Iron – carbon system II - NPTELnptel.ac.in/courses/113105023/Lecture24.pdf · Iron – carbon system II . ... In iron carbon system there is a ... Cast iron having such a structure

NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | |

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Module 24

Iron – carbon system II

Lecture 24

Iron – carbon system II


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Keywords: Iron – graphite phase diagram, morphology of graphite in cast iron, grey cast iron,

malleable cast iron, nodular cast iron, mechanical properties of iron‐carbon alloys, effect of

super cooling on decomposition of austenite, time temperature transformation (TTT) diagram,

bainite, martensite

Introduction In the last lecture we looked at Fe‐Fe3C meta‐stable phase diagram. It was mentioned that iron

carbide is not a stable phase although it can exist in this form at room temperature for

indefinite periods. However on prolonged exposure to high temperature (around 600°C) it can

decompose into ferrite and graphite. We would therefore look at iron graphite phase diagram

and learn about alloys where carbon is present as graphite. We would also look at the effect of

carbon content on the mechanical properties of binary iron carbon alloys. We are also familiar

with the limitation of phase diagram. It can only predict evolution structure in metals and alloys

under equilibrium rate of cooling which is often too slow. In iron carbon system there is a

transformation where all the three phases involved are solid. This is the eutectoid reaction.

During this carbon atom in austenite gets partitioned between ferrite and cementite. This is

possible only through solid state diffusion of carbon which is slow. It would be interesting to

see what could be the effect of super cooling (or cooling rate) on such a transformation.

Iron Carbon (Graphite) Phase Diagram:

Iron – carbon binary phase diagram

Fe3C = 3Fe + C (graphite)

a b c d

T

Hypo Hyper

L

G

L + GL

G

0.02 0.8 2.0 4.3

910

1539

1400

727

738

11461158

+ L

100

Slide 1


3

The Fe‐Fe3C phase diagram has been reproduced in slide with firm line. The iron graphite phase

diagram is very much the same. Solubility of carbon in the two forms of ferrite is still the same.

There is a minor change in the upper limit of the solubility of carbon in austenite and in liquid.

The broken (dashed) lines are the boundaries between various phase fields in the case of Fe‐C

phase diagram. Note that the compositions of the critical points marked as a, b, c, & d are

nearly the same as those in Fe‐Fe3C phase diagram. The eutectoid and the eutectic

temperatures are a little higher. The diagram extends till 100% C. Fe3C is not a stable phase. It

decomposes into Fe and C. There are several grades of cast iron having graphite as one of the

constituents. However graphite is rarely present in steel. The density of graphite is 2.2gm/cc

whereas that of iron is 7.87gm/cc. This is why iron carbon alloys where carbon is present as

graphite expands on solidification. They are easy to cast as there is no problem associated with

shrinkage during solidification. Figure 1 shows different morphologies of graphite in cast iron.

Fig 1: Shows a few common shapes of graphite present in cast iron

The most common forms of graphite present in cast iron are flakes of various sizes and shapes.

Figure 1 (a) and (b) are the two common morphology. It is visible under microscope in even if

the specimen is not etched. Flakes act like stress raisers. Therefore cast irons having flakes of

graphite are brittle. The appearance of its fracture surface is grey. This is why it is known as

grey cast iron. Long flakes usually develop as primary phase during solidification. This is possible

only in hyper‐eutectic cast iron. In hypo‐eutectic grey cast iron the primary phase that forms

during solidification is austenite. The last liquid to solidify is the eutectic which is a mixture of

austenite and graphite. The graphite flakes that form during this stage are much smaller. Its

appearance may look like that in Fig 1(b). % graphite in eutectic can be estimated from the

phase diagram using lever rule. This is approximately equal to 100 x (4.3 – 2.0)/(100 – 2.0) = 2.4.

The balance is austenite. The excess carbon from austenite can precipitate as graphite as it

cools. This continues till the composition of austenite reaches that of the eutectoid point. On

further cooling it may transform into pearlite if the cooling rate is not slow enough. If the

cooling rate is extremely slow it should transform into a mixture of ferrite and graphite. Under

Large flakes

of graphite

Tiny flakes of

graphite

Nodules of

graphite Irregular patches

of graphite

(a) (b) (c) (d)


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such a condition grey cast iron would consist of ferrite grains and graphite flakes. The matrix of

such cast iron can vary from 100% ferrite to 100% pearlite.

The morphology of graphite shown in fig 1 (c) develops when white cast iron is held at around

950°C for long hours. This shape is usually known as temper carbon. White cast iron is

extremely brittle. However such treatment results in substantial improvement in its ductility.

Cast iron having such a structure is known as malleable cast iron. Flakes of graphite behave as

long cracks. This makes grey cast iron brittle. Temper carbons appear as collections of irregular

shaped patches of graphite in a microstructure. Unlike flakes such a form of carbon does not

act as notches or stress raisers. Therefore cast iron where graphite exits in this form is ductile.

The morphology shown in fig 1(d) can be obtained by special treatment during solidification. If

molten iron having high superheat is inoculated with magnesium, graphite nucleates in nodular

shapes. This too has good ductility. It is known as SG (Spheroidal Graphite) or nodular iron. Like

grey cast iron both nodular and malleable cast iron can have a matrix of 100 % ferrite or 100%

pearlite or a mixture of the two. Commercial cast iron has significant amount of silicon. Truly

cast iron is not a binary alloy. It is often considered as a ternary alloy of Fe‐C‐Si.

Mechanical properties of iron – carbon alloys: Phase diagram helps us interpret the structure of an alloy belonging to a system. The properties

of an alloy depend on its microstructure. Most of these follow the rule of mixture. Let us look at

the effect of carbon content on the properties of iron carbon alloys. Slide 2 presents a sketch of

iron carbon phase diagram. Recall that it tells us about the structure of alloys under equilibrium

rate of cooling. Pure iron at room temperature consists of grains of ferrite. It has the lowest

hardness but the highest ductility. If it has less than 0.002 % carbon, the structure is expected

to be primarily ferrite with a few precipitates of cementite. As a consequence its hardness and

tensile strength is likely to increase with carbon content. When % C exceeds the solubility limit

pearlite appears as one of the constituents. Since it is an intimate mixture of ferrite and

cemetite it has significantly higher strength and hardness. The amount of pearlite keeps

increasing from 0% at 0.002% C to 100% at 0.8%C. The strength and hardness of the alloy

increase linearly with the amount of pearlite. If %C is in the range 0.8 to 2.0 the alloy consists of

pearlite and pro‐eutectoid cementite. Its strength and hardness keep increasing with the

amount of cementite since it has higher hardness. However ductility decreases with increasing

amount of cementite. The trend continues even beyond 2.0% carbon.


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Mechanical properties of Fe-C alloys

0.02 0.8 2.0 4.3

T

6.67

9101146

727

LL+cm+L

+cm

+cm

% %

100P

TS TS PTS

TS

BHN%El

Pearlite being an intimate mixture of ferrite and cementite has significantly higher strength and

hardness. The strength (TS) of hypo‐eutectoid steel is directly proportional to the amount of

pearlite in the structure. It can be described by the expression given in slide 2. A set of curves

representing tensile strength, hardness and ductility are superimposed on the phase diagram

given in slide 2. TS = tensile strength. BHN = Brinell Hardness Number and %El = ductility

(elongation). Note that it has been shown till about 1.5%C. This is because iron – carbon alloys

become extremely brittle with increasing amount of cementite. It may break (fail) before

yielding. Cast iron has very high amount of cementite. They have poor tensile strength. It is

used mostly under compressive loading. However its hardness shown by the plot marked BHN

keeps increasing with carbon content. Slide 3 shown typical variations of the properties of steel

as a function of % carbon. Data were taken from metals handbook. Note that the YS increases

as % C approaches that of the eutectoid (0.8%C). Thereafter the effect of %C is negligible. The

ultimate tensile strength (UTS) shows similar trend till 0.8%C. Beyond this UTS drops. This is due

to the loss of ductility because of the presence of brittle cementite. The increase in hardness is

significant till 0.8%C. Beyond this the increase in hardness is marginal. In short the strength of

steel to a great extent is controlled by the amount of pearlite. Therefore it is worthwhile to look

at the structure of pearlite in details.

Slide 2


6

0

100

200

300

400

500

600

700

800

900

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

% Carbon

BH

N /

YS

/ U

TS YS Mpa

UTS Mpa

BHN

Effect of temperature on the pearlitic transformation: Pearlite is a product of eutectoid transformation. It has a lamellar structure consisting of

alternate layers of ferrite and cementite plates. The thicknesses of the plates are in the ratio

7:1. The formation of pearlite takes place through a process called nucleation and growth. It

begins with the nucleation of a tiny plate of cementite preferably at one of the grain

boundaries of austenite. It has 6.67% carbon whereas the %carbon in the eutectoid austenite is

only 0.8. This suggests that cementite can nucleate only if there is a very large fluctuation of

carbon content in certain areas. The probability of such fluctuation depends on the degree of

super‐cooling. The eutectoid transformation temperature in the phase diagram is often

referred to as the lower critical temperature. It is denoted as A1. If the reaction takes place at A1

it may take a very long time for nucleation to take place. However if the reaction is forced to

take place at a lower temperature the nucleation rate is likely to be higher. In short the

nucleation rate increases with the degree of super‐cooling. However the reaction would also

depend on the rate of growth. This occurs through the diffusion of carbon atoms in austenite.

The diffusivity is a strong function of temperature. It decreases with decreasing temperature.

Therefore the growth rate is expected to decrease with decreasing temperature. This is

schematically shown in fig 2.

Slide 3


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Figure 2 (a) presents a schematic diagram for the nucleation of a cementite platelet. It takes

carbon from the surrounding area. This develops a concentration gradient at the interface. The

sketch shows the carbon concentration profile at the tip of the cementite along one of the

direction of its growth. Figure 2(b) gives the expected nature of the rate of nucleation (N) and

that of growth (G) of a pearlite nodule. N increases with the degree of super‐cooling (�T)

whereas Gdecreases with the degree of super‐cooling. This suggests that there must be a

critical degree of super‐cooling where the growth of pearlite nodule will be the fastest. Let us

look at a hypothetical experiment which would help us visualize the same.

Hypothetical experiment: Heat a set of steel specimens having 0.8% carbon to a temperature around 760°C which is a

little higher than the A1 temperature of the steel. Keep them long enough to transform them to

homogeneous austenite. Transfer one of these to a bath maintained at a temperature T1 which

is lower than A1. After a time t1 take it out and quench in water maintained at room

temperature. Transfer another specimen from the furnace at 760C to the bath at T1, keep it at

this temperature for time t2 and then quench in water. Repeat the step to get a set of samples

that were kept at T1 for different lengths of time. Polish and etch the samples for

metallographic examination. Estimate the amount of pearlite present in the each of the

samples. Find the hardness of the sample in Rockwell C scale. Plot % pearlite (P) found in the

structure as a function of the hold time at temperature T1. Also plot the hardness (Rc) of these

against hold time at T1. Figure 3 shows the main feature of these plots.

Grain boundary

of austenite

Fe3C

platelet

%C

0.8

x

% Carbon near the

tip of the nucleus

A1

T

T = degree of super‐cooling

(a) (b)

Fig 2

% P

t 0

100

ts tf

Rc

t 0

64

ts tf

Fig 3 (a) (b)


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Note the S‐shaped feature of the curve representing the extent of transformation in fig 3(a). It

shows that pearlite forms only in specimens that were held at T1 for a period greater than ts,

which is a measure of the time at which the formation of pearlite begins. The amount of

pearlite that forms at this temperature increases with hold time till it becomes 100% pearlite at

tf. It denotes the time at which the transformation ends. It is very easy to identify pearlite in the

microstructure. It has a distinct lamellar feature. If the cementite plates are very closely spaced

the lamellar feature may not get resolved. Nevertheless the area would appear to be much

darker than the rest. What does the brighter region represent we would know very shortly.

Figure 3(b) represents the variation in hardness as a function of hold time at temperature T1.

Look at the shape of the curve. It appears to be a mirror reflection of the S‐shape in fig 3 (a).

The hardness of the samples which were kept at T1 for a period less than ts is very high. As the

hold time increases the hardness keeps dropping till it reaches a limiting value. This happens at

tf, the time at which the transformation is complete.

The times at which the transformation starts or ends can thus be estimated either by looking at

the microstructure or from the hardness. The two are expected to be very close. Note the initial

and the limiting value of the hardness. Repeat the same experiment at several temperatures

below the lower critical temperature A1. The data may be compiled in a tabular form as shown

in table 1. Plot these as shown in Fig 4. This gives two C‐shaped curves for ts and tf.

Table 1: A record of ts & tf as a function of transformation temperature

Transformation Temperature ts tf

T1 ts1 tf1

T2 ts2 tf2

‐ ‐ ‐

Ti tsi tfi

‐ ‐ ‐

‐ ‐ ‐

Tn tsn tfn

Ms

Mf

T

0

A1 T1

t

T2

Tc

Tn

Rc

+ P

+ B

+ M

Pearlite

(P)

Bainite

(B)

Martensite

(M) t1s t1f tcs tcf tnf tns

Fig 4

ts tf


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Time Temperature Transformation (TTT) Diagram: The plot shown in Fig 4 represents the time at which the transformation of austenite to a

mixture of ferrite and carbide starts (ts) and the time at which it is finished (tf) as a function of

temperature. It is commonly known as the time temperature transformation diagram. Often it

is referred to as the TTT diagram. This has been obtained by an experiment where the

transformation takes place at a constant temperature. Therefore it is also known as the

isothermal transformation diagram of the steel.

The transformation of austenite into an aggregate of ferrite and carbide takes place by

nucleation and growth. The formation of a new constituent in austenite is associated with the

creation of a new interface. At the eutectoid temperature both austenite and the ferrite

carbide aggregate are equally stable. The free energy of transformation is zero. Therefore

nucleation that needs creation of new interface cannot take place. Super‐cooling (�T) is

essential for this to happen. The rate of nucleation increases with �T (Fig 2). The growth of a

stable nucleus needs continuous supply of solutes through the austenite grains surrounding it.

This is controlled by the diffusion of carbon atoms in austenite. It decreases with decreasing

temperature (Fig 2). As a consequence as the transformation temperature drops the rate of

transformation becomes fasters. Both ts and tf keep decreasing. During this stage the

transformation is dominated by nucleation. However as the temperature drops further, the

diffusion of carbon becomes very slow. This is when the transformation begins to be dominated

by the growth. The ts and tf hereafter start increasing. There is a critical temperature at which

the magnitudes of ts and tf are the lowest. This is often called the knee (or the nose) of the

curve. The rate of eutectoid transformation is the highest at this temperature. The ferrite

carbide aggregate that forms above the knee of the TTT diagram is called pearlite. It has a

lamellar structure consisting of alternate layers of ferrite and cementite. The product of the

eutectoid transformation occurring at temperatures below the knee of the TTT diagram but

above the line marked Ms is known as Bainite. This too is a fine aggregate of ferrite and

carbide. The hardness or the strength of the product is a function of the size and the

distribution of hard cementite within ferrite matrix. Lower the temperature the finer is the

structure and finer structure means higher hardness. This is why fig 4 shows that the hardness

of the final structure (usually reported in Rockwell C scale: Rc) increases with decreasing

transformation temperature.

When the sample is quenched below a temperature marked Ms in fig 4, a totally different kind

of transformation takes place. This does not involve diffusion at all. Therefore the

transformation takes place at a very high speed approaching that of the sound. It occurs

through shear displacement. It is known as Martensitic transformation. The product is known

as Martensite. It is a phase. Carbon does not precipitate out of austenite. It is retained within


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the lattice by force. This results in a distorted body centered unit cell. Ferrite has BCC structure

where carbon occupies the octahedral interstitial sites.

Since the solubility of carbon in ferrite is very low (0.002% at room temperature) only a very

small fraction of these sites are occupied. The octahedral sites are not symmetrical. Whenever

these sites are occupied by carbon the iron atoms are pushed apart along one of the 3 axes.

This results in a tetragonal (asymmetric) distortion. The effect is not felt when only an

infinitesimally small fractions of these sites are filled up by carbon atoms. However if 0.8%

carbon is retained within the lattice (it is not allowed to precipitate out as carbide) a significant

number of these sites are occupied by carbon atoms. The lattice gets highly strained. In fact due

to the asymmetric distortion the crystal structure becomes body centered tetragonal. The c/a

ratio is a little higher than 1. In BCC the c/a ratio equal to 1. The c/a ratio of martensite is a

function of its carbon content. In eutectoid steel this may be around 1.08. Therefore for all

practical purpose, the martensite may be assumed to be a supersaturated solid solution of

carbon in ferrite having BCC structure. The presence of excess carbon is the main reason for the

tetragonal lattice distortion. This also makes the structure very hard. The hardness of

martensite is a function of its carbon content. For eutectoid steel it is around Rc 64.

Since the transformation of austentite to martensite does not involve any diffusion, the

transformation occurs only as long as the temperature keeps dropping. Such a transformation is

known as an athermal transformation. It starts at a characteristic temperature called Ms and it

continues till a characteristic temperature called Mf when it is over. Ms is the temperature at

which maternsitic transformation starts and Mf is the temperature at which it finishes.

Refer to fig 3 (b). Note that the hardness of the samples that were held at the transformation

temperature for less than ts have hardness equal to Rc 64. Thereafter it decreases because a

part of the austenite transforms into ferrite carbide aggregate having lower hardness. The

hardness approaches a limiting value which is the characteristic of the final structure. This can

be as low as Rc 20 if the structure is coarse pearlite. It can also be very high (~Rc 55) for bainite

that forms at temperatures close to the Ms. Thus depending on the transformation

temperature eutectoid steel can have hardness as low as Rc 20 to as high as Rc 64.

Structures of Pearlite, Bainite & Martensite: Pearlite is a mixture of ferrite and cementite. The two are arranged as alternate layers. The

plates of cementite are thin whereas those of the ferrite are thick. The widths of the two are in

the ratio of 1:7. The structure is often referred to as lamellar. The evolution its structure is

controlled by the diffusion of carbon atoms in austenite. At a higher temperature the diffusion

distance ( √Dt ) is longer. Therefore the plates of cementite and ferrite are wider. The inter‐

lamellar spacing is longer. Such a structure is called coarse pearlite. At lower temperature the


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distance covered by the carbon atoms during the transformation is much shorter. Therefore the

inter‐lamellar spacing is shorter. This is called fine pearlite. The hardness of coarse pearlite can

be as low as Rc 20 whereas that fine pearlite can be as high as Rc 35. A sketch of pearlite is

shown in Fig 5.

Cm

Pearlite

Coarse Fine

Bainite

Upper Lower

Cm Fe2.4C

Martensite

M or ’

Fig 5


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Figure 5 shows the main features of the three different types of microstructures that could

form in 0.8% C steel (eutectoid). If the transformation temperature is less than A1 but greater

than Tc (see fig 4) the austenite transforms into pearlite. Whereas if the temperature is less

than Tc but greater than Ms the austenite transforms into bainite. It is also a mixture of ferrite

and carbide. There are two variants of bainite. These are called upper and lower bainite. The

former consists of fine broken platelets of cementite in a matrix of ferrite. Figure 5 presents a

sketch of the microstructure of upper bainite. Note the orientation of bainite platelets. These

are aligned along the direction of growth of the ferrite lath. The lower bainite consists of tiny

rod like carbides aligned at a definite angle to the direction of growth of ferrite grains. The

shape of the ferrite grains is different from that in upper bainite. It is acicular (needlelike). The

carbide too has a different composition. It is represented as Fe2.4C and is commonly known as

epsilon carbide. A sketch of the microstructure of lower bainite is given in Fig 5. The structure

of bainite is extremely fine. The lamellar structure of pearlite particularly those of coarse

pearlite can be seen under an optical microscope. However you need an electron microscope to

reveal the finer details of the structure of bainite. The lengths of the carbides in bainite are

much shorter than those in pearlite. This is because bainite forms at a lower temperature. The

diffusivity of carbon is less. Therefore the average distance covered by carbon is also less. This

is why the lengths of carbide are much shorter than those in pearlite.

Figure 5 shows the main features of martensite in eutectoid steel. The transformation of

austenite to martensite occurs without any diffusion of carbon atoms. It is extremely fast.

Plates or needles of martensite form within austenite by shear mechanism. It only involves

nucleation. The needle that forms first is the longest. It does not extend beyond the boundary

of austenite. The subsequent needles are shorter. They too do not cross any boundary. Note

that there is no precipitate. The structure is homogeneous. This is why it is difficult to etch. The

martensitic region always appears to be much brighter than the pearlitic or bainitic regions.

Fig 6

Ps

Pf

Ms

Mf

T

0

A1

t

Bs

+ P

+ B

+ M

Coarse Pearlite (Rc5‐20)

Upper Bainite (Rc40‐50)

Martensite (Rc64)

Bf

Fine Pearlite (Rc20‐30)

Lower Bainite (Rc50‐60)


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Figure 6 gives a schematic time temperature transformation diagram of eutectoid steel. The

structure and properties of the final product is a function of the temperature at which the

transformation takes place. Coarse pearlite has the lowest hardness. The details of its structure

can be easily seen under optical microscope. The lower the transformation temperature finer is

the pearlite. Just below the knee of the diagram the transformation product is upper bainite. It

is also known as feathery bainite. The structure is finer than that of fine pearlite. It is harder as

well. The bainite that forms at temperatures nearer the Ms is called lower bainite. It is also

known as acicular bainite. Its hardness is higher than that of upper bainite. On direct quenching

to room temperature you get 100% martensite. This has the highest hardness.

Experimental techniques for determining TTT diagram of steel: The method described above can be used to generate the TTT diagram of steel. However this is

a very tedious method of determining the same. It takes a long time as well. There are several

alternative methods of determining the TTT diagram. One of the most popular techniques uses

the dimensional change that takes place during the decomposition of austenite into low

temperature stable structures. This is known as dilatometer. Austenite is a close packed

structure. Its density is higher than that of its transformation product. If the length of a small

test piece is continuously monitored after it is quenched from its austenitic state to a

temperature below A1 and held there you would notice that the length initially remains

constant but it begins to increase soon after. The trend continues till it reaches a limiting value.

A typical dilatometer plot at a constant temperature is shown in fig 7. The plot gives the time

at which the transformation starts (ts) and also the time at which it finishes (tf) at a given

transformation temperature. Thus by performing the experiment over a range of temperature

you can get a set ts & tf as function of temperature needed to construct the TTT diagram.

L L & T

0

T1

ts1 tf1

Fig 7: A schematic diagram showing typical plots of

temperature and dilation as a function of time.

When the sample is quenched to T1 there is an

initial drop in the length due to thermal

contraction. Thereafter L remains constant. The

change sets in with the onset of transformation at

ts1. The trend continues until the transformation is

over at tf1. time


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In principle the TTT diagram can be generated by continuous measurement of any physical characteristic of steel (e.g. dilation, electrical resistance or magnetic property, thermal analysis). A wide range of commercial equipment is available which can readily be used for this purpose. Summary: In this lecture we looked at Fe‐C equilibrium diagram. Fe3C is only a meta‐stable phase. Under

prolong thermal exposure at high temperature it decomposes into Fe & C. Traces of graphite

have been detected in several high temperature components of power plants and oil refineries.

The tendency of graphitization increases with increasing carbon content. Graphite is a major

constituent in several grades of cast iron. We did look at the different types of morphology of

graphite in cast iron. We also learnt about the correlation between structure and properties of

steel and cast iron. The steel is strong yet ductile whereas cast iron is brittle. Steel is strong

under tension as well as under compression. Whereas cast iron has good compressive strength

but it has poor tensile strength. In the end we looked at the effect of transformation

temperature on the nature of transformation and the structure that evolves in eutectoid steel.

Depending on the temperature of transformation the steel can have pearlite bainite or

martensite. We learnt about the main features of three different structures and their

properties. We also learnt how to construct time temperature transformation diagram for this

steel. The evolution of microstructure so far was illustrated with the help of a set of schematic

sketches. Here are a few microstructures taken from the work of one my colleague and his

students. These would help you have an idea about the main morphological features of

martensite and bainite in steel. Finer details get revealed only when these are examined under

SEM and TEM.

Fig 8: Microstructure of steel quenched from austenitic state consists of martensite (M), bainite (B) and retained austenite (RA). (Curtsey S Samanta & D Chakrabarti, MME

Fig 9: Microstructure of steel quenched from ferrite ‐ austenite state consists of martensite (M) and ferrite. (Curtsey M Mandal & D Chakrabarti, MME Department, I I T


15

Department, I I T Kharagpur) Kharagpur)

Fig 10 : SEM image of upper bainite (UB) in steel. Note that the carbides in UB are aligned along the growth direction of ferrite. (Curtsey S Samanta & D Chakrabarti, MME Department, I I T Kharagpur)

Fig 11 : SEM image of steel having martensite (M) and lower bainite (LB). Note that the carbides in LB are aligned at an angle with the growth direction of ferrite. (Curtsey D Chakrabarti, MME Department, I I T Kharagpur)

Fig 12 : SEM image of steel having mostly lower bainite (LB). The finer details of carbides within a ferrite plate are clearly visible. (Curtsey D Chakrabarti, MME Department, I I T Kharagpur)

Fig 13: TEM image of an air hardening steel after it has been tempered. It consists of lath martensite with carbides along lath boundaries. (Curtsey G Krishna et al, CSIR‐NML Jamshedpur)


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Exercise:

1. If an eutectoid steel is kept at 700⁰C what change do you expect? 2. What is the limitation of phase diagram? 3. If a piece of steel having 0.8 % carbon has martensitic stucture can it be converted to

fully pearlite structure by holding it at 700⁰ C? 4. Suggest a method of getting a mixture of Pearlite, Bainite & Martensite in an eutectoid

steel. 5. Which microstructure in eutectoid steel has maximum hardenss? Give reason.

Answer:

1. Lamellar structure is unstable as it has large surface area. Initially the cementite plates would break down in to globular structure. Size of the globule will grow with time. Cementite is also metastable on prolonged thermal exposure it breaks down in to ferrite and graphite.

2. Phase diagram does not show the effect of cooling rate. It gives the expected phases in an alloy at a given temperature under equilibrium condition.

3. No. To get pearlitic structure it must be heated back to austenite state then cooled slowly.

4. This is possible through two step isothermal transformation in Pearlitic & Bainitic region followed by queching. The cooling scheme is shown below. Note that no transformation takes place during quenching (fast cooling: the virtical step in cooling curve) if temperature is above Ms.

5. Martenstic structure has the maximum hardness. This is because of the presence of carbon atoms in the interstices is far in excess of its normal solubility in ferrite lattice. This results in tetragonal lattice distortion. This makes dislocation movement very

T

t

P

B

+P

+M

Ms

Mf


17

difficult. This in conjuction with extremely fine microstructure account for the high hardness of Martensite.

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Iron – carbon system II - NPTELnptel.ac.in/courses/113105023/Lecture24.pdf · Iron – carbon system II . ... In iron carbon system there is a ... Cast iron having such a structure