1

STEEL

8.1CLASSIFICATION OF STEEL

Steels can be classified by a variety of different systems depending on:

The composition, such as the chemical composition percentage of carbon.

The manufacturing methods, such as open hearth, basic oxygen process, or

electric furnace methods.

The finishing method, such as hot rolling or cold rolling

The product form, such as bar, plate, sheet, strip, tubing or structural shape

The deoxidation practice, such as killed, sime-killed and rimmed steel

The microstructure, such as ferritic, pearlitic and martensitic

The required strength level, such as high or low tensile strength steel

The heat treatment, such as annealing, quenching and tempering steel.

8.2 CARBON STEEL

Carbon steel is an iron-based metal containing carbon up to 2%, and

small amounts of other elements(Mn, Si, S, and P). variations in carbon have

the greatest effect on mechanical properties, with increasing carbon content

leading to increased hardness and strength. As such, carbon steels are

generally categorized according to their carbon content. Generally speaking,

carbon steels can be subdivided into low-carbon steels, medium-carbon

steels and high-carbon steels

8.2.1 Low carbon steel

Of all the different steels, those produced in the greatest quantities fall

within the low-carbon classification. These 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. As a consequence, these alloys are

relatively soft and weak, but have outstanding ductility ; in addition, they are

machinable, weldable, and, of all steels, are the least expensive to produce.

Typical applications include automobile body components, structural shapes

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

rivets, nails and others. Plain low-carbon steels have a yield strength between

180 and 260 MPa , tensile strengths between 325 and 485 MPa , and a

ductility reach to 25%EL.

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8.2.2 Medium-Carbon Steels

The medium-carbon steels have carbon concentrations between about

0.25 and 0.60 wt%. These alloys may be heat treated by quenching, and then

tempering to improve their mechanical properties. They are most often

utilized in the tempered condition, having microstructures of tempered

martensite. The plain medium-carbon steels have low hardenabilities and can

be successfully heat treated only in very thin sections and with very rapid

quenching rates. Applications include railway wheels and, gears, crankshafts,

and other machine parts.

8.2.3 High-Carbon Steels

The high-carbon steels, normally having carbon contents between

0.60 and 1.4 wt%, are the hardest, strongest, and yet least ductile of the

carbon steels. They are almost always used in a hardened and tempered

condition and, as such, are especially wear resistant and capable of holding a

sharp cutting edge. Application include blacksmith tools and wood working

tools.

8.3 Impurities in Steel

Most ordinary steels contain amounts of manganese, silicon,

sulphur and phosphorus are. The effect of such impurities on mechanical

properties will depend largely upon the way in which these impurities are

distributed throughout the structure of the steel. If a troublesome impurity is

heavily cored in the structure it can be expected to have a far more

deleterious effect than if the same quantity of impurity were evenly

distributed throughout the structure.

Figure 8.1 Effect of freezing range

on segregation of impurities

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Excessive coring concentrates the impurity in the grain-boundary

regions often producing the effect of very brittle inter granular films. The

extent to which coring of a particular element is indicated by the distance

between the compositions intersected with solidus and liquidus lines at any

temperature on the appropriate equilibrium diagram. Thus in Fig. 6.1A the

relative compositions of solid (S) and liquid (L) are very far at any

temperature and this may lead to excessive coring. Since relatively pure

metal is solidifying it follows that the bulk of the impurity element becomes

concentrated in the metal which solidifies last—in the grain boundary

regions. In Fig. 6.1B, however, the compositions of the solid (S) and the

liquid (L) remain close to each other throughout solidification and this will

lead to a relatively even distribution of the impurity element throughout the

microstructure and a consequent lack of dangerous crystal-boundary

concentrations of brittle impurity.

Manganese

It is not only soluble in austenite and ferrite but also forms a stable

carbide, Mn3C. manganese increases the depth of hardening' of a steel. It also

improves strength and toughness. Manganese should not exceed 0.3% in

high-carbon steels because of a tendency to induce quench cracks particularly

during water-quenching.

Silicon

Silicon imparts fluidity to steels intended for the manufacture of

castings, and is present in such steels in amounts up to 0.3%. In high carbon

steels silicon must be kept low, because of its tendency to render cementite

unstable and liable to decompose into graphite (which precipitates) and

ferrite.

Phosphorus

Phosphorus has a considerable hardening effect on steel but it must

be rigidly controlled to amounts in the region of 0.05% or less because of the

brittleness it imparts, particularly if Fe3P (brittle compound) should appear as

a separate constituent in the microstructure.

Sulphur

It is the most deleterious impurity commonly present in steel. If

precautions were not taken to render it harmless it would tend to form the

Figure 6.1

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brittle sulphide, FeS. Sulphur is completely soluble in molten steel but on

solidification the solubility falls to 0.03% sulphur. If the effects of extensive

coring, referred to above, are also taken into account it will be clear that

amounts as low as 0.01% sulphur may cause precipitation of the sulphide at

the crystal boundaries. In this way the austenite crystals would become

virtually coated with brittle films of iron sulphide. Since this sulphide has a

fairly low melting point, the steel would tend to crumble during hot-working.

Being brittle at ordinary temperature. It would be very difficult, and certainly

very expensive, to reduce the sulphur content to an amount less than 0.05%

in the majority of steels. To nullify the effects of the sulphur present an

excess of manganese is therefore added during deoxidation. Provided that

about five times the theoretical manganese requirement is added, the sulphur

then forms manganese sulphide, MnS, in preference to iron sulphide. The

manganese sulphide so formed is insoluble in the molten steel, and some is

lost in the slag. The remainder is present as fairly large globules, distributed

throughout the steel, but since they are insoluble, they will not be associated

with the structure when solidification takes place. Moreover, manganese

sulphide is plastic at the forging temperature, so that the tendency of the steel

to crumble is removed. The manganese sulphide globules become elongated

by the subsequent rolling operations (Fig. 6.2A and B).

Figure 8.2 (A) The segregation of

iron sulphide (FeS), at the crystal

boundaries in steel. (B) The formation

of isolated manganese sulphide (MnS)

globules when manganese is present in

a steel

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The Heat-treatment of Steel

9.2Introduction

Steels are among the relatively few engineering alloys which can be

heat-treated in order to vary their mechanical properties. This statement

refers, of course, to heat-treatments other than simple stress-relief annealing

processes. Heat-treatments can be applied to steel not only to harden it but

also to improve its strength, toughness or ductility. The type of heat-

treatment used will be governed by the carbon content of the steel and its

subsequent application.

The various heat-treatment processes can be classified as follows:

(a) annealing;

(b) normalising;

(c) hardening;

(d) tempering;

(e) treatments which depend upon transformations taking place at a

single predetermined temperature during a given period of time (isothermal

transformations).

In all of these processes the steel is heated fairly slowly to some

predetermined temperature, and then cooled, and it is the rate of cooling

which determines the resultant structure of the steel and, hence, the

mechanical properties associated with it.

Annealing

The term 'annealing' describes a number of different thermal

treatments which are applied to metals and alloys. Annealing processes for

steels can be classified as follows:

1-Stress-relief Annealing

The recrystallisation temperature of low carbon steel is about 500°C,

so that, during a hot-rolling process recrystallisation proceeds simultaneously

with rolling. Thus, working stresses are relieved as they are set up.

Frequently, however, we must apply a considerable amount of cold work to

low carbon steel, as, for example, in the drawing of wire. Stress-relief

annealing then becomes necessary to soften the metal so that further drawing

operations can be carried out. Such annealing is often referred to as 'process'

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annealing, and is carried out at about 650°C. It should be noted that process

annealing is a sub-critical operation, that is, it takes place below the lower

critical temperature (A1). In this treatment, there is no phase change and the

constituents ferrite and cementite remain in the structure throughout the

process.

2-Spheroidising Annealing

Spheroidising used to improve the machinablity of carbon steel. This

process included spheroidisation of pearlitic cementite of high carbon steel.

The spheroidised condition is produced by annealing the steel at a

temperature between 650 and 700°C, that is, just below the lower critical

temperature (A1) for duration between 6-20 hour depending on chemical

composition, grain size of pearlite and on the size of the specimen. Whilst no

basic phase change takes place, surface tension causes the cementite to

assume a globular form (Fig. 9.8). If the layers of cementite are relatively

coarse they take rather a long time to break up, and this would result in the

formation of very large globules of cementite. This in turn would lead to

tearing of the surface during machining. To obviate these effects it is better to

give the steel some form of normalizing treatment prior to annealing in order

to refine the distribution of the cementite. It will then be spheroidised more

quickly during annealing and will produce much smaller globules of

cementite.

Figure9.8 The spheroidisation of pearlitic cementite.

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3-Annealing of Castings

The cast structure of a large body of steel is extremely coarse. This

is due mainly to the slow rates of solidification and subsequent cooling

through the austenitic range. Thus, a 0.35% carbon steel will be completely

solid in the region of 1450°C(see Fe-Fe3c diagram),but, if the casting is large,

cooling, due to the lagging effect of the sand mould, will proceed very slowly

down to the point (approximately 820°C) where transformation to ferrite

begins. By the time 820°C has been reached, therefore, the austenite crystals

will be extremely large. Ferrite, which then begins to precipitate in

accordance with the equilibrium diagram, deposits first at the grain

boundaries of the austenite. The remainder of the ferrite is then precipitated

along certain crystallographic planes within the lattice of the austenite. This

gives rise to a directional precipitation of the ferrite, as shown in Fig. 9.9

representing typically what is known as a Widmanstatten structure. The

mesh-like arrangement of ferrite in the Widmanstatten structure tends to

isolate the stronger pearlite into separate patches, so that strength, and more

particularly toughness, are impaired. The main characteristics of such a

structure are, therefore, weakness and brittleness, and steps must be taken to

remove it. Heat-treatment must therefore be used to effect what limited

refinement of grain is possible.

The most suitable treatment for a large casting involves heating it

slowly up to a temperature about 40°C above its upper critical (thus the

annealing temperature depends upon the carbon content of the steel) , holding

it at that temperature only just long enough for a uniform temperature to be

attained throughout the casting and then allowing it to cool slowly in the

furnace. As the temperature rises, the Widmanstatten type plates of ferrite are

dissolved by the austenite until, when the upper critical temperature is

reached, the structure consists entirely of fine grained austenite. Cooling

causes reprecipitation of the ferrite, but, since the new austenite crystals are

small, the precipitated ferrite will also be distributed as small particles.

Finally, as the lower critical temperature is reached, the remaining small

patches of austenite will transform to pearlite. The structural changes taking

place during annealing are illustrated diagrammatically in Fig. 9.9. Whilst the

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tensile strength is not greatly affected by this treatment, both toughness and

ductility are improved.

Normalizing

In normalizing steel is heated at about 30-50°C above its upper

critical (thus the normalising temperature depends upon the carbon content of

the steel) , holding it at that temperature only just long enough for a uniform

temperature to be attained throughout the workpiece, then steel is removed

from the furnace and allowed to cool in still air. This relatively rapid method

of cooling limits grain growth . Moreover, the surface finish of a normalised

steel is often superior to that of an annealed one when machined , and the

strength and hardness of this steel are higher than the annealed steel. The

type of structure obtained by normalising will depend largely upon the

thickness of cross-section, as this will affect the rate of cooling. Thin sections

will give a much finer grain than thick sections, the latter differing little in

Figure 9.9 Structural changes occurring during the annealing of a steel casting (approx

0.35% carbon). The as-cast Widmanstatten structure is reheated to some temperature above

its upper critical and then allowed to cool in the furnace.

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structure from an annealed section. Since cooling of steel in still air is

relatively rapid cooling method, so that, normalized steel has several internal

stresses. Tempering process is used after normalizing process in order to

relive these stresses.

Hardening

When a piece of steel, containing sufficient carbon, is cooled rapidly

from above its upper critical temperature it becomes considerably harder than

it would be if allowed to cool slowly. The degree of hardness produced can

vary, and is dependent upon such factors as the initial quenching

temperature; the size of the work; the constitution, properties and temperature

of the quenching medium; and the degree of agitation of the quenching

medium. Water quenching of a steel containing sufficient carbon produces an

extremely hard structure called martensite which appears under the

microscope as a mass of uniform needle-shaped crystals (figure 9.10).

Carbon is still in solution in the iron and has not been precipitated as iron

carbide as it would have been if the steel had been cooled under equilibrium

conditions. However, X-ray crystallographic examination of martensite

shows that despite very rapid cooling which has prevented the precipitation

of iron carbide, the lattice structure has nevertheless changed from FCC

(face-centred cubic) to something approaching the BCC (body-centred cubic)

structure which is normally present in a steel cooled slowly to ambient

temperature. This BCC type structure is considerably supersaturated with

carbon since at ambient temperatures only 0.006% carbon is retained in

solution under equilibrium conditions. Consequently the presence of

dissolved carbon in amounts of, say, 0.5% can be expected to cause

considerable distortion of the structure and in fact produces one which is

body-centred tetragonal BCT.

Generally no attempt is made to harden plain carbon steels which

contain less than 0.25% carbon since the increase in hardness produced

would be so small as shown in figure(9.11) and for anther reasons which will

be discussed later. Large masses of steel of heavy section will obviously cool

more slowly than small work of thin section when quenched, so that whilst

the surface skin may be martensitic, the core of a large section may be

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pearlitic or bainitic because it has cooled more slowly. If, however, small

amounts of such elements as nickel, chromium or manganese are added to the

steel, it will be found that the martensitic layer is much thicker than with a

plain carbon steel of similar carbon content and dimensions which has been

cooled at the same rate. Alloying elements therefore 'increase the depth of

hardening', and they do so by slowing down the transformation rates(γ to

pearlite). The liability to produce quench-cracks, which are often the result of

water-quenching, is reduced in this way. So the low alloy steel could be

hardened by using oil medium. The quenching medium is chosen according

to the rate at which it is desired to cool the steel. The following list of media

is arranged in order of quenching speeds:

5% Caustic soda, 5-20% Brine, Cold water, Warm water, Mineral oil,

Animal oil, and Vegetable oil

To harden a piece of steel, then, it must be heated to between 30 and

50°C above its upper critical temperature for hypoeutectoid steel ,and it must

be heated to between 30 and 50°C above its lower critical temperature for

hypereutectoid steel for several time, then quenched in some medium which

will produce in it the desired rate of cooling. The medium used will depend

upon the composition of the steel and the ultimate properties required.

Symmetrically shaped components are best quenched, and all components

should be agitated in the medium during quenching.

Figure 9.10 Photomicrograph showing the martensitic

microstructure. The needleshaped grains are the

martensite phase, and the white regions are austenite that

failed to transform during the rapid quench.

Figure 9.11 relationship

between carbon content

and hardness

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Tempering

In this process quenched component heats or tempers in order to

relieve the internal stresses and reduce the brittleness. During tempering,

which is always carried out below the lower critical temperature, martensite

tends to transform to the equilibrium structure of ferrite and cementite. The

higher the tempering temperature the more closely will the original

martensitic structure revert to this ferrite and cementite mixture and so

strength and hardness fall progressively, whilst toughness and ductility

increase (Fig. 9.12). Thus by choosing the appropriate tempering temperature

a wide range of mechanical properties can be achieved in carbon steels. The

structural changes which occur during the tempering of martensite containing

more than 0.3% carbon, take place in three stages:

First Stage

First stage occurs at temperatures below 200 °C. This stage involves

conversion of the martensite to low carbon martensite (0.25%C) plus epsilon

carbide (є). Є-carbide is metastable and richer in carbon than cementite and

is described by the formula Fe2.5C (or Fe5C2). low carbon martensite retains

some degree of tetragonality because it still contains more carbon in solid

solution than would ferrite; there are no changes in the morphology of the

martensite crystals. At this stage a slight increase in hardness may occur

because of the presence of the finely-dispersed but hard є-carbide.

Brittleness is significantly reduced as quenching stresses disappear in

consequence of the transformation. At 100°C the transformation proceeds

very slowly but increases in speed up to 200°C.

Second Stage

The second stage of tempering occurs between 200 and 300°C. It

refers directly to the conversion of retained austenite to bainite (ferrite+ є

carbide) .this stage is important when there are significant quantities of

retained austenite.

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Third Stage

Third stage start at 300°C. At this stage є-carbide begins to transform

to ordinary cementite and this continues as the temperature rises. In the mean

time the remainder of the carbon begins to precipitate from the low carbon

martensite also as cementite and in consequence the martensite structure

gradually reverts to one of ordinary BCC ferrite. Above 500°C the cementite

particles coalesce into larger rounded globules in the ferrite matrix. This

structure was formerly called sorbite or tempered martensite. Due to the

increased carbide precipitation which occurs as the temperature rises the

structure becomes weaker but more ductile, though above 550°C strength

falls fairly rapidly with little rise in ductility (Fig. 9.12).

Figure 9. 12 The relationship between mechanical properties and tempering temperature for a

steel containing 0.5% carbon and 0.7% manganese in the form of a bar 25mm diameter,

previously water quenched from 830°C.

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9.3 Isothermal Transformation(IT orTTT) Diagram

Introduction

The relationship, between structure and time at constant temperature,

can be studied by using isothermal transformation IT curves which are

known as TTT (Time-Temperature-Transformation) curves. The TTT curves

indicate the time necessary for transformation to take place and the structure

which will be produced when austenite is supercooled to any predetermined

transformation temperature.

Building of TTT curves

Such curves are constructed by taking a large number of similar

specimens of the steel and heating them to just inside the austenitic range.

These specimens are divided into groups each of which is quickly transferred

to an 'incubation' bath at a different temperature. At predetermined time

intervals individual specimens are removed from their baths and quenched in

water. The microstructure is then examined to see the extent to which

transformation had taken place at the holding temperature. Let us assume, for

example, that we have heated a number of specimens of eutectoid steel to just

above 727°C and have then quenched them into molten lead at 500°C (Figs.

9.13 and 9.14). Until one second has elapsed transformation has not begun,

and if we remove a specimen from the bath in less than a second, and then

quench it in water, we shall obtain a completely martensitic structure,

proving that at 500°C after one second ('A' on Figs. 9.13 and 9.14) the steel

was still completely austenitic. If we allow the specimen to remain at 500°C

for ten seconds ('B' on Figs. 9.13 and 9.14) and then water-quench it, we shall

find that the structure is composed entirely of bainite in feather-shaped

patches. If we quenched a specimen after it had been held at 500°C for five

seconds ('C on Figs.9.13 and 9.14) we would obtain a mixture of bainite and

martensite By repeating such treatments at different holding temperatures we

are able, by interpreting the resulting microstructures, to construct TTT

curves of the type shown in Fig. 9.13.

TTT Description for eutectoid steel

The horizontal line at the upper of the diagram representing the

eutectoid temperature Below this line austenite is unstable, and the two

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approximately C-shaped curves indicate the time necessary for the austenite

to ferrite + cementite transformation to begin and to be completed following

rapid quenching to any predetermined temperature. Transformation is

sluggish at temperatures just below the eutectoid temperature, because the

difference in free energy between austenite and pearlite at these temperatures

is small, the structure formed will be coarse pearlite. Then the time required

for completion, decrease as the temperature falls towards 550°C. The rate of

transformation reaches a maximum at 550°C. At region just above 550°C,

structure formed will be fine pearlite. At temperatures between 550 and

220°C transformation becomes more sluggish as the temperature falls

because the slower rate of diffusion of carbon atoms in austenite at lower

temperatures. In this temperature range the transformation product is bainite.

The appearance of this phase may vary between a feathery mass of fine

cementite and ferrite for bainite formed around 450°C; and dark acicular

(needle-shaped) crystals for bainite formed in the region of 250°C. The

horizontal lines at the foot of the diagram are, not part of the TTT curves, but

represent the temperatures at which the formation of martensite will begin

(Ms) and end (Mf) during cooling of austenite through this range. It will be

noted that the Mf line corresponds approximately to -50°C. Consequently if

the steel is quenched in water at room temperature, some 'retained austenite'

can be expected in the structure since at room temperature transformation is

incomplete.

Figure 9.13 Time-temperature-

transformation (TTT). Curves for a

plain carbon steel of eutectoid

composition.

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6.9 Continuous Cooling Transformation Diagrams CCT

Most heat treatments for steels involve the continuous cooling of a

specimen to room temperature. An isothermal transformation diagram is

valid only for conditions of constant temperature, which diagram must be

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 as

indicated in Figure 9.15 for an iron–carbon alloy of eutectoid composition. A

plot containing such modified beginning and ending reaction curves is

termed a continuous cooling transformation (CCT) diagram. On this diagram

are superimposed four curves, A, B, C and D, which represent different rates

of cooling. Curve A represents a rate of cooling of approximately 5°C per

second such as might be encountered during normalising. Here

transformation will begin at X and can be completed at Y, the final structure

being one of fine pearlite. Curve B, on the other hand, represents very rapid

cooling at a rate of approximately 400°C per second. This is typical of

conditions prevailing during a water-quench, and transformation will not

begin until 220°C, when martensite begins to form. The lowest rate at which

this steel (of eutectoid composition) can be quenched, in order to obtain a

structure which is almost wholly martensitic, is represented by curve C

(140°C per second). This is called the critical cooling rate for the steel, and

if a rate lower than this is used some fine pearlite will be formed. For

example, in the case of the curve D, which represents a cooling rate of about

50°C per second, transformation would begin at P with the formation of some

fine pearlite. Transformation, however, is interrupted in the region of Q and

Figure 9.14 The

thermal treatment

sequence used in the

derivation of a set of

TTT curves.

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does not begin again until the MS line is reached at R, when the remaining

austenite begins to transform to martensite. Thus the final structure at room

temperature is a mixture of pearlite, martensite and traces of retained

austenite.

TTT Curves for Hyper and Hypo eutectoid Steel

Several differences are evident between the eutectoid TTT diagram

and non-eutectoid TTT diagram as shown in figure 9.16. one major

difference is that the curves of the hypoeutectoid steel have been shifted to

the left. While the curves of hypereutectoid steel have been shifted to the

right. A second major difference is that another transformation line has been

added, that indicates the start of the formation of proeutectoid ferrite for

hypoeutectoid and proeutectoid cementite for hypereutectoid.As shon in

figure 9.16 the Ms and Mf temperatures decreases as the weight percent

carbon increases.

Figure9.15The effect of

cooling rate on the structure

for eutectoid TTT diagram.

Figure 9.16 modified TTT

diagrams for hypoeutectoid

and hypereutectoid steels.

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Hardenability (Mass Effect)

Let us examine the conditions under which a large body of steel will

cool, when quenched. The core will cool less quickly than the outside skin,

and since its cooling curve B (Fig. 9.17A) cuts into the nose of the

'transformation-begins' curve, we can expect to find some fine pearlite in the

core, whilst the surface layer is entirely martensitic. This feature is usually

referred to as the 'mass effect' or hardenability. Even if we are able to cool

the component quickly enough to obtain a completely martensitic structure,

as indicated in Fig. 9.17B there will be such a considerable time interval CD

between both core and surface reaching a martensitic condition that this will

lead to quench-cracks. This difficulty may be remedied to some extent by

adding alloying elements to the steel. These reduce the critical rates of

austenite transformation and make it possible to get a martensitic structure

throughout quite thick sections even when the less drastic oil- or air-

quenching processes are used.

6.11Martempering or Marquenching

When we cool the steel under conditions of the kind indicated in Fig.

9.17c. Here the steel is quenched into a bath at temperature E and left there

long enough to permit it to reach a uniform temperature throughout. It is then

removed from the bath and allowed to cool so that martensite will begin to

form at F. The net result is that, by allowing the core to attain the same

temperature as the surface whilst in the bath at temperature E. The final air-

cooling will not be rapid enough to allow a large temperature gradient to be

set up, and both core and surface will become martensitic at approximately

the same time. thus minimizing the tendency towards quench-cracking.

6.12 Austempering

This process illustrated in Fig.9.17D. Here the steel is quenched into a

bath at a temperature above that at which martensite can be formed and

allowed to remain there long enough for transformation to be complete at G.

Since transformation to bainite is complete at G. Austempering is

importance when heat-treating components of intricate section. Such

components might distort or crack if they were heat-treated by the more

conventional methods of quenching and tempering.

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6.13 Isothermal Annealing

In this process the large body of steel is heated into the austenitic

range and then allowed to transform as completely as possible in the pearlitic

range. The object of such treatment is generally to soften the steel sufficiently

for subsequent cold-forming or machining operations. The nature of the

pearlite formed during transformation is influenced by the temperature at

which isothermal transformation takes place. Transformation just below the

lower critical temperature leads to the formation of spheroidal pearlitic

cementite since precipitation is slow, whilst at lower temperatures

transformation rates are higher and lamellar cementite tends to form. A

structure containing spheroidal cementite is generally preferred for lathe

work and cold-forming operations, whilst one with lamellar cementite is

often used where milling or drilling are involved.

Figure 9.17 (A) and

(B) illustrate the

effects of mass

during normal

quenching. (C) and

(D)

show how these

effects may be

largely overcome in

martempering and

austempering.

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Ruling Section

It became necessary for manufacturers to specify the maximum

diameter or ruling section of a bar, up to which the stated mechanical

properties would apply following heat-treatment. If the ruling section is

exceeded then the properties across the section will not be uniform since

hardening of the core will not be complete.

The Jominy end-quench test is of great practical use in determining

the hardenability of steel. Here a standard test piece is made, (Fig.9.18A)

and heated up to its austenitic state. It is then dropped into position in a

frame, as shown in( Fig. 9.18B), and quenched at its end only, by means of a

pre-set standard jet of water at 25°C. Thus different rates of cooling are

obtained along the length of the bar. After the cooling, a 'flat', 0.4 mm deep,

is ground along the side of the bar and hardness determinations made every

millimetre along the length from the quenched end. The results are then

plotted as in Fig.9.19. These curves show that the depth of hardening of a

nickel-chromium steel is greater than that of a plain carbon steel of similar

carbon content, whilst the depth of hardening of a chromium-molybdenum

steel is greater than that of the nickel-chromium steel.

Figure 9.18 The Jominy end-quench test.

(A) The standard form of test piece used. (B) A simple

type of apparatus for use in the test.

Figure 9.19 The relative depth of

hardening of three different steels as

indicated by the Jominy test.