University of Santo Tomas

Faculty of Engineering

Department of Chemical Engineering

MSE300 GROUP REPORT:

METALS

3 ChE - A Group One

Alday, Joy Angelique

Arpon, Jolyn Dominique

Balagtas, Fracelyn

Baluyot, Allen Joseph

Bello, Jean Raynell

Beronio, Lemuel John

Bustillo, Khazel

Dr. Ma. Natalia Dimaano

MSE300 Professor

INTRODUCTION TO METALS

What are Metals?

Metals are materials that are composed of one or more metallic elements (often also

nonmetallic elements in relatively small amounts). Atoms in these materials are arranged in

a very orderly manner and relatively dense compared to the ceramics and polymers. With

regard to mechanical characteristics, metals are relatively stiff and strong yet it is capable of

large deformations without fracture. It is resistant to fracture making it more common for

structural applications. They have a large numbers of electrons that are not bound to any

particular atoms which contributes to the many properties of metals, like good conductors,

opaque and lustrous appearance.

Metallic Structures

The atomic bonding in this group of material is metallic, and thus, non-directional in

nature

Relatively large numbers of nearest neighbors and dense atomic packings for most

metallic crystal structures

It is because there are minimal restrictions as to the number and position of nearest-

neighbor atoms

For metals, in the hard-sphere model for the crystal structure, each sphere

represents an ion core

IMPORTANT CHARACTERISTICS OF A CRYSTAL STRUCTURE

1. Coordination Number - number of nearest-neighbor or touching atoms; for metals, each

atom has the same coordination number

2. Atomic Packing Factor (APF) - The sum of the sphere volumes of all atoms within a unit

cell divided by the unit cell volume

APF = Volume of atoms in a unit cell / total unit cell volume

FACE – CENTERED CUBIC (FCC) CRYSTAL STRUCTURE

Atoms are located at each of the corners and the

centers of all the cube faces

This structure is found many metals (Ex: Copper,

Aluminum, Silver, Gold, etc.)

Each corner atom is shared among 8 unit cells

Face-centered atoms only belong to two unit cells

The total number of atoms per FCC unit cell is

o [1/8 * 8] + [1/2 * 6] = 4 atoms

The coordination number is 12

The atomic packing factor for FCC is 0.74

o Maximum packing possible for spheres all having the same diameter

BODY – CENTERED CUBIC (BCC) CRYSTAL STRUCTURE

Atoms are located at all eight corners and a single atom at the

cube center

This structure is found in some metals (Ex: Chromium, Iron,

Tungsten)

Each corner atom is shared among 8 unit cells

The total number of atoms per BCC unit cell is

o [1/8 * 8] + [1 * 1] = 2 atoms

The coordination number is 8

The atomic packing factor for BCC is 0.68

THE HEXAGONAL CLOSE-PACKED (HPC) CRYSTAL STRUCTURE

Not all metals have cubic symmetry

This structure is found in Titanium, Cobalt, Magnesium, Zinc, etc)

Atoms per HCP unit cell = [1/6 * 12] + [1/2 * 2] + [1 *3] = 6 atoms

Coordination number = 12

APF = 0.74

POLYMORPHISM AND ALLOTROPY

Some metals as well as nonmetals, may have more than one crystal structure, this is a

phenomenon called polymorphism

When found in elemental solids, the condition is often termed as allotropy

The prevailing crystal structure depends on both the temperature and external

pressure

Example: carbon: graphite is the stable polymorph at ambient conditions, whereas

diamond is formed at extremely high pressures

Types of Metal Alloys

Metal alloys are often grouped into two classes depending on its composition. It can either

be ferrous or nonferrous metal alloys.

I. Ferrous Alloys

- Iron is the prime constituent and are produced in large quantities

- widespread use is accounted for by three factors:

o iron-containing compounds exist in abundant quantities within the earth‘s

crust

o metallic iron and steel alloys may be produced using relatively economical

extraction, refining, alloying, and fabrication techniques

o ferrous alloys are extremely versatile, in that they may be tailored to have a

wide range of mechanical and physical properties

- major advantage is susceptibility to corrosion

A. STEELS

- iron–carbon alloys that may contain appreciable concentrations of other alloying

elements

- mechanical properties are sensitive to the content of carbon (less than 1 wt. %)

- subclasses according to alloying elements:

o plain carbon steels

contain only residual concentrations of impurities other than

carbon and a little manganese

o alloy steels

more alloying elements are intentionally added in specific

concentrations

- subclasses according to carbon concentration:

o low-carbon steels

produced in greatest quantity

generally contain less than about 0.25 wt% C and are unresponsive

to heat treatments intended to form martensite

strengthening is accomplished by cold work

microstructures consist of ferrite and pearlite constituents

relatively soft and weak, but have outstanding ductility and

toughness

also machinable, weldable, and the least expensive to produce

applications include automobile body components, structural

shapes (I-beams, channel and angle iron), and sheets that are used

in pipelines, buildings, bridges, and tin cans

high-strength, low alloy steels (HSLA)

contain other alloying elements such as copper, vanadium,

nickel, and molybdenum in combined concentrations as

high as 10 wt%, and possess higher strengths than the plain

low-carbon steels

more resistant to corrosion than the plain carbon steels,

which they have replaced in many applications where

structural strength is critical

o medium-carbon steels

have carbon concentrations between about 0.25 and 0.60 wt%

alloys may be heat treated by austenitizing, quenching, and then

tempering to improve their mechanical properties

additions of chromium, nickel, and molybdenum improve the

capacity of these alloys to be heat treated, giving rise to a variety of

strength–ductility combinations

heat-treated alloys are stronger than the low-carbon steels, but at a

sacrifice of ductility and toughness

applications include railway wheels and tracks, gears, crankshafts,

and other machine parts and high-strength structural components

calling for a combination of high strength, wear resistance, and

toughness

Society of Automotive Engineers (SAE), the American Iron and Steel

Institute (AISI), and the American Society for Testing and Materials

(ASTM)

responsible for the classification and specification of steels

as well as other alloys

designation for these steels is a four-digit number:

o the first two digits indicate the alloy content; the last

two, the carbon concentration

o for plain carbon steels, the first two digits are 1 and 0

o alloy steels are designated by other initial two-digit

combinations

o third and fourth digits represent the weight percent

carbon multiplied by 100

Unified Numbering System (UNS)

uniformly indexing both ferrous and nonferrous alloys

consists of a single-letter prefix followed by a five-digit

number

o letter is indicative of the family of metals to which an

alloy belongs.

o UNS designation for these alloys begins with a G,

followed by the AISI/SAE number

o the fifth digit is a zero

o high-carbon steel

having carbon contents between 0.60 and 1.4 wt%

hardest, strongest, and yet least ductile of the carbon steels

wear resistant and capable of holding a sharp cutting edge

chromium, vanadium, tungsten, and molybdenum are added to

form very hard and wear- carbide compounds

utilized as cutting tools and dies for forming and shaping materials,

as well as in knives, razors, hacksaw blades, springs, and high-

strength wire

o stainless steels

highly resistant to corrosion which may be enhanced by addition of

nickel and molybdenum

predominant alloying element is chromium, concentration of at

least 11 wt% Cr

resist oxidation and maintain mechanical integrity

strengthened by precipitation hardening heat treatment

classes on the basis of the predominant phase constituent of the

microstructure:

martensitic stainless steels

o capable of being heat treated in such a way that

martensite is the prime microconstituent

o magnetic

austenitic stainless steels

o austenite phase field is extended to room

temperature

o most corrosion resistant

o strengthened by cold work

austenitic stainless steels

o composed of ferrite (BCC) phase

o magnetic

o strengthened by cold work B. CAST IRONS

- class of ferrous alloys with carbon contents above 2.14 wt%

- easily melted and brittle

- casting is the most convenient fabrication

o gray iron

carbon and silicon contents vary between 2.5 and 4.0 wt% and 1.0

and 3.0 wt%, respectively

the graphite exists in the form of flakes, which are normally

surrounded by an ferrite or pearlite matrix

because of these graphite flakes, a fractured surface takes on a

gray appearance

weak and brittle in tension as a consequence of its microstructure

tips of the graphite flakes are sharp and pointed

strength and ductility are much higher under compressive loads

molten state they have a high fluidity at casting temperature, which

permits casting pieces having intricate shapes

casting shrinkage is low

least expensive of all metallic elements

o ductile or nodular iron

produced by the addition of small amount of magnesium and/or

cerium to the gray iron before casting

castings are stronger and much more ductile than gray iron

applications for this material include valves, pump bodies,

crankshafts, gears, and other automotive and machine components

o white and malleable iron

extremely hard and very brittle---unmachinable

heating for a prolonged time period causes the decomposition of

cementite forming graphite

microstructure is similar to that of nodular iron

high strength and appreciable ductility or malleability

applications include connecting rods, transmission gears, and

differential cases for the automotive industry, and also flanges,

pipe fittings, and valve parts for railroad, marine, and other heavy-

duty services II. Nonferrous Alloys

- Cast alloys

o Alloys that are brittle and cannot be formed or shaped with appreciable

deformation

- Wrought alloys

o Amendable to mechanical deformation A. COPPER AND ITS ALLOYS

- improve mechanical and corrosion-resistance properties

- brass

o zinc is the predominant alloying element

o FCC crystal structure

o For alloys with 35wt% Zn--- soft, ductile and easily cold worked

o uses are for brass alloys include costume jewelry, cartridge casings,

automotive radiators, musical instruments, electronic packaging, and

coins

- bronze

o alloys of copper and other elements including tin, aluminum, silicon and

nickel

o stronger than brass and has a high degree of corrosion resistance

- beryllium copper

o most common precipitation hardenable copper alloys

o remarkable combination of properties: tensile strengths, excellent

electrical and corrosion properties, and wear resistance when properly

lubricated

o may be cast, hot worked, or cold worked

o high strengths are attained by precipitation-hardening heat treatments

o costly because of the beryllium additions, which range between 1.0 and

2.5 wt%

o applications include jet aircraft landing gear bearings and bushings,

springs, and surgical and dental instruments B. Aluminum and its Alloys

- Have low density, high electrical and thermal conductivities, resistance to

corrosion, high ductility

- Low melting temperature at around 660 C

- Principal alloying elements include copper, magnesium, silicon, manganese, and

zinc

- Composition for both types is designated by a four-digit number that indicates

the principal impurities, and in some cases, the purity level

- For cast alloys, a decimal point is located between the last two digits followed by

a hyphen and temper designation

- Temper designation---- a letter and possibly a one- to three-digit number, which

indicates the mechanical and/or heat treatment to which the alloy has been

subjected

- Specific strength

o quantified by the tensile strength–specific gravity ratio

o even if an alloy of one of these metals may have a tensile strength that is

inferior to a more dense material, on a weight basis it will be able to

sustain a larger load C. Magnesium and its Alloys

- Has Low density and used where light weight is an important consideration

- Difficult to deform

- has a moderately low melting temperature 651 °C

- unstable and especially susceptible to corrosion in marine environment

- used in aircraft and missile application

- magnesium alloys have replaced engineering plastics that have comparable

densities inasmuch as the magnesium materials are stiffer, more recyclable, and

less costly to produce D. Titanium and its Alloys

- Pure metal has low density, high melting point and high elastic modulus

- Alloys are strong, has high tensile strength, highly ductile and easily forged and

machined

- Highly reactive at elevated temperature

- commonly utilized in airplane structures, space vehicles, surgical implants, and in

the petroleum and chemical industries E. Refractory Metals

- Have extremely high melting temperatures

- niobium (Nb), molybdenum (Mo), tungsten(W), and tantalum (Ta)

- Interatomic bonding in these metals is extremely strong, large elastic moduli and

high strength and hardness F. Superalloys

- Have superlative combinations of properties

- are used in aircraft turbine components, which must withstand exposure to

severely oxidizing environments and high temperatures for reasonable time

periods

- classified according to the predominant metal in the alloy, which may be cobalt,

nickel, or iron

- utilized in nuclear reactors and petrochemical equipment G. Noble Metals

- noble or precious metals are a group of eight elements that have some physical

characteristics in common

- expensive and characteristically soft, ductile, and oxidation resistant

- silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium H. Miscellaneous Nonferrous Alloys

- Nickel and its alloys

o Highly resistant to corrosion

o coated or plated on some metals that are susceptible to corrosion as a

protective measure

o Monel

a nickel based alloy containing approximately 65 wt% Ni and 28

wt% Cu

has very high strength and is extremely corrosion resistant

used in pumps, valves, and other components that are in contact

with some acid and petroleum solution

- lead, nickel, tin and their alloys

o mechanically soft and weak, have low melting temperatures, are quite

resistant to many corrosion environments, and have recrystallization

temperatures below room temperature

o applications for lead and its alloys include x-ray shields and storage

batteries

o primary use of tin is as a very thin coating on the inside of plain carbon

steel cans that are used for food containers

- Unalloyed Zinc

o relatively soft metal having a low melting temperature and a subambient

recrystallization temperature

o susceptible to corrosion

o applications of zinc alloys include padlocks, automotive parts and office

equipment

- zirconium and its alloys

o ductile and have other mechanical characteristics that are comparable to

those of titanium alloys and the austenitic stainless steels

o primary asset of these alloys is their resistance to corrosion in a host of

corrosive media, including superheated water

o transparent to thermal neutrons, so that its alloys have been used as

cladding for uranium fuel in water-cooled nuclear reactors

o materials of choice for heat exchangers, reactor vessels, and piping

systems for the chemical-processing and nuclear industries

ALLOYS FOR HIGH TEMPERATURE USE

- melting temperature, elastic modulus, and grain size affect the creep

characteristics of metals

- the higher the melting temperature, the greater the elastic modulus, and the

larger the grain size, the better is a material‘s resistance to creep

- smaller grains permit more grain-boundary sliding, which results in higher creep

rates

- stainless steels, refractory metals and superalloys are resilient to creep and

employed in high-temperature service applications

- creep resistance of the cobalt and nickel superalloys is enhanced by solid-

solution alloying, and also by the addition of a dispersed phase which is virtually

insoluble in the matrix

- directional solidification produces either highly elongated grains or single-

crystal components

- controlled unidirectional solidification of alloys having specially designed

compositions wherein two-phase composites result

Point Defects on Metals

Point defects are where an atom is missing or is in an irregular place in the lattice structure.

Vacancy

The simplest point defects.

Empty spaces where an atom should be, but is missing.

The number of vacancies, Nv, for a given quantity of material depends on and

increases with temparature

Self- Interstitial

An atom from the crystal is crowded into an interstitial site

Self-Interstitial introduces relatively large distortion in the surroundings, because the

atom is much larger than the interstitial site

IMPURITIES

Alloys

metal composed of more than one element.

combining it with one or more other metals or non-metals that often enhance its

properties. Solid Solution

―new second phase‖

It is formed when solute atoms are added to the host material, and the crystal

structure is maintained.

IMPURITY POINT DEFECT FOUND IN SOLID SOLUTION:

Substitutional

The host atoms are replaced by the solute or impurity atom.

kT

vQ

vNeN

Where: Nv = number of vacancies

N = total number of atomic site

Qv = Energy required for the formation of vacancy

T = Absolute temperature

k = Boltzmann’s constant

Interstitial

Solute or impurity atoms takes up sites that are normally unfilles

Features of solute and solvent atoms that determine the degree of impurity.

1. Atomic size factor. Difference of atomic radii of +,-15%.

2. Crystal structure. For appreciable solid solubility.

3. Electronegativity. The more electropossitive one element the more electronegative

the other.

4. Valences.

Fracture

Fundamentals of Fracture

A fracture, in its simplest form, can be descride as a single body beig separated into piceis

by an imposed stress ( maybe tensile, compresiive, shear, etc.,) and temperatures lower

than the melting piont.

Fracture Mode

Ductile - exhibit substantial plastic deformation with high energy absorption before fracture.

Brittle- little or no plastic deformation with low energy absorption

Ductile Fracture

have their own distinctive features on both macroscopic and microscopic level.

Ductility is often characterized by the material's ability to be stretched into a wire

Stages in Cup and Cone Fracture

Brittle Fracture

Without any appreciable deformation, and by rapid crack propagation

The crack motion is nearly perpendicular to the direction of the applied tensile

stress.

Type of Brittle Fracture

Transgranular (or transcrystalline)

The fracture cracks pass through the grains.

Crack propagation corresponds to the successive and repeated breaking of atomic

bonds along specific crytallographic planes, cleavage.

Fracture surface have faceted texture because of different orientation of cleavage

planes in grains.

Intergranular Fracture – fracture crack propagation along its grain boundary

Forms Of Corrosion In Metals

Uniform Attack

Occurs with equivalent intensity over the entire exposed surface

Leaves behind a scale or deposit

Examples: Rusting of steel and iron & tarnishing of silverware

Galvanic Corrosion

Occurs when two metals or alloys having different compositions are electrically

coupled while exposed on an electrolyte

Example: steel screws corrosion with brass in a marine environment

Crevice Corrosion

Occurs in crevices and recesses or under deposits of dirt or corrosion products

where the solution become stagnant and a localized depletion of dissolved oxygen

Crevice must be wide enough to penetrate yet narrow enough to make the solution

stagnant

Prevented by using welding using non-absorbing gasket, removing accumulated

deposits frequently and designing containment vessels to avoid stagnancy

Pitting

A very localized corrosion attack where small pits or holes are formed

An extremely insidious type of corrosion (often undetected with little material lose)

Intergranular Corrosion

Disintegration occurs on grain boundaries for some alloys in a specific environment

Prevalent in stainless steel

Weld decay occurs on the welding of stainless steel

Prevention: (1) subjecting to high temperature heat treatment (2) lowering the

carbon content below 0.03 wt% C and (3) alloying the stainless steels with niobium

or titanium

Selective Leaching

Found in solid solution alloys

Occurs when one element/constituent is preferentially removed as a consequence of

corrosion processes that significantly impaired the mechanical properties of the

metal/alloy

Example: dezincification of brass

Erosion – Corrosion

Arises from the combined action of chemical attack and mechanical abrasion or wear

as a consequence of fluid motion

Characterized by surface grooves and waves having contours from a fluid flow

Found in piping especially the positions where the fluid changes direction or

turbulent flow

Prevention: changed the design to eliminate fluid turbulence, removal of particulates

and bubbles from the flowing fluid and used other materials that resist corrosion

Stress Corrosion (Cracking)

the combined action of an applied tensile stress and a corrosive environment

small cracks form and propagate in a direction perpendicular to the stress with the

result of failure behavior like a brittle material even though the material is

intrinsically ductile

Example: stainless steels corrosion in solution containing chloride ions and brass

corrosion on contact with ammonia

Prevention: lower the magnitude of stress by reducing the external load or

increasing the cross-sectional area perpendicular to the applied stress and heat

treatment

Hydrogen Embrittlement

A significant reduction in ductility and tensile strength when an atomic hydrogen

penetrates into the some metal alloys (like steel)

Maybe called ―hydrogen-induced cracking‖ or ―hydrogen stress cracking‖

Brittle fracture occurs catastrophically

High strength steels are susceptible to this type of corrosion.

Martensitic steels and FCC alloys are vulnerable to this type of corrosion because of

their resiliency, for the former and high ductility, for the latter. However, strain

hardening increases their susceptibility to the corrosion

Prevention: reducing the tensile strength via heat treatment, removal of H source,

baking of alloy or substitution of a more embrittlement-resistant alloy

Phase Diagram and Phase Transformations

Alloy Phase diagrams

The importance of strength in materials is to allow greater loads to be carried with

smaller cross sections and, thus, lower structural weight. Also, the one of the most effective

and widely used means of making metallic elements stronger is to alloy them by adding

other elements or solutes to produce solid-solution strengthening.

Alloying metals can have the following effects, any one or more of which may be

considered useful or essential to an engineer to achieve desired properties to meet design

performance requirements.

Yield and tensile strength can be increased to allow structural weight to be reduced.

1. Hardness can be increased to resist wear or penetration.

2. The melting point of the solvent or base metal can be increased – or, at least, the

strength of the solvent at elevated temperatures can be dramatically increased –

to allow service at elevated temperature.

3. The melting point of the solvent or base metal can be lowered (often significantly)

to make casting easier or to provide filler metals for brazing or soldering,

because these joining processes minimally disrupt base metal structure and

properties from heat.

4. Solid phase transformations can be induced to produce second phase solid

solutions, intermetallic compounds, or, occasionally, ceramics that increase

strength beyond single-phase solid-solution strengthening (e.g., phase boundary

strengthening and precipitation hardening).

5. Solid phase martensite transformations may occasionally be induced.

6. Corrosion resistance of the base metal can be enhanced, often dramatically.

7. Fabricability can be enhanced through improved machinability (e.g., easier chip

formation), improved castability (e.g., better fluidity, lower melting

temperature), improved formability (e.g., less tendency to crack), or improved

weldability (e.g., less susceptibility to cracking).

Fig 1. The solubility of sugar in water in varying temperature is shown.

A phase on a material is a region that differs in its microstructure and/or composition

from another region. Phase diagrams are graphical representations of what phases are

present in a materials system at various temperatures, pressures, and compositions. Most

phase diagrams are constructed by using equilibrium conditions and are used by engineers

and scientists to understand and predict many aspects of the behaviour of materials.

For solids: Chemically and structurally distinct

For liquids: Miscibility

For gases: Always 1 phase

The Language of Phase Diagrams

A component is a pure substance that is the sole ingredient or one of several

ingredients that a phase diagram refers or relates to. The number of components involved in

a phase diagram over the full range of possible compositions (i.e., concentrations of one in

the other) constitutes the system. A phase refers to a substance that is chemically

homogenous (i.e., has a distinct and unique set of physical properties, such as crystal

structure, density, and even hardness and strength, etc.)

When two (or more) solid phases combine to form an intimate mechanical mixture in

which two distinct phases are on such a fine (microscopic) scale that they cannot be

distinguished from on another under an optical microscope, the result is called a

microstructural constituent. Microstructure refers to the aggregate physical entity seen

under a microscope. Phase equilibrium refers to the state of the thermodynamic equilibrium

as it applies to systems in which more than one phase exists.

Solubility Limits and Hume-Rothery’s Rules

Many times, there is a limit to how much solute can be dissolved into a solvent.

Fig 2. P-T Diagram for Water

In the mid-1930s, the English metallurgist William Hume-Rothery proposed some

rules he obtained from empirical observations of real alloy systems that explained and

allowed the prediction of the solid solubility of substitutional solutes in metals.

Hume Rothery Rules:

1. Relative Size Ratio ±15%

2. Crystal Structure-must be the same

3. Electronegativity Difference – within ± 0.4 e.u.

4. Valence must be the same

One-Component Pressure-Temperature Phase Diagrams:

The map of a pure material is called a one-component phase diagram. The simplest

case is Water, also known as a P-T diagram.

Gibbs Phase Rule

From thermodynamics considerations, J.W. Gibbs derived an equation that computes

the number of phases that can coexist in equilibrium in a chosen system. This equation,

called Gibbs phase rules, is

P + F = C + 2

Where P = number of phases that coexist in a chosen system

Fig 3. Nickel-copper phase diagram

C = number of components in the system

F = degrees of freedom

The degrees of freedom is the number of variable (pressure, temperature, and

compositions that can be changed independently without changing the number of phases in

equilibrium in the chosen system.

Binary Isomorphous Alloy Systems

A mixture of two metals is called a binary alloy and constitutes a two-component system

since each metallic element in an alloy is considered a separate component. In some binary

metallic systems, the two elements are completely soluble in each other in both the liquid

and solid states. In these systems, only a single type of crystal structure exists for all

compositions of the components, and therefore they are called isomorphous systems.

Fig 4. Tin-Lead Phase Diagram

The area above

the upper line in the

diagram, called the

liquidus, corresponds

to the region of

stability of the liquid

phase, and the area

below the lower line,

or solidus, represents

the region of stability

for the solid phase.

Binary Eutectic Alloy Systems

Many binary alloy

systems have components

that have limited solid

solubility in each ither as,

for example, in the lead-tin

system. The regions of

restricted solid solubility at

each end of the Pb-Sn

diagram are designated as

alpha and beta phases and

are called terminal solid

solutions since they appear

at the ends of the diagram.

Fig 6. Gold-Germanium Phase Diagram

Fig 7. Proof of the Lever Rule

In simple binary

eutectic systems like the Pb-

Sn one, there is a specific alloy

composition known as the

eutectic composition that

freezes at a lower temperature

than all other compositions.

This low temperature, which

corresponds to the lowest

temperature at which the

liquid phase can exist when

cooled slowly, is called the

eutectic temperature. In the

Pb-Sn system, the eutectic

composition and the eutectic

temperature determine a

point on the phase diagram

called the eutectic point.

When liquid of eutectic

composition is slowly cooled

to the eutectic temperature, the single liquid phase transforms simultaneously into two solid

forms (solid solutions α and β). This transformation is known as the eutectic reaction.

The Lever Rule

The weight percentages of the

phases on any two-phase region of a

binary equilibrium phase diagram can be

calculated by using the lever rule. By using

the lever rule, the weight percent liquid

and weight percent solid for any particular

temperature can be calculated for any

average alloy composition in the two-

phase liquid-plus-solid region of the binary

copper-nickel phase diagram.

Microstructure in Eutectic System

There are several different types of microstructure can exist when binary eutectic

alloys are cooled slowly depending on the composition. Alloys which are to the left of the

eutectic concentration (hipoeutectic) or to the right (hypereutectic) form a proeutectic phase

Fig 8. Cooling of liquid Pb-Sn system at different

composition.

Fig 9. Formation of the eutectic structure in the lead-tin system. In the

micrograph, the dark layers are lead-reach α phase, the light layers are

the tin-reach β phase.

Fig 10. Although the eutectic structure consists of two

phases, it is a microconstituent with distinct lamellar

structure and fixed ratio of the two phases.

before reaching the eutectic temperature, while in the solid + liquid region. The eutectic

structure then adds when the remaining liquid is solidified when cooling further. The

eutectic microstructure is lamellar (layered) due to the reduced diffusion distances in the

solid state.

At compositions between

the room temperature solubility

limit and the maximum solid

solubility at the eutectic

temperature, β phase nucleates as

the α solid solubility is exceeded

upon crossing the solvus line.

Compositions of α and β phases

are very different → theeutectic

reaction involves redistribution of

Pb and Sn atoms by atomic

diffusion. This simultaneous

formation of α and β phases result

in a layered (lamellar)

microstructure.

Fig 11. Schematic representations of the microstructures for an iron–

carbon alloy of hypoeutectoid composition C0 (containing less than 0.76

wt% C) as it is cooled from within the austenite phase region to below the

eutectoid temperature.

Iron-Carbon/Iron-Iron Carbide Phase Diagram

No phase diagram, none, is more important in metallurgy in particular and to

engineering in general than that of the iron-carbon (Fe-C) system. It is in the Fe-Fe3-C

system that steels, as well as cast irons, are found. It is a simple fact that it is not possible for

a country to be a major player in the modern world without the ability to produce

quality steel. The ability to make high-quality steel in one measure - if not the measure – of

the technological sophistication and economic strength of an industrialized society.

Some key points about the iron-iron carbide phase diagram are :

Pure iron (Fe), at the extreme left-hand vertical axis, exhibits three solid-state phases

(i.e., allotropic forms) before melting at 1538oC – namely, BCC α-Fe below 912oC,

FCC γ-Fe between 912oC and 1394oC, and BCC δ-Fe above 1394oC until melting

occurs at 1538oC.

Metastable iron carbide, Fe3C exist at 6.67 wt.% C, at the extreme right side of the

diagram.

The interstitial solid solubility of carbon (C) in a α-Fe is around 0.021 wt.% at 740oC

and one hundred times higher at 2.11 wt.% in γ-Fe at 1153oC. The solid solubility at

room temperature drops to less than 0.008 wt.%.

There is eutectic reaction at 4.2 wt.% C at 1153oC, a peritectic reaction at 0.2 wt.% C

at 1493oC, and a eutectoid reaction at 0.76 wt.% C at 740oC.

Cast irons are centered around the eutectic reaction at 4.3 wt.% C and range from

about 2.5 to 5.5 wt.% C.

Steels are centered around the eutectoid reaction at 0.77 wt.% C and range from

about 0.005 to 1.4 wt.% C.

Hypoeutectic case

Consider a mixture of A and B so that

the overall composition of the alloy places it

to the left of the eutectic point. Initially the

alloy is at a high enough temperature to

ensure that the mixture is fully liquid. When

the composition of an alloy places it to the

left of the eutectic point it is called hypo-

eutectic. The mixture is slow cooled,

undergoing no change in state until it

reaches temperature, when it reaches the

liquidus line.

As the alloy continues to cool the

existing nucleation sites will grow and

further nucleation sites will continue to form

Fig.12 Hypereutectoid alloys contain proeutectoid cementite (formed

above the eutectoid temperature) plus perlite that contain eutectoid

ferrite and cementite.

within the liquid parts of the mixture. These nucleating and growing regions of solid alloy

form grains and when these meet grain boundaries are formed. The primary alpha

dendrites grow, which accounts for the shapes the alpha forms in cross sectional samples.

The existing eutectic nucleation sites will grow, adding alpha to the stripes of alpha and beta

to the stripes of beta in the eutectic regions. New sites will continue to form. It is not

necessary to continue decreasing the temperature to achieve full solidification. The eutectic

liquid solidifies in the same way as a pure solid, at a specific temperature.

When alloy of eutectoid composition (0.76

wt % C) is cooled slowly it forms perlite, a

lamellar or layered structure of two phases: α-

ferrite and cementite (Fe3C). The layers of

alternating phases in pearlite are formed for the

same reason as layered structure of eutectic

structures: redistribution C atoms between ferrite

(0.022 wt%) and cementite (6.7 wt%) by atomic

diffusion.

Hypereutectic Cases

Consider a mixture of A and B so that the overall composition of the alloy places it to

the right of the eutectic point. Initially the alloy is at a high enough temperature to ensure

that the mixture is fully liquid. When the composition of an alloy places it to the right of the

eutectic point it is called hyper-eutectic. Note, though, that this is merely convention. If the

phase diagram had been drawn the other way around, with 100%A on the right and 100%B

on the left, then the same alloy would be called hypo-eutectic, as it would be to the left of the

eutectic point.

The mixture is slow cooled, undergoing no change in state until it reaches

temperature T1, when it reaches the liquidus line. Here, as the labelling suggests, beta starts

to solidify at any favourable nucleation sites. The beta solidifies as dendrites which grow to

become grains of beta. The first solid to form is called the primary solid and so, in this case,

primary beta is formed.

As the alloy continues to cool the existing nucleation sites will grow and further

nucleation sites will continue to form within the liquid parts of the mixture. These nucleating

and growing regions of solid beta alloy form grains and when these meetgrain boundaries

are formed. The primary beta dendrites grow, which accounts for the shapesthe beta forms

in cross sectional samples. The existing eutectic nucleation sites will grow, adding alpha to

the stripes of alpha and beta to the stripes of beta in the eutectic regions. New sites will

continue to form.

Note that, unlike the beta solidification, it is not necessary to decrease the

temperature to achieve full solidification. The eutectic solidifies in the same way as a pure

solid, at a specific temperature.

Microstructural And Property Changes In Iron-Carbon Alloys

Time and temperature are huge factors in the way we are able to manipulate the

properties of steels. We will look at some of the phase transformations that happen within

steels.

I. Isothermal Transformation Diagrams (Time-Temperature-Transformation or T-

T-T plots)

A. Pearlite - a lamellar constituent of steel consisting of alternate layers of ferrite

(alpha-iron) and cementite (iron Carbide Fe3C) and is formed on cooling austenite at

723°C. This produces a tough structure and is responsible for the mechanical

properties of unhardened steel.

Iron-Iron Carbide Eutectoid Reaction

Temperature plays an important role in the rate of the austenite-to-pearlite

transformation. The temperature dependence for an iron–carbon alloy of eutectoid

composition is indicated in the graph above which plots S-shaped curves of the percentage

transformation versus the logarithm of time at three different temperatures. For each curve,

data were collected after rapidly cooling a specimen composed of 100% austenite to the

temperature indicated; that temperature was maintained constant throughout the course of

the reaction. The S-shaped curves are shifted to longer times at higher T showing that the

transformation is dominated by nucleation and not by diffusion.

Two solid curves are plotted; one represents the time required at each temperature

for the start of the transformation; the other is for the transformation conclusion.

The dashed curve corresponds to 50% completion.

The S-shaped curve illustrates how the data transfer is made.

Note that the eutectoid temperature (727°C)is indicated by a horizontal line; at

temperatures above the eutectoid only austenite will exist.

The austenite-to-pearlite transformation will occur if an alloy is supercooled to below

the eutectoid.

To the left of the transformation start curve, only austenite will exist whereas to the

right of the finish curve, only pearlite will exist. In between, the austenite is in the

process of transforming to pearlite and thus, both microconstituent will be present.

Temperatures just below the eutectoid, very long times are required for the 50%

transformation, and thus, the reaction rate is very slow.

The transformation rate increases with decreasing temperature such that at 540°C

only about 3 s is required for the reaction to go to 50% completion.

Constraints are imposed on using

diagrams like this. First, this particular

plot is valid only for Fe-C alloy of

eutectoid composition. Second, these

plots are accurate only for

transformations in which the temperature

of the alloys is held constant throughout

the duration of the reaction.

The transformation of austenite to pearlite begins at the intersection, point C and has

reached completion by about 15 s, corresponding to point D.

The thickness ratio of the ferrite and cementite layers in pearlite is approximately 8

to 1.

The absolute layer thickness depends on the temperature of the transformation. The

higher the temperature, the thicker the layers.

At temperatures just below the eutectoid, relatively thick layers of both the α-ferrite and Fe3C phases are produced; this microstructure is called coarse pearlite. Longer

cooling times at higher temperatures, high diffusion rates allow for larger grain

growth and formation of thick layered structure of coarse pearlite.

The thin-layered structure produced in the vicinity of 540°C is termed fine pearlite.

Slow diffusion at low temperatures leads to fine-grained microstructure with thin-

layered structure of fine pearlite.

Photomicrographs of (a) coarse pearlite and (b) fine pearlite. 3000x

For iron–carbon alloys of

other compositions, a proeutectoid

phase (either ferrite or cementite)

will coexist with pearlite. Thus

additional curves corresponding to

a proeutectoid transformation also

must be included on the isothermal

transformation diagram

B. Bainite – another microconstituents that are products of the austentic transformation.

An acicular aggregate of ferrite and carbide particles formed when austenite is

transformed on cooling at temperatures in the intermediate (200-450°C) range, i.e.

above the martensite and below the pearlite range. If transformation temperature is

low enough 540°C) bainite rather than fine pearlite forms.

Bainite forms as needles or

plates, depending on the

temperature of the transformation;

the microstructural details of bainite

are so fine that their resolution is

possible only using electron

microscopy.

It is composed of a ferrite

matrix and elongated particles of

Fe3C. Furthermore, no proeutectoid

phase forms with bainite.

All three curves are C-

shaped and have a nose at

point N, where the rate of

transformation is at

maximum.

Note the top half (above the

neck of point N) is the

region where pearlite (P) is

formed, range of about 540

to 727°C. Below the point N,

bainite (B) is formed, range

of about 215 to 540°C,

instead of pearlite.

Once some portion of an

alloy has transformed to

either pearlite or bainite,

transformation to other

microconstituent is not

possible without reheating

to form austenite.

C. Spheroidite

If a steel alloy having either pearlitic or bainitic microstructures is heated to, and left

at, a temperature below the eutectoid for a sufficiently long period of time, for

example, at about 700°C for between 18 and 24 h—this microstructure will form.

The Fe3C phase appears as spherelike particles embedded in a continuous α-phase

matrix.

Composition of relative amounts of ferrite and cementite are not changing in this

transformation, only shape of the cementite inclusion is changing.

Transformation proceeds by C diffusion – needs high T. Driving force for the

transformation – reduction in total ferrite – cementite boundary area.

Photomicrograph of a steel having a spheroidite microstructure. 1000x

D. Martensite

Another microconstituent that is formed when austenitized iron–carbon alloys are

rapidly cooled (or quenched) to a relatively low temperature (in the vicinity of the

ambient).

The austenite-martensite does not involve diffusion – no thermal activation is needed,

this is called athermal transformation (not time dependent).

The martensitic transformation occurs when the quenching rate is rapid enough to

prevent carbon diffusion.

It can co-exist with other phases and/or microstructures in Fe-C system

It is the strongest and hardest phase in steel and is very important in the heat

treatment and hardenability of steels.

The martensitic transformation involves the sudden orientation of C

and Fe atoms from the FCC solid solution of -Fe (austenite) to a

body-centered tetragonal (BCT) solid solution (martensite).

The body-centered tetragonal unit cell for martensite steel showing

iron atoms (circles) and sites that may be occupied by carbon atoms

(Xs). For this tetragonal unit cell, c > a.

Photomicrograph showing the martensitic

microstructure. The needle-shaped grains are the

martensite phase, and the white regions are

austenite that failed to transform during the rapid

quench. 1220x.

The complete isothermal transformation

diagram for an iron-carbon alloy of eutectoid

composition: A, austenite; B, bainite; M, martensite;

P, pearlite.

The presence of alloying elements other than

carbon may cause significant changes in the position

and shapes of the curves in the isothermal

transformation diagrams.

Steels in which carbon is the prime alloying

element are termed plain carbon steels, whereas

alloy steels contain appreciable concentrations of

other elements.

Isothermal transformation

diagram for an alloy steel: A,

austenite; B, bainite; M, martensite;

P, pearlite; F, proeutectoid ferrite.

II. CONTINUOUS COOLING TRANSFORMATION DIAGRAMS

Continuous cooling transformation diagram is modified for transformations that occur

as the temperature is constantly changing. For continuous cooling, the time required for a

reaction to begin and end is delayed. Thus the isothermal curves are shifted to longer times

and lower temperatures. It is a plot containing such modified beginning and ending reaction

curves.

Superimposition of isothermal and continuous

cooling transformation diagrams for a eutectoid iron-carbon alloy.

Moderately rapid and slow cooling curves

superimposed on a continuous cooling

transformation diagram for a eutectoid iron–

carbon alloy.

The transformation starts after a time

period corresponding to the intersection of the

cooling curve with the beginning reaction curve

and concludes upon crossing the completion

transformation curve.

Bainite will not form when an alloy of

eutectoid composition or, for that matter, any

plain carbon steel is continuously cooled to room

temperature.

The microstructural products for the

moderately rapid and slow cooling rate curves

are fine and coarse pearlite, respectively.

With continued cooling, the unreacted

austenite begins transforming to martensite upon

crossing the M (start) line.

Continuous cooling transformation

diagram for a eutectoid iron–carbon alloy and

superimposed cooling curves, demonstrating

the dependence of the final microstructure on

the transformationsthat occur during cooling.

Only martensite will exist for

quenching rates greater than the critical; in

addition, there will be a range of rates over

which both pearlite and martensite are

produced. Finally, a totally pearlitic structure

develops for low cooling rates.

Continuous cooling transformation diagram for an alloy

steel and several superimposed cooling curves demonstrating

dependence of the final microstructure of this alloy on the

transformations that occur during cooling.

The continuous cooling transformation diagram for the

same alloy steel for which the isothermal transformation

diagram is presented.

Alloys and carbon shift the nose of the diagram to the

right (more time so the critical cooling rate can be slower).

This improves the hardenability of steels.

It is difficult to cool steels with less than about 0.25%

carbon (and no alloys) fast enough to form any martensite.

These mild steels are typically not heat treated to produce

martensite.

In summary, isothermal and continuous cooling transformation diagrams are, in a

sense, phase diagrams in which the parameter of time is introduced. Each is experimentally

determined for an alloy of specified composition, the variables being temperature and time.

These diagrams allow prediction of the microstructure after some time period for constant

temperature and continuous cooling heat treatments, respectively.

III. MECHANICAL BEHAVIOR OF IRON-CARBON ALLOYS

We shall now discuss the mechanical behavior of iron–carbon alloys having the

microstructures discussed namely, fine and coarse pearlite, spheroidite, bainite, and

martensite. For all but martensite, two phases are present (ferrite and cementite), and so an

opportunity is provided to explore several mechanical property– microstructure

relationships that exist for these alloys.

A. Pearlite

Cementite is much harder but more brittle than ferrite. Thus, increasing the fraction

of Fe3C in a steel alloy while holding other microstructural elements constant will

result in a harder and stronger material.

The tensile and yield strengths as well as the Brinell hardness number increase with

increasing carbon concentration.

Increasing cementite content will result in a decrease in both ductility and toughness

(or impact energy).

Fine pearlite is harder and stronger than coarse pearlite.

For fine pearlite there are more boundaries through which a dislocation must pass

during plastic deformation. Thus, the greater reinforcement and restriction of

dislocation motion in fine pearlite account for its greater hardness and strength.

Coarse pearlite is more ductile than fine pearlite.

B. Bainite

Because bainitic steels have a finer structure (i.e., smaller _-ferrite and Fe3C

particles), they are generally stronger and harder than pearlitic ones; yet they

exhibit a desirable combination of strength and ductility.

Bainite has desirable strength –ductility combination but is not strong as tempered

martensite.

C. Spheroidite

Alloys containing pearlitic microstructures have greater strength and hardness than

do those with spheroidite.

There is less boundary area per unit volume in spheroidite, and consequently plastic

deformation is not nearly as constrained, which gives rise to a relatively soft and

weak material. In fact, of all steel alloys, those that are softest and weakest have a

spheroidite microstructure.

As would be expected, spheroidized steels are extremely ductile, much more than

either fine or coarse pearlite. In addition, they are notably tough because any crack

can encounter only a very small fraction of the brittle cementite particles as it

propagates through the ductile ferrite matrix.

D. Martensite

Martensite is the hardest and strongest and, in addition, the most brittle; it has, in

fact, negligible ductility.

Its hardness is dependent on the carbon content, up to about 0.6 wt%.

In contrast to pearlitic steels, strength and hardness of martensite are not thought to

be related to microstructure. Rather, these properties are attributed to the

effectiveness of the interstitial carbon atoms in hindering dislocation motion, and to

the relatively few slip systems (along which dislocations move) for the BCT structure.

Austenite is slightly denser than martensite, and therefore, during the phase

transformation upon quenching, there is a net volume increase. Consequently,

relatively large pieces that are rapidly quenched may crack as a result of internal

stresses; this becomes a problem especially when the carbon content is greater than

about 0.5 wt%.

IV. TEMPERED MARTENSITE

In the as-quenched state, martensite, in addition to being very hard, is so brittle that

it cannot be used for most applications; also, any internal stresses that may have been

introduced during quenching have a weakening effect. consists of very small cementite

particles within a ferrite matrix. Heating martensite at temperatures within the range of

about 250 to 650°C will result in its transformation to tempered martensite. It is very strong

but relatively ductile. The ductility and toughness of martensite may be enhanced and these

internal stresses relieved by a heat treatment known as tempering. Tempering is

accomplished by heating a martensitic steel to a temperature below the eutectoid for a

specified time period. Diffusion is the key process in tempering. The single phase BCT

martensite transforms to two phase (α-Fe3C) tempered martensite. Tempered martensite is

strong with some ductility restored. The smaller the cementiteparticles, the greater the

strength due to the larger boundary area of the particles (similar to fine pearlite). The

particles grow larger as the tempering temperature is increased, making the material more

ductile, but less strong. If tempered for a long time, the structure becomes spheroiditic

(cementite particles are large).

An electron micrograph

showing the microstructure of

tempered martensite at a very

high magnification.

Small cementite particles

distributed within a ferrite

matrix. Similar to spheroidite

except the cementite particles in

martensite are much smaller.

A. Temper Embrittlement

The tempering of some steels may result in a reduction of toughness as measured by impact

tests this is termed temper embrittlement. The phenomenon occurs when the steel is

tempered at a temperature above about 575°C followed by slow cooling to room

temperature, or when tempering is carried out at between approximately 375 and 575°C.

Steel alloys that are susceptible to temper embrittlement have been found to contain

appreciable concentrations of the alloying elements manganese, nickel, or chromium and,

in addition, one or more of antimony, phosphorus, arsenic, and tin as impurities in relatively

low concentrations. Embrittlement of some steel alloys results when specific alloying and

impurity elements are present and upon tempering within a definite temperature range.

Possible transformations involving the decomposition of austenite. Solid arrows,

transformations involving diffusion; dashed arrow, diffusionless transformation.

Fabrication Of Metals

Fabrication Techniques are methods in which the material is manufactured into components

that may be incorporated in useful products. It also requires some processing treatment to

achieve the desired properties the consumer asked for the material. For metals, it is

normally preceded by refining, alloying and heat-treatment processes just to produce the

alloy with the desired properties one is asking for. Two or more fabrication techniques in

metals are used for the production of a single product. The technique to be used depends on

different factors such as the desired property the metal shall exhibit and the size, shape and

cost of the finished product.

A. FORMING OPERATIONS - the shape of the metal piece is changed by plastic

deformation induced by an external force/stress which is greater than the yield strength of

the material

Hot Working

Happens when deformation is achieved at a temperature above the point where

recrystallization occurs

Large deformations are possible and be repeated because the metal remains soft

and ductile

The drawback is since metals experience surface oxidation, materials may be loss

and a poor final surface finish is observed

Cold Working

Happens when deformation is achieved at a temperature below the point where

recrystallization occurs

Deformation energy requirements are less than compared to hot working

Produces an increase in strength with the attendant decrease in ductility since the

metal strain hardens

The advantage is it produces a higher quality surface finish, better mechanical

properties and a closer dimensional control of the finished piece

Deformation may be accomplished by series of steps in which the material is

successively cold worked on a small amount and then annealed

The disadvantage is that it is expensive and inconvenient

Metal Facrication Techniques

Forming

Operations

Forging Rolling Extrusion Drawing

Casting

Sand Die Investment Lost

Foam Continous

Miscellaneous

Powder Metallurgy

Welding

KINDS

a. Forging

Mechanically working or deforming a single piece of a normally hot metal

Accomplished by application of successive blows or continuous squeezing of

the material

Have outstanding grain structures and the best combination of mechanical

properties

CLOSED DIE – a force is brought to bear

on two or more die halves having the

finished shape such that the deformation

is in the cavity between them

OPEN DIE – two dies having simple

geometric shapes are employed normally

on large workspaces

Articles formed: wrenches, automative crankshafts and piston connecting

rod

b. Rolling

most widely used deformation process,

consists of passing a piece of metal between two rolls

(causing a compressive stress) which will result in a

reduction of the metal‘s thickness

Cold rolling is used in the production of

sheet, strip and foil while grooved rolling is used in

circular shapes, I-beams and railroad rails

c. Extrusion

a bar of metal is forced through a die orifice by a

compressive force that is applied to a ram

the extruded piece has the desired shape and a

reduced cross-sectional area

Products are rods and tubing with complicated

cross-sectional geometries and seamless tubing

d. Drawing

the pulling of a metal piece through a die having a tapered bore by means of

a tensile force that is applied on the exit side

A reduction in cross section with a corresponding increase in length occurs

Products are rod, wire and tubing products

Figure 1. Open-die forging and Close-die forging

Figure 2. Metal deformation during rolling

Figure 3. Metal deformation during Extrusion

B. CASTING – a fabrication process whereby a totally molten metal is poured into a mold

cavity having the desired shape then upon solidification, the metal assumes the shape of the

mold but experiencing some shrinkage. Usually employ in the last step of refining of ductile

materials.

When does casting employed?

1. The finished shape is so large or complicated

2. A particular alloy has a low ductility

3. Most economical in all techniques

Kinds

a. Sand Casting

most commonly used Casting Process because of the cost effectiveness of the

process and the east availability of raw materials.

ordinary sand is used as a molding material

a two-piece mold formed by packing sand around a pattern that has the shape

of the intended casting

a gatting system is employed to guide the flow of molten metal into the cavity

and minimize internal casting defects

Products are automotive cylindrical blocks, fire hydrants and large pipe

fittings b. Die Casting

the liquid metal is forced into a mold under pressure and high velocity and

then solidify in that pressure

a two-piece permanent steel mode or die that once clamped contains the

desired shape

Rapid casting rates are possible and one single set of dies may be used for

thousands of castings making it inexpensive c. Investment or Lost Wax Casting

The pattern is made from a wax or plastic of low melting temperatures where

a fluid slurry is poured and served as a solid mold or investment (plaster of

paris)

the mold is heated until the pattern melts and burn out which leave a cavity of

the desired shape

Used for high dimensional accuracy, reproduction of fine detail and excellent

finish

Products are jewelry, dental crowns and inlays, blades for gas turbines and

jet engine impellers d. Lost Foam or Expendable Pattern Casting

A variation of the investment casting where the expendable pattern is a foam

that can be formed by compressing polystyrene beads into the desired shape

and bonding them together by heating

Sand is packed around the pattern to form the mold then the molten metal is

poured and replaces the pattern (which will vaporize) and upon solidification,

the metal assumes the shape of the mold

Used for complex geometries and tight tolerances

Simpler, quicker and less expensive compared to investment casting and

most commonly used for cast irons and aluminum alloys

Products are engine blocks, cylinder heads, crankshafts and electric motor

frames e. Continuous or Strand Casting

at the extraction processes, many molten metals solidified by casting into

large ingot molds (normally subjected to hot-rolling operation with a product

of flat sheet/slab

the casting and rolling steps are combined in the process

refined and molten metal is cast directly in a continuous strand (having either

rectangle or circular in terms of cross section) and solidification occurs at a

water cooling die

chemical composition and mechanical properties are more uniform

throughout the cross sections than ingot-cast products

highly automated and more efficient

TYPE OF CASTINGS Advantages

Sand Casting Easy availability of raw material

Smooth surface finish and

dimensional accurate castings

Cost effective casting process.

Die Casting Economical process that can be used

for a wide range of complex

application

Parts have longer service life,

dimensional accuracy and close

tolerance

Post machining can be totally

eliminated

A process that can be fully automated

Mold can be use repeatedly

Investment or Lost Wax Casting

Produces impossible and intricate

designs

Less surface finishing and machining

required for the castings

Produces near net shaped as desired

by the customer

Casting have excellent dimensional

tolerances

Lost Foam or Expendable Pattern Casting

No cores are required.

Reduction in capital investment and

operating costs.

Closer tolerances and walls as thin as

0.120 in.

No binders or other additives are

required for the sand, which is

reusable.

Flasks for containing the mold

assembly are inexpensive, and

shakeout of the castings in unbonded

sand is simplified and do not require

the heavy shakeout machinery

required for other sand casting

methods.

Need for skilled labor is greatly

reduced.

Casting cleaning is minimized since

there are no parting lines or core fins.

Continuous or Strand Casting

Cost effective, time saving, casting

process

A highly productive process that can

be fully automated

High quality castings can be done.

C. MISCELLANEOUS TECHNIQUES

Powder Metallurgy (P/M)

the compaction of powdered metal followed by a heat treatment to produce a more

dense piece

produce a virtually nonporous piece having properties almost equivalent to the fully

dense parent material through diffusional process during heat treatment

suitable for metals of low ductility, having high melting temperatures and requiring a

very close dimensional tolerance

Welding

metal parts are joined to form a single piece when one of the component is expensive

or inconvenient

the joining bond is metallurgical rather mechanical

Arc and Gas Welding – workpieces are to be joined and the filler (welding rod) is

heated to cause both to metal and then once solidifies, the filler acts as a joint. The

Figure 4. The conventional P/M process sequence

heat affected zone (HAZ) is the region adjacent to the weld that is experiencing a

microstructural and property alterations. Possible alterations are:

1. If the workpiece was previously cold worked, recrystallization and grain

growth occurs that may diminish the strength, hardness and toughness.

2. Upon cooling, residual stresses may form that weakens the joint

3. For steels, HAF may form austentite due to high heating temperature. Upon

cooling, microstructural products depends on the cooling rate and alloy

composition. For steels, martensite is formed which is too brittle

4. Some stainless steels may be sensitized which renders the material

susceptible to intergranular corrosion

Laser Beam Welding

A modern joining technique where a highly focused and intense laser beam

as the heat source

No filler required

Advantages: (1) a noncontact process (eliminates mechanical distortion of the

workpiece) (2) automated (3) low energy input in workpiece (minimal HAZ)

(4) welds may be done in small and precise sizes (5) a large variety of metals

and alloys can be used (6) porosity-free welds with strength equal or greater

than the base metal

Used in the automotive and electronic industries (for high quality and rapid

welding rates)

FORMS OF CORROSION IN METALS

Uniform Attack

Occurs with equivalent intensity over the entire exposed surface

Leaves behind a scale or deposit

Examples: Rusting of steel and iron & tarnishing of silverware

Galvanic Corrosion

Occurs when two metals or alloys having different compositions are electrically

coupled while exposed on an electrolyte

It is caused by the existence of a galvanic cell - essentially two metals submersed in

an electrolyte - that results in an attack on one metal at the expense the other.

Example: steel screws corrosion with brass in a marine environment

Crevice Corrosion

Occurs in crevices and recesses or under deposits of dirt or corrosion products

where the solution become stagnant and a localized depletion of dissolved oxygen

Crevice must be wide enough to penetrate yet narrow enough to make the solution

stagnant

Prevented by using welding using non-absorbing gasket, removing accumulated

deposits frequently and designing containment vessels to avoid stagnancy

Pitting

A very localized corrosion attack where small pits or holes are formed

An extremely insidious type of corrosion (often undetected with little material lose)

Intergranular Corrosion

Disintegration occurs on grain boundaries for some alloys in a specific environment

Prevalent in stainless steel

This often occurs due to impurities in the metal, which tend to be present in higher

contents near grain boundaries. These boundaries can be more vulnerable to

corrosion than the bulk of the metal.

Weld decay occurs on the welding of stainless steel

Prevention: (1) subjecting to high temperature heat treatment (2) lowering the

carbon content below 0.03 wt% C and (3) alloying the stainless steels with niobium

or titanium

Selective Leaching

Found in solid solution alloys

Occurs when one element/constituent is preferentially removed as a consequence of

corrosion processes that significantly impaired the mechanical properties of the

metal/alloy

Example: dezincification of brass

Erosion – Corrosion

From Carbon steel pipe

Arises from the combined action of chemical attack and mechanical abrasion or wear

as a consequence of fluid motion

Characterized by surface grooves and waves having contours from a fluid flow

Found in piping especially the positions where the fluid changes direction or

turbulent flow

Prevention: changed the design to eliminate fluid turbulence, removal of particulates

and bubbles from the flowing fluid and used other materials that resist corrosion

Stress Corrosion (Cracking)

the combined action of an applied tensile stress and a corrosive environment

small cracks form and propagate in a direction perpendicular to the stress with the

result of failure behavior like a brittle material even though the material is

intrinsically ductile

Example: stainless steels corrosion in solution containing chloride ions and brass

corrosion on contact with ammonia

Prevention: lower the magnitude of stress by reducing the external load or

increasing the cross-sectional area perpendicular to the applied stress and heat

treatment

Hydrogen Embrittlement

A significant reduction in ductility and tensile strength when an atomic hydrogen

penetrates into the some metal alloys (like steel)

Maybe called ―hydrogen-induced cracking‖ or ―hydrogen stress cracking‖

Brittle fracture occurs catastrophically

High strength steels are susceptible to this type of corrosion.

Martensitic steels and FCC alloys are vulnerable to this type of corrosion because of

their resiliency, for the former and high ductility, for the latter. However, strain

hardening increases their susceptibility to the corrosion

Prevention: reducing the tensile strength via heat treatment, removal of H source,

baking of alloy or substitution of a more embrittlement-resistant alloy

Thermal Processing in Metals

ANNEALING PROCESSES

Annealing

- heat treatment in which a material is exposed to an elevated temperature for an

extended time period and then slowly cooled

- done to relieve stresses, increase softness, ductility, and toughness; and/or to

produce a specific microstructure

- stages in annealing process

o heating to a desired temperature

o holding or ―soaking‖ at that temperature

o cooling, usually at room temperature

Process Annealing

- heat treatment that is used to negate the effects of cold work,

- soften and increase the ductility of a previously strain-hardened metal

- utilized during fabrication procedures that require extensive plastic deformation,

to allow a continuation of deformation without fracture or excessive energy

consumption

- Ordinarily a fine-grained microstructure is desired, therefore, the heat treatment

is terminated before appreciable grain growth has occurred

Stress Relief

- the piece is heated to the recommended temperature, held there long enough to

attain a uniform temperature, and finally cooled to room temperature in air

- eliminates distortion and warpage

- prevents internal residual stresses developed from:

o plastic deformation processes such as machining and grinding

o nonuniform cooling of a piece that was processed or fabricated at an

elevated temperature, such as a weld or a casting

o a phase transformation that is induced upon cooling wherein parent and

product phases have different densities

Annealing of Ferrous Alloys

Normalizing

- used to refine grains and produce a more desirable size distribution

- fine-grained pearlitic steels are tougher than coarse-grained ones

- accomplished by heating at approximately 55 to 85 °C above the upper critical

temperature, which is, of course, dependent on composition

- after sufficient time has been allowed for the alloy to completely transform to

austenite, the treatment is terminated by cooling in air

Full Anneal

- utilized in low- and medium-carbon steels that will be machined or will

experience extensive plastic deformation during a forming operation

- alloy is austenitized by heating to 15 to 40 °C

- alloy is then furnace cooled

- microstructural product of this anneal is coarse pearlite which is soft and ductile

- time consuming but results to small and uniform grain structure

Spheroidizing

- consists of heating the alloy at a temperature just below the eutectoid

- have a maximum softness and ductility and are easily machined or deformed

- done on medium- and high-carbon steels having a microstructure with even

coarse pearlite which may still be too hard to conveniently machine or plastically

deform

Treatment of Steels

- factors that affect heat treatment are:

o composition of the alloy

o type and character of quenching medium

o size and shape of the specimen

Hardenability

- influence of alloy composition on the ability of a steel alloy to transform to

martensite for a particular quenching treatment

- ability of an alloy to be hardened by the formation of martensite as a result of a

given heat treatment

- a qualitative measure of the rate at which hardness drops off with distance into the

interior of a specimen as a result of diminished martensite content

The Jominy End-Quench Test

- except for alloy composition, all factors that may influence the depth to which a

piece hardens are maintained constant

- cylindrical specimen 25.4 mm (1.0 in.) in diameter and 100 mm (4 in.) long is

austenitized at a prescribed temperature for a prescribed time

- The lower end is quenched by a jet of water of specified flow rate and

temperature

- the cooling rate is a maximum at the quenched end and diminishes with position

from this point along the length of the specimen

- After the piece has cooled to room temperature, shallow flats 0.4 mm (0.015 in.)

deep are ground along the specimen length

- Rockwell hardness measurements are made for the first 50 mm (2 in.) along each

flat

The Hardenability Curve

- quenched end is cooled most rapidly and exhibits the maximum hardness

- cooling rate decreases with distance from the quenched end, and the hardness

also decreases

- With diminishing cooling rate more time is allowed for carbon diffusion and the

formation of a greater proportion of the softer pearlite

INFLUENCE OF QUENCHING MEDIUM, SPECIMEM SIZE AND GEOMETRY

- ‗‗Severity of quench‘‘ is a term often used to indicate the rate of cooling

- three most common quenching media—water, oil, and air

- Increasing the velocity of the quenching medium across the specimen surface

enhances the quenching effectiveness

- During the quenching of a steel specimen, heat energy must be transported to the

surface before it can be dissipated into the quenching medium

- the cooling rate within and throughout the interior of a steel structure varies with

position and depends on the geometry and size

- rate of cooling for a particular quenching treatment depends on the ratio of

surface area to the mass of the specimen

- larger this ratio, the more rapid will be the cooling rate and, consequently, the

deeper the hardening effect

PHASE TRANSFORMATON

- these transformations are divided into three classifications.:

o In one group are simple diffusion-dependent transformations in which

there is no change in either the number or composition of the phases

present

- Alteration in phase compositions and often in the number of phases present

- third kind of transformation is diffusionless, wherein a metastable phase is

produced

KINETCS OF SOLID STATE REACTIONS

- do not occur instantaneously because obstacles impede the course of the reaction

and make it dependent on time.

- Nucleation

o formation of very small particles, or nuclei, of the new phase, which are

capable of growing.

o First stage

- Growth

o Second stage

o nuclei increase in size

o The transformation reaches completion if growth of these new phase

particles is allowed to proceed until the equilibrium fraction is attained

- In kinetics

- With many kinetic investigations, the fraction of reaction that has occurred is

measured as a function of time, while the temperature is maintained constant

MULTIPHASE TANSFORMAIONS

- Vary temperature, in composition, and the external pressure; however,

temperature changes by means of heat treatments are most conveniently utilized

to induce phase transformations

- Most phase transformations require some finite time to go to completion, and the

speed or rate is often important in the relationship between the heat treatment

and the development of microstructure

- Supercooling

o transformations are shifted to lower temperatures

- superheating

o the shift is to higher temperature

-