When 11.5% or more chromium is added to iron, a fine film of chromium oxide forms

spontaneously on the surfaces exposed to air. The film acts as a barrier to retard further

oxidation, rust or corrosion. As this steel cannot be stained easily, it is called stainless steel.

All stainless steels can be grouped into three metallurgical classes, i.e.,

A. Austenitic B. Ferritic C. Martensitic

Based on their microstructures each of the classes has different welding requirements.

A. Austenitic stainless steels

- They possess austenitic structure at room temperature.

- They possess the highest corrosion resistance of all the stainless steels.

- They possess greatest strength and scale resistance at high temperatures.

- They retain ductility at temperatures approaching absolute zero.

- They are non-magnetic so that they can be easily identified with a magnet.

- They have the following composition

C 0.03-0.25%

Mn 2-10%

Si 1-2%

Cr 16-26%

Ni 3.5-22%

P and S Normal

Mo and Ti in some cases.

- They may find uses in

Dairy industry (milk cans)

Aircraft industry (engine parts)

Food processing (kettles, tanks)

Household items (cooking utensils)

Chemical processing (heat exchangers)

Transportation industry (Trailers and railways cars), etc.

B. Ferritic stainless steels

- They possess a microstructure which is primarily ferritic.

- Ferritic stainless steels have a low carbon-to-chromium ratio. This eliminates the





effects of thermal transformation and prevents hardening by heat treatment.

- These steels are magnetic and have good ductility.

- Such steels do not work harden to any appreciable degree.

- Ferritic steels are more corrosion resistant than martensitic steels.

- Ferritic steels develop their maximum softness, ductility and corrosion resistance in

the annealed condition.

- Ferritic stainless steels have the following chemical composition:

C 0.08-0.20%

Si 1%

Mn 1 to 1.5%

Cr 11-27%

- Ferritic stainless steels have the following uses:

Lining for petroleum industry

Heating elements for furnaces

Interior decorative work

Screws and fittings

Oil burner parts.

C. Martensitic stainless steels

- Martensitic stainless steels are identified by their martensitic microstructure in the

hardened condition.

- Because of the higher carbon-to-chromium ratio, martensitic stainless steels are the

only types hardenable by heat treatment.

- These steels are magnetic in all conditions and possess the best thermal conductivity

of the stainless types.

- Hardness, ductility and ability to hold an edge are characteristics of martensitic steels.

- Martensitic stainless steels can be cold worked without difficulty, especially with low

carbon content, can be machined satisfactorily, have good toughness, show good

corrosion resistance to weather and to some chemicals and are easily hot worked.

- Martensitic stainless steels have the following composition:

C 0.15-1.2%

Si 1%

Mn 1%

Cr 11.5 to 18%

- A few typical uses of martensitic stainless steels are as follows:

Pumps and valve parts

Rules and tapes

Turbine buckets

Surgical instruments, etc.

The structural condition known as austenite is favourable to the production of a tough and

ductile weld, capable of withstanding considerable stress without fracture. Hence, for

corrosion resistance and for a high degree of heat resistance Austenitic stainless steels are





used in welded assemblies in preference to Ferritic orMartensitic stainless steels. (Austenitic

stainless steels, except for the free-machining grades, are more weldable than the ferritic and

martensitic stainless steels).

4.17 high speed steel

- As named, the high speed steel removes metal (e.g., on a turning lathe) at much

higher speeds or rates than ordinary carbon steels.

- High speed steels are characterized by being heat-treatable to very high hardness

(usually Rockwell C64 or over) and of retaining their hardness and cutting ability at

temperatures as high as 540°C, thus permitting truly high-speed machining. Above

540°C, they rapidly soften and lose their cutting ability.

Properties

1. Excellent red hardness.

2. Good wear resistance.

3. Good shock resistance.

4. Fair machinability.

5. Good non-deforming property.

6. Poor resistance to decarburization.

Types

- Two main types of high speed steels are: (A)Tungsten base (B) Molybdenum base

- Both the types have similar important properties, but as compared to tungsten,

molybdenum being adequate and cheap, molybdenum high speed steel is more

common.

- Besides tungsten or molybdenum (or both) as the primary heat resisting additive,

there are other elements also present in the high speed steel. Carbon for high hardness,

chromium for ease of heat-treating, vanadium for grain refining and, in amounts over

1% for abrasion resistance, and sometimes cobalt for additional hardness and

resistance to heat softening.

Uses

1. For making all types of cutting tools such as drills, taps, reamers, milling cutters,

broaches, power-saw blades, lathe, shaper and planer tool bits etc.

2. In forming dies, drawing dies, inserted heading dies, knives, chisels, high temperature

bearings and pump parts.

4.18 Non Ferrous Material





Ferrous alloys are consumed in exceedingly large quantities because they have such a wide range of

mechanical properties, can be fabricated with relative ease and are economical to produce. However,

they possess some distinct limitations, chiefly

1. A relatively high density; therefore heavier in weight,

2. A comparatively low electrical conductivity, and

3. An inherent susceptibility to corrosion in some common on environments.

Thus, for many applications it is advantageous or even necessary to utilize other alloys e.g., non-

ferrous alloys, having more suitable property combinations. On occasion, a distinction is made (in

non-ferrous alloys) between cast and wrought alloys. Alloys that are so brittle that forming or

shaping by appreciable deformation is not possible ordinarily are cast; these are classified as cast

alloys. On the other hand, those that are amenable to mechanical deformation are termed wrought

alloys.

In addition, the heat treatability of an alloy system is mentioned frequently. Heat-treatable designates

an alloy whose mechanical strength is improved b y precipitation hardening etc.

Non-ferrous metals/alloys are not iron-based. The more common non-ferrous materials are the

following metallic elements and their alloys:

1. Copper

2. Aluminium

3. Magnesium

4. Lead

5. Nickel

6. Tin

7. Zinc

8. Cobalt etc.

4.19 Copper and Copper alloys

The main grades of raw copper used for cast copper base alloys are:

a) High conductivity copper (electrolytic) having not less than 99.9% Cu. The oxygen content

may be of the order 0.40%, Pb and Fe less than 0.005% each. Ag 0.002% and Bi less than

0.001%. Electrolytic copper is used for electrical purposes.

b) Deoxidized copper having not less than 99.85% Cu, less than 0.05% As, 0.03% Fe, and

0.003% Bi. Other elements may be of the order of 0.05% P, 0.01% Pb, 0.10% Ni, 0.003%

and 0.005% Ag and Sb respectively.

c) Arsenical deoxidized copper having; 0.4% At, 0.04% P and remaining copper. It is used for

welded vessels and tanks.

d) Arsenical touch pitch copper containing 0.4% As, 0.065% oxygen, 0.02% Pb, 0.15% Ni,





0.006% Ag, 0.01% Sb and less than 0.005% Bi, less than 0.020% Fe and remaining copper.

e) Oxygen free copper contains 0.005% Pb, 0.001% Ni, 0.001% Ag and less than 0.0005% and

0.001% Fe and Bi respectively. It is melted and cast in non-oxidising atmosphere.

Properties:

- Excellent resistance to corrosion.

- Non-magnetic properties.

- Easy to work, it is ductile a d malleable.

- Moderate to high hardness and strength.

- High thermal and electrical conductivity.

- It can be easily polished, plated and possesses a pleasing appearance.

- Resistance to fatigue, abrasion and corrosion.

- It can be soldered, brazed or welded.

- Very good machinability.

- Ease of forming alloys with other elements like Zn, Sri, Al, Pb, Si, Ni, etc.

Uses

- Copper is used for the Electrical parts, Heat exchangers, Screw machine products, for making

various copper alloys, such as brass and bronze, household utensils, etc.

Copper Alloys

- Copper alloys normally possess excellent corrosion resistance, electrical and thermal

conductivities and formability.

- Some copper alloys combine high strength and corrosion resistance, a combination desirable

for marine applications.

- Some copper alloys because of their wearing properties, high hardness or corrosion resistance

are used as surfacing metals.

- Some copper alloys are selected for decorative applications because of appearance.

- Elements such as aluminium, zinc, tin, beryllium, nickel, silicon, lead etc., form alloys with

copper.

- High copper alloys contain 96.0 to 99.3% copper.

- They possess enhanced mechanical properties due to the addition of small amounts of

alloying elements such as chromium, zirconium, beryllium and cadmium.

- A few typical high copper alloys are:





1. Cu, 1% Cd

2. Cu, 0.8% Cr

3. Cu, 0.12-0.30% Zr

4. Cu, 1.5-2.0% Be

- Such alloys are used for electrical and electronic components, as resistance welding

electrodes, wire conductors, diaphragms and pump parts.

4.20 Brass and Brass Alloys

- Brasses contain zinc as the principle alloying element.

- Brasses are subdivided into three groups;

1. Cu-Zn alloys,

2. Cu-Pb-Zn alloys or leaded brasses, and

3. Cu-Zn-Sn alloys or tin brasses.

- Brass has high resistance to corrosion and is easily machinable. It also acts as good bearing

material. Zinc in the brass increases ductility along with strength.

- Brass possesses greater strength than copper, however, it has a lower thermal and electrical

conductivity. Various types of brasses are discussed below:

(1) Gilding Metal

- The gilding metals cover a range from 5% to 15% Zn (balance Cu) and possess shades of

colour from the red of copper to a brassy yellow.

- They are supplied mainly in the form of sheet strip and wire for jewellery and many other

decorative purposes. Like copper, gilding metal is hardened and strengthened by cold work.

Gilding metal is used for making coins, medals, tokens, fuse caps etc.

(2) Cartridge Brass

- Cartridge brass normally contains 70% Cu and 30% Zn. In the fully annealed condition it has

a tensile strength of over 300 N/mm2.

- Greater % elongation and tensile strength make this brass satisfactory for cold deformation in

presses and by spinning or other means, and have led to its almost universal adoption for

cartridge and shell cases, W, well as for countless cupped articles like the caps of electric

lamp bulbs, door furniture etc.

- Cartridge brass work hardens when deformed in the cold, and must be annealed if many

successive operations are to be performed. An annealing temperature of about 600°C is

satisfactory in most cases.





(3) Admiralty brass

- Admiralty brass contains Cu 71%, Zn 28%, and Sri 1%.

- The small amount of tin added to brass improves its resistance to certain types of corrosion.

- Admiralty brass, though, it has been to a greater extent superseded by better materials for the

exacting conditions of marine condensers, it is still widely used for the tubes and other parts

of condensers coded by fresh water and for many other purposes. For such applications, the

modern alloy contains about 0.04% Arsenic, which improves resistance to a penetrative form

of corrosion known as dezincification.

(4) Aluminium brass

- Aluminium brass contains 76% Cu, 22% Zn and 2% Al; a little arsenic is added to inhibit

dezincification.

- In contact with sea water, a protective film builds up on the surface of this alloy in the early

stages of corrosion and prevents further attack. Moreover, if the film is damaged, by the

abrasive action of sand particles, for instance, it is self-healing.

(5) Basis brass

- Basis brass contains copper 61.5-64%, the remainder being zinc.

- Basis brass is used for press work where a relatively cheap material is required, and the main

commercial forms are sheet, strip and wire.

(6) Muntz metal

- Muntz metal or yellow metal contains 60% of copper and 40% of zinc and is essentially a hot

working material. It is manufactured in the form of hot rolled plate, and hot rolled rod or

extruded sections in a great variety of shapes and sizes. Yellow metal is frequently used as a

brazing alloy for steel.

- Other applications of muntz metal are as: Ship sheathing, Perforated metal, Valve stems,

Condenser tubes, Architectural work etc.

(7) Leaded 60:40 brass

- Leaded 60:40 brass is the chief material fed to automatic lathes and similar machines, usually

in the form of extruded bar.

- Lead is added to Cu-Zn alloy to promote machinability. The lead content ranges from about

0.5% to as much as 3.5%. Where machinability is the chief consideration, the lead content is

high, but alloys relatively low in lead are preferred for hot forging operations. Lead has no





marked effect on the tensile strength, which is approximately the same as that of straight 60 :

40 brass, though lead tends to impair the weldability, ductility and impact strength.

- Leaded brass is used for: Keys, Lock-parts, Gears, Clock parts, Valve parts, Pipe unions,

Items for automatic water heaters.

(8) Naval brass

- Naval brass contains Cu 60%, Zn 39.25% and Sn 0.75%. The purpose of tin is to improve the

resistance to corrosion.

- Naval brass is used for structural applications and for forgings, especially in cases where

contact with sea water is likely to induce corrosion.

- Naval brass is obtainable as hot-rolled plate particularly for marine condenser plates, and in

the form of extruded rod for the production of machined or hot forged components. Both sand

and die castings are also available.

- Other applications of naval brass are: Propeller shafts, Valve stems, Pump impellers etc.

(9) Admiralty brass

- Admiralty brass contains 71% Cu, 28% Zn, and 1% Sn.

- It is used for decorative and architectural applications, screw machine products, heat

exchanger components, pump impellers etc.

4.21 Bronze and Bronze Alloys

- Bronze is a broad term defining an alloy of copper and elements other than nickel or zinc.

- Bronze is basically an alloy of copper and tin. Bronze possesses superior mechanical properties

and corrosion resistance than brass. Bronze is comparatively hard and it resists surface wear.

- Bronze can be shaped or rolled into wire, rod and sheets. Types of bronzes are:

(1) Phosphor Bronze

- Phosphor Bronze containing approximately 4% each of tin, lead and zinc has excellent free-

cutting characteristics.

- The most important copper-tin alloys are those which have been deoxidized with phosphorus

during the refining process and hence are known as phosphor bronze.

- The amount of residual phosphorus may range from a trace to about 0.35% or even higher in

some special grades.

- The excess phosphorus, which exists in solid solution, materially increases the hardness and





strength of the alloy, but it does so at the expense of ductility.

- In amounts greater than 1.0% phosphorus causes excessive brittleness and impairs surface

appearance but affords a good bearing surface, as is evident by the use of high phosphorus

bronze compositions for gears and other machine parts subject to wear.

- Standard Phosphor bronze for bearing applications contains 90% Cu, 10% Sri (min), and

0.5% P (min). In sand cast condition it has a tensile strength of 220-280 N/mm2 and %

elongation 3 to 8%.

- Phosphor bronze for gears contains 88% Cu, 12% Sri, 0.3% (max) Zn, 0.50% (mw.) Pb and

0.15% (min) P. In sand cast condition, it has a tensile strength of 220-310 N/mm2 and 5-15%

elongation. This alloy is also utilised for general bearings, where its rigidity is of advantage.

- Leaded phosphor bronze contains 87% Cu, 7.5% Sn, 2.0% (max)Zn, 3.5% Pb, 0.3% (min) P

and 1.0% (max) Ni. In sand cast condition, it has a tensile strength of 190-250 N/mm2 and %

elongation is 3-12. This material is satisfactory for many bearing applications.

- In general phosphor bronze

Has high strength and toughness

Is resistant to corrosion

Has good load bearing capacity and

Has low coefficient of friction.

- Phosphor bronze is used for bearing applications, making pump parts, linings, springs,

diaphragms, gears, clutch discs, bellows etc.

(2) Aluminium bronzes

Aluminium bronzes have the following compositions:

Cu Al Fe Sn Mn(%)

89 7 3.5 0.35 -

91 6-8 1.5-3.5 - 1(max)

Aluminium bronzes possess the following properties:

- Good strength

- High corrosion resistance

- Good heat resistance

- Good cold working properties, etc

Aluminium bronze finds the following uses:

- Bearings

- Valve seats

- Gears

- Propellers

- Slide valves

- Cams

- Imitation jewellery

- Pump parts etc.

(3) Silicon bronze





Silicon bronzes have compositions:

- Si 1-4%

- Fe 0.5-1.0%

- Mn 0.25-1.25%

- Cu Balance

- Lead when added as 0.5% improves machinability.

- Silicon bronzes possess high strength and toughness as that of mild steel and corrosion resistance

as that of copper.

- Silicon bronze can be cast, rolled, forged and pressed hot or cold.

- Silicon bronzes find the following uses:

Bearings

Ways and gibs

Roll mill sleepers

Screw down nuts

Turntable bushings

Boiler parts

Marine hardware

Die cast parts etc.

4.22 Gun metal

- Gun metal is an alloy of copper, tin and zinc.

- Zinc cleans the metal and increases its fluidity.

- A small amount of lead may be added to improve castability and machinability.

- Admirality gun metal contains 10% Sri, 2% Zn, 1.5% max. Pb, 1.5% max Ni and balance Cu.

It has tensile strength of 260-340 N/mm2. It is widely used for pumps, valves and

miscellaneous castings and is also used for statuary.

- Nickel gun metal contains 7% Sri, 2.25% Zn, 0.3% Pb, 5.5% Ni and balance copper. When

sand cast and heat treated it has a tensile strength of 430-480 N/nun2.

- Leaded gun metal contains 5% Sri, 5% Zn, 5% Pb, 2.0% max Ni. When sand cast it has a

tensile strength of 200-270 N/mm2. This is among the most widely used grades, particularly

where pressure tightness is required.

- In general gun metal is used for Bearings, Steam pipe fittings, Marine castings, Hydraulic

valves, Gears, etc

4.23 Bearing Materials

Bearings support moving parts, such as shafts and spindles, of a machine or mechanism.

Bearings may be classed as

(1) Rolling contact (i.e., Ball and roller) bearings, (2) Plain bearings.





Copper-based alloys

- The term bronze covers a large number of copper alloys with varying percentages of Sri, Zn

and Pb.

- Typical compositions of bearing bronze are: Cu 80%, Sn 10%, Pb 10%

- Bronze is one of the oldest known bearing materials.

- Bronze (a) is easily worked; (b) has good corrosion resistance; (c) is reasonably hard.

- Tin bronze (10 to 14% tin remainder copper) is used in the machine and engine industry for

bearing bushes made from thin walled drawn tubes.

- Copper-based alloys are employed for making bearings required to heavier pressures such as

in railways.

4.24 Aluminium and Aluminium Alloys

Aluminium is a silvery white metal and it possesses the following characteristics:

- It is a light metal, with a density about a third that of steel or brass.

- Aluminium is a very good conductor of electricity. On a weight-for-weight basis aluminium

is a better conductor than copper.

- Aluminium has a higher resistance to corrosion than many other metals, owing to the

protection conferred by the thin but tenacious film of oxide which forms on its surface.

- Aluminium is a good conductor of heat.

- Aluminium is very ductile.

- Aluminium is non-magnetic.

- Melting point of pure aluminium is about 650°C and the fusion range of most of the

aluminium alloys varies between 520 and 650°C.

Aluminium Alloys

- Although pure aluminium is not particularly strong, it forms high strength alloys in

conjunction with other metals such as Cu, Cr, Ni, Fe, Zn, Mn, Si and Mg.

- Some of these aluminium alloys are more than 4 times as strong as the same weight of mild

steel.

- They are malleable and ductile.

- They exhibit toughness and become stronger at temperatures below the ordinary atmospheric

range.





- They do not work well at temperatures of the order of 300 to 400°C.

- Aluminium and its alloys can be Cast, Forged, Welded, Extruded, Rolled, etc.

Uses of Al and Al-alloys

The intensive demand for Al and its alloys arises chiefly from their attractive physical, mechanical

and chemical properties. Aluminium and its alloys are frequently used for

- Transportation industry-structural frame-work, engine parts, trim and decorative features,

hardware, doors, window frames, tanks, furnishing and fittings.

- Trains, trucks, buses, automobile cars and aeroplanes use many component parts made up of

aluminium alloys.

- Overhead conductors and heat exchanger parts.

- In food industry, aluminium alloys find applications as food preparation equipments,

refrigeration, storage containers, bakery equipment, shipping containers, etc.

- In architectural field, aluminium alloys find uses such as window frames, doors hardware,

roofing, coping sills, railings, fasteners, lighting fixture, solar shading, grills, etc.

- Cryogenic applications.

- As heavy duty structures such as dragline booms, travelling cranes, hoists, conveyor supports,

bridges, etc.

- In process industries, parts made up of aluminium and its alloys are used to handle organic

chemicals, petrochemicals and drugs. Tanks, drums, pipes, heat exchangers, gratings, smoke-

stacks, precipitators, centrifuges, valves, fittings, etc. are produced from aluminium alloys.

- Types of aluminium alloys are:

1. Al-Mn

2. Al-Mg

3. Al-Mg-Mn

4. Al-Mg-Si

5. Al-Cu-Mg

6. Al-Cu-Si

7. Al-Cu-Mg-Pb

8. Al-Mg-Si-Pb

9. Al-Zn-Mg-Cu

- Aluminium alloys can be classified as follows:

1. Wrought alloys

2. Cast alloys

3. Heat-treatable alloys

4. Non-heat-treatable alloys.

4.25 Magnesium and Magnesium Alloys

- Magnesium is a silvery white metal and has the lowest density of the common structural

materials.





- It has a specific gravity of 1.74 and weighs only about 1 oz/cu in (1.73 gm/cu cm).

- Aluminium weighs 1.5 times more, iron and steel 4 times more, and copper and nickel alloys

5 times more.

- Magnesium has a melting point of 650°C.

- Magnesium is not employed in its pure state for use for engineering purposes because it is not

sufficiently strong. Usually it is necessary to use considerable thickness or utilize deep

sections so as to obtain adequate stiffness.

- Magnesium corrodes badly under many conditions and therefore need to be painted or given

some surface finish to avoid corrosion.

- Magnesium being costly, finds applications where light weight is a very important

requirement, such as in aircraft industry.

Magnesium Alloys

Magnesium is alloyed with elements such as Al, Zn, Mn, Zr, etc., to make it useful for engineering

applications. Alloying increases strength and corrosion resistance.

Properties

- High strength-to-weight ratio.

- Good fatigue strength.

- Good dimensional stability.

- Good damping capacity.

- High thermal conductivity.

- Relatively high elect. Conductivity.

Uses

- For making parts such as airframes, engines, gear boxes, flooring, seating, etc., for

aeroplanes, helicopters, missiles and satellites.

- For parts such as engines, transmission pumps, differentials, floors and body panels of ground

transportation vehicles (trucks, etc.).

- For material handling equipments such as hand trucks, barrel skids, grain shovels, gravity

conveyors, foundry equipment, etc.

- For storage tanks and hoppers, ladders and scaffolds, electric drills, chain saws, power

hammers, etc.

- Moving parts of textile machines and printing equipment.

- Furniture, griddles, ladders and lawn movers.

- Typewriters, dictating machines, calculators, etc.

- Binocular and camera bodies.





- In the production of Uranium, beryllium, zirconium, titanium, etc.

Types of Magnesium Alloys

(1) Cast alloys of magnesium

Sr. No. % Composition (Balance Mg) Characteristics

1. Rare earth metal 3.0, Zn 2.5, Zr

0.6

Creep resistant upto 250°C, Excellent

castability. Pressure tight and weldable.

2. Al 8, Zn 0.5, Mn 0.3 General purpose alloy, Good founding

properties. Good ductility, strength and shock

resistance.

3. Al 8, Zn 0.5, Mn 0.3, Be 0.0015 General purpose pressure die casting alloy.

4. Al 7.5-9.5, Zn 0.3-1.5, Mn 0.15

min.

General purpose alloy with good average

properties.

(2) Dow metal

- It contains 90% magnesium, 10% Aluminium and a small addition of manganese.

- Dow metal finds applications in auto and aircraft industries.

- Dow metal is extremely light and can be welded and machined.

(3) Wrought Magnesium Alloys

Sr. No. % Composition (Balance Mg) Characteristics

1. Zn 3.0, Zr 0.6 High strength sheet, extrusion and forging

alloy. Weldable under good conditions.

2. Th 0.8, Zn 0.6, Zr 0.6 Creep resistant up to 350°C, fully weldable.

3. Al 8.5, Zn 0.5, Mn 0.12 Min High strength alloy for forgings of simple

design.

4. Al 6.0, Zn 1.0, Mn 0.3 General purpose alloy, gas and arc weldable.

4.26 lead and its alloys

Lead is the oldest of the commonly used metals and the softest of the heavy metals. When it is cast

or cut, it is a lustrous silvery colour to begin with. After standing for a time, however, the surface

turns a dull bluish grey due to oxidation. Lead is poisonous and should not be brought into contact

with food. Lead has a F.C.C. crystal structure.





Properties

- It has a low melting point of 327°C and density is 11.34 kg/dm3.

- It is very resistant to corrosion, against most acids, but not against HCl-HNO3 mixture.

- Its strength, hardness and elasticity are low, e.g., tensile strength 15 NNmm2, extensibility up

to 60%.

- It has low resistance to deformation but high formability, cold forming is preferred.

- Lead can be easily soldered, welded and cast. It can be spread over other metal surfaces.

- In addition lead has: Heavy weight, High density, Softness, Malleability, Lubricating

properties, High coefficient of expansion, Low electrical conductivity and poisnous.

Uses and applications of lead

- As an alloying element to improve the machinability of bronzes, brasses and free machining

steels.

- Tank linings for corrosion protection.

- Manufacture of storage batteries.

- Pipe and drainage fittings.

- Bearing metals.

- Lead compounds in paints.

- Lead sheathing of electric cable.

- Low melting solders.

- Terne plate (lead-tin coated), etc.

- Radiation protection (from x-rays).

Lead Alloys

- Lead alloys containing 8% to 10% Pb are used as bearing (antifriction) metals, in cable

sheaths, accumulation plates, etc.

- Antimony makes the alloy hard.

- Lead compounds include red lead and white lead.

- Lead glass (lead crystal glass) refracts light strongly.

Alloy Composition in % Uses

Lg Pb Sb 12

(Bearing hard lead)

Sb 10.5-13, Cu 0.3-1.5, Ni

0-0.3, As 0-1.5, Rest lead

In mechanical engineering, easily

soldered to steel, for lining

bearings.

Pb (Sb) Sb 0.2-0.3, Rest lead Drain pipes

Pb Sb As Sb 2.0-3.8, As 1.2-1.7, Rest

lead

Buckshot





4.27 Nickel and Nickel Alloys

- The element nickel (along with the elements Fe and Co) constitute the transition

group in the fourth series of the periodic table.

- It has an atomic number of 28, an atomic weight of 58.71, density 8.908 g/cu cm at

20°C and melting point of 1453°C.

- The normal crystallographic system of nickel is F.C.C. at all temperatures.

- The usual commercial grade of wrought nickel ('A' nickel) nominally contains 99.0%

nickel + up to 0.4% cobalt, which traditionally are combined and reported as 99.4%

nickel.

- Commercially pure nickel is almost as hard as low-carbon steel.

- Nickel work-hardens rapidly when cold worked.

- With suitable modifications in temperatures, tools, pressures, rates etc., wrought

nickel is amenable to most of the fabrication processes used for mild steel. It can be

forged, rolled, bent, extruded, sheared, punched, spun, deep drawn, machined, ground,

polished and buffed.

Properties of Nickel

- Is a hard lustrous white metal.

- Possesses good corrosion and oxidation resistance.

- Has high tensile strength and can be easily formed hot or cold.

- Can take up high polish.

- Can be fabricated using processes similar for mild steel.

- Is ferromagnetic at ordinary and low temperatures but becomes paramagnetic at

elevated temperatures.

- Melting point ... 1453°C

- Density, gm/cu cm, at 20°C ... 8.908

- Coefficient of Thermal expansion x 10-6/°C (25 toJ00°C) … 13.3

- Tensile strength 65000 - 115000 psi (From hot (4565 to 8075 kg/cm2) to cold drawn)

- Modules of Elasticity ... 30 x 106 psi (2.11 x 106 kg/cm2)

- Hardness RB 40 to 100 (depends on whether Ni is hot rolled, annealed or cold drawn)

Uses of Nickle

- For corrosion protection of iron and steel parts and Zn-base die castings used in the

automotive field.





- In the chemical, soap, caustic and allied industries for the construction of evaporators,

tanks, jacketed kettles, heating coils, tubular condensers and many other processing

types of equipment.

- As an alloying element in both ferrous and non-ferrous alloys. Nickel is a strong

austenite stabilizer and with chromium is used to form the important AISI 300 series

of non-magnetic austenitic stainless steels.

- As a coating for parts subjected to corrosion and wear. Therefore the second

important use of nickel is in electroplating.

- In the incandescent lamp and radio industries.

- In electronic (vacuum electronic tubes) and low-current electrical applications.

- As permanent magnets.

Nickel Alloys:

Various nickel alloys are:

A. Nickel-Iron alloys

B. Nickel-Copper alloys

C. Nickel-Copper-Zinc alloys

D. Nickel-Chromium alloys

E. Nickel-Molybdenum alloys

F. Super alloys

(A) Nickel-Iron alloys

- Nickel and iron form a series of alloys with thermal-expansion and magnetic

characteristics of commercial importance.

- Invar is the Trademark for an iron-nickel alloy containing 40-50% nickel and is

characterized by an extremely low coefficient of thermal expansion. Invar is used for

making precision instruments, measuring tapes, weights etc.

- The addition of 12% chromium, in lieu of some of the iron, produces an alloy

(Elinvar) with an invariable modulus of elasticity over a considerable temperature

range as well as a fairly low coefficient of expansion.

(B) Nickel-Copper alloys

1. The major nickel-based alloy with copper is Monel which nominally contains 66%

Ni, 31.5% Cu, 1.35% Fe, 0.90% Mn, plus residuals.

- Monel has a brighter appearance than nickel, is stronger and tougher than mild steel,

has excellent resistance to atmospheric and sea-water Corrosion and generally is more

resistant than nickel to acid, less resistant to alkalies and equally resistant to salts.

- Monel is used in architectural and marine applications where appearance and





corrosion resistance is important and in specialized equipment used by the food

pharmaceutical, paper, oil and chemical industries.

2. Several variations of monel have been produced to obtain an additional characteristic.

These include:

- An age-hardenable grade (K Monel),

- A free machining grade (R Monel),

- A hard-casting grade (H Monet),

- An age-hardenable casting grade (S Monel), etc.

3. Constantan, another alloy of nickel and copper contains 45% Ni and 55% Cu.

- Constantan has highest electrical resistivity, lowest temperature coefficient of

resistance, and highest thermal emf against platinum, of any of the copper-nickel

alloys.

- Uses in Electrical resistors, Thermocouples, Wheatstone bridges, etc.

(C) Ni-Cu-Zn alloys (Nickel-Silver)

- Nickel-copper-zinc alloys though known as nickel-silver, do not contain silver, and in

actuality they are brasses with sufficient nickel added to give a silvery white colour,

improved corrosion resistance and high strength.

- These alloys are used as low cost substitutes for silver in tableware and jewellery,

usually with silver or gold electroplate on the surface.

- Nickel silvers are also construction materials for many musical, drafting and scientific

instruments and are also used for marine and architectural applications.

(D) Nickel-Chromium alloys

- Nickel-chromium alloys with or without iron, form a series of corrosion and heat-

resistant materials.

- In this group the 80% Ni, 14% Cr, 6% Fe alloy (Inconel) with many modifications

resists progressive oxidation below 1093°C and is used in applications such as

furnace chambers, salt pots, aircraft exhaust manifolds, and high-temperature springs.

- It was originally developed for use in the milk industry as a corrosion-resistant alloy

and now is much used in many chemical industries because of its excellent corrosion

resistance.

- The 80% Ni, 20% Cr alloy (Chromel A, Nichrome V, Tophet A) and the 60% Ni,

16% Cr, 24% Fe alloy (Nichrome, Chromel C, Tophet C) form the bulk of materials

used for heater elements.





- The 90% Ni, 10% Cr (Chromel P) alloy in combination with alumel is much used as a

dependable base-metal thermocouple.

(E) Nickel-Molybdenum alloys

- Nickel-Molybdenum alloys such as Hastelloy A, Hastelloy C and Hastelloy D possess

very good resistance to corrosion.

Type of

Alloy

% Composition Properties and Uses

Ni Mo Fe Cr W

Hastelloy A 57 20 20 - - High strength and ductility. Can be forged and

rolled. Resists attack of acids. Used in chemical

industry for equipments such as storage tanks

and for material handling and transportation.

Hastelloy C 54 17 5 15 4 Can he cast and machined. Resistant to HNO3,

free chlorine and acid solutions of salts such as

cupric and ferric. Used in chemical industry for

pumps, valves, spray nozzles etc.

(F)Super alloys

- The super alloys have superlative combinations of properties.

- These materials are classified according to the predominant metal in the alloy, which

may be cobalt, nickel or iron.

- Other alloying elements include the refractory metals (Nb, Mo, W, Ta), chromium

and titanium.

- Super alloys are at least five times as strong as steels routinely used for making

bridges and large buildings. They can withstand enormous strains and exhibit

remarkable resistance to metal fatigue. They possess high impact strength and

superior strength-to-mass ratio. They are probably the toughest materials ever

produced.

- Super alloys are used in aircraft turbine components, which must withstand exposure

to severely oxidizing environments and high temperatures for reasonable time

periods.

- Mechanical integrity under these conditions is critical; in this regard, density is an

important consideration because centrifugal stresses are diminished in rotating





members when the density is reduced.

4.28 Tin and Tin Alloys

Tin is it non-toxic white, soft and pliable metal adaptable to cold working such as rolling,

extrusion and spinning. Tin melts at a low temperature (231.9°C), is highly fluid when

molten and has a high boiling point (2270°C), which facilitates its use as a coating for other

metals. It can be electrodeposited readily on all common metals.

Properties:

- Boiling point ... 2270°C

- Specific gravity - α-form (gray tin) ... 5.77, β -form (white tin) ... 7.29

- Coefficient of linear expansion, 0°C ... 19.9 x 106

- Solidification shrinkage, % ... 2.8

- BHN 10 kg/5m, 180 sec - 20°C ... 3.9, 220°C ... 0.7

- Tensile strength, as cast 200°C 0.65 x 103 psi (45.65 kg/cm2)

15°C 2.1 x 103 psi (147.4 kg/cm2)

- 40°C 2.9 x 103 psi (204 kg/cm2)

- Tin has good resistance to acid corrosion. Therefore, it is used as a coating on steel

containers for food.

- Tin alloys with Cu, Sb, Bi, Cd and Ag and its hardness increases.

- Tin is usually sold as ingots; however, it can be obtained in a number of forms such as

granulated, mossy, fine powder, sheet, foil and wire.

Tin Alloys

(1) Soft Solders 20 to 70% tin and remaining lead.

Symbols Composition in % by weight

and melting point range

Uses

L-Pb Sn 35 Sb containing

antimony

Sn 34.5-35.5, Sb 0.5-2.0, Rest

Pb 186°C-225°C

Solder for copper, tin, zinc,

used with soldering iron or

flame

L-Sn 50 Ph (Sb) Tin-lead

solder low antimony

Sn 49.5-50.5, Sb 0.12-0.5,

Rest Pb 183°C-215°C

Tinning, fine soldering,

electrical engineering

L-Sn 60 Pb Ag Sn 59.5-60.5, Ag 3.0-4.0, Rest

Pb 178°C -180°C

Electronics, precision

mechanics, production of





electrical appliances

- The eutectic alloy, with 63% tin is used extensively in electrical industry.

- Alloy with 50% tin is general-purpose solder.

- Lower tin solders (15-20% tin) are used as dipping solders for sealing automotive

radiator cores.

- Bi, Cd, Pb and tin alloys are used as fuses for fire-extinguishing equipment, boiler

plugs, etc.

(2) Copper-tin alloys (bronzes)

- Phosphor bronze containing 8-12% tin with small additions of phosphorus is used for

springs, condenser tubes, bearings, bushings and diaphragms.

- The gun metals, tin-bronze casting alloys with 1 to 6% Zn are used for valves and

fittings for water and steam lines.

- Babbitt bearing metals are essentially tin-base alloys.

(3) Aluminium-tin alloys

- The 6% tin-aluminium alloys can be bonded to steel by rolling.

- Low tin-aluminium alloys possess high fatigue strength and thus can carry fluctuating

loads.

(4) Pewter,

- It’s tarnish resistant alloy containing 7% antimony and 2% copper, is easy to cast and

to work into complicated shapes by spinning and hammering and is available as

plates, trays, bowls, pitchers, etc.

(5) Alloys of titanium

- Contains 2.5% Sn and 5% Al (or 13% Sn with 2.75% Al) find use in aircraft industry.

4.29 zinc and zinc base alloys

- Zinc is a blue to gray metallic element.

Characteristics:

1. Relatively low melting point, 419.5°C (die-casting).

2. Good resistance to atmospheric corrosion.

3. Solubility in copper (brass).

4. Inherent ductility and malleability.

5. Volume coef. of Ther Exp. 20-400°C ... 8.9 x 10-5/°C.

6. Thermal conductivity at 18°C ... 0.27 cal/sec/cm/°C





- Zn is produced commercially as slab, Anode, strip, wire, Powder and rod.

Applications:

- Stampings

- Die castings

- Anodes for electro-galvanising

- Coating on steel (to protect it)

- Making different alloys

- Fabricated (and rolled) shapes.

- Shells for dry batteries.

- Building materials (as for

flashings, roofing, gutters, etc.)

- Engravers' plates.

- Wire for metallizing.

- Lithographers' sheets, etc.

Typical chemical composition (%):

Pb Fe Cd Cu (Pb + Cd +

Fe)

Zn

Rolled zinc 0.05 to

0.12 max

0.012

Max

0.005

Max

0.65 to

1.25

- Remainder

High grade Slab zinc 0.07 0.02 0.07 - < 0.10 Remainder

Selected grade slab zinc 0.80 0.04 0.75 - < 1.25 Remainder

Zinc Alloys

Zinc alloys can be fabricated by Drawing, Bending, Extrusion, Spinning, Rolling, Soldering.

Zn makes alloys with:

1. Al

2. Pb

3. Cu

4. Cd

5. Mg

6. Sn

7. Fe, and

8. Ni

- Pb stimulates intercrystalline corrosion in Zn alloy die castings.

- Cd exerts an important hardening effect and raises the recrystallization temperature.

Zn containing Cd can absorb considerable amounts of work hardness during cold

rolling or drawing.

- Fe, some iron is desirable but excessive amounts are harmful to properties.

- Cu, Cu-Zn alloys are more ductile and easier to roll. Copper enters into solid solution

in zinc up to approximately 1%.

- Mg, when added in the presence of copper, increases resistance to creep.





- Al-Zn alloys containing Al in uncontrolled compositions are unstable when rolled.

4.30 Cobalt and Cobalt Alloys

Cobalt is a silvery-white metal with a faint bluish tinge, closely resembling nickel in

appearance and mechanical properties. Its chemical properties resemble, in part, those of both

nickel and iron. Below 421°C, cobalt is H.C.P.; above, it is F.C.C. structure.

Cobalt Alloys

%, Composition Application

Cr 19-27, Ni 0-32, W 0-15, M 0-5.5 Rest Cobalt High temperature alloy

Cr 26-33, W 5-14, C 1-2.5, Rest Cobalt Hard facing alloys (stellites)

Cr 30-32, Mo 1.3-3.5, C 0.05-2, Rest Cobalt Wear resistant alloys

Co 49-50, V 0.2, Rest Fe High permeability

Ni 18 -29, Co 17-18, Rest Fe Glass-metal seals (Kovar wrought)

Co 64, Cr 30, Mo 5 Dental Prosthesis and osteosynthesis

- Cobalt alloys can be Cast, Forged, Extruded, Rolled, Swaged, drawn, Welded, brazed,

Shaped by powder metallurgy.

Properties:

- The desirable high-temperature properties of the first alloy – high stress rupture,

creep, thermal shock resistance and resistance to carburization-may be the result of

the allotropic change of cobalt from a close-packed hexagonal at room temperature to

a face-centered cubic lattice at high temperatures.

- The stellite alloys are immune to all ordinary corroding media and are highly resistant

to many corrosive acids and chemicals, stellites combine high reflectivity,

permanence of finish and resistance to abrasion.

- Cobalt's high curie temperature (1121°C) imparts high damping characteristics useful

for alloys subjected to vibe ation, such as in high pressure, high-temperature steam

turbines.

- The interesting engineering property of cobalt-containing permanent, soft and

constant permeability magnets are a result of the electronic configuration of the metal

and its high curie temperature.

- In cutting alloys and high speed steel, cobalt seems to contribute to low coefficients of

friction and to maintaining red hardness.

- The Co-Cr-base alloys for dental and surgical applications are not attacked by the





body fluids and hence do not set up an electromotive force in the body to cause

irritation of the tissue; thus the Vitallium alloys are also used as bone replacements.

They are ductile enough to permit anchoring of dentures on neighboring teeth.

- Cobalt reduces the hardenability of steel. When dissolved in ferrite, cobalt provides

resistance to softening at elevated temperatures.

Applications

- High temperature alloys.

- Permanent magnets.

- Hardfacing purposes.

- Searchlight reflectors

- Dies and cutting tools.

- Nuclear reactors.

- Wear resistant components in nuclear submarines.

- Rocked and motor cases for the space age.

- Components of jet aircraft engines such as gas turbines, superchargers, turbojet

nozzles and vanes etc.

References






Questionnaire

1. What is cast iron? Give composition, typical properties and uses of any three of the following

cast irons: (i) Grey cast iron (ii) White iron (iii) Malleable iron (iv) S.G. iron

2. Distinguish between plain carbon steel and alloy steel.

3. Give composition, important properties and uses of: Mild steel, medium carbon steel and high

carbon steel.

4. Give effects of following elements on cast iron: (i) Silicon (ii) Sulphur (iii) Phosphorus (iv)

Manganese (v) carbon.

5. Describe the effects of the following alloying elements on the properties of steel: (i)

Chromium (ii) Cobalt (iii) Manganese (iv) Nickel (v) Phosphorus.

6. What are the needs for alloying steels?

7. Give composition typical properties and uses of: stainless steel and high speed steel.

8. Write short notes on any three of the following: Heat resisting steels and Nickel steels

9. Give properties and applications of copper as an engineering material.

10. Describe important properties and uses of aluminium.

11. Discuss important properties and uses of any two of the following: (i) Nickel (ii) Tin

(iii) Zinc.

12. Give composition properties and uses of any three of the following: (i) brass (ii)

Muntz metal (iii) Manganese bronze (iv) Phosphor bronze.

13. (b) Distinguish between the Brasses and bronzes.

14. Discuss composition, properties and uses of any three of the following alloys: (i) Gun-

metal (ii) Phosphor bronze (iii) Aluminium bronze (iv) Nickel silver.

15. Briefly discuss the effects of the following as alloying elements on the properties of

aluminum: (i) Copper (ii) Silicon (iii) Magnesium (iv) Manganese.

16. Describe briefly any five heavy non-ferrous metals.

17. Write short notes on any Duralium.

18. What are the important requisite qualities of a satisfactory bearing alloy?

19. Write a short note on Nickel alloys.

20. Write short notes on Cobalt base alloys

Iron Carbon System





Objective

To understand the concept of iron carbon diagram and TTT curve.

6.1 Introduction

- Alloys of the Iron-carbon system include steel and cast iron.

- Alloys with carbon content up to 2% are known as Steels whereas those having

carbon above 2% are called Cast-Irons.

- Alloys of the Iron-carbon system are of the most vital important to modern industry

due to their extensive, versatile applications.

- The Iron-carbon system provides the most prominent example of heat treatment and

property alteration based on polymorphic transformation and eutectoid

decomposition.

- Because of its outstanding commercial importance, the Iron-carbon system has been

studied in more detail than most all systems.

- The primary constituent of Iron-carbon system is the metal Iron.

6.2 Iron, Allotropy

- Iron is a relatively soft and ductile metal. Iron has a melting point of 1539°C.

- Iron is allotropic metal, which means that it exists in more than one type of lattice

structure (e.g., B.C.C. /F.C.C.) depending upon temperature.

Iron Carbon System




Fig. 6.1 cooling curve for pure iron

In- its normal room temperature state, iron is B.C.C. in lattice arrangement, whereas at 908°C

it changes to F.C.C. and then at 1403°C back to B.C.C. again and vice versa.

One another change occurs at about 770°C (called the Curie point) at which the room

temperature magnetic properties of iron disappear and it becomes non-magnetic.

The iron remains non-magnetic until the temperature drops back below the Curie point upon

which its magnetic properties reappear.

- Fig. shows a cooling curve for pure iron with allotropic forms of iron marked over it.

- Iron is molten above 1539°C. It solidifies in the B.C.C. δ (delta) form.

On further cooling at 1400°C, a phase change occurs and the atoms rearrange themselves into

the γ (Gamma) form which is F.C.C. and nonmagnetic.

On still further cooling at 910°C, another phase change occurs from F.C.C. non-magnetic y

iron to B.C.C. non-magnetic α (alpha) iron.

Finally at 768°C, the α-iron (B.C.C.) becomes magnetic without a change in lattice structure.

6.3 Micro-Constituents of Iron and Steel

- When steel is heated above the austenitic temperature (Refer Fig. 6.4) and is allowed

to cool under different conditions (e.g., different rates of cooling), the austenite in

steel transforms into a variety of micro constituents discussed below.

- The study of these microconstituents is essential in order to understand Fe-C

equilibrium diagram and T.T.T. diagrams.

- Various micro-constituents are:

A. Austenite

B. Ferrite

C. Cementite

D. Ledeburite

E. Pearlite

F. Bainite

G. Martensite

H. Troostite

I. Sorbite

(A) Austenite

- Austenite is the solid solution of carbon and/or other alloying elements (e.g., Mn, Ni,

etc.) in gamma iron.

- Carbon is in interstitial solid solution whereas Mn, Ni, Cr, etc., are in substitutional

solid solution with iron.

- Austenite can dissolve maximum 2% C at 2066°F, the left hand corner of Fig. 6.4

- Austenite has:

Tensile strength 10500 kg/cm2

Iron Carbon System




Elongation 10% in 50 mm

Hardness Rockwell C 40 (Approx)

Fig. 6.2 Microstructure of austenite at room temperature

- Austenite is normally not stable at room temperature. Under certain conditions,

however, it is possible to obtain austenite room temperature (as in austenite stainless

steels).

- Austenite is non-magnetic and soft.

(B) Ferrite

- Ferrite is B.C.C. iron phase with very limited solubility for carbon.

- The maximum solubility is 0.025% carbon at 1.333°F at extreme left hand corner of

Fig. 6.4, and it (i.e., ferrite) dissolves only 0.008% carbon at room temperature.

- Ferrite is the softest structure that appears on the Fe-C equilibrium diagram.

- Ferrite has:

Tensile strength 2800 kg/cm2 (Approx)


Hardness less than Rockwell C 0 or Rockwell B 90

Fig. 6.3 Microstructure of ferrite at room temperature

(C) Cementite

Iron Carbon System




- Cementite or iron carbide, chemical formula Fe3C, contains 6.67% carbon by weight.

- It is a typical hard and brittle interstitial compound of low tensile strength (approx.

350 kg/cm2) but high compressive strength.

- Cementite is the hardest structure that appears on the iron-carbon equilibrium

diagram. Its crystal structure is orthorhombic.

(D) Ledeburite.

- Ledeburite is the eutectic mixture of austenite and cementite. It contains 4.3% carbon.

It is formed at about 1130°C (2065°F).

(E) Pearlite

- The pearlite microconstituent consists of alternate lamellae of ferrite and cementite.

- Pearlite is the product of austenite decomposition by an eutectoid reaction. Thus, pearlite

is an eutectoid mixture containing about 0.8% carbon and is formed at 1333°F (723°C),

point C in Fig. 6.4.

- Pearlite is shown in Fig. 40.5; the white ferrite back-ground or matrix which makes up

most of the eutectoid mixture contains thin plates of cementite (black).

- Pearlite has


Hardness Rockwell C 20

(F) Bainite

- Bainite is the constituent produced in a steel when austenite transforms at a

temperature below that at which pearlite is produced and above that at which

martensite is formed. Bainite is produced by Austempering.

- Thus bainite is a decomposition product of austenite, consisting of an aggregate of

ferrite and carbide.

- Bainite forms on isothermal transformation at temperatures below the nose of TTT

diagram (Refer Fig. 6.10)

- Bainite is an isothermal transformation product and cannot be produced by continuous

cooling.

- If bainite is formed in the upper part of the temperature range, its appearance is

feathery and it is called Feathery Bainite; it is known as Acicular Bainite. if it is

formed in the lower part of the temperature range. Acicular bainite resembles

tempered martensite because it has somewhat needle like structure.

(G) Martensite

Iron Carbon System




- Martensite is a metastable phase of steel, formed by transformation of austenite below

the MS temperature (refer Fig. 40.10).

- Martensite is an interstitial supersaturated solid solution carbon in a-iron and has a

body-centered-tetragonal lattice.

- Martensite is considered to be highly stressed a-iron which is supersaturated with

carbon.

- Martensite forms as a result of shear-type transformation with virtually no diffusion.

- Martensite, normally, is a product of quenching.

- Martensite possesses an acicular or needle-like structure, Fig. 6.13

(H) Troostite

- Troostite (Nodular) is a mixture of radial lamellae of ferrite and cementite and

therefore differs from pearlite only in the degree of fineness and carbon content which

is the same as that in the austenite from which it is formed

- In steel heat treatment, the troostite, i.e., the microstructure, consisting of ferrite and

finely divided cementite is produced on tempering martensite below approximately

450°C.

- The constituent also known as troostite pearlite is produced by the decomposition of

austenite when cooled at a rate slower than that which will yield a martensitic

structure and faster than that which will produce a sorbitic structure.

(I) Sorbite

- Sorbite is the microstructure consisting of ferrite and finely divided cementite,

produced on tempering martensite, above approximately 450°C.

- The constituent also known as Sorbitic Pearlite, is produced by the decomposition of

austenite when cooled at a rate slower than that which will yield a troostitic structure

and faster than that which will produce a pearlitic structure.

Difference between Pearlite, Sorbite and Troostite

Pearlite, sorbite and troostite are all ferrite-cementite mixtures having a lamellar structure and

distinguishable from each other in eutectoid steel only by their degrees of dispersion.

The lower the decomposition temperature (higher degree of super cooling), the more

dispersed the ferrite-cementite mixture will be.

Pearlite is obtained at low degrees of supercooling. Sorbite, a finer mixture, is obtained at

higher degrees of supercooling.

Iron Carbon System




At subcritical temperatures in the region of 500 to 550°C, troostite, .in even more dispersed

mixture, is obtained. Under an optical microscope, troostite is observed as a dark mass

because the ferrite and cementite particles cannot be resolved. Thus, unlike pearlite, troostite

is difficult to differentiate. However, structure of troostite is sufficiently clearly revealed

under an electron microscope.

6. 4 Iron-Carbon Equilibrium Diagram

Introduction

- An equilibrium, phase or constitutional diagram is a graphic representation of the

effects of temperature and composition upon the phases present in an alloy.

- An equilibrium diagram is constructed by plotting temperature along they-axis and

percentage composition of the alloy along the x-axis. This diagram shows ranges of

temperatures and compositions within which the various phase changes are stable and

also the boundaries at which the phase changes occur.

Iron Carbon System




Fig. 6.4 Iron carbon equilibrium diagram

- Iron-carbon equilibrium diagram (refer Fig. 6.4) indicates the phase changes that

occur during heating and cooling and the nature and amount of the structural

components that exist at any temperature. Besides, it establishes a correlation between

the microstructure and properties of steel and cast irons and provides a basis for the

understanding of the principles of heat-treatment.

- An iron-carbon equilibrium diagram forms a basis for differentiating among iron

(0.008% C or less), hypoeutectoid steels (0.008 to 0.8%C), hypereutectoid steels (0.8

to 2.0% C), hypoeutectic cast irons (2 to 4.3% C) and hypereutectic cast irons (above

4.3% carbon).

- The iron carbon equilibrium diagram has a peritectic (point J) an eutectic (point C)

and an eutectoid (point S).

- Peritectic reaction equation may be written as

Iron Carbon System




Delta (δ) + liquid

Austenite

- The horizontal line at 2720°F shows the peritectic reaction.

- The eutectic reaction takes place at 2066°F and its equation may be written as

Lliquid

Austenite + cementite [Eutectic Mixture (ledeburite)]

- Eutectic point is at 4.3% carbon. Eutectic mixture is not usually seen in the

microstructure, because austenite is not stable at room temperature and must undergo

another reaction during cooling.

- The eutectoid reaction is represented by the horizontal line of 1333°F and (point) S

marks the eutectoid point. The eutectoid equation may be written as

Solid

Ferrite + cementite [Eutectoid Mixture (pearlite)]

6.5 Transformation in Different Steel Structure

Transformation which takes place in the structures of steels containing 0.4%, 0.83% and

1.2% carbon respectively (refer Fig. 6.4) when heated to a temperature high enough to make

them austenitic and then allowed to cool slowly (under equilibrium conditions), have been

explained below.

(1) Steel containing 0.4% C

Steel containing 0.4% carbon is a hypoeutectoid steel and is completely austenite aboveA3,

i.e., upper critical temperature line. As it is cooled below A3 line the iron begins to change

from F.C.C. to B.C.C. As a result, small crystals of body centered cubic (B.C.C.) iron begin

to separate out from the austenite (F.C.C.).

The B.C.C. crystals retain a small amount of carbon (less than (1.03%) and are referred as

crystals of ferrite.

As the cooling proceeds, ferrite crystals grow in size at expense of austenite.

By the time the steel has reached Al line, i.e., 1333°F (called lower critical temperature) it is

composed of approximately half ferrite and half austenite. At this stage the austenite contains

0.83% carbon and since austenite can hold no more than 0.83% carbon in solid solution at

1333°F (or 723°C) thus as the temperature drops further, carbon begins to precipitate as

cementite.

This cementite and still separating ferrite form alternate layers until all the remaining

austenite is consumed. The lamellae structure, i.e., eutectoid of ferrite and cementite contains

0.83% carbon and is known as Pearlite (refer Fig. 40.5).

Iron Carbon System




All hypoeutectoid steels when cooled from austenite state will tra form into ferrite and

pearlite in the same way as explained above.


Consider the transformation of a eutectoid steel containing 0. 83% carbon. It will remain

austenite up to the point S. The transformation will begin and end at the same temperature,

i.e., 1333°F (or 723°C). Since eutectoid steel contains 0.83% carbon initially, it follows that

the final transformed structure will be completely pearlite (see Fig. 6.5).

Fig. 6.5 Typical appearance of lamellar pearlite containing 0.83% carbon by weight, the

eutectoid composition


Consider the transformation of hypereutectoid steel containing 1.2% carbon.

As the temperature drops and steel crosses Acm (i.e., upper critical temperature) line at point

d and moves towards e, the excess carbon above the amount required to saturate austenite

(i.e. 0.83%) is precipitated as cementite primarily along the grain boundaries (Fig. 6.4).

Thus above 1333°F, i.e., lower critical temperature line, the structure insists of austenite and

cementite.

As the temperature drops below 1333°F, the austenite has become less rich in carbon

(because of cementite precipitation), it contains only 0.83% carbon and it transforms to

pearlite as it does so in the cases of hypoeutectoid and eutectoid steels explained earlier.

The structure of hypereutectoid steel at room temperature consists of cementite and pearlite

(Fig. 6.4).

6.6 Transformation in Cast Iron

So far the discussions were only with regard to structures produced in steels by slow cooling

from austenite under equilibrium conditions.

Iron Carbon System




In normal foundry practice, the rate of cooling is slightly faster and as a result more cementite

plates are nucleated and individual lamellae of pearlite become thinner and the structure is

called fine pearlite.

If castings are cooled at still faster rate to prevent transformation of austenite above

(approximately) 600°F, nartensite forms on further continuous cooling. Martensite is a hard,

strong and brittle constituent. It is a super-saturated solution of carbon in ferrite and the

presence of excess carbon distorts the normally cubic ferrite to a body-centered tetragonal

structure which is produced by a shear mechanism and is strained.

Transformations which take place in the structure of a cast iron containing 3% carbon is

shown in Fig. 6.4 and explained as under:

Case (a):

Cast iron containing 3% carbon is when cooled under rapid rate as a thin section of a sand

casting, from a temperature of about ?500°F, it begins to solidify with the formation of grains

of austenite. Austenite continues to solidify until the cast iron reaches the temperature of

2066°F. At this stage the alloy consists of 50% austenite and 50% liquid of eutectic

composition (austenite and cementite, i.e., ledeburite).

As the alloy cools below solidus, i.e., 2066°F, lledeburite (a form of eutectic consisting of

spheres of austenite embedded in cementite) freezes and cementite precipitates from austenite

because of the decreasing solubility of carbon in the austenite. This occurs between 2066 and

1333°F.

Cooling of the alloy below 1333°F involves the transformation of remaining austenite of

eutectoid composition (i.e., 0.83% C) to pearlite as explained earlier for steels.

Thus, the structure of alloy at room temperature consists of cementite, pearlite and

transformed ledeburite.

Cast iron of any composition between 2.0 to 4.3% carbon will solidify in exactly the same

way as has been described above.

Case (b):

If the above very cast iron is cooled at a slow rate, as usual, austenite will first form from the

melt (i.e., liquid) but eutectic freezing being slow, products of eutectic reaction will be

austenite and graphite [Fig. 6.6 (a)]. This is between 2066 and 1333°F.

As cooling continues, austenite gets depleted in carbon content and graphite flakes grow. At

1333°F, remaining austenite transforms to pearlite and the structure of the alloy at room

temperature looks as shown in Fig. 6.6 (b). It is pearlitic gray cast iron.

Iron Carbon System




(a) (b)

Fig. 6.6 Microstructures of pearlitic gray cast iron

Case (c):

Phase changes in the same alloy when cooled at a very slow rate will be similar to case (b)

above, except that, at the eutectoid, (i.e., 1333°F) cooling will be sufficiently slow to permit

graphite to precipitate rather than pearlite. Although no new graphite flakes form, the one

present, grow in size. Instead of pearlite as in case (b), the matrix of the alloy solidified in

this case is ferrite and graphite flakes are embedded in it.

6.7 Effect of Alloying Elements on Founding and other Properties of Steels

- An alloying element is one which is added to a metal to effect changes in properties

and which remains within the metal.

- Common alloying elements which are added to steel for the purpose are C, Ni, Mo,V

W, Mn, Cu, Bo, Al, Co, Si, Ti, Cr, etc.

Carbon:

- Hardness

- Tensile strength

- Machinability

- Melting point

Nickel:

- Increases toughness and resistance to impact and Strengthens steels

- Lessens distortion in quenching

- Lowers the critical temperatures of steel and widens the range of successful heat

treatment

- Renders high-chromium iron alloys austenitic

- Does not unite with carbon.

Chromium:

Iron Carbon System




- Joins with carbon to form chromium carbide, thus adds to depth hardenability with

improved resistance to abrasion and wear

- Helps preventing corrosion and oxidation

- Adds some strength at high temperatures.

Molybdenum:

- Promotes hardenability of steel

- Makes steel fine grained

- Makes steel unusually tough at various hardness levels

- Counteracts tendency towards temper brittleness

- Raises tensile and creep strength at high temperatures

- Enhances corrosion resistance in stainless steels

- Forms abrasion resisting particles.

Vanadium:

- Promotes fine grains in steel

- Increases hardenability (when dissolved)

- Imparts strength and toughness to heat-treated steel.

- Resists tempering and causes marked secondary hardening.

Tungsten:

- Increases hardness (and also red-hardness)

- Promotes fine grain

- Resists heat

- Promotes strength at elevated temperatures.

Manganese:

- Contributes markedly to strength and hardness (but to a lesser degree than carbon)

- Counteracts brittleness from sulphur

- Lowers both ductility and weldability if it is present in high percentage with high

carbon content in steel.

Aluminium:


- Produces fine austenite grain size

- If present in an amount of about 1%, it helps promoting nitriding.

- Cobalt:

- Contributes to red-hardness by hardening ferrite.

Iron Carbon System




Copper: Copper (0.2 to 0.5%) added to steel

- increases resistance to atmospheric corrosion

- Acts as a strengthening agent.

Boron:

- Increases hardenability or depth to which steel will harden when quenched.

Silicon:

- Improves oxidation resistance

- Strengthens low alloy steels

- Acts as a deoxidizer.

Titanium:

- Prevents localized depletion of chromium in stainless steels during long heating

- Prevents formation of austenite in high chromium steels

- Reduces martensitic hardness and hardenability in medium chromium steels.

6.8 Effect of alloying elements on founding and other properties of ast iron

- Alloying elements are added to cast iron for attaining special properties such as

resistance to corrosion, heat and wear and to improve mechanical properties.

- Many alloying elements in cast iron will accelerate or retard graphitization and this is

one of the important reasons for alloying.

The Host common alloying elements in cast iron are, Cr, Cu, Mo, Ni, V, Mg, Mn and Zr.

Chromium: Chromium (0.15 to 1.0%) is used for

- Hardness

- Chilling power, i.e., depth of chill

- Improvement of wear resistance

- Strength

- Resistance to heat

- Chromium reduces and refines the graphite. However, chromium decreases

machinability.

Copper: Copper (0.5 to 2.0%)

- Toughens the matrix

- Increases the fluidity

- Tends to break up massive cementite and strengthens matrix

- Assists in control of chill depth

Iron Carbon System




- Tends to increase and refine the graphite.

Molybdenum:

- In conjunction with Ni, Cu and Cr, Mo is used to produce high strength cast irons.

Molybdenum (0.3 to 1.0%)

- Improves mechanical properties like tensile strength, fatigue strength and hardness.


- Is a mild stabilizer of carbides

- Improves heat resistance

- Retards the transformation of austenite and thus increases hardenability and freedom

from cracking and distortion.

Vanadium: Vanadium (0.15 to 0.5%)

- Is a powerful carbide former

- Stabilizes cementite and improves the structure of the chill

- Reduces graphitization

- Improves tensile strength, transverse strength and hardness

- Adds resistance to wear and heat.

Nickel: Nickel (0.1 to 3.0%)

- Stabilizes austenite

- Refines pearlite and graphite

- Is a mild graphitizer

- Improves toughness and density of castings

- Minimizes extremes of hardness between light and heavy sections.

Magnesium: Magnesium (0.04 to 0.08%)

- Produces graphite in spheroidal form in as cast alloys

- Increases ductility.

Manganese: Manganese (0.3 to 1.25%)

- Stabilizes austenite



- Refines grains

- Increases fluidity and density in castings.

Zirconium: Zirconium (0.10 to 0.3%)

- Assists formation of graphite

Iron Carbon System




- Deoxidizes and improves the fluidity and density of castings

- Reduces hardness.

6.9 The Pearlite Transformation

- The microstructure Pearlite is very important in iron and steel technology, because it

may be formed in almost all steels by means of suitable heat treatments.

- Pearlite is a specific mixture of two phases formed by transforming austenite of

eutectoid composition to ferrite and cementite.

- When viewed in white light under a microscope, pearlite is readily recognised by its

lustrous appearance and its structure of alternate plates of ferrite and cementite.

- The transformation of austenite to pearlite starts by formation of cementite crystal

nuclei at austenite grain boundaries.

- Carbon diffuses from the surrounding austenite to the cementite and the growth of

(cementite) carbide begins. As carbon diffuses, t adjacent austenite is depleted in

carbon and transforms to ferrite [ref Fig. 6.7 (a) and (b)].

- With formation of ferrite, there is rejection of carbon from t ferrite region, (because

ferrite can dissolve much less carbon than austenite) i.e., effective enrichment of the

adjacent austenite. Th form the additional nuclei of cementite [Fig. 6.7 (c)].

- Because of the alternate formation of cementite and ferrite, cementite can only grow

away from the boundary of original austenite grain as a platelet, somewhat like the

filling in a sandwich [Fig. 6.7 (d)].

(a) (b) (c) (d)

Fig. 6.7 Pearlite-formation by nucleation and growth

- Nucleation and growth of alternate plates of cementite and ferrite occur at several

points along the austenite grain boundaries [Fig. 40.8 (a)]. This forms pearlite

colonies which are approximately hemispherical regions of alternate plates of

cementite and ferrite.

- Usually several such colonies form within any one austenite grain.

Iron Carbon System




- These pearlite colonies grow until the entire austenite grain has been consumed and

has become a pearlite nodule.

- The process of pearlite formation is sometimes referred to as Side-Wise nucleation

and Edgewise growth [Fig. 6.8 (b)].

(a) (b)

Fig. 6.8 (a) Pearlite colonies (b) Edgewise growth

6.10 T.T.T. Diagram

- T.T.T. (Time-Temperature-Transformation) diagram is also known as S-Curve, C-

Curve, Bain's Curve or Isothermal Transformation diagram.

- A T.T.T. diagram shows the relationship between temperature and time (Fig. 40.12)

taken for a decomposition transformation to take place in a metal when the

transformation is isothermal, i.e., transformation is allowed to occur at constant

temperature.

- T.T.T. diagram is used more particularly in the assessment of de-Composition of

austenite in heat treatable steel.

Difference from Fe-C Equilibrium Diagram (and Importance of TTT Diagram)

- Many heat treatments of steels involve reaction conditions so far removed from

equilibrium that Fe-C equilibrium diagram is of only limited use in the study of steels

cooled under non-equilibrium conditions.

- The iron-carbon equilibrium diagram shows only the phases and the resulting

microstructures corresponding to equilibrium conditions.

The usefulness of Fe-C diagram is restricted to fixing the austenitizing temperature and

predicting the phases that are eventually obtained at a given composition (C%age) and

temperature.

- Many metallurgists realized that time and temperature of austenite transformation had

a profound influence on the transformation products and the subsequent properties of

Iron Carbon System




the steel. For example, the microstructure and properties of (quenched) steel are

dependent upon the rate of cooling which prevails during quenching.

As the cooling rate increases, the experimentally observed transformation temperatures are

lowered and metastable (non-equilibrium) phases may be formed. For example, at very high

rates of cooling in the Neel, a metastable phase called rnartensite can develop which, of

course, h,a.s no place in the Fe-C equilibrium diagram.

The principal source of information on the actual process of austenite decomposition under

non-equilibrium conditions is the TTT Diagram, which relates the transfonnation of austenite

to the time and temperature conditions to which it is subjected.

Steps to Construct a T.T.T. Diagram

1. Obtain a large number of relatively small specimens (cut from the same bar).

2. Place the samples in a molten salt bath held at the proper austenitizing temperature, [refer

Fig. 6.9]. For 1080 (eutectoid) steel, this temperature is approximately 1425°F. Specimens

are kept in the molten salt bath for long enough to form complete austenite

Fig. 6.9 Steps in processing specimens

3. When austenitized, the samples are quickly transferred to another molten salt bath held at the

desired reaction temperature below A1, l or example at 1300°F.

4. After a given specimen has been allowed to react isothermally for a certain time, it is

quenched in cold water or iced brine.

The first specimen may be allowed to react isothermally for 2 secs., second specimen for 4

secs., third for 8 secs., fourth for 15 secs., and so on up to say 15 hours (Fig. 6.9)

The more the time is given to a specimen to react isothermally the more pearlite is formed

(Fig. 6.10).

5. As the specimen is quenched in water, this stops the isothermal reaction (or heat treatment)

by causing the remaining (untransformed), austenite to change almost instantly to martensite.

Iron Carbon System




In microstructures shown in Fig. 6.10, both pearlite and martensite (white portion) can be

seen. Pearlite is the result of isothermal heat treatment and its amount depends upon the time

permitted for isothermal reaction to continue. Martensite is the result of water quenching of

the specimen after the isothermal heat treatment.

6. When a large number of specimens isothermally reacted for varying time periods are

metallographically examined, the result is the Reaction Curve as shown in Fig. 6.10

Fig. 6.10 Reaction curve found by metallographic examine of processed specimens

7. When the data obtained from a series of isothermal reaction curves over the whole

temperature range of austenite instability for a given composition of steel is summarized, the

result is TTT diagram for that steel.

Iron Carbon System




\Fig. 6.11 TTT diagram for the decomposition of austenite in eutectoid (0.8% C) steel

- Austenite is stable above A1 temperature line, and below this line, austenite is

unstable, i.e., it can transform into pearlite, bainite or martensite.

- In addition to the variations in the rate of transformation with temperature, there are

variations in the structure of the transformation products also.

- Transformations at temperatures between approximately 1300°F and 1020°F (550°C)

result in the characteristics lamellar microstructure of pearlite. At a temperature just

belowAI line, nucleation of cementite from austenite will be very slow, but diffusion

and growth of nuclei will proceed at maximum speed, so that there will be few large

lamellae and the pearlite will be coarse.

However, as the transformation temperature is lowered i.e., it is just above the nose of the C-

curve, the pearlite becomes fine.

- At temperatures between 1020°F and 465°F (the approximate, Ms temperature line),

transformation becomes more sluggish as the temperature falls, for, although austenite

becomes increasingly unstable, the slower rate of diffusion of carbon atoms in

austenite at lower temperatures outstrips the increased urge of the austenite to

transform. In this temperature range the transformation product is bainite.

Bainite consists, (like pearlite) of a ferrite matrix in which particle of cementite are

embedded. The individual particles are much finer than in pearlite.

Iron Carbon System




The appearance of bainite may vary between feathery mass [Fig. 40. 16 (c)] of fine cementite

and ferrite for bainite formed around 900°F; and dark acicular (needle-shaped) crystals [Fig.

40.16 (d)] for bainite formed in the region of around 600°F.

At the foot of the TTT diagram, there are two lines Ms, (240°C or 465°F) and Mf (-50°C).

Ms, represents the temperature at which the formation of martensite will start and MJ, the

temperature at which the formation of martensite will finish during cooling of austenite

through this range. Mf is a fairly low temperature.

- Martensite is formed by the diffusionless transformation of austenite on rapid cooling

to a temperature below 465°F (approximately) designated as Ms Temperature.

The martensitic transformation differs from the other transformations in that it is not time

dependent and occurs almost instantaneously, the proportion of austenite transformed to

martensite depends only on the temperature to which temperature to which it is cooled. For

example the approximate temperature transforms to martensite are 330°F and 240°F

respectively.

Fig. 6.13 shows the effect of cooling rate on the formation of different reaction products e.g.,

pearlite, bainite and martensite

Fig. 6.12 cooling curves

Iron Carbon System




Fig. 6.13 effect of cooling rate on formation of different reaction product

Cooling Curve-a:

- Very slow cooling rate, typical of conventional annealing. Transformation product is

coarse pearlite with low hardness.

Cooling Curve-b:

- Transformation will start at 3 with the formation of coarse pearlite and finish at 4,

with the formation of medium pearlite. Since there is a greater temperature difference

between point 3 and 4 than there is between 1 and 2, the structure will show a greater

variation in the fineness of pearlite and a smaller proportion of coarse pearlite as

compared to that of curve-a.

- Curve-b involves a faster cooling rate than curve a (annealing) and may be considered

typical of normalizing.

Cooling Curve-c:

- This curve is typical of a slow oil quench and the microstructure will be a mixture of

medium and fine pearlite.

Cooling Curve-d:

- This curve is typical of an intermediate cooling rate and austenite will start to

transform (at point 5) to fine pearlite. As Ms Line is crossed, the remaining austenite

will transform to martensite. The final structure at room temperature will thus consist

of martensite and fine perlite.

Cooling Curve-e:

- This curve is typical of a drastic quench, the substance remains austenitic until the MS

line is reached, and changes to martensite between the Al, and Mf lines.

Cooling Curve-ef:

Iron Carbon System




- It is possible to form 100% perlite or 100% martensite by continuous cooling, but it is

not possible to form 100% Bainite.

- Cooling Curve-ef obtains a bainitic structure by cooling rapidly enough to miss the

nose of curve and then holding in the temperature range at which bainite is formed

until transformation is complete.

Cooling Curve-g:

- This curve is tangent to the nose of TTT curve. The cooling rate associated with

curve-g is approximate critical doling rate (CCR) for this steel.

Any cooling rate equal to or faster than CCR (e.g., cooling rate-e) will form only martensite

and any cooling rate slower than CCR (e.g., cooling rates a, b and c) will form some softer

transformation products such as pearlite or bainite.

6.11 critical cooling rate

Concept

Refer Fig. 6.13, T.T.T, diagram, where critical cooling rate has been marked as "CCR",

Critical Cooling rate is that slowest rate of cooling at which, all the austenite is transformed

into 100% martensite.

Importance

- Critical cooling rate is important when hardening the steels (having proper carbon

content). In order to obtain fully martensitic (a very hard) structure, the cooling rate

must be more than the critical cooling rate.

- It is actually the cooling rate (refer Fig. 40.13 T.T.T. diagram) that determines the

type of structure which will be obtained on cooling. Slow cooling will produce a

pearlitic structure and higher cooling rate can form a martensitic structure.

- The final properties of the steel component thus cooled will depend upon, therfore, on

the magnitude of cooling rate.

Factors Affecting

Various factors affecting the critical cooling rate are:

1. Composition of steel

The critical cooling rate varies with the carbon content of the steel (Fig. 6.14). The critical

cooling rate is minimum when carbon is around 0.9%.

Iron Carbon System




Fig. 6.14 CCR versus carbon content of steel

2. Temperature of hardening

The higher is the quenching (hardening) temperature, the more uniform becomes the

austenite, the lower is the critical cooling time and more will be the critical cooling rate (Fig.

6.15).

Fig. 6.15 hardening temperature versus time

3. Purity of steel

The purer is the steel, the lesser will be the critical cooling rate for quenching.

6.12 The Bainite Transformation

- Bainite is an isothermal transformation product that forms at temperature below the nose

of TTT diagram(refer to cooling curve-ef ), it cannot be produced by continuous cooling.

- Bainite is an intimate mixture of ferrite and cementite, as is pearlite. Whereas pearlite has

alternate plates of ferrite and cementite, in bainite cementite apparently exists as tiny

spheroids or stringers [Fig. 6.16 (a)] uniformly distributed throughout the ferrite matrix.

- Fig. 6.16 (a) and (b) show respectively the upper (feathery) bainite formed at

temperatures just below the nose of the TTT diagram and the lower (acicular) (needlelike)

bainite formed at temperatures approaching Ms.

Iron Carbon System




- Between about 1000° and 750°F, initial nuclei are ferrites which are coherent with the

austenite matrix. Cementite then precipitates from the carbon-enriched layer of austenite,

allowing further growth of the ferrite [Fig. 6.16 (a)].

- The carbides tend to lie parallel to the long axis of the bainite needle to form the typical

open feathery structure of upper bainite [Fig. 6.16 (a)].

- Below about 750°F, coherent ferrite, supersaturated with carbon, forms first and is then

followed by the precipitation of carbide within the ferrite needle, transversely at an angle

of 55°.

- A proportion of the carbide is Fe24C and the ferrite contains a little dissolved carbon. This

is lower bainite in which cementite is too fine f rr resolution and the structure or pattern is

acicular (needlelike) [refer Fig. 6.16 (d)].

- Due to differences in size, shape and distribution of cementite, bainite is normally harder,

stronger and tougher than fine pearlite of the same chemical composition.

- Moreover, whereas pearlite is initiated by cementite precipitation, hainite is initiated by

ferrite precipiiation.

- It is thought that bainite forms initially as supersaturated ferrite by a lattice shearing

process (somewhat similar to martensite).

- It is agreed that diffusion-controlled processes are involved in bainite formation.

(a) Upper Bainite (Feathery) Refer Fig. (c) also

Iron Carbon System




(b) Lower Bainite (Acicular) Refer Fig. (d) also

(c) Upper Bainite for eutectoid steel partially reacted at 540°C.

(d) Lower Bainite in eutectoid steel formed upon reaction at temp. near the martensite range

Fig. 6.16 (a) & (c) Formation of upper bainite (b) & (d) Formation of lower bainite

6.13 The Martensite Transformation

- Martensite is a metastable phase of steel, formed by transformation of austenite below Ms

temperature

- Martensite is an interstitial supersaturated solid solution of carbon in iron having a body-

centered-tetragonal lattice (Fig. 6.18).

- Martensite is normally a product of quenching.

Iron Carbon System




- During most phase transformations, diffusion occurs to produce a difference in

concentration between the two (or more) phases involved. However, at low temperatures,

where diffusion occurs slowly, greatly super cooled solids can transform by the

martensitic mechanism, in which diffusion may be completely absent. In this type of

transformation the second phase has the same composition as the matrix, but it forms a

different crystal structure by shearing action within the matrix.

- Martensite is the result of a shear-type transformation with virtually no diffusion.

- Martensite is characterized by an acicular or needlelike pattern (Fig. 6.17).

Fig. 6.17 Martersite (Needle-like pattern)

- Since martensite has a non-cubic structure and since carbon is still present in the lattice,

slip does not occur readily and therefore martensite is very hard, strong and brittle.

- The carbon content of martensite may be the same as of the original austenite.

- Under relatively slow cooling, austenite transforms to ferrite, cementite and/or pearlite;

but a very rapid cooling rate is essential to transform austenite into martensite.

- The cooling rate must exceed a critical value depending on (steel) composition and

metallurgical history.

- The austenite-martensite transformation starts at temperature Ms and continues as the

temperature decreases to Mf, at which transformation reaction ceases even though

retained austenite may still exist.

- Thus, martensite forms in the range Ms - Mf only with decreasing temperature.

Iron Carbon System




- Ms and Mf temperatures are influenced by the carbon content steel and both decrease

with increasing carbon content. Ms and Mf temperatures do not depend upon the cooling

rate.

- One may visualise the transformation of austenite to martensite as a transition stage in

the change from the F.C.C. structure of austenite to B.C.C. structure of ferrite.

- The low temperature at which martensite forms greatly decreases the chance for

carbon atoms to diffuse out of the lattice; hence they remain in solution in the

transition lattice. The excessive supersaturation of carbon distorts the normal BCC

structure to body-centered tetragonal(Fig. 6.18).

Fig. 6.18 Crystal lattice of martensite

- Since martensite formation from austenite is accompanied by an increase in specific

value (about 3%); this may be a solid reason why large stresses are set up in

hardening that distort the article being hardened and develop cracks.

- The lattice distortion in martensite is reflected in mechanic properties of high tensile

strength and hardness and low ductility.

- In fact, martensite being brittle, by itself, has limited application; the major interest

lies in the desirable combinations of properties which can be obtained by heat treating

(i.e., tempering) this (martensitic) structure.

- Formation of martensite is similar to slip and twinning since it is a shear

transformation, but differs since there is a change in crystal structure. Martensite

transformation is diffusionless, the sheared region remains coherently joined to the

matrix and shear propagates at about the speed of sound through the crystal.

- There are a number of important characteristics of martensite transformation such as:

1. The martensite transformation is diffusionless and there is change in chemical

composition.

Iron Carbon System




2. The transformation proceeds only during cooling and ceases if cooling is interrupted.

Thus martensite transformation depends only upon the decrease in temperature and is

independent of time.

The amount of martensite formed with decreasing temperature is not linear. The number of

martensite needles produced at first is small, then the number increases and finally near the

end, it decreases again (Fig. 6.19).

Actually, martensite forms from austenite as individual platelets (needles) as temperature

decreases. Each platelet is formed in a short time interval (perhaps less than a micro-second).

Additional transformation on continued cooling is by formation of additional plates rather

than by growth of existing plates.

3. Martensite transformation of a given alloy cannot be suppressed, nor can the ms

temperature be changed by varying the cooling rate.

4. Martensite is probably never in a condition of real equilibrium, although it may persist

indefinitely at or near room temperature.

5. Martensite transformation associates high rates of nucleation and crystal growth at

low temperatures.

Fig. 6.19 percent of martensite formed as a function of temperature

- The mechanism of martensite transformation is perhaps nucleation and shear, which

implies a progressive shear wave rather than a homogeneous shear.

- It is believed that the strain energy of misfit between the martensite formed and the

untransformed austenite matrix (between Ms and Mf temperatures) is minimized by

formation of platelets (as observed) upon which a non-homogeneous shear is

imposed. This shear cancels the macroscopic changes in dimensions in the plane of

the plate.

Iron Carbon System




- The martensite transformation, for many years, was believed to be unique for steel.

However, in recent years, martensite transformation has been found in a number of

other alloy systems such as Fe-Ni, Cu-Zn, and Cu-Al.

- The martensite transformation is therefore recognized as a basic type of reaction in

the solid state and the term martensite is no longer confined only to the metallurgy of

steel.

References


Questionnaire

1. Explain any five of the following terms: (I) Ferrite (ii) Pearlite (iii) Cementite (iv)

Austenite (v) Ledeburite (vi) Martensite (vii) Troostite (viii) Bainite (ix) Sorbite (x)

Spheroidite.

2. Draw simplified iron-carbon diagram and briefly explain it. Interpret various

transformations when a liquid solution containing 0.5%C in iron is cooled to room

temperature.

3. Explain the Pearlite transformation.

4. Explain importance of TTT Diagram?

5. Describe various steps to construct a TTT diagram.

6. Explain bainaite transformation

7. Explain martensite transformation.

Non Destructive Tests





Objective

To understand the procedure of Non Destructive tests.

6.1 Introduction

A nondestructive test is an examination of an object in any manner which will not impair the

future usefulness of the object. Although in most cases nondestructive tests do not provide a

direct measurement of mechanical properties, they are very valuable in locating material

defects that could impair the performance of a machine member when placed in service. Such

a test is used to detect faulty material before it is formed or machined into component parts,

to detect faulty components before assembly, to measure the thickness of metal or other

materials, to determine level of liquid or solid contents in opaque containers, to identify and

sort materials, and to discover defects that may have developed during processing or use.

Parts may also be examined in service, permitting their removal before failure occurs.

Fig. 6.1 Typical S-N (stress-cycle) diagrams

Nondestructive tests are used to make products more reliable, safe, and economical.

Increased reliability improves the public image of the manufacturer, which leads to greater

sales and profits. In addition, manufacturers use these tests to improve and control

manufacturing processes.

There are five basic elements in any nondestructive test.

1. Source: A source which provides some probing medium, namely, a medium that can

be used to inspect the item under test.

2. Modification: This probing medium must change or be modified as a result of the

variations or discontinuities within the object being tested.

3. Detection: A detector capable of determining the changes in the probing medium.





4. Indication: A means of indicating or recording the signals from detector.

5. Interpretation: A method of interpreting these indications.

While there are a large number of proven nondestructive tests in use. This section will

concentrate on the most common methods and on one recent development.

The most common methods of nondestructive testing or inspection are:

Radiography

Magnetic-particle inspection

Fluorescent-penetrant inspection

Ultrasonic inspection

Eddy current inspection

6.2 Radiography of metals

The radiography of metals may be carried out by using x-rays or gamma rays-short-

wavelength electromagnetic rays capable of going through relatively large thicknesses of

metal. Gamma rays may be obtained from a naturally radioactive material such as radium or a

radioactive isotope such as cobalt-60. Gamma radiation is more penetrating than that of x-

rays, but the inferior sensitivity limits its application. There is no way that the source may be

regulated for contrast or variable thickness, and it usually requires much longer exposure

times than the x-ray method.

Fig. 6.2 Schematic representation of the use of x-rays

X-rays are produced when matter is bombarded by a rapidly moving stream of electrons.

When electrons are suddenly stopped by matter, a part of their kinetic energy is converted to

energy of radiation, or x-rays.

The essential conditions for the generation of x-rays are

1. A filament (cathode) to provide the source of electrons proceeding toward the target,





2. A target (anode) located in the path of electrons,

3. A means of regulating tube current to control the no. of electrons striking the target.

4. A voltage difference between the cathode and anode which will regulate the velocity

of the electrons striking the target and thus regulate the wavelength of x-rays

produced.

The first two requirements are usually incorporated in an x-ray tube. The use of x-rays for the

examination of a welded plate is shown schematically in Fig. X-rays are potentially

dangerous, and adequate safeguards must be employed to protect operating personnel.

A radiograph is a shadow picture of a material more or less transparent to radiation. The x-

rays darken the film so that regions of lower density which readily permit penetration appear

dark on the negative as compared with regions of higher density which absorb more of the

radiation. Thus a hole or crack appears as a darker area, whereas copper inclusions in

aluminum alloy appear as lighter areas.

While the radiography of metals has been used primarily for the inspection of castings and

welded products, it may also be used to measure the thickness of materials. Fig. 10.3 shows a

simple radiation thickness gauge

Fig. 6.3 a simple radiation thickness gauge

The radiation from the source is influenced by the material being tested. As the thickness

increases, the radiation intensity reaching the detector decreases. If the response of the





detector is calibrated for known thicknesses, the detector reading can be used to indicate the-

thickness of the inspected material. With a suitable feedback circuit, the detector may be used

to control the thickness between predetermined limits.

6.3 Magnetic-Particle Inspection (Magnaflux)

This is a method of detecting the presence of cracks, laps, tears, seams, inclusions, and

similar discontinuities in ferromagnetic materials such as iron and steel. The method will

detect surface discontinuities too fine to be seen by the naked eye and will also detect

discontinuities which lie slightly below the surface. It is not applicable to nonmagnetic

materials.

Magnetic-particle inspection may be carried out in several ways. The piece to be inspected

may be magnetized and then covered with fine magnetic particles (iron powder). This is

known as the residual method. Or, the magnetization and application of the particles may

occur simultaneously. This is known as the continuous method. The magnetic particles may

be held in suspension in a liquid that is flushed over the piece, or the piece may be immersed

in the suspension (wet method). In some applications, the particles, in the form of a fine

powder, are dusted over the surface of the workpiece (dry method). The presence of a

discontinuity is shown by the formation and adherence of a particle pattern on the surface of

the workpiece over the discontinuity. This pattern is called an indication and assumes the

approximate shape of the surface projection of the discontinuity. The Magnaglo method

developed by the Magnaflux Corporation is a variation of the Magnaflux test. The suspension

flowed over the magnetized workpiece contains fluorescent magnetic particles. The

workpiece is then viewed under black light, which makes the indications stand out more

clearly.

Fig. 10.4 Principle of the Magnaflux test





When the discontinuity is open to the surface, the magnetic field leaks out to the surface and

forms small north and south poles that attract the magnetic particles (see Fig. 10.4). When

fine discontinuities are under the surface, some part of the field may still be deflected to the

surface, but the leakage is less and fewer particles are attracted, so that the indication

obtained is much weaker. If the discontinuity is far below the surface, no leakage of the field

will be obtained and consequently no indication. Proper use of magnetizing methods is

necessary to ensure that the magnetic field set up will be perpendicular to the discontinuity

and give the clearest indication.

Fig. 6.5 illustrating two kinds of magnetization:

(a) Longitudinal magnetization; (b) circular magnetization.

As shown in Fig. 10.5 for longitudinal magnetization, the magnetic field may be produced in

a direction parallel to the long axis of the workpiece by placing the piece in a coil excited by

an electric current so that the long axis of the piece is parallel to the axis of the coil. The

metal part then becomes the core of an electromagnet and is magnetized by induction from

the magnetic field created in the coil. Very long parts are magnetized in steps by moving the

coil along the length. In the case of circular magnetization, also shown in Fig. 10.5, a





magnetic field transverse to the long axis of the workpiece is readily produced by passing the

magnetizing current through the piece along this axis.

Direct current, alternating current, and rectified alternating current are all used for

magnetizing purposes. Direct current is more sensitive than alternating current for detecting

discontinuities that are not open to the surface. Alternating current will detect discontinuities

open to the surface and is used when the detection of this type of discontinuity is the only

interest. When alternating current is rectified, it provides a more penetrating magnetic field.

The sensitivity of magnetic-particle inspection is affected by many factors, including strength

of the indicating suspension, time in contact with the suspension, time allowed for indications

to form, time subject to magnetizing current, and strength of the magnetizing current.

All machine parts that have been magnetized for, inspection must be put through a

demagnetizing operation. If these parts are placed in service without demagnetizing, they will

attract filings, grindings, chips, and other steel particles which may cause scoring of bearings

and other engine parts. Detection of parts which have not been demagnetized is usually

accomplished by keeping a compass on the assembly bench.

6.4 Fluorescent-Penetrant inspection (zyglo)

This is a sensitive nondestructive method of detecting minute discontinuities such as cracks,

shrinkage, and porosity that are open to the surface. While this method may be applied to

both magnetic and nonmagnetic materials, its primary application is for nonmagnetic

materials. Penetrant techniques can be used for inspecting any homogeneous material that is

not porous, such as metals, glass, plastic, and some ceramic materials.

Parts to be tested are first treated with a penetrant. Penetrants are usually light, oil-like liquids

which are applied by dipping, spraying, or brushing, or in some other convenient manner.

The liquid penetrant is drawn into cracks and other discontinuities by strong capillary action.

After the penetrant has had time to seep in, the portion remaining on the surface is removed

by wiping or washing. This leaves the penetrant in all surface connected discontinuities. The

test part is now treated with a dry powder or a suspension of powder in a liquid. This powder

or developer acts like a sponge drawing the penetrant from the defect and enlarging the size

of the area of penetrant indication. In order for the inspection process to be completed, the

penetrant must be easily observed in the developing powder. One method is to use

contrasting colors for the penetrant and developer. A combination of white developer and red

penetrant is very common.





Another method is to use a fluorescent penetrant. The major steps in fluorescent penetrant

inspection are shown in Fig. 10.6. The steps are exactly the same as described previously

except that the penetrating liquid contains a material that emits visible light when it is

exposed to ultraviolet radiation. Lamps that emit ultraviolet are called black lights, because

the visible light they might normally emit is stopped by a filter, making them appear black or

dark purple. When the part to be inspected is viewed under black light, the defect appears as a

bright fluorescing mark against a black background

Fig. 6.6 Major steps in fluorescent-penetrant inspection

Fluorescent penetrant inspection is used to locate cracks and shrinkage in castings, cracks in

the fabrication and regrinding of carbide tools, cracks and pits in welded structures, cracks in

steam and gas-turbine blading, and cracks in ceramic insulators for spark plugs and electronic

applications.

6.5 Ultrasonic Inspection

The use of sound waves to determine defects is a very ancient method. If a piece of metal is

struck by a hammer, it will radiate certain audible notes, of which the pitch and damping may

be influenced by the presence of internal flaws. However, this technique of hammering and

listening is useful only for the determination of large defects.

A more refined method consists of utilizing sound waves above the audible range with a

frequency of 1 to 5 million Hz (cycles per second)-hence the term ultrasonic. Ultrasonic is a

fast, reliable nondestructive testing method which employs electronically produced high-

frequency sound waves that will penetrate metals, liquids, and many other materials at speeds

of several thousand feet per second. Ultrasonic waves for non destructive testing are usually

produced by piezoelectric materials. These materials undergo a change in physical dimension

when subjected to an electric field. This conversion of electrical energy to mechanical energy





is known as the piezoelectric effect. If an alternating electric field is applied to a piezoelectric

crystal, the crystal will expand during the first half of the cycle and contract when the electric

field is reversed. By varying the frequency of the alternating electric field, we can vary the

frequency of the mechanical vibration (sound wave) produced in the crystal. Quartz is a

widely used ultrasonic transducer. A transducer is a device for converting one form of energy

to another.

Fig. 6.7 The through-transmission and pulse-echo methods of ultrasonic inspection

Two common ultrasonic test methods, the through-transmission and the pulse-echo methods,

are illustrated in Fig. 6.7. The through-transmission method uses an ultrasonic transducer on

each side of the object being inspected. If an electrical pulse of the desired frequency is

applied to the transmitting crystal, the ultrasonic waves produced will travel through the

specimen to the other side. The receiving transducer on the opposite side receives the

vibrations and converts them into an electrical signal that can be amplified and observed on

the cathode-ray tube of an oscilloscope, a meter, or some other indicator. If the ultrasonic

wave travels through the specimen without encountering any flaw, the signal received is

relatively large. If there is a flaw in the path of the ultrasonic wave, part of the energy will be

reflected and the signal received by the receiving transducer will be reduced.

Fig. 6.8 Oscilloscope pattern for the pulse-echo method of ultrasonic inspection





The pulse-echo method uses only one transducer which serves as both transmitter and

receiver. The pattern on an oscilloscope for the pulse-echo method would look similar to that

shown in Fig. 6.8. As the sound wave enters the material being tested, part of it is reflected

back to the crystal where it is converted back to an electrical impulse. This impulse is

amplified and rendered visible as an indication or pip on the screen of the oscilloscope. When

the sound wave reaches the other side of the material, it is reflected back and shows as

another pip on the screen farther to the right of the first pip. If there is a flaw between the

front and back surfaces of the material, it will show as a third pip on the screen between the

two indications for the from and back surfaces. Since the indications on the oscilloscope

screen measure the elapsed time between reflection of the pulse from the front and back

surfaces, the distance between indications is a measure of the thickness of the material. The

location of a defect may therefore be accurately determined from the indication on the screen.

In general, smooth surfaces are more suitable for the higher-frequency testing pulse and

thereby permit detection of smaller defects. Proper transmission of the ultrasonic wave has a

great influence on the reliability of the test results. For large parts, a film of oil ensures proper

contact between the crystal searching unit and the test piece. Smaller parts may be placed in a

tank of water, oil, or glycerin. The crystal searching unit transmits sound waves through the

medium and into the material being examined. Close examination of the oscilloscope screen

in this picture shows the presence of three pips. The left pip indicates the front of the piece,

the rights pip the back of the piece, and the smaller center pip is an indication of a flaw.

Ultrasonic inspection is used to detect and locate such defects as shrinkage cavities, internal

bursts or cracks, porosity, and large nonmetallic inclusions. Wall thickness can be measured

in closed vessels or in cases where such measurement cannot otherwise be made.

6.6 Eddy Current Inspection





Fig.6.7 Eddy Current Test

Eddy current techniques are used to inspect electrically conducting materials for defects,

irregularities in structure, and variations in composition. In eddy current testing, a varying

magnetic field is produced if a source of alternating current is connected to a coil.

When this field is placed near a test specimen capable of conducting an electric current, eddy

currents will be induced in the specimen. The eddy currents, in turn, will produce a magnetic

field of their own. The detection unit will measure this new magnetic field and convert the

signal into a voltage that can be read on a meter or a cathode-ray tube. Properties such

hardness, alloy composition, chemical purity, and heat treat condition influence the magnetic

field and may be measured directly by a single coil.

An important use for eddy current testing is sorting material for heat treat variations or

composition mix-ups. This application requires the use of two coils (see Fig. 10.9). A

standard piece is placed in one coil and the test piece in the other coil. Acceptance or

rejection of the test piece may be determined by comparing the two patterns on the

oscilloscope screen. Eddy current testing may be used to detect surface and sub-surface

defects, plate or tubing thickness, and coating thickness.

References



Questionnaire

1. What is non-destructive testing? Describe any two tests for detecting surface defects

of materials or components.

2. Name amd explain any two tests for detection of internal defects.

3. Write short notes on: (i) Magnetic particle test. (ii) Gamma-ray radiographic test. (iii)

Acid pickling test. (iv) Dye-penetrant test.

4. What factors may be varied in taking a radiograph with x-rays?

5. In a radiograph, what will be the difference in appearance of gas cavities, cracks, and

impurities?

6. What are the limitations of magnetic-particle inspection?

7. What are the limitations of fluorescent-penetrant inspection?





8. What are the limitations of ultrasonic inspection?

9. Explain the difference between through-transmission and pulse-echo methods of

ultrasonic inspection.

10. Which nondestructive testing method is best suited to determine the wall thickness at

the bottom of a steel tank?

11. Which nondestructive testing method should be used to sort out bars of mixed steel?

Heat Treatment Processes





Objective

To understand the concept of heat treatment and case hardening processes.

7.1 Definition

Heat treatment is a stage in the fabrication of structures and is often forgotten; but it has

perhaps more wide-reaching and important ramifications than many of the other stages in the

fabrication of structures or components.

Heat treatment may be defined as: An operation or combination of operations involving

heating and cooling of a metal/alloy in solid state to obtain desirable

1. Conditions, e.g., that of relieved stresses

2. Properties, e.g., better machinability, improved ductility, Homogeneous structure etc.

7.2 Classification of heat-treatment processes

Various heat treatment processes can be classified as

1. Annealing

a) Stress-relief annealing

b) Process annealing

c) Spheroidising (annealing)

d) Full annealing

2. Normalising

3. Hardening (by quenching)

4. Tempering

5. Martempering

6. Austempering





7.3 purpose of heat treatment

- One or the other heat treatment process is carried out in order to

- Cause relief of internal stresses developed during cold working, welding, casting,

forging etc

- Harden and strengthen metals.

- Improve machinability.

- Change grain size.

- Soften metals for further (cold) working as in wire drawing or cold rolling.

- Improve ductility and toughness.

- Increase, heat, wear and corrosion resistance of materials.

- Improve electrical and magnetic properties.

- Homogenise the structure; to remove coring or segregation.

- Spheroidize tiny particles, such as those of Fe3C in steel, by diffusion.

7.4 Principles (fundamentals) of heat treatment

- Steel heat treatments are made possible by the eutectoid reaction in the iron-carbon

system. All basic heat-treating processes for steel involve the transformation or

decomposition of austenite.

- The nature and appearance of thus obtained transformation products develop a variety of

useful physical and mechanical properties in steels.

- Cooling rate plays an important role in the transformation of austenite to pearlite or

martensite, etc.

- Heat treatment is effective only with certain alloys (e.g., Fe-C, Aluminium bronze, etc.)

because it depends upon, one element being soluble in another in the solid state in

different amounts under different circumstances.

- The theory of heat treatment is based on the principle that an alloy experiences change in

structure when heated above a certain temperature and it undergoes again a change in

structure when cooled to room temperature. Cooling rate is an important factor in

developing different (soft or hard) structures.

- Slow cooling from above the critical range in steel will produce a pearlitic (soft) structure

whereas rapid cooling (depending upon steel composition) will give rise to a martensitic

(hard) structure.





Stages of Heat Treatment Process

1. Heating a metal/alloy to definite temperature.

2. Holding (or soaking) at that temperature for a sufficient period to allow necessary

changes (e.g., austenitizing) to occur.

3. Cooling at a rate necessary to obtain desired properties associated with changes in the

nature, form, size and distribution of micro-constituents (such as ferrite, pearlite.

martensite, etc.).

7.5 annealing

Definition

Annealing, primarily is the process of heating a metal which is in a metastable or distorted

structural state, to a temperature which will remove the instability or distortion and then

cooling is (usually at a slow rate) so that the room temperature structure is stable and/or strain

free.

Purpose

- Inducing a completely stable structure (full annealing)

- Refining and homogenising the structure.

- Reducing hardness.

- Improving machinability.

- Improving cold working, characteristics (process annealing) for facilitating further

cold work.

- Producing desired microstructure.

- Removing residual stresses.

- Removing gases.

- Improving mechanical, physical, electrical and magnetic properties.

Concept

- When applied to ferrous alloys, the term annealing, generally implies fill annealing.

- When applied to non-ferrous alloys, the term annealing implies a heat treatment

designed to soften an age-hardened alloy by causing a nearly complete precipitation

of the second phase in relatively coarse form.

- Any process of annealing will usually reduce stresses, but if the treatment is applied

for the sole purpose of such relief, it should be designated stress relieving.

(A) Stress relieving





- Stress relief annealing (or stress relieving) relieves (essentially eliminates) stresses

produced by casting, quenching, machining, cold working, welding, etc.

- Stress relief annealing applies equally well to ferrous and nonferrous metals.

- Stress relief is often desirable when a casting is liable to change dimensions to a

harmful degree during machining or use. Stresses if not relieved might later cause

warpage or even failure of the casting.

- Thermal stress relieving requires heating the casting to a temperature at which

relaxation of the elastic (residual) stress is brought about by plastic deformation

corresponding to the elastic strain.

- Stress relieving does not affect the metallurgical structures of the castings and is

essentially one of creep; the temperature required for stress relief of a casting varies

from 0.3 MP to 0.4 MP where MP is the melting point of the cast metal or alloy

- Stress relief is also known as Recovery.

(B) Process annealing

- Process annealing is usually subcritical (operation takes place below the lower critical

temperature) annealing and is applied to remove the effects of cold work, to soften

and permit further cold work as in sheet and wire industries.

- Ferrous alloys are heated to a temperature close to, but below the lower limit of the

transformation range (550-650°C), are held at that temperature and then cooled

usually in air in order to soften the alloy for further cold working as in wire drawing.

- Process annealing associates with it only partial recrystallisation of the distorted

ferrite and since mild steel contains only a small volume of strained pearlite, a high

degree of softening is induced.

- Process annealing does not involve any phase change and the constituents ferrite and

cementite remain present in the structure throughout the process.

- Process annealing is generally carried out in either batch-type or continuous furnaces,

usually with an inert atmosphere of burnt coal gas; cast-iron annealing pots may be

used, with their lids being luted on with clay.

- Fig. 7.1 shows temperatures for various heat-treating processes.





Fig. 7.1 Heat treating temperature for carbon steel

(C) Spheroidise annealing

Fig. 7.2 Spheroidising

Spheroidise annealing or spheroidizing involves subjecting steel to a selected temperature

cycle, usually within or near the transformation range in order to produce a spheroidal or

globular form of carbide in steel (Fig. 7.2).

Spheroidizing:

- Improves machinability.

- Facilitates a subsequent cold working operation

- Obtains a desired structure for subsequent heat treatment.

- Softens tool steels and some of the air hardening alloy steels.

- Improves surface finish during machining; the steels can be machined freely.

- Prevents cracking of steel during cold forming operations.

A spheroidized steel has a lower hardness and tensile strength and a correspondingly higher

relative elongation and reduction of area than steel subjected to normal annealing.

Spheroidizing is extensively employed for high carbon (tool) steels to transform lamellar

pearlitic cementite into spheroidal type (Fig. 7.2). Cementite spheroids are embedded in a

matrix of ferrite (Fig. 7.3).





Fig. 7.3 Spheroidized annealed (1%) steel showing spheroidized cementite in a ferrite matrix

The spheroidized condition is produced by one of the following methods:

1. Heating steel and then its prolonged holding at a temperature just below the lower

critical line (between 650 and 700°C).

- Whilst no basic phase change takes place, surface tension causes the cementite part of

pearlite to assume a globular form (Fig. 43.2), in a similar way to which droplets of

mercury behave when mercury is spilled on a flat surface.

- If some form of quenching treatment is given to steel prior to spheroidizing, the steel

will be sphroidized more quickly and will produce much smaller globules of

cementite.

- Small cementite globules tend to improve surface finish during machining and also

are dissolved more quickly when a carbon steel tool is ultimately heated for

hardening.

2. Heating and cooling steel, alternately between temperatures that are just above and

just below the lower critical line.

3. Heating to a temperature above the lower critical line (i.e., between 730-770°C) with

subsequent holding at this temperature followed by slow cooling, at a rate of 25 to

30°C per hour, to 600°C.

(D) Full annealing

Full annealing implies annealing a ferrous alloy by austenitizing (heating to austenite

condition) and then cooling slowly (in the furnace itself) through the transformation range.

The austenitizing temperature for hypoeutectoid steels is usually between 723°C (1333°F)

and 910°C (1670°F); and for hypereutecoid steels, austenitizing temperature is between

723°C (1333°F) and 1130°C (2066°F).

Full annealing, thus, involves:

- Heating steel to proper annealing temperature in the austenitic zone.

% C in steel Approx Annealing Temperatures (°C)





0.10 to 0.50 950 to 815

11.50 to 0.80 815 to 760

0.90 to 1.50 760 to 780

- Holding the steel object at that temperature for a definite period of time depending

upon its thickness or diameter (about 2.5 to 3 minute per mm thickness) so that it

becomes completely austenitic; and then

- Cooling very slowly the steel object through the transformation range, preferably in

the furnace or in any good heat-insulating material, till the object acquires a low

temperature. Because of very slow cooling involved, annealing comes close to

following the Fe-C equilibrium diagram.

Slow cooling associated with full annealing enables the austenite to decompose at low

degrees of super cooling so as to form

- A pearlite + ferrite structure in hypoeutectoid steels,

- A pearlite + cementite structure in hypereutectoid steels

Full annealing

- Refines grains.

- Removes strains (from forgings and castings)

- Induces softness and machinability.

- Improves formability.

- Improves electrical and magnetic properties.

7.6 Normalizing

Normalising or air quenching consists in heating steel to about 40-50°C above its upper

critical temperature (i.e., A3 and Acm line) and , if necessary, holding it at that temperature for

a short time and then cooling in still air at room temperature (Fig. 7.1)

Normalising differs from full annealing in that the rate of cooling is more rapid and there is

no extended soaking period.

The type of structure obtained by normalising will depend largely on the thickness of cross

section as this will affect the rate of cooling. Thin sections will give a much finer grain than

thick sections.

Normalising produces microstructures consisting of ferrite (white network) and pearlite (dark

areas) for hypoeutectoid (i.e., up to about 0.8% C) steels (Fig. 7.4).

For eutectoid steels, the microstructure is only pearlite and it is pearlite and cementite for

hypereutectoid steels.





Fig. 7.4 Normalized 0.5%, C steel (heated to 982"C and air cooled)

Purpose

- Produces a uniform structure.

- Refines the grain size of steel, which may have been unduly coarsened at the forging

or rolling temperature.

- May achieve the required strength and ductility in steel that is too soft and ductile for

machining.

- Reduces internal stresses.

- Improves structures in welds.

- Produces a harder and stronger steel than full annealing,

- Eliminates the carbide network at the grain boundaries of hypereutectoid steels.

- In general, improves engineering properties of steels.

7.7 hardening (by quenching)

Introduction

Hardening is that heat treatment of steel which increases its hardness by quenching (and

tempering). Tools and machine parts required to undergo heavy duty service are oftenly

hardened.

- The hardening of steel requires the formation of martensite.

- Quench hardening and tempering are confined in application to the so-called heat

treatable steels (i.e., steels containing C in excess of 0.3% and, perhaps, containing alloy

additions), nodular graphite irons and alloy cast irons.

- However, in steels, the maximum % increase of hardness by quenching is obtained if they

contain between 0.35 and 0.60 % carbon.

- Hardening followed by tempering





- Hardens steel to resist wear.

- Enables steel to cut other metals.

- Improves strength, toughness and ductility.

- Develops best Combination of strength and notch-ductility.

Hardening procedure

Steel with sufficient carbon (0.35 to 0.70%), Is heated 30 to 50°C above A3 line (Fig. 7.1), Is

held at that temperature from 15 to 30 minutes per 25 mm of cross-section, Is cooled rapidly

or quenched in a suitable medium (e.g., brine, water, oil, etc), To produce desired rate of

cooling, and A suitably hardened steel.

The degree of hardness produced in steel depends upon

- Composition of steel (0.35 to 0.50% C steels harden more).

- Nature and properties of quenching medium.

- Quenching temperature.

- Size of the objective to be quenched.

- Homogeneity of austenite.

- Degree of agitation.

- Rate of cooling.

Alloying elements (e.g., Ni, Cr, Mn, etc.) when added to steel increase the depth of

hardening; they do so by slowing down the transformation rates. Thus alloy steel can be

hardened by much less drastic quenching than is necessary for plain carbon steel.

Structure of hardened steels

Depending upon the factors affecting hardness (and listed above):

1. A very rapid rate of cooling forces carbon to remain in solution and austenite

transforms to martensite.

2. Comparatively slower rate of cooling produces fine pearlie and still slower cooling

rate gives rise to coarse pearlite.

Quenching

Media

Rate Of

Cooling

Structure

Produced

Hardness

Obtained Rc

Ultimate Tensile

Strength MN/m2

Water Very fast Martensite 65 1725

Oil Low Very fine pearlite 35 1100

Air Medium Fine pearlite 25 865

Furnace Cool Very slow Coarse pearlite 15 520





Pearlite is harder than austenite (or ferrite) and martensite is harder than pearlite. This

accounts for the increased hardness of the (quenched) steel.

Quenching Medium

- A quenching medium is one into which heated metal objects are plunged in order to

withdraw heat from the objects rapidly.

- The quenching medium (for hardening) must provide for a cooling rate above the

critical value (such as curves g and e in Fig. 40.13) to prevent austenite decomposition

in the pearlite, etc.

- In the martensite transformation temperature range, cooling should be slower to avoid

high internal stresses, warping of the hardened part and cracking.

Types Quenching Media

Given below are some industrial quenching media, in order of decreasing quenching severity;

- 5 to 10% Caustic soda......very drastic quench.

- 5 to 20% Brine (NaCI).

- Cold / Warm water

- Mineral oil (Obtained during the refining of crude petroleum).

- Animal oil (Produced by boiling the blubber of seal and whale or by rendering down

other animal tissue to obtain neats foot or lard oils).

- Vegetable oil (such as linseed, cottonseed and rapeseed).

- Water is preferred when hardening plain carbon steels and oils are suitable for

quenching alloy steels and Air... ...Least drastic quench.

Quenching characteristics of (liquid) coolants are controlled by the following factors:

1. Temperature of quenching medium.

2. The specific heat (i.e., the amount of heat necessary to raise the temperature one

degree per unit weight).

3. The heat of vaporization (i.e., the amount of heat necessary to vaporize unit weight)

4. Thermal conductivity of the quenching medium.

5. Viscosity (i.e., coefficient of resistance to flow, reciprocal fluidity).

6. Agitation (the rate of movement of work piece or flow of coolant).

Relative cooling rates for some quenching media





Quencing Medium Cooling rate relative to that of water at 18°C

720 to 550°C 200°C

10% Caustic soda (NaOH) 2.06 1.36

Water at 0°C 1.60 1.02

Water at 18°C 1.00 1.00

Oil (Rapseed) 0.30 0.055

Water at 100°C 0.044 0.71

Air 0.028 0.007

Quenching severity relative to still water as 1.0 for various conditions of quench.

Method of cooling Oil Water Brine

No agitation of piece or No circulation of liquid 0.25 – 0.30 0.9 – 1.0 2

Mild circulation or agitation 0.30 – 0.35 1.0 – 1.1 2 – 2.2

Good circulation 0.40 – 0.50 1.4 – 1.5

Violent circulation 0.80 – 1.10 4 5

The rate of heat transfer from a hot metallic body into the quenching fluid depends upon the

body itself, i.e., the body’s

1. Size and shape: thin section will cool fast and transform to martensite completely.

2. Temperature.

3. Thermal conductivity.

4. Specific heat.

5. Surface condition.

Stages of Quenching

Stage-l: Vapour-blanket cooling stage.

- The temperature of the metal is so high that the quenching medium is vaporized at the

surface of the metal and a thin stable film of vapour surrounds the hot metal.

- Job is cooled by conduction and radiation through the gaseous film and since vapour

films are poor heat conductors, the cooling rate is relatively slow through this stage.

Stage 2: Vapour-transport cooling stage.

- This stage starts when the metal has cooled to a temperature at which the vapour film

is no longer stable.





- Vapour-blanket is broken intermittently, allowing liquid to touch the hot metal at one

instant but soon being pushed away from it by vapour bubbles. TI e bubbles escape

from the surface and the liquid touches the hot metal again.

- In the stage, since hot metal surface is wetted by the quenching liquid, violent boiling

occurs that generates bubbles on the surface of the metal being cooled.

- Very rapid cooling takes place in this stage that soon brings the metal surface

temperature below the boiling point of the liquid.

Stage-3: Liquid Cooling Stage.

- Third stage begins when surface temperature just reaches the boiling point of the

quenching liquid.

- Cooling in this stage takes place by simple convection and conduction.

- The rate of cooling decreases as the temperature of the metal falls.

- The rate of cooling is slowest in this stage.

7.8 Tempering

- Quench hardening produces structures – martensite and retained austenite.

- The martensitc formed in quench hardened steel is exceedingly brittle, hard and highly

stressed; the cracking and distortion of the hardened article is liable to occur after

quenching. For this reason, the use of steel in this condition is inadvisable except in cases

where extreme hardness is required.

- Secondly, quench hardened steel besides containing martensite has some retained

austenite too. Retained austenite is, also, an unstable phase and as it changes with time,

dimensions may alter e.g., dies may alter 0.125 mm.

- It is therefore necessary to return towards equilibrium after quench hardening, by heating

the (hardened) steel to a temperature below the lower critical temperature (A1); this is

tempering.

- Tempering requires

1. Heating hardened steel below the lower critical temperature

2. Holding it at that temperature for 3 to 5 min. for each mm of thickness or diameter;

3. Cooling the steel (in water, oil or air) either rapidly or slowly except in case of steels

susceptible to temper brittleness.

[Note: Temper brittleness is generally used to describe the notch impact intergranular

brittleness induced in some steels by slow cooling after tempering above about 600°C and

also from prolonged soaking of tough material between about 400°C and 550°C. Temper





brittleness seems to be due to the segregation of solute atoms to the grain boundaries on slow

cooling from 600°C]

Essentially the tempering reaction can be pictured as the change from carbon atoms dispersed

in the martensite to precipitated carbide particles of increasing size.

Purpose:

- Relieve residual stresses.

- Improve ductility.

- Improve toughness.

- Reduce hardness.

- Increase % elongation.

Theory of Tempering

Quench hardened steel produces structures-martensite and retained austenite. These being

unstable phases have a tendency to change to a stable structure. This change cannot take

place at room temperature because of the low mobility of atoms at this temperature. As the

temperature of hardened steel is raised (for tempering) the mobility of atoms increases and

the phases tend to pass into a stable/equilibrium condition. For this change to occur both

temperature and time are important variables, but temperature is far the more important

variable, and the time of tempering usually adopted is about one hour.

- Tempering consists of heating the hardened steel to some temperature below A1 for

about one hour to produce tempered martensite.

- Essentially the tempering mechanism/reaction can be pictured as the change from

carbon atones dispersed in the martensite to precipitated carbide particles of

increasing size Fig. 7.5 indicates a number of stages by which this change is believed

to occur

- When the fresh martensite is heated to below the critical temperature, it becomes

softer and more ductile and internal stresses are relieved. Little benefit is derived

below 150°C.

Stages of Tempering

1. At low tempering temperatures (Approx. 80-200°C) a hexagonal close-packed carbide

(called epsilon carbide) begins to form, and with this rejection of carbon the crystal

structure of martensite changes ultimately from tetragonal to the body-centered-cubic

characteristic of ferrite.





2. The second stage at about 200 to 300°C, depending upon the steel is characterized by

transformation of the retained austenite to bainite.

Fig. 7.5 Changes in structure and hardness that accompany the tempering of eutectoid

carbon steel.

3. At low tempering temperatures (Approx. 80-200°C) a hexagonal close-packed carbide

(called epsilon carbide) begins to form, and with this rejection of carbon the crystal

structure of martensite changes ultimately from tetragonal to the body-centered-cubic

characteristic of ferrite.

4. The second stage at about 200 to 300°C, depending upon the steel is characterized by

transformation of the retained austenite to bainite.

5. During third stage from 300 to 475°C (Approx.) there is formation of (i) Fe3C

(cementite) from epsilon carbide and (ii) change from low-carbon martensite to cubic

ferrite.

6. From 450 to 705°C (Approx.) the cementite (Fe3C) agglomerates and coalesces. The

structure becomes an aggregate of ferrite with cementite in quite fine spheres, referred

to as tempered martensite and tempered bainite.





Fig. 7.6 Quench hardening and tempering cycle.

Tempering may be classified into following types:

1. Low temperature tempering

- This treatment is carried out in the temperature range from 150 to 250°C

- Internal stresses are reduced.

- Toughness and ductility get increased without any appreciable loss in hardness.

- The structure still contains martensite.

- Low temperature tempering is applied to cutting tools of carbon steels and low alloy

steels and to the components that are surface hardened and carburized.

2. Medium Temperature Tempering

- Medium temperature tempering is carried out in the temperature range from 350°C to

450°C.

- This treatment develops troostite structure.

- With this treatment, hardness and strength of steels decrease, while % elongation and

ductility increase.

- Medium temperature tempering imparts to steels the highest (attainable) elastic limit

with sufficient toughness.

- Medium temperature tempering is applied to objects such as coil, springs, laminated

springs, hammers, chisels, etc.

3. High Temperature Tempering

High temperature tempering is carried out in the temperature range from 500°C to 650°C.

High temperature tempering:

- Eliminates internal stresses completely,

- Imparts high ductility in conjunction with adequate hardness, and





- Is used for components such as (i) Connecting rods, (ii) Shafts, (iii) Gears, etc.

Temper Colours

Temp., °C Colour Type of Component

220 Pale yellow Hack saws, scrapers.

230 Straw Planning and slotting tools.

240 Dark straw Shear blades, drills, paper cutters.

250 Light brown Metal shears, punches, dies.

270 Purple Axes, gimlets, augers.

280 Deeper purple Cold chisels (for steel & C.I.) chisel for woods

290 Bright blue Screw drivers

300 Dark blue Springs, wood saws.

- The tempering of quenched steel, in fact, is sometimes controlled by observation of

the temper colours (e.g. straw at 230"C, blue at 300°C, etc.) produced on the clean

(steel) metal surface when heated.

- Temper colours are produced due to interference effects of thin films of oxide formed

during tempering.

7.9 Interrupted quenching

The continuous rapid cooling (Quenching) to room temperature ordinarily used to cause

martensite formation has the disadvantages of

- Setting up severe quenching stresses. (which are the result of contraction during cooling

and expansion caused by martensite)

- warping and distorting the object, and

- Promoting crack formation in the steel.

If instead of rapid cooling, slow air cooling is employed, definitely the quenching stresses

will become minimum, but one will have to go for steel with sufficient hardenability to form

martensite on air cooling; and such steels, i.e., air hardening steels are expensive.

In other words, to obtain desired hardness in a given object either one should select a less

costly steel, rapidly cool it and take the risk of creating severe quenching stresses or one

should go for expensive air hardening steel and decrease the stresses by cooling the steel in

air.

For example, air hardening steels are selected in making complex dies for which preliminary

machining cost is high therefore it is economical to protect the investment in machining





operations by using an air hardening steel that can be hardened with a minimum danger of

developing stresses, distortion or cracking.

One more, comparatively less costlier, alternative to achieve the desired hardness with

reduced stresses is to use steels of moderate hardenability and subjecting them to an

interrupted Quenching Procedure.

Interrupted quenching is a two-stage process. The piece is first quenched in a medium which

cools it rapidly past the nose of the TTT diagram (Fig. 7.7).

After that, the piece is further quenched in a second medium which cools it rapidly enough

(but much more slowly than the first medium) to avoid the bainite transformation.

The most generally useful type of interrupted quenching is called Martempering.

7.10 Martempering

In Martempering, steel is

1. Heated to above the critical range to make it all austenite; is then

2. Quenched into a salt bath maintained at a temperature above the Ms and is held at this

temperature long enough until the temperature is uniform across the section of the

workpiece (i.e., from surface to core) without transformation of the austenite; and

3. subsequently cooling the workpiece in air through the martensite range (Fig. 7.7).

Fig. 7.7 Martempering and Austempering

- The result is the formation of martensite with a minimum of stresses, distortion and

cracking.

- The steel thus obtained may be further tempered in order to increase ductility.





- Large sections cannot be heat treated by martempering because the time required to

obtain temperature uniformity (of workpiece surface and its core) exceeds the start of

transformation of austenite into bainite.

- Actually, in practice, in order to utilize benefits of martempering, alloying elements

are added to steel. Otherwise the critical cooling rate is too fast and the benefits of the

martensite hardness cannot be realized in parts that are large or even medium in size.

7.11 Austempering

- Austempering is not a hardening treatment.

- Austempering is another type of interrupted quenching, that forms Bainite (and not

martensite). In structure and properties, however, the bainite thus formed closely

resembles tempered martensite.

- In general, steels treated thus are tougher and more ductile than steels of tempered

martensite having equal hardness and tensile strength.

- Fig. 7.7. Shows the austempering process.

- However, a major limitation is that size is restricted to relatively thin selections so

that the entire piece can quickly attain the temperature of quenching bath.

- For steels of high hardenability, larger sections can be used.

- An additional disadvantage is the relatively long time required for the isothermal

transformation of austenite to bainite.

- Tempering is rarely needed after austempering.

Austempering consists in

- heating the steel above the critical range to make it all austenite; is then

- quenched at a critical cooling rate into a salt bath or lead bath held in the bainite range

(usually between 205°C and 425°C);

- the steel piece remains in the bath until the austenite is completely transformed to

bainite and then it is

- Allowed to cool to room temperature, the rate being immaterial.

Advantages of Austempering

- Greater ductility and toughness (impact strength) along with high hardness.

- Less distortion.

- Less danger of quenching cracks because the quench is not very drastic.





7.12 Case hardening and surface treatment processes

Introduction

Numerous industrial applications such as cams, gears, etc., require a hard wear resistant

surface called the case and a relatively soft, tough and shock resistant inside, called the core.

No plain carbon steel can possess both these requirements at the same time; because a low

carbon steel, containing about 0.1% carbon, will be tough, whilst a high carbon steel of 0.9%

or more carbon will possess adequate hardness when suitably heat treated.

However, both these requirements may be met by employing a low carbon steel with suitable

core properties and then adding (or penetrating) Carbon, Nitrogen or both to the surface of

the steel part in order to provide a hardened case (or layer) of a definite depth. These

treatments are known as Case Hardening.

The processes used to create hardened cases are:

1. Carburizing 3. Cyaniding

2. Nitriding 4. Carbonitriding

Another method to obtain tough core and hard case in a steel part is to take medium carbon

steel in the normalized condition and then introduce local hardness at the surface by some

hardening processes such as.

3. Flame hardening.

4. Induction hardening.

Surface treatments involve applying coatings to the surface of metals/alloys in order to

- Improve corrosion, heat or wear resistance,

- Rebuild worn or undersized parts,

- Serve as an ornamental finish,

- Lengthen the useful life of a part manufactured from a low cost material having

surface characteristics unsuited for a given installation.

Coatings may be broadly classified as

(a) Metallic coatings obtained by

- Hot dipping

- Electroplating

- Impregnating

- Spraying

- Facing

- Cladding

(b) Non-metallic coatings, which include

- Oxide coating

- Phosphate coating

- Vitreous enamel coating.





7.13 Carburizing

Carburizing is a method of introducing, carbon into solid iron-ha ,r alloys such as low carbon

steels in order to produce a hard case (surface). Carburizing is also called cementation.

- Carburizing increases the carbon content of the steel surface by it process of

absorption and diffusion.

Process

- Low carbon steel (about 0.20% carbon or lower) is heated at 87() l 925°C in contact

with gaseous, solid or liquid carbon containing sub stances for several hours.

- The high carbon steel surface (thus obtained) is hardened by quenching from above

the AI temperature.

Characteristics

- Case depth is about 0.05 inch (1.27 mm).

- Hardness after heat treatment is Re 65.

- Carburizing causes negligible change in dimensions.

- Distortion may occur during heat treatment.

Uses:

In the case hardening of Gears, Camshafts, Bearings.

Methods

There are three general methods of carburizing, depending upon the form of the carburizing

medium, namely

A. Pack Carburizing employing solid carburizing medium.

B. Gas Carburizing employing suitable hydro-carbon gases

C. Liquid Carburizing employing fused baths of carburizing salt

The choice of a particular method out of the three listed above depends upon:

1. The characteristics of the case which are desired,

2. The equipment available,

3. The quantity of parts which is to be handled.

A. Pack Carburizing

Pack carburizing involves packing the components into cast iron or eel boxes along with the

carburizing material so that a space of approximately 50 mm exists between the components.





Carburizing medium consists essentially of wood, or bone charcoal or charred leather,

together with an energizer (a mixture of sodium carbonate and barium carbonate) which may

account for up to 40% of the total composition.

The energizer accelerates the carburizing process.

After the components and the carburizing medium have been packed in the boxes, the lids are

luted on to them and the boxes are then slowly hated to the carburizing temperature, between

900 and 950°C, and then kept at that temperature up to five hours depending upon the depth

of case desired. When carburizing is complete, the parts are quenched or cooled slowly in the

boxes, depending upon the nature of the subsequent heat-treatment to be applied.

Fig. 7.1 shows the relationship between Time and Temperature of carburizing treatment and

the depth of case produced.

Fig. 7.1 Relationship between depth of case, carburizing temperature and time of treatment.

Fig. 7.2 schematic diagram of pack carburizing process





The mechanism by which carburization proceeds involves following stages:

1. Formation of carbon monoxide (CO) in the box containing carburizing material when

heated to between 900 and 950°C.

2. Dissociation of CO with the evolution of atomic carbon

2CO

CO2 + C atm

3. Enrichment of the steel surface layer with carbon

2CO + 3Fe

Fe3C + CO2

(CO2 thus liberated reacts with the carburizing material to form more CO at the

expense of the charcoal)

Although the reaction may be represented in this manner, in this manner carbon is not

in the form of iron carbide, but instead is in the form of elemental carbon dissolved in

gamma iron, forming austenite.

4. Diffusion of carbon, absorbed by the steel surface, deep into the metal. The rate of

diffusion of carbon in austenite, at a given temperature, is dependent upon the

diffusion coefficient and the carbon concentration gradient. Of course the higher the

temperature, the more rapidly carbon will be diffused.

If it is desired to prevent any area of a component from being carburized, that area

1. May be electroplated with copper to a thickness of 0.075-0.10 mm because carbon is

insoluble in copper at the carburizing temperature;

2. Should be coated with a mixture of fireclay and ignited asbestos made into a paste

with water and then allowed to dry on the surface before the component is carburized.

B. Gas Carburizing

Principles

- Gas carburizing can be applied in mass production. It is readily adaptable to all types and

configurations of steel parts in either batch or continuous operation.

- In gas carburizing, components are heated at about 900°C for three or four hours in an

atmosphere containing gases which will deposit carbon atoms at the surface of the

components.

- The most commonly used atmosphere for gas carburizing consists of approximately 20%

CO, 40% hydrogen and 40% Nitrogen.

- This endothermic atmosphere is applied both as a source of carbon from CO and as a

carrier gas to dilute hydrocarbon gas such as methane (or propane) which is used both as

a source of carbon and to control 'ace chemical reactions.





- The gaseous atmosphere can be generated as follows: A neutral carrier gas is first

prepared by burning town gas with the correct amount of air to give complete

combustion. The resulting combustion gases are cooled and passed over silica gel to

remove water vapours. CO2 is removed by passing over heated coke or charcoal at

1000°C or by absorption in an organic solvent such as ethanolamine or tetramine.

- The product thus left, which is mainly CO, N, and H2 is termed the carrier gas and a

proportion of propane or butane or methane is added to it to give the necessary

carburizing mixture.

- Gas carburizing requires specially designed atmosphere-tight furnace capable of

maintaining a positive pressure of atmosphere to prevent the infiltration of air. Furnace is

provided with a fan to circulate the atmosphere throughout the heating chamber.

- Following major reactions take place during the process of Gas carburizing.

3Fe + 2CO

Fe3C + CO2

Fe3C + 2H2

3Fe + CH4

3Fe + CO + H2

Fe3C + H2O

CO2 + CH4

CO + 2H2

CH4 + 3Fe

Fe3C + 2H2

- By suitable adjustment of time, temperature and gas composition, the case depth can be

varied to suit the work in hand.

- The depths of case vary from 0.25 mm on articles for light work, 0.5-1.0 mm for much

automobile work, to 0.37 mm for roller bearings and ball races where compressive

stresses are high.

Advantages of Gas Carburizing

- Pack carburizing cannot be accurately controlled with regard to case depth and it

requires considerable labour in the packing, loading and unpacking of boxes. Gas

carburizing eliminates these handicaps and in addition eliminates pack material,

which results in more favourable heat treatment economies.

- Labour costs are lower than in pack carburizing.

- Heating is more rapid and uniform than in pack carburizing.

- The carbon potential can be closely controlled.

- Less time is required than pack carburizing, since there is no need to pack and handle

boxes containing workpieces to be treated. For the same reason, the floor space

required is less.





- Gas carburizing provides cleaner surroundings, closer quality control and greater

flexibility of operation in comparison with pack carburizing.

- Gas carburizing may be feasibly mechanised.

Disadvantages of Gas Carburizing

- As compared to pack carburizing, higher skilled personnel arc required to maintain

the necessary controls.

C. Liquid carburizing

Principles

- Carburizing in liquid baths is of comparatively recent origin and is an outgrowth of the

older process of cyaniding.

- Liquid carburizing is employed principally for relatively shallow cases (0.104.25 mm)

which can be produced at lower cost by this process than with pack or gas carburizing.

- Liquid carburizing is carried out in baths containing 20 to 50% Sodium Cyanide, together

with up to 40% Sodium carbonate and varying amounts of Sodium or Barium Chloride.

- This cyanide-rich mixture is heated to a temperature of 870-950°C and the workpieces

contained in wire baskets are immersed into the liquid bath for periods varying from

about 5 minutes to one hour, depending, upon the depth of case required.

- The following chemical reaction perhaps occurs in liquid carburizing

2NaCN + 2O2 Na2CO3 + 2N + CO

- Dissociation of CO at the steel surface then takes place with the same result as in pack-

carburizing.

- Moreover, nitrogen, in atomic form, also dissolves in the surface and produces an

increase in hardness by the formation of nitrides.

- Thus in liquid carburizing, like cyaniding (or carbonitriding) both C and N are added

to the steel surface.

- However, liquid carburizing may be distinguished from Cyaniding by the character

and composition of the case produced.

- In cyaniding, case is higher in nitrogen and lower in carbon; the reverse is true in

liquid carburizing.

- Cyanide cases are seldom to a depth greater than 0.25 mm; liquid carburizing permits

cases as deep as 6.25 mm.

Advantages of liquid carburizing:

Liquid carburizing offers advantages of





- Rapid heat transfer

- Low distortion

- Negligible surface oxidation or decarburization

- Rapid absorption of carbon and nitrogen

- Uniform case depth and carbon content

- Reduced time for steel to reach the carburizing temperature.

- Flexibility to handle a wide range of parts of varied design and varied case depths.

Disadvantages of liquid carburizing

- Cyanide salts are highly poisonous when taken internally (even as fumes) or when in

contact with open wounds.

- Molten cyanide explodes on contact with water, so all work should be dried carefully

before it is placed in the liquid bath.

- Parts need thorough washing after treatment to prevent rusting.

Applications

- Gas carburizing is particularly suitable for mass production of thin cases in small and

medium size parts.

Heat treatment after carburizing

- Because of prolonged heating at a high temperature in the carburizing operation, both

the core and the case exhibit overheated structures, which are unsatisfactory for

severe service. The carburized parts are therefore heat-treated

1. To refine the core.

2. To refine and harden the case.

- To refine the grain of the core and consequently toughening it, the part is heated to just

above its upper critical temperature (about 870°C for the core) when the coarse ferrite-

pearlite structure will be replaced by refined austenite crystals. The component is then

water or oil quenched to give a fine dispersion of ferrite in martensite.

- Then, for refining the case, the component is again heated to about 760°C so that the

coarse martensite of the case changes to fine grained austenite. Quenching then gives a

fine-grained martensite in the case.

- Finally, the component is tempered at about 200°C to relieve any quenching strains

present in the case.

- The above treatments, are highly desirable from a theoretical approach, but generally the

heat treatment of a case-harden, it steel is modified as under:





1. Component may be quenched direct from the carburizing temperature and then given

a low temperature tempering to remove quenching strains. This treatment serves well

in case of steels with slow grain growth at the carburizing temperature.

2. Component may be slowly cooled from the carburizing temperature and then reheated

to 760°C and water quenched.This treatment will give a soft ductile core and hard

case. OR

3. Component may be slowly cooled from the carburizing temperature, reheated to

820°C, water quenched and then tempered at 200°C to give a fairly (not maximum)

tough core and hard case.

Case hardening of steels

Composition, characteristics and uses of a few case hardening steels are given below:

Composition Characteristics and uses

1. 0.1% C, 0.8% Mn Max. Toughness of core. Machine parts requiring shock

resisting properties.

2. 0.17% C, 0.7% Mn Very hard surface and a tough core. Gears, shafts, cams etc.

3. 0.25%C, 0.5% Mn High duty ball and roller bearings.

4. 0.15% C, 0.5% Mn,

3.0% Ni

For combined hardness and toughness as in gear wheels,

crank pins etc.

5. 0.15% C, 0.4% Mn,

3.5% Ni, 0.8% Cr

Parts requiring a glass hard surface and toughness of core-

high duty gears, worm gears, clutch gears, crown wheels etc.

6. 0.15% C, 0.4% Mn,

4.0% Ni, 1.2% Cr

For a combination of surface hardness, stress-bearing and

shock resisting properties. Used for crown wheels, bevel

pins, aero reduction gears etc.

7.14 Nitriding

Nitriding accompanies the introduction of nitrogen into the surface of certain types of steels

(e.g., containing Al and Cr) by heating it and holding it at a suitable temperature in contact

with partially dissociated ammonia or other suitable medium. This process produces a hard

case without quenching or any further at treatment.

Process Characteristics

- Case depth is about 0.381 mm.





- Extreme hardness (Vickers 1100),

- Growth of 0.025-0.050 mm occurs during nitriding.

- Case has improved corrosion resistance.

Uses:

- Valve seats

- Guides

- Gears

- Gauges

- Bushings

- Ball races

- Aircraft engine parts

- Aero engine cylinderst

- Crank pins and journals

- Moulds for plasters

- Aero crankshafts, air screw shafts

Table shows that steels suitable for nitriding must contain such alloying elements as Al, Cr

and V to form hard nitrides because iron nitride does not confer hardness to any extent.

Composition (%)

Heat-treatment temp (°C) Tensile

strength

(N/mm2)

Hardne

ss core

(VPN)

case Oil quench Tempering

1 C 0.39, Cr 1.6Mo 0.2, Al 1.1 900 650 880 269 1050

2 C 0.39, Cr 3.2,Mo 1.0, V 0.12 940 625 1310 380 900

3 C 0.5, Cr 1.3,Mo 1.0, Ni 1.8 870 620 1450 - 700

4 C 0.2, Mn 0.45,Cr 3.0, Mo 0.4 900 600 – 700 772 - 800 – 850

5 C 0.3, Mn 0.45,Cr 3.0, Mo 0.4 900 600 – 700 1000 - 800 – 850

6 C 0.4, Mn 0.5, Cr 3.0,Mo 1.0, V

0.25 900 550 – 650 1390 - 850 – 900

7 C 0.35, Mn 0.5, Cr 2.0,Mo 0.25, V

0.15 900 600 – 700 741 - 750 - 800

Nitriding process

Before being nitrided, the components are heat treated to produce the required

properties in the core. The normal sequences of operations are:

1. Oil quenching from between 850 and 900°C followed by tempering at between 600

and 700°C.

2. Rough machining followed by a stabilizing anneal at 550°C for five hours to remove

internal stresses.

3. Finish machining, followed by nitriding.





Fig 7.3 principle of nitriding process

- The components are placed in a heat-resistant metal container which is then filled

with ammonia whilst cold. When it is completely purged, it is sealed, placed in a

furnace and raised to a temperature of approximately 500°C.

- At this temperature the ammonia dissociates.

- NH3

3H + N, and N is absorbed in the surface layer of steel.

- Parts are maintained at 500°C for between 40 to 100 hours (40 hours for 025 mm and

100 hours for 0.75 ntm case depth, (approximate values)) depending upon the depth

of case required; after which the parts are allowed to cool in the container.

Advantages of Nitriding

- Very high surface hardness of the order of 1150 VPN may be obtained.

- Since nitrided parts are not quenched, this minimizes distortion or cracking.

- Good corrosion and wear resistance.

- Good fatigue resistance.

- Whereas in a carburized part, hardness begins to fall at about 200°C, a nitrided part

retains hardness up to 500°C.

- No machining is required after nitriding.

- Some complex parts which are not carburized satisfactorily can be nitrided without

difficulty.

- The process is economical when large numbers of parts are to be treated.

Disadvantages of Nitriding

- Long cycle times (40 to 100 hours).

- The brittle case.

- Only special alloy steels (containing Al, Cr and V) can be satisfactorily treated.





- High cost of the nitriding process.

- Technical control required.

- If a nitrided component is accidentally overheated, the surface hardness will be lost

completely and the component must be nitrided again.

7.15 Cyaniding

In cyaniding, carbon and nitrogen are introduced into the surface of steeI by heating it to a

suitable temperature and holding it in contact with molten cyanide to form a thin skin or case

which is subsequently quench hardened.

Characteristics of the process


- Hardness is about Re 65.

- Negligible dimension change is caused by cyaniding.

- Distortion may occur during heat treatment.

Uses

- Screws

- Nuts and bolts

- Small gears

- Plain carbon or alloy steels containing about 0.20% C usually hardened by cyaniding

Process

- Low carbon steel is heated between 800 and 870°C in a molten sodium cyanide bath

for a period of between 30 min and 3 hour, depending upon the depth of case

required.

- Quenching in oil or water from this bath hardens the surface of steel.

- In cyaniding, the bath usually-contains 30% NaCN, 40% Na2CO3 and 30% NaCl.

- This mixture has a melting point of 1140°F (615.6°C) and remains to stable under

continuous operating conditions.

- This mixture, when used at temperatures ranging from 787 to 898°C, composes to

free carbon and nitrogen which are then absorbed into the steel to form a hardened

carbide-nitride case.

2NaCN + 2O2

Na2CO3 + CO + 2N

2CO

CO2 + C

2NaCN +O2

2NaCNO (Sod. cyanate)





NaCN + CO2

NaCNO + CO

3NaCNO

NaCN + Na2CO3 + C + 2N

- In order to obtain hardness after cyaniding, it is necessary to quench directly into oil

or water from the cyaniding bath.

- In cyaniding, nitrogen imparts inherent hardness, whereas the increased carbon

content makes the surface of steel respond to a quenching treatment.

- Probably the greatest use of cyaniding is for parts that arc to be subjected to relatively

light loads and that require improvement in the surface-wear resistance.

7.16 Carbonitriding

- Cases (surfaces) that contain both carbon and nitrogen are whereas produced by liquid

salt baths in cyaniding, they are produced by the use of gas atmospheres in

carbonitriding.

- Carbonitriding implies introducing carbon and nitrogen into a solid ferrous alloy by

holding above Act in an atmosphere that contains suitable gases such as hydrocarbon,

carbon monoxide and ammonia.

- The carbonitrided alloy is usually quench-hardened.

- Metals Usually Hardened by Carbonitriding. Plain carbon steels containing about

0.20% carbon.

Process characteristics


- Hardness after heat treatment Re 65.

- Negligible dimensional changes.

- Distortion is less than in carburizing or cyaniding.

Uses: Carbonitriding mostly uses for Gears, Nuts, Bolts

Process

Carbonitriding is a modification of gas carburizing process because anhydrous ammonia gas

is added to the furnace atmosphere to cause both Carbon and Nitrogen to be absorbed by the

surface of steel at the carbonitriding temperature.

- The atmospheres used in carbonitriding usually comprise a mixture of carrier gas,

enriching gas (about 5%) and ammonia (about 15%).





Fig 7.4 principle of carbonitriding process

The carrier gas is usually a mixture of nitrogen, hydrogen and carbon monoxide produced in

an endothermic generator, as in gas carburizing. The carrier gas is supplied to the furnace

under positive pressure to prevent air infiltration and acts as a diluent for the active gases

(hydrocarbons and ammonia), thus making the process easier to control.

The enriching gas is usually propane or natural gas and is the primary source for the carbon

added to the surface.

At the furnace temperature, the added ammonia breaks up to provide the nitrogen to the

surface of the steel.

Carbonitriding is carried out at lower temperatures than gas carburizing. The temperature

range from 650 to 885°C with 845°C being most common. Low carbon steel is heated at this

temperature for several hours in the gaseous atmosphere discussed above.

Nitrogen in the surface layer of steel increases hardenability and permits hardening by oil

quench (instead by water quench) and thus reduces distortion and minimizes danger of

cracking.

Since nitrogen increases the hardenability, carbonitriding the less expensive carbon steels for

many applications will provide properties equivalent to those obtained in gas carburized alloy

steels.

7.17 Flame hardening

Principle

1. Rapid heating of the surface of heat-treatable steel by means of a flame, to a

temperature within or above transformation range (austenite range).

2. Followed immediately by quenching (Fig. 7.5). The highly heated surfaces became

hard but the core remains soft and tough.





Fig. 7.5 principle of flame hardening

- Objects are heated by an oxyacetylene flame.

- Steels having 0.3 to 0.6% carbon are hardened by flame hardening. Small amounts of

nickel (up to 4%) and chromium (up to 1%) can be added with advantage.

- Flame hardening is essentially a shallow hardening method.

- Depth of the hardened zone may be controlled by an adjustment of the flame

intensity, heating time or speed of travel.

- Overheating may result in cracks after quenching.

- The heating time is 7y2

seconds (where y is the depth of the hardens, I layer in mm);

the torch is traversed along with the work-piece (or vice versa) at a speed of 72/y mm

per second.

Equipment

- A flame hardening unit comprises the source of acetylene supply, an oxygen plant,

quenching devices, hardening control desk, instruments and set of torches and tips. A

torch may have single flame, slit type or multiple flame tips.

Methods

Four methods are in general use for flame hardening.

1. Stationary. Both workpiece and torch are stationary. This method is used for spot

hardening of small parts, e.g., valve stems and open end wrenches.

2. Progressive. Torch moves over stationary workpiece. Large parts such as the guide

ways of lathes can be hardened by this method. This technique is also suitable for

hardening teeth of large gears.

3. Spinning. Torch is stationary while the workpiece rotates. The method hardens

circular parts such as precision gears, pulley etc.

4. Progressive Spinning. Torch moves over a rotating workpiece. Long shafts and rolls

are hardened by this method.





- Before hardening by any of the above methods, the work piece. Is generally

normalized, so that the final structure will consist of a martensite case about 3.75 mm

thick and a tough ferrite pearlite core.

- After hardening (i.e., quenching) the workpiece should be stress relieved by heating in

the range of 180 to 205°C and then cooling in air. Stress relieving does not

appreciably reduce surface hardness.

Advantages

- Flame hardening is a useful and economical method of surface hardening.

- The hardened zone is generally much deeper than that obtained by carburizing; it

ranges from 3 to 6 mm in depth.

- Thinner cases (about 1.5 mm) can be obtained by increasing the speed of heating and

quenching.

- Large machine parts can be surface hardened economically.

- Surfaces can be selectively hardened with minimum warping and with freedom from

quench cracking.

- Flame hardening is easily adaptable and involves portable equipment.

- Electronically controlled equipment provides precise control of case properties.

- Flame hardening can treat workpieces even after their surfaces have been finished,

because there is little sealing, decarburization or distortion.

Disadvantages

- To obtain optimum results, a technique of flame hardening must be established for

each design.

- Overheating can damage the components.

- It is difficult to produce hardened zones less than 1.5 mm in depth.

Applications

- Ways of lathes

- Spindles

- Teeth of gears

- Valve stems

- Worms

- Open end

wrenches

- Shafts

- Pulleys

- Mill rolls, etc.

7.18 Induction hardening





The danger of either overheating or burning the surface of the metal by flame hardening may

be avoided by inducing heat electrically in the surface of the metal. Induction hardening

involves:

- Heating medium carbon steel by means of an alternating magnetic field to a temperature

within or above the transformation range (the hardening temperature is about 750 to

800°C). Followed immediately by quenching.

- This process may be applied for both surface hardening and full annealing.

- The principle of induction hardening is similar to that employed in the induction melting

of steels by the high frequency induction process in which heat is produced by currents

induced in the metal charge itself.

- Heat generated in the metal by induction is mostly confined to the outer surface of the

component to be induction hardened.

- The higher the frequency of current, the closer the heat is to the surface of the component.

At 50 cycles/second, the effective current flow through a surface layer of about 7.5 mm

deep, whilst at 10,000 cycle/second, the surface layer is reduced to 0.5 mm deep.

Procedure

High frequency currents are generated using

- Motor generators with frequencies of 1,000 to 10,000 cycles/second and capacities to

10,000 kW.

- Spark gap oscillators with frequencies of 100,000 to 400,000 cycles/second and

capacities to 25 kW.

- Vacuum-tube oscillators operating at 500,000 cycles per second with output

capacities of 20 to 50 kW.

The component part to be induction hardened (i.e., heated) is placed in the so called Inductor

or Inductor coil or Work coil (Fig. 7.6).

- Inductor coil comprises one or several turns of copper tube or busbar and is water

cooled. When high frequency (alternating) current is passed through the inductor coil,

it sets up a magnetic field (the intensity of which varies periodically in magnitude and

direction).

- As the alternating magnetic lines thread through the surface of the component (being

heated) placed in the inductor coil, they induce in component's surface an alternating

current of the same frequency but reversed in direction. Heating results from the

resistance of the metal (of the component) to passage of these currents.





- The high frequency induced currents tend to travel at the surface of the metal. This is

known as skin effect.

Fig. 7.6 Operation of induction hardening

- The component is held stationary in the inductor coil and the whole of the surface of

the component is heated simultaneously. The temperature of the surface layer rises to

above its upper critical temperature (the hardening temperature is about 755°C for a

0.5% carbon steel approximate values i.e., austenite range) in a few seconds.

- The surface of the component (after it has been heated to the required temperature) is

then quenched by pressure jets of water which pass through the holes existing in the

inductor block (Fig. 7.6). As after flame hardening, the induction hardened

component also needs be stress relieved.

Advantages

- Time required for hardening a component is sharply reduced. The heating time varies

between 1 and 5 seconds.

- It can be applied to both external and internal surfaces.

- Components may be heated with practically no scaling and distortion.

- Higher hardness can be' obtained in a given (%C) steel than with thermal heating.

- A hardness of about 60 Re may be obtained in certain types of steels to a depth of

about 3 mm.

- Through proper design of the heating coils, the shape of the hardened portion can be

controlled very closely.





- Depth of hardening can be controlled by selecting current of appropriate frequency

Frequency, cycles/second 1000 4000 10,000 120,000 500,000

Depth of hardening (mm) 6.0 3.0 2.5 1.5 0.751'

- Hard case and tough core is obtained.

- Induction hardening process can be made nearly automatic so that it can be carried out

with unskilled labour.

Disadvantages

- Cost of equipment is high.

- Steels having less than 0.40% carbon cannot be induction hardened.

- Irregular shaped components cannot be handled easily and economically.

- It is beneficial in mass production only.

- It associates high maintenance costs.

- Before induction hardening the component needs some treatment, e.g., normalising.

Applications

Typical parts hardened by this method are

- Piston rods

- Crankshafts

- Pump shafts

- Camshafts

- Spur gears

- Cams

- Automobile parts

7.19 Clad coating (metal cladding)

- A clad metal is a composite metal containing two or three layers that have been

bonded together. The bonding may have been accomplished by rolling, welding,

casting, heavy chemical deposition or heavy electroplating.

- Cladding is the method by which coating (or layer of another material) becomes an

integral part of base metal. The base metal gets sandwiched between pieces of the

coating metal.

- Cladding may be done on one or both surfaces of the base medal However, two layer

combinations are most common.

- In cladding, the surface coating exceeds about 3% of the total mass of the base metal.

Purpose of cladding

1. To obtain the properties of the cladding (coating) in the bars material at lower cost.





2. To obtain a combination of properties not available in either metal alone. For example

in copper-clad steel wire, copper exterior provides high electrical conductivity and

good corrosion resistance, while the steel core provides high tensile strength.

3. The principal objective for cladding is to produce a corrosion resistant surface. For

example Alclad is the name applied to aluminium alloys which are clad with pure

aluminium to improve corrosion resistance.

4. Cladding is also employed for making bimetal strips for temperature control devices.

Available forms

Clad metals are available in strip, sheet, foil, plate, wire, rod, tube form etc.

Methods of cladding

1. Rolling

a) Roll bonding, cold

b) Roll bonding, Hot

Cladded duralumin is obtained by placing the duralumin ingot between two sheets of

pure aluminium and rolling this pack (or sandwich together). This pack is hot rolled to

a thickness of 6 mm and then cold rolled with intermediate process annealing

operations.

2. Extrusion. This method is used for making tubing. Mechanical assemblies (or

electroplated metals) are extruded together.

3. Continuous brazing. In this method the brazing alloy distributed between strips is

fused as strips are continuously heated and rolled.

4. Casting. This method consists of one metal being cast against another, generally by

centrifugal casting process.

5. Welding. In this method, one material is deposited on the other and it may be rolled

afterwards.

6. Explosive bonding. Here the detonation action of a sheet of explosive drives the base

metal and coating metal together and bonds them. This method has made available

titanium, nickel and stainless clad steel in large plates.

Applications for clad metals

Cladding Base metal Applications

1. Copper Stainless steel To improve thermal properties for

heaters and heat exchangers.

2. Pure aluminium Wrought Reflectors, cooking utensils, chemical





aluminium (Alclad) equipment, marine applications.

3. Stainless steel Carbon and low

alloy steel

For improved corrosion resistance of

storage and mixing tanks and process

equipment.

4. Stainless Steel Aluminium Cookware, automotive trim and

chemical process vessels.

5. Titanium Steel Chemical process equipment.

6. Silver Unlimited variety For use as an electrical contact.

7. High chrome carbide

stainless steel

Carbon and low

alloy steel

Wear plates in chutes, conveyors and

earth-moving equipment.

7.20 Faced coatings

Metal facing or hard surfacing is the operation of welding metal to the surface of a part in

order to:

1. Improve abrasion, corrosion or heat resistance of the surface;

2. Improve resistance to wear, erosion, galling, cavitation and chemical reactions;

3. Rebuild worn or eroded parts.

Facing materials

1. High carbon and high alloy steels.

2. Cobalt and Nickel-base alloys.

3. Carbides of tungsten, tantalum and boron mounted in suitable alloy binders to

increase their toughness.

4. Copper base alloys, etc.

Base metals

Almost any steel or cast iron, but usually a medium carbon steel.

Surfacing methods

1. Oxyacetylene process. It is used where it is necessary to achieve a surface with the

minimum of required finishing. It is especially good in applying crack-free

applications of non-ferrous alloys.

Tungsten carbide particles are put in a mild steel tube and in the tube melts under the

flame, the tungsten carbide particles become fused to the surface in a matrix of mild

steel.

2. Inert gas process. It is used where a flawless deposit is required, mainly on new

construction.





3. Atomic hydrogen process. It is preferred where a large mass of metal requires a

small overlay of hard surface.

4. Submerged arc process. It is used where, in large quantities the facing material is to

be deposited.

5. Manual metal arc process. It is quick, economical, easily adaptable and the one most

frequently used for short jobs.

6. Plasma arc process. Because of its very high temperatures, this process greatly

extends the capabilities of surfacing.

Applications

References


Metal Coating Thickness Applications

1. Resistance to wear

- Molybdenum

- Babbit

0.25 mm

1.00 mm

Splined shaft (53 mm dia.)

Refrigeration pistons 990 mm dia.)

2. Resistance to corrosion

- Aluminium

- Zinc

0.20 mm

0.20 mm

Bridge dams.

Overhead crane.

3. Resistance to corrosion and wear

- Monel

- Phosphor bronze

1.0 mm

1.0 mm

Turbine shaft (100 mm dia.)

Gas compressed piston (380 mm dia.)

4. Resistance to heat and corrosion

- Aluminium

- Nikrome and Al (separate Layer)

0.15 mm

0.50 mm

Exhaust stack in chemical plants

Lead-annealing pans.





Questionnaire

1. Define heat treatment. List various objectives of heat treatment.

2. Explain what you understand by the following regarding heat-treatment of steel: (i)

Lower critical temperature (ii) upper critical temperature (Ui) critical range.

3. Write short notes on important factors in the heat-treatment of steel.

4. Give definition, purposes and process for the following heat-treatment operations for

steel: (i) Annealing (Ii) Normalising.

5. Distinguish between normalising and annealing in the following respect: (i)

Temperature for heating (ii) Heating and soaking time (Iii) Rate of cooling (iv)

Manners of cooling (v) Structural changes involved (vi) Applications.

6. Give purposes, process and structural changes involved in the fallowing heat-

treatment processes for steel: (i) Hardening (Ii) Tempering

7. Give reason why low carbon steel cannot be hardened practically by hardening

operation.

8. Describe any two of the following surface hardening processes used for low-carbon

steels: (i) Case- hardening (ii) Nitriding (iii) Cyaniding.

9. Differentiate between 'hardening' and 'case-hardening' processes.

10. Discuss the following surface hardening processes used for medium and high carbon

steels: (i) Induction hardening (ii) Flame hardening.

11. What are the requisites of a good quenching medium? Discuss various quenching

media used in the heat treatment of steel.

Powder Metallurgy





Objective

To understand the concept of powder metallurgy.

8.1 Definition and Concept

- Powder metallurgy is an art and science of producing fine metal powders and then

making objects from individual, mixed or alloyed metal powders with or without the

inclusion of non-metallic constituents.

Components are produced in their final form by pressing metal powders into the desired

shape, usually in a metal mold, and then heating the compacted powder, either concurrently

or subsequently, for a period of time at a temperature below the melting point of the major

constituent.

For making a component by powder metallurgy,

1. The metal in the powder form must be able to respond to solid phase welding;

2. The metal powder must be capable of sufficiently close packing under pressure to

permit welding to take place and, in case of alloying, be capable of being sufficiently

intimately mixed.

8.2 APPLICATIONS OF POWDER METALLURGY

- Porous products, e.g., bearings and filters.

- Refractory parts, e.g., components made out of Tungsten, Tantalum and Molybdenum

are used in electric bulbs, radio valves, oscillator valves, X-ray tubes in the form of

filament, cathode, anode, control grids, etc.

- Products of complex shapes that require considerable machining when made by other

processes, e.g., toothed components such as gears.

- Automotive components such as electrical contacts, crankshaft drive or camshaft

sprocket, piston rings and rocker shaft brackets, door mechanisms, connecting rods

and brake linings, etc.

- Products made from materials that are very difficult to machine, e.g., tungsten

carbide, etc. Components are: gauges, wire drawing dies, wire guides, deep drawing,

stamping and blanking tools, stone hammers, rock drilling bits, etc.

- Products where the combined properties of two metals or of metals and non-metals

Powder Metallurgy




are desired: non-porous bearings, electric motor brushes, etc.

- Tungsten parts are employed in plasma jet engines, etc., which are operated at about

1850°C. Silver infiltrated tungsten is used in nozzles for rockets and missiles.

- Use as parts in military and defence systems, e.g., in military arms.

- Atomic energy applications.

- Parts made by powder metallurgy have also been used in clocks and timing devices,

typewriters, adding machines, calculators, permanent magnets, laminated bimetallic

strips, etc.

- Grinding wheels that incorporate steel and diamond powder may be manufactured by

powder metallurgy.

8.3 ADVANTAGES OF POWDER METALLURGY

- The dimensional accuracy and surface finish obtainable are such that for many

applications all machining can be eliminated.

- Cleaner and quieter operation and longer life of the components.

- High production rates.

- Control of grain size, relatively much uniform structure and defect (e.g., voids,

blowholes, etc.) free components.

- No material is wasted as scrap; the process makes use of 100% raw material unlike

casting, press forming, etc.

- Quite complex shapes can be produced.

- Components shapes obtained possess excellent reproducibility.

- Porous parts can be produced that could not be made in any other way.

- Parts with wide variations in compositions and materials can be produced.

- Structure and properties can be controlled more closely than in other fabricating

processes.

- Highly qualified or skilled labour is not required.

- Impossible parts (e.g., super-hard cutting tool bits) can be produced.

- The use of diamond in industry has been made possible mainly through powder

metallurgy.

- Powder metallurgy is free from the limitations imposed by phase diagram. For

example, it is difficult to produce copper-lead bearing alloys containing large amounts

of lead, since the two metals are insoluble as liquids. However, mixed powders of

Powder Metallurgy




copper and lead can be successfully shaped by powder metallurgy.

8.4 LIMITATIONS OF POWDER METALLURGY

- Complicated shapes, such as produced by casting, cannot be made by powder

metallurgy, because metallic powders lack the ability to flow to the extent of molten

metals.

- Parts made by powder metallurgy, in most cases, do not have as good physical

properties as wrought or cast parts.

- Relatively high tool and die cost is associated with the process.

- The size of products (as compared to casting) is limited because of the large presses

and expensive tools which would be required for compacting.

- Powdered metals are considerably more expensive than those in wrought forms.

- Extreme care is required in handling pyrophoric powders (e.g.,Mg, Th, Zr) to prevent

fires or explosions and with toxic powders (e.g., U, Be, Th) to minimize health

hazards.

- Powder metallurgy is not economical for small scale production.

- It may be difficult, sometimes, to obtain particular alloy powders.

- Parts pressed from the top tend to be less dense at the bottom.

- Some metals are difficult or impossible to compress, since they tend to cold-weld to

the walls of the die and thus cause excessive wear on the die.

- There are design limitations as regards the parts being made by powder metallurgy.

A part must be of such a configuration that it can be ejected from the die. This means that re-

entrant angles, featheredges or deep and narrow splines must be handled as secondary

operations after the part has been made by powder metallurgy; this adds additional cost

factors.

8.5 POWDER METALLURGY PROCESS

The principal steps in powder metallurgy process include:

a. Obtaining/producing metal powders in a suitable degree of fineness and purity.

b. Weighing and mixing of the necessary powders (and lubricants) to arrive at a composition

which processes satisfactorily and which produces desired properties in the fabricated part.

c. Pressing the powder (mixture) in a suitable mold (of required size and shape) to cause

cohesion to occur between the powder particles.

d. Presintering the powder compact by heating and holding it at a moderate temperature.

Powder Metallurgy




Presintering develops additional green strength and drives off mixing lubricants and/or

moisture.

e. Sintering the compacted mass at a temperature high enough to cause diffusion and

intergranular crystal growth to occur.

f. Finishing and sizing the final product.

g. Annealing.

h. Repressing for greater density or closer dimensional control.

i. Machining.

j. Polishing.

k. Rolling, forging or drawing.

l. Surface treatments to protect against corrosion.

Different steps in a powder-metallurgy process are shown in Figure. 8.1

Fig. 8.1 Powder metallurgy processess

8.6 CHARACTERISTICS OF METAL POWDERS

The performance of metal powders during processing anal the properties of powder

Powder Metallurgy




metallurgy products are highly depend upon the characteristics of metal powders used.

Most important characteristics of metal powders are:

1. Purity,

2. Chemical composition,

3. Particle size,

4. Size distribution,

5. Particles shape,

6. Particle microstructure

7. Apparent density, and

8. Flow rate.

Chemical Composition implies the type and percentage of alloying elements and impurities

and usually determines the particle hardness and compressibility. The chemical composition

of a powder can be determined by chemical analysis methods.

Particle Size is expressed by the diameter for spherical shaped particles and by the average

diameter for non-spherical particles as determined by sieving method or microscopic

examination. Metal powders used in powder metallurgy usually vary in size from 4 to 200

microns. Particle size influences mold strength, density/porosity of the compact,

permeability, flow and mixing characteristics, dimensional stability etc.

Particle-size Distribution is specified in terms of a sieve analysis. i.e., the amount of powder

passing through 100-, 200-, etc., mess sieves. Particle-size distribution influences packing of

the powder and its behaviour during molding and sintering.

Particle Shape influences the packing and flow characteristics of powders. There are various

shapes of metal powders, e.g.

- Spherical (Condensed zinc)

- Rounded (Atomized copper)

- Angular (Mechanically atomized antimony).

- Acicular, dendritic, flakes, irregular, etc.

Spherical particles have excellent sintering qualities, however, irregular-shaped particles are

superior for practical molding.

Particle Microstructure reveals various phases, impurities, inclusions, fissures and internal

porosity.

Apparent Density is defined as the weight of a loosely heaped quantity of powder necessary

to fill a given die cavity completely. Apparent density is influenced by chemical composition,

particle shape, size, size distribution, method of manufacture, etc. The apparent density of

iron powder (electrolytic) having average particle size of 63 microns is 2.56 gm/cc.

Flow Rate is defined as the rate at which a metal powder will now under gravity from a

container through an orifice both having the specific shape and finish.

Powder Metallurgy




Flow rate measures the ability of a powder to be transferred. Flow rate is an important

characteristic because the die must be filled rapidly with powder to achieve high rate of

production and economy.

Flow rate depends upon particle size, shape, apparent density, etc. Spherical shaped metal

powders possess maximum flow rates whereas dendritic ones the least.

8.7 PRODUCTION OF METAL POWDERS

The particle size of powders falls into a range of 1 to 100µ (1µ = 10-6

metre), with the range

of 10 to 20µ (micron) being predominant. There are various methods of manufacturing

powders of this size, but those commonly used are:

1. Atomization

2. Reduction

3. Electrolysis

4. Crushing

5. Milling

6. Condensation of metal vapours.

7. Hydride and carbonyl processes.

Fig. 8.2 Methods of producing metal powders

(1) In Atomization, the molten metal is directed through an orifice and as it emerges, a high

pressure stream of gas or liquid impinges on it causing it to atomize into fine particles.

Frequently an inert gas is employed in order to improve the purity of the powder.

Powder Metallurgy




Atomization is used mostly for low melting point metals because of the corrosive action of

the metal on the orifice (or nozzle) at high temperatures.

(2) In Reduction process, the compounds of metals (usually oxides, e.g., iron oxide) are

reduced with CO or H2 at temperatures below the melting point of the metal (e.g., iron) is an

atmosphere-controlled furnace. The reduced product is then crushed and ground.

Sponge-iron powder is produced this way.

Fe3O4 + 4C = 3Fe + 4CO

Fe3O4 + 4CO = 3Fe + 4CO2

Copper powder can be produced by the same method i.e., by heating copper oxide in a stream

of hydrogen,

Cu2O + H2 = 2Cu + H2O

Powders of W, Mo, Ni and CO are also manufactured by reduction process. Reduction

process is a convenient, economical and flexible method and perhaps the largest volume of

metallurgical powders is made by the process of oxide reduction.

(3) Electrolysis is principally used for the production of extremely pure powders of copper

(and Iron). Electrolysis is similar to electroplating. For making copper powder, copper plates

are placed as anodes in a tank of electrolyte, whereas aluminium plates are placed into the

electrolyte to act as cathodes. High amperage produces a powdery- deposit of anode metal on

the cathodes.

After a definite time period, the cathode plates are taken out from the tank, are rinsed to

remove electrolyte and are dried. The (Cu) deposit on the cathode plates, is then scraped off

and pulverized to produce powder of the desired grain size.

(4) Crushing requires equipments such as stamps, hammers, jaw crushers or gyratory

crushes. Various ferrous and non-ferrous alloys can be heat-treated in order to obtain a

sufficiently brittle material which can be easily crushed into powder form.

(5) Milling operation is carried out by using equipments such as ball mill, impact mill, eddy

mill, disk mill, vortex mill, etc. Milling (or grinding) can be classified as comminution of

brittle, friable, tough and hard materials and pulverization of malleable and ductile metals.

A ball mill is a horizontal barrel-shaped container holding a quantity of balls which, being

Powder Metallurgy




free to tumble about as the container rotates, crush and abrade any powder particles that are

introduced into the container.Generally, a large mass to be powdered, first of all, goes

through heavy crushing machines, then through crushing rolls and finally through a ball mill

to produce successively finer grades of powder.

(6) Condensation of metal powders. This technique can be applied in the case of metals,

such as Zn, Cd, and Mg, which can be boiled and the vapour are condensed in a powder form.

A rod of metal (say Zn) is fed into a high temperature flame. The vaporized droplets of metal

are allowed to condense on to a cool surface of a material to which they will not adhere.

This method is not very useful for large-scale production of powder.

(7) Hydride and Carbonyl Processes

- Tantalum, niobium and zirconium, when made to combine with hydrogen form

hydrides that are stable w. room temperature, but begin to dissociate into hydrogen

and the pure metal (e.g., tantalum or niobium) when heated to about 350°C.

- In the same way, nickel and iron can be made to combine with CO to form volatile

carbonyls. The carbonyl vapour is then decomposed in a cooled chamber so that

almost spherical particles of very pure metal are deposited.

Ni + 4CO

Ni (CO) 4 (Nickel carbonyl)

8.8 BLENDING AND MIXING OF POWDERS

Before the powders are pressed into shape, they are usually blended for the following

reasons:

1. To add lubricants (such as stearic acid, graphite, oils, paraffin, glycerin, etc.) (to

powder) to reduce friction during the pressing operations. Powder particles get coated

with lubricants. This reduces die wear and lowers the pressure required for pressing.

2. To mix powders of different materials (i.e., alloying action), in order to obtain

properties of heat resistance, friction, heavy weight and hardness.

3. To obtain uniform distribution of particle sizes.

4. To add volatilizing agents to give a desired amount of porosity.

Different powders in correct proportions are thoroughly mixed either wet or dry, in a ball

mill. In wet mixing, water or a solvent is used to obtain better mixing. Moreover, wet mixing.

1. Reduces dust,

2. Lessens explosion hazards which are present with some finely divided powders, and

Powder Metallurgy




3. Prevents surface oxidation.

Proper blending and mixing of the powders are essential uniformity of the finished product.

However, over-mixing should be avoided since it may decrease particle size and work

hardens the particles.

Typical flow sheets showing steps in the manufacture of powder metal parts are shown in

Figure 8.3

8.9 COMPACTING

After blending and mixing, the next step is that of compacting or pressing the powders into

their semi-finished form preparatory to sintering.

The purpose of compacting is to consolidate the powder into the desired shape and as closely

as possible to final dimensions, taking into account any dimensional changes that result from

sintering. Compacting also imparts

1. The desired level and type of porosity, and

2. Adequate strength for handling.

Powders are compacted by using high pressures. The degree of pressure depends upon:

1. The required density of the final product, and

2. The ease with which the powder particles will weld together.

Powder Metallurgy




Compacting pressures may be applied in the following ways:

1. Die Pressing

2. Roll pressing

3. Extrusion

1. Die pressing is done in special presses that include a feed hopper for the powder, the

shaping die to form the product, an upper punch and a lower punch to apply correct pressures

onto the powder being compacted.Weighed- quantity of powder is placed in the die through

the hopper and is compressed under pressure ranging from 8 to 158 kg/sq. mm.

Fig. 8.3 Die pressing.

2. Roll pressing is used for production of continuous strip section, using a system as shown

in Fig. 8.3There are two rolls of appropriate size into which a regulated stream of powder is

guided, so that the rolls are able to apply the necessary compacting pressure in a continuous

sequence.

Fig. 8.4 Roll compacting

3. The Extrusion method of compacting does not give such efficient control as that given by

pressing or by rolling. It is difficult to obtain high densities and some porosity is always left.

Powder Metallurgy




Fig. 8.5 Extrusion method

8.10 PRESINTERING AND SINTERING

Presintering

- Frequently, powder metallurgy is used to make parts from materials that are very

difficult to machine.

When some machining is required on such parts, one goes lot presintering before the actual

sintering operation.

After presintering operation, the compacted part acquires sufficient strength to be handled

and machined without difficulty. Moreover, very little dimensional change takes place, then

in the final sintering, therefore, machining after final sintering may be eliminated.

- For presintering, the compacted parts are heated for a shot time at a temperature

considerably below the final sintering temperature.

- Presintering is necessary when holes are to be drilled in hard to machine parts.

- Presintering, in addition, removes lubricants and binders added to the powders during

the blending operation.

- Presintering can be eliminated if no machining of the final product is required.

Sintering

- After being compressed into a briquette of the shape required in the finished

component, the agglomerated metals are sintered. Sintering is done to achieve all

possible final strength and hardness needed in the finished product.

- Sintering consists of heating pressed metal or cermet compacts in batch or continuous

furnaces` to a temperature below the melting point of the major constituent in an inert

or reducing atmosphere (of hydrogen, dissociated ammonia or cracked hydrocarbon),

where time, temperature, heating rate and cooling rate are automatically controlled.

- Most metals are sintered at 70 to 80% of the melting temperature. Certain refractory

materials may be sintered at 90% of the melting point.

Powder Metallurgy




The sintering time varies from thirty minutes up to several hours. Sintering temperatures and

times vary considerably with different materials, e.g., Porous bronze bearings require

treatment for only a few minutes at 800°C; iron base compacts and cemented carbides require

treatment for up to 2 hours at 1200-1250°C, etc.

The relationship between time, t, and temperature, T, is given by

Log (t1/t2) = Qs/R (1/T1 – 1/ T2)

Where QS is the activation energy of sintering

and R is the gas constant.

As the sintering proceeds, volatile materials are driven off. Advantage is taken of this

phenomenon is making certain types of (i.e., porous) bearings and fine filters by the powder

metallurgy process.

- Sintering is essentially a process of bonding solid bodies (particles) by atomic forces.

Bonding of powder particles during sintering can take place in any of the three ways:

1. Melting of a minor constituent,

2. Volume diffusion, although surface diffusion, evaporation and condensation also

contribute to bonding.

3. Mechanical bonding or entrapment of non-metallic particles in a metal matrix.

8.11 HOT PRESSING

Concept

Hot pressing involves applying pressure and temperature simultaneously so that molding

(compacting) and sintering of the powder takes place at the same time in the die.

The effect of time is important, due to the viscous nature of the compact material, and the

higher the temperature the shorter the time.

Die materials include steels, graphite and ceramics. High speed steels may be used for short

times at 600°C and graphite for 700-3000°C. However, graphite dies are limited to pressures

of 140 kg/cm2 and also react with metals to produce carbides.

Advantages of Hot pressing

- Reduction in gas content and shrinkage effect.

- Higher strength and hardness.

- Higher density and sound compacts.

- Hot pressing breaks-down any oxide film and exposes fresh surfaces.

Disadvantages of Hot pressing

Powder Metallurgy




- High cost of dies (to withstand pressures at elevated –temperatures).

- Process takes a considerable time to complete.

Applications of Hot pressing

- Hot pressing is used for the production of very hard cemented carbide parts.

8.12 SECONDARY OPERATIONS

In many cases, the metal parts may be used in the as-sintered condition, but in other cases

where desired surface finish, tolerance or metal structure cannot be obtained by briquetting

(Briquetting is the act of converting loose powder into a green compact of accurately defined

size and shape. The briquette is considered fairly fragile, but it can be handled), certain

additional operations must follow. They are

1. Sizing

2. Coining

3. Machining

4. Impregnation

5. Infiltration

6. Plating

7. Heat treatment

8. Joining

1. Sizing.

Sintering process produces some distortion and alterations in size. After the part has been

sintered, in order to make it dimensionally correct, it is placed in a die and is repressed.

Sizing improves surface finish of the component also. However, a slight change in density

occurs during sizing.

2. Coining.

The sintered part is repressed in the die to reduce the void space and impart the required

density.

3. Machining.

Features such as threads, under cuts, grooves etc., are usually not practical for powder

metallurgy fabrications, and are generally machined on parts after they have been presintered.

Boring, turning, drilling, tapping, etc., can be done on presintered parts using tungsten

carbide cutting tools.

4. Impregnation

Sintered parts may be impregnated with oil, grease, wax or other lubricating materials, in

case self-lubricating properties are desired. Parts are immersed in lubricants heated to

approximately 93°C. The porous structure gets completely (about 90%) impregnated in 10 to

20 minutes. The lubricant is retained in the part by capillary action.

The sintered part may be impregnated with plastics also. This is done in order to,

Powder Metallurgy




1. Improve corrosion resistance,

2. Seal prior to plating.

3. Improve machinability.

4. Introduce pressure tightness.

5. Infiltration

A part is first pressed and sintered from iron powder to about 77% of theoretical density.

Then a replica (or infiltration) blank of copper (or brass) is placed over the part which is sent

through the furnace. The infiltrant melts and soaks through the porous part, producing a

density close to 100%.

Infiltration provides increased strength, hardness and density not obtainable by straight

sintering.

6. Plating

Plating is carried out in order to,

1. Impart a pleasing appearance (Cr plating).

2. Protect from corrosion (Ni plating).

3. Improve wear resistance (Ni or Cr plating).

4. Improve frictional (Tin plating) and hardness characteristics (Cr plating)

5. Improve electrical conductivity (Cu and Ag plating).

- Before plating, the part is impregnated with plastic resin so that the electrolyte is not

entrapped in the porous structure during plating.

- Sintered parts, then, may be plated with Cr, Ni, Co, Cd, Zn, brass, etc.

7. Heat treatment

Sintered parts are heat treated in order to improve:

1. Wear resistance.

2. Grain structure.

3. Hardness.

4. Strength.

To prevent oxidation of the internal structure, the heat treatment is carried out in controlled

atmosphere. The porosity of sintered parts decreases the heat conductivity, therefore longer

heating and shorter cooling periods are required. Following heat treatment processes are

usually applied to parts made by powder metallurgy.

1. Stress relieving

2. Carburizing

3. Carbonitriding

4. Nitriding

5. Through hardening

6. Induction hardening

7. Precipitation hardening.

Powder Metallurgy




8. Joining of sintered parts

Parts may be joined after they have been sintered or the joining may be incorporated into

sintering operation. Various joining, techniques are

1. Soldering (on Al and Cu based sintered parts).

2. Brazing (carried out in vacuum or controlled atmosphere. High frequency heating is

preferred).

3. Welding (TIG welding, projection welding, friction welding, electron beam welding,).

References


Questionnaire

1. Briefly describe various steps in the production of a product through powder-

metallurgy process.

2. Briefly describe any five engineering products produced by powder metallurgy.

3. Briefly give advantages and disadvantages of powder metallurgy process.

4. Why is particle-size distribution important in the packing of powders?

5. Discuss the importance of particle shape on the properties of sintered compacts?

6. List the three common methods of powder production and discuss their influences on

the properties of the final product.

7. Contrast mechanical and hydraulic compacting presses with regard to advantages,

disadvantages, and applications.

Powder Metallurgy




8. Why is sintering carried out in a controlled-atmosphere furnace?

9. Why do elevated temperatures tend to favor the sintering process although sintering

forces tend to decrease with increasing temperature?

10. What are the advantages and disadvantages of hot pressing as compared with cold

compacting and sintering?

11. Give' three specific applications of powder metallurgy parts. Describe how these parts

may be manufactured by other methods, and give the advantages of the powder

metallurgy method.

12. Why is pore size important in the manufacture of self-lubricating bearings? How may

pore size be controlled?

Jomeny end quench test





Objective

To understand the concept of hardenability and obtain the hardness distribution curve with

the help of Jomeny End quench test apparatus.

9.1 Hardenability

When a steel piece of a larger cross section is heated to austenite temperature and then

quenched, the cooling rate decreases from the surface to the interior. Martensite is obtained at

the surface due to the highest cooling rate. But it is not possible to get a martensitic structure

at the centre due to the relatively slow cooling rate. Hence, a gradient of hardness exists from

the surface to the centre. Since every grade of steel has its own transformation characteristics,

the depth of penetration of hardness across the cross section differs. The measure of this

property is termed as hardenability of steel.

Hardenability is defined as the relative ability of steel to be hardened by quenching and it

determines the depth and distribution of hardness across the cross section. Hardenability

should not be confused with maximum hardness of steel.

Hardenability is a very useful and important property of steel. It determines the rate at which

the given steel should be quenched. Maximum hardness is mainly a function of carbon

content. Hardenability of steel depends on

1. Composition of steel

2. Method of manufacture

3. Section of the steel

4. The quenching media

5. Quenching method

In industry, a simple experiment called Jominy end quench test (named after Walter Jominy,

American metallurgist) is used to determine the hardenability of steel.

9.2 Equipment

1. Electric Furnace

2. Jominy End Quench Test Fixture

3. Jominy Specimens (Made as per ASTM standard)

4. Rockwell Hardness Testing Machine





9.3 Test Procedure

1. Preheat the furnace to 1700°F.

2. Place the Jominy specimen in the furnace and soak for one hour.

3. Turn the water on at the Jominy sink. Adjust the free water column to about 2.5 in.

Swivel the baffle plate to block the water column so that there is no contact between

water and the test specimen when the test specimen is initially placed on the fixture.

4. Remove the Jominy specimen from the furnace and place it in the fixture as shown in

Figure 10.10. Swivel the baffle out of position so that the water impinges on the bottom

of the specimen without wetting the sides of the specimen. Leave water running for about

15 minutes.

Fig 9.1 Apparatus used in the test and Standard form of test piece

5. Remove the Jominy specimen from the fixture and grind a flat on the side of the

specimen.

6. Mark points on the ground surface at an interval of 1.6 mm distance from the

quenched end as shown in Figure 10.11.

7. Take readings at an interval of 1.6 mm intervals. Near the quenched end, this interval

is reduced to 0.8 mm as hardness values vary rapidly.

8. The results are expressed as a curve of hardness value versus distance from the

quenched end (Jominy distance). This curve is called the Jominy hardenability curve.





Fig 9.2 Hardness variation in quenched Jominy bar

9.4 REPRESENTATION OF HARDENABILITY DATA

Hardenabiiity curves

A typical hardenability curve is shown In Fig. 10.11. The quenched end is cooled

rapidly and exhibits more hardness. This is because the product is 100% martensite at

this position. Cooling rate decreases with the distance from the quenched end and the

hardness also decreases accordingly. The diminishing cooling rate leads, to the

formation of a softer pearlite, which is mixed with martensite and bainite.

Fig. 9.3 Hardenability curves for five different steels, each containing 0.4% C and

varying alloy compositions





High hardenable steel retains large hardness values, for a relatively long distance. Fig. 10.11

shows the typical Jominy curves for different steels having the same carbon content, but

differing in alloy composition.

All these steels have identical hardness at the quenched end (57 HRC) because hardness is a

function of carbon content. But hardenability is low for plain carbon steel because the

hardness drops off more steeply after a short Jominy distance. By contrast, the changes in

hardness for other alloys are more gradual. That is, the high quenched hardness would persist

to a much greater depth. Comparatively, alloy A is more hardenable than others.

Hardenability Band

A Jominy curve is strictly valid only for steel having a particular chemical composition and

average grain size. But, during industrial production of steel, there is always a slight variation

in chemical composition and average grain size from one batch to another. This result in

some scatter in the measured hardenability data, and these data are plotted as a band rather

than as a single curve. Hardenability curves of all batches lie in this range. These bands are

termed as hardenability bands.

Cooling rate curves

Distance from

quenched end, IN

Cooling rate °F/s

at 1300°F

Distance from

Quenched end, In

Cooling rate, °F/s

at 1300°F

1/16 490 11/16 19.5

1/8 305 3/4 16.3

3/16 195 13/16 14.0

1/4 125 7/8 12.4

5/16 77.0 15/16 11.0

3/8 56.0 1 10.0

7/16 42 1 ¼ 7.0

1/2 33 1 ½ 5.1

9/16 26 1 ¾ 4.0

5/8 21.4 2 3.5

Cooling rates at distances from the water-cooled end of the standard end-quench

hardenability test bar





Cooling rate at different locations inside the bar of varying cross sections such as round, flat

or square is correlated with the cooling rate at different distances along the quenched end of

the Jominy bar. Fig. 10.13 shows cooling rate as a function of diameter at the surface, three

quarter radius, mid radius and centre position for cylindrical bars quenched in mildly agitated

water and oil. The equivalent Jominy distances are included along the bottom axis.

(a) (b)

Fig. 9.4 CRC for cylindrical bars quenched in mildly agitated (a) Water (b) Oil

Use of Hardenability Data

1. By using the hardenability curves and cooling rate curves, the hardness existing at a

particular section in a steel specimen after quenching could be predicted. For example, the

centre of the 75mm diameter round bar quenched in oil has a cooling rate of 5.6°C/s. Since

the centre of the round bar has the same cooling rate as a Jominy test bar at a point 25mm

from the quenched end, the hardness at the two positions are the same. The hardness at 25mm

from the quenched end is found from the hardenability curve of that steel. This is equal to the

centre hardness of a 75mm bar after quenching.

2. These curves are also used to select a particular type of steel to meet a minimum hardness at a

given location in a part quenched under given conditions.

References



Darshan Institute of Engineering & Technology · 2019. 11. 19. · specimen preparation for...

Documents

Transcript of Darshan Institute of Engineering & Technology · 2019. 11. 19. · specimen preparation for...