Module 23 Iron Carbon System I Lecture 23 Iron ... … -...

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

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Module 23

Iron Carbon System I

Lecture 23

Iron Carbon System I


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Keywords : Ferrite (), Austenite (), Ferrite (d): three different forms of iron, solubility of carbon in

different forms of iron, cementite, graphite, Fe – Fe3C meta‐stable phase diagram, pearlite, ledeburite,

hypo‐eutectoid steel, hyper‐eutectoid steel, hypo‐eutectic white cast iron, hyper‐eutectic white cast

iron

Introduction

The last five modules were devoted to the solidification behavior of binary alloys. This is best

represented by its phase diagram. We are now familiar with a wide range of simple and complex phase

diagrams. However while introducing this we only considered hypothetical cases. Now let us consider a

specific binary alloy of considerable importance. We are going to talk about iron ‐ carbon system. Iron

can exist in two different crystalline forms. These are BCC and FCC. This is known as allotropy. Carbon

too has several allotropic forms. However the form which is of relevance to this system is graphite. It has

a hexagonal structure. This is considered to be the most stable state of carbon. An atom of carbon atom

is significantly smaller than that of iron. Therefore it can be accommodated within the interstitial sites of

iron lattice resulting in an interstitial solid solution. Solubility carbon is likely to be limited. Apart from

this iron has a strong affinity to form carbide. The most common form is cementite. Its chemical formula

is Fe3C. This is truly a meta‐stable. However the rate of decomposition of Fe3C into Fe and C is extremely

slow. In most of the common grades iron – carbon alloy excess carbon is present as cementite. Let us

first look at the Fe‐Fe3C meta‐stable phase diagram and in a later module we shall discuss about iron –

graphite phase diagram as well. The alloys belonging to this system is popularly known as steel or cast

iron. We would soon learn about the difference between the two.

Solidification of pure iron:

Fe : crystal structure

Solubility of carbon in Fe = f(structure, temperature)

Where is the carbon located in iron lattice?

T

1539

910

time

Liquid

BCC

FCC

BCC

1394

770Ferromagnetic

Paramagnetic

Slide 1


3

The sketch in slide 1 is a typical cooling curve of pure iron. Solidification begins with nucleation and

growth of crystals of iron at 1539°C. It is BCC (body centered cubic). At 1394°C it transforms into FCC

(face centered cubic) structure. This is stable till 910°C where it again transforms into BCC. Each of these

transformations appears as steps on the cooling curve. Apart from this there is another transformation

which may not get detected by thermal analysis. This is the transformation from paramagnetic to

ferromagnetic state. It occurs at 770°C. This is known as its Curie temperature. The property which is

most sensitive to detect it, is magnetic permeability. The three different forms of iron are known as

ferrite (), stable until 910°C, austenite (), stable from 910°‐1394°C and ferrite (), stable from1394°–

1539°C. Note that the BCC form of iron is known as ferrite. Therefore in order to distinguish between

the two, the high temperature form is termed as delta ferrite. If carbon atoms are introduced into iron

these are likely to occupy the interstitial sites because the atoms carbon are much smaller than those of

iron atoms. The interstitial sites in BCC and FCC are shown in slide 2. The solubility of carbon in iron is a

function of temperature and crystal structure.

Solubility of carbon is higher in BCC or FCC?

a

b

c

a

b

c

Interstitial sites in iron lattice

BCC FCC

Lattice site

Interstitial site

Slide 2 shows the lattice sites occupied by iron atoms and possible interstitial sites for carbon atoms in

both BCC & FCC structures. The interstitial sites shown here are known as octahedral sites. The sketches

in slide 2 show only the positions of iron atoms and the interstitial sites in one unit cell. The sites located

within the cell belong exclusively to a unit cells but those lying on the faces, the edges or the corners are

shared by the neighboring unit cells as well. BCC has eight corner sites for iron atoms. Each of these is

shared by 8 neighboring unit cells. The contribution of corner site is thus 1/8. The atom at the centre

belongs exclusively to this unit cell. Therefore the number of iron atoms / unit cell = 8 x (1/8) + 1 = 2. Let

Slide 2


4

us use the same approach to estimate the number of interstitial sites /unit cell for BCC structure. The

sketch in slide 2 shows the locations of interstitial sites. There are 6 sites at the centers of 6 faces. Each

face is shared by two unit cells. There are 12 sites at each of the 12 edges. Each of these is shared by 4

neighboring cells. Therefore the number of interstitial sites / unit cell in BCC crystal = 12 x (1/4) + 6 x

(1/2) = 6. Note that it is 3 times the number of Fe atoms in a unit cell. Look at the sketch for FCC unit cell

in slide 2. Count the number of lattice sites. Follow the same approach to show that the number of Fe

atoms / unit cell = 6 x (1/2) + 8 x (1/8) = 4. In the same way the number of interstitial sites / unit cell in

FCC structure = 1 + 12 x (1/4) = 4. Note that in FCC structure the ratio of the number of interstitial site to

the number of lattice sites = 1. Now that we know about the possible sites carbon atoms could occupy

is it possible to guess which of these is likely to have higher solubility?

Fig. 1(a): Shows the lattice and the interstitial sites in a BCC unit cell. Circles are the lattice sites. The

black dots are the interstitial sites. In a lattice several of such cells are stacked one after the other. F

represents the body center site in a unit cell just above the one shown in the sketch. The figure ABCDEF

formed by joining the lattice sites as illustrated is an octahedron. There is an interstitial site at its center

marked as O. All the interstitial sites are identical. Note that AB = BC = CD = DA = = the lattice

parameter. The other edges of the octahedron AE = BE = CE = DE = AF = BF = CF = DF = √3/2. It is not a regular octahedron. The interstitial site in BCC structure is not symmetrical. The gap along EF is shorter

than that along AC or DB. It is 2 √3 /2. The lattice parameter of BCC iron called ferrite is

0.286nm. The shortest gap is equal to 0.038nm.

Fig. 1(b): Shows the lattice and the interstitial sites in a FCC unit cell. Circles are the lattice sites. The

black dots are the interstitial sites. In a lattice several of such cells are stacked one after the other. The

figure ABCDEF formed by joining the lattice sites as illustrated is an octahedron. There is an interstitial

site at its center marked as O. All the interstitial sites are identical. Note that AB = BC = CD = DA = AE =

BE = CE = DE = AF = BF = CF = DF = √ , where = the lattice parameter. It is a regular octahedron. The

interstitial site in FCC structure is symmetrical. The gap along EF is equal to that along AC or DB. It

is 2 √2 /2. The lattice parameter of FCC iron called austenite is 0.362nm. The interstitial gap is

equal to 0.106nm. It is much larger than the interstitial gap in ferrite.

B A

D C

E

O

(a)

F

(b)

B C

D

A

F

E

O Fig 1


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FCC has higher packing density than BCC. Yet the solubility of carbon is FCC is higher. This is because the

interstitial sites in FCC are bigger than that in BCC. There are two types of interstitial sites known as

octahedral and tetrahedral. In FCC the tetrahedral sites are surrounded by four close packed atoms. The

gap is extremely small, whereas in BCC the four atoms are more widely spaced. The tetrahedron is not

symmetric. The shortest dimension (size) of tetrahedral sites in BCC lattice was derived in module 5. This

is nearly same or marginally lower (0.036nm) than that of the octahedral site (0.038nm). The number of

such sites is much more than that of the octahedral sites. Although the packing density in BCC is lower

than that of FCC, the gaps are equally distributed between two types of interstices making these

significantly smaller than that of FCC. This is why the solubility of carbon which is an interstitial solute is

significantly lower in ferrite (BCC) than that in austenite (FCC). Apart from this the interstitial site in BCC

is asymmetric. When a carbon atom goes into the interstices the atoms along one of the axes are

pushed further apart. This results in a tetragonal distortion. We shall talk about it in one of the

subsequent modules.

Phases in iron – carbon binary system:

Iron can exist in three different crystalline forms each having limited solubility of carbon. The stability of

these depends on temperature and composition. The two high temperature forms of iron are ferrite which is BCC (stable above 1394°C) and austenite (stable above 910°C) which is FCC. The room

temperature form of iron is ferrite which is BCC. The solubility of carbon in ferrite is limited. The

maximum solubility is around 0.025wt% as against this the solubility of carbon in austenite is a little

more. It is about 2wt%. Apart from this iron carbon system may have iron carbide (Fe3C) called

cementite. It has 6.67% carbon. It is considered as an inter‐metallic compound having relatively more

complex crystal structure than those of ferrite and austenite. It is a meta‐stable phase. It may exist for

indefinite periods of time at room temperature. However on prolonged thermal exposure at 600°C or

beyond it transforms into ferrite and graphite. Therefore iron carbon alloys of commercial importance

may be considered as a binary alloy of iron and cementite. Let us first look at its phase diagram. It is

also known as iron cementite meta‐stable phase diagram. Although it is a binary system there are 5

different phases including the liquid. This is likely to have more than one invariant reaction involving 3

phases.


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Iron – cementite phase diagram

1148

L

Hypoeutectic Hyper-eutectic

% C0.8 2.0 4.30

727

910

1394

1539

Hypo eutectoid

Hyper eutectoid

6.67

+ L L + Cm

+ Cm

+ L

+

Eutectoid isotherm

Eutectic isotherm

Peritecticisotherm

Slide 3 gives a schematic Fe‐Fe3C phase diagram. It has 3 invariant reactions (transformation). These are

given in slide 4. The one occurring at 1495°C is the peritectic reaction. The delta ferrite reacts with liquid

to form austenite. The one at 1148°C is known as the eutectic reaction where the liquid transforms into

a mixture of austenite and cementite. The eutectic is known as Ledeburite. The one at 727°C is known as

eutectoid transformation where austenite decomposes into a mixture of ferrite and cementite. This is

known as Pearlite. On the basis of this diagram iron – carbon alloys having less than 2.0% carbon are

known as steel, whereas those having more than 2.0% carbon are known as cast iron. This classification

is based on their ability to undergo large plastic deformation. Steel is ductile but cast iron is brittle.

Steel:

It is an iron carbon alloy where most of the carbon is present as meta‐stable iron carbide called

cementite. The upper limit of carbon content is 2%. Phase diagram helps us guess the structure of alloys

and their properties. Let us look at what kinds of structure steel could have depending on its

composition. We would only consider the structure that develops under equilibrium rate of cooling. The

steel on solidification is expected to have fully austenitic structure. It may be assumed to be

Slide 3

Slide 4


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homogeneous since the rate of cooling is considered to be slow. Depending on its composition we may

have 3 types of structures. (i) % carbon < 0.02 (ii) 0.02 < % carbon < 0.8 (iii) 0.8 < % Carbon < 2.0.

(i) Figure 2 explains the solidification behavior of steel having less than 0.02% carbon with the

help of a set of schematic diagrams. The sketch on the left shows a part of the equilibrium

diagram (Fe‐Fe3C) with the location of the alloy by a vertical dotted line. It intersects the

liquidus, solidus, and a set of solvus curves. These are projected on to the cooling curve

shown on the right with the help of a set of horizontal lines. The cooling curve exhibits

inflection points at each of these intersections. Solidification begins with precipitation of a

few grains of ferrite. The top most microstructure corresponds to this stage. The

solidification takes place by nucleation and growth. The composition of the liquid and the

solid keeps changing during this stage. When solidification is complete the entire liquid is

replaced by ferrite having the same composition as that of the alloy. This is shown by the

second schematic structure from the top in Fig 2. The structure remains unchanged until the

temperature crosses the boundary between / phase fields. Thereafter austenite precipitates from ferrite. The grain corners and boundaries are the preferred sites where grains of austenite nucleate. The third microstructure from the top in Fig 2 represents its

main features. It consists of grains of (white) and a few grains of (grey). There is partition of carbon between these two phases. Bulk of the carbon goes into austenite. The

composition of the two keeps changing as the temperature drops. The volume fraction of increases at the cost of . When the %carbon in austenite becomes equal to that of the steel

ferrite disappears. The structure now consists of 100% austenite. Note the main features

of the fourth microstructure from the top in Fig 2. The grain size is finer than that of 100% ferrite. The structure remains as 100% austenite until the temperature drops below the line

representing the boundary between and phase fields of the equilibrium diagram.

This is where ferrite starts precipitating from austenite. The grain boundaries and the

T°C

Wt% C0.8 2.0 0.02 0

1539

1495

1394

727

910

L

+ L

+ L

+ Cm

+ Cm

L

+ Cm

time

T

L +

L

Fig 2


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grain corners are the preferred sites for precipitation. The fifth sketch from the top of Fig 2

is a typical representation of its microstructure at this stage. Ferrite grains are shown as

white and austenite grains are shown as grey. This continues through nucleation of new

grains and growth of the existing ones until the temperature drops below the line between

and phase fields of the phase diagram. At this stage the structure is 100% ferrite (). The 6th sketch in Fig 2 is a typical representation of the microstructure. This remains

unchanged till the temperature drops below the solvus. At this stage excess carbon

precipitates as cementite. The last sketch in Fig 2 is a typical representation of its

microstructure. The amount of cementite keeps increasing as the room temperature drops.

It can be estimated by lever rule. From the phase diagram it is evident that the steel at room

temperature would consist of ferrite with a few specks of cementite. If % carbon in the steel

is 0.01 the amount of cementite is given by (0.01/6.67)x100 = 0.15%. The grains are

relatively finer than that after solidification.

(ii) Figure 3 explains the solidification behavior of steel having greater than 0.02% carbon but

less than 0.8% carbon with the help of a set of schematic diagrams. The sketch on the left

shows a part of the equilibrium diagram (Fe‐Fe3C) with the location of the alloy by a vertical

dotted line. It has around 0.18% carbon. It intersects the liquidus and a set of lines denoting

either phase field boundaries or isotherms representing 3 phase equilibrium. These are

projected on to the cooling curve shown on the right with the help of a set of horizontal

lines. The cooling curve exhibits inflection points or steps (discontinuity) at each of these

intersections. Solidification begins with the precipitation of a few grains of ferrite. The first microstructure from the top corresponds to this stage. Solidification takes place by the

nucleation of new grains and the growth of the existing ones. The composition of the liquid

and the solid keeps changing during this stage. When the temperature reaches 1495°C

peritectic reaction sets in. This is an invariant reaction. The liquid having 0.55% carbon

reacts with ferrite having 0.08% carbon to form austenite () having 0.18% carbon. The amount of is just enough to consume the remaining liquid. The alloy on solidification

L

Fig 3

T°C

Wt% C0.8 2.00.02 0

1539

1495

1394

727

910

L

+ L

+ L

+ Cm

+ Cm

L

L

time

T

Pearlite


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consists of austenite () having 0.18% carbon. This is shown by the second schematic

structure from the top in Fig 3. The structure remains unchanged until the temperature

crosses the boundary between / phase fields. Thereafter ferrite () precipitates from

( austenite. The grain corners and boundaries are the preferred sites where grains of ferrite nucleate. The ferrite precipitating from austenite is known as pro‐eutectoid ferrite.

The third microstructure from the top in Fig 3 represents its main features. It consists of

grains of pro‐eutectoid (white) and a few grains of (grey). There is partition of carbon between these two phases. Bulk of the carbon goes into austenite. The composition of the

two keeps changing as the temperature drops. The volume fraction of increases at the cost of . When % carbon in austenite reaches 0.8% eutectoid reaction sets in. This is an

invariant reaction. As long as it continues the temperature remains constant. During this

stage both cementite and ferrite start precipitating from austenite at the same time. The

product is an intimate mixture of two phases. It is known as pearlite. It consists of parallel

layers (plates) of ferrite and cementite. Figure 4 gives a series of sketches showing various

stages involved in the formation of pearlite. When a cementite plate nucleates as in (Fig.

4(a)) at an austenite boundary it takes carbon from its neighboring area. This results in a

change in local carbon profile as shown in the sketch. Just beside cementite % C = 0.02

whereas at a distance far from this it approaches 0.8 % C (this is the composition of the

eutectoid). When this becomes low enough a ferrite plate nucleates. Figure 4 (b) illustrates

this. Note the change in carbon concentration as a function of distance. Figure 4 (c) shows

the arrangement of ferrite cementite plates in a colony of pearlite. The width of the two is

approximately proportional to % ferrite and % cementite in eutectoid steel having 0.8%

carbon. Using lever rule % cementite in pearlite =100 x (0.8‐0.025) / (6.67‐0.025) = 12%

(approximately). Therefore % ferrite = 88%. The ratio of the two is approximately equal to

1:7. This means the width of the ferrite plate will be seven times the width of cementite

plate. On completion of the eutectoid reaction the structure consists of pro‐eutectoid ferrite

and pearlite. Note the main features of the fourth microstructure from the top in Fig 3.

When the temperature drops below the eutectoid temperature, excess carbon from ferrite

() precipitates as cementite. This is known as tertiary cementite (Why this is called tertiary

will be clear from subsequent part of this module). However the change in structure below

the eutectoid temperature is too little to detect. Steels having carbon within 0.02 – 0.8

consist of ferrite and paerlite. They are known as hypo‐eutectoid steel.

Fig 4

Austenite

boundary

Fe3C

% C

6.67

0.8 0.8

x 0.02 0.02

Austenite

boundary

Fe3C

% C

6.67

0.8 0.8

x0.02 0.02 Ferrite

Austenite

boundary

Pearlite

colonyFe3C

(a) (b)

(c) (d) Structure


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Hyper-eutectoid steel (0.8-2.0% C)

L

% C0.8 2.00

723

910

1394

1539 L

L+

Cm

Time

(a)

L

(a)(a)(a)(a)(a)

Cm

P

Cm

(iii) Slide 5 explains the solidification behavior of steel having greater than 0.8% carbon but less

than 2.0% carbon with the help of a set of schematic diagrams. Such steels are known as

hyper‐eutectoid steel. The sketch on the left shows a part of the equilibrium diagram (Fe‐

Fe3C) with the location of the alloy as a vertical dotted line. It has around 1.0% carbon. It

intersects the liquidus, solidus, solvus and the eutectoid reaction isotherm representing 3

phase equilibrium. These are projected on to the cooling curve shown on the right with the

help of a set of horizontal lines. The cooling curve exhibits inflection points or a step

(discontinuity) at each of these intersections. Solidification begins with precipitation of a

few grains of austenite. The top most microstructure corresponds to this stage. The

solidification takes place by nucleation of new grains and growth of the existing ones. The

composition of the liquid and the solid keeps changing during this stage. When the

temperature reaches that of the solidus the composition of the solid becomes equal to that

of the steel. The alloy on solidification consists of 100% austenite () having 1.0% carbon (say). This is shown by the second schematic structure from the top in slide 5. The structure

remains unchanged until the temperature crosses the solvus, the boundary between / + Cm phase fields. At this stage cementite starts precipitating from austenite. It grows at the

cost of austenite. The % carbon in austenite keeps decreasing as the amount of cementite

increases. The grain boundary is the most favored site for precipitation. The fourth structure

from the top in slide 5 gives a typical structure of steel at this stage. When % carbon in

austenite decreases to 0.8% eutectoid reaction sets in. This is an invariant reaction. As long

as it continues the temperature remains constant. During this stage both cementite and

Slide 5


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ferrite start precipitating from austenite at the same time. The product is an intimate

mixture of two phases. It is known as pearlite. The details about the pearlitic structure are

shown in Fig 4. Cementite is a hard and brittle phase. The presence of a continuous network

of cementite makes steel extremely brittle. There are methods to avoid the formation of

such a network to make it suitable for engineering applications. We shall learn about in one

of the subsequent modules.

Estimation of the amount of micro‐constituents in steel from phase diagram:

The micro‐constituents in steel are ferrite, austenite, cementite and pearlite. Out of these austenite

is not stable at room temperature. When we look at microstructures of steel we get an idea about

the distribution of various constituents. If the composition or % carbon in steel is known we can

estimate the volume fraction (weight fraction) of each the likely constituents.

Figure 5 gives the relevant part of Fe‐Fe3C phase diagram. For practical purpose we may assume that the

solubility of carbon in ferrite is negligible. The general expressions for the amount of Pearlite (P),

Cementite (Cm), Pro‐ eutectoid Ferrite & Total ferrite () in a hypo‐eutectoid steel having x% C are as follows.

% 100.

100

% 100.

.100

% 100.

100

% .

100.

.100

This truly gives weight % of various constituents. When you look at the microstructure you get an

estimate of volume %. However if the density of the phases are known weight % can be converted to

volume %. In the case of steel the density of ferrite is 7.87 g/cc and that of cementite is 7.662. Although

the difference is not much the amount of Cm seen in a microstructure (volume fraction) will be a little

more than weight fraction obtained from the lever rule.

T

L

0

910

0.8 2.0 4.3 6.67 0.02

727

1145

+ Cm

+ Cm

a b c d e

(x) (y) Fig 5: Shows how to estimate the amounts of

micro‐constituents in steel at room temperature.

The phase diagram gives approximate % C for

various points of interest. These are denoted as a,

b, c, d, & e. % C in a hypo‐eutectoid steel is x and

that in hyper‐eutectoid steel is y.


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Microstructure of Fe‐Fe3C eutectic:

The slide 6 explains the solidification behavior of Fe‐Fe3C eutectic. This is the binary iron‐carbon alloy

having the lowest melting point. It solidifies as eutectic at 1148°C. It is known as Ledeburite.

Structure of eutectic (Ledeburite)

L

cm

cm

time

T

% in eutectic at TE = (de/ce) x100

a b c d

T

e

% P in eutectic at RT = (de/be) x100

% carbon in the eutectic is 4.3. Solidification occurs at a constant temperature. Immediately after

solidification the alloy consists of a mixture of austenite and cementite. % austenite = 100 x (e‐d)/(e‐c) =

100 x (6.67 – 4.3)/(6.67‐2.0) =52% (approx). The solubility of carbon in austenite decreases with

decreasing temperature. At the eutectoid temperature the solubility of carbon in austenite is only 0.8%.

Therefore as the alloy cools cementite precipitates from the austenite in the eutectic. % austentite at

the eutectoid temperature (just before the decomposition of austenite starts) = 100 x (e ‐ d)/(e – b) =

100 x (6.67 – 4.3)/(6.67 – 0.8) = 41% (approx). When the eutectoid reaction sets in the austenite having

0.8% carbon transform into pearlite. The eutectic after this transformation is known as transformed

Ledeburite. It approximately consists of 41% Pearlite in a matrix of cementite. Figure 6 shows the main

features of the eutectic that forms at 1148°C. It is initially a mixture of cementite and austenite. On

cooling the % cementite increases and the carbon content of austenite decreases till it reaches 727°C.

This is where the austenite having 0.8% carbon transforms into pearlite. Thereafter as per the phase

diagram % ferrite within the pearlitic region would decrease as some cementite would precipitate out of

it. This is due to the decrease in the solubility of carbon in ferrite with decreasing temperature. However

the change is too little to be detected from the microstructure.

Slide 6


13

White cast iron:

If % carbon is greater than 2.0 but less than 6.67 the Fe‐Fe3C alloy is known as white cast iron. Such an

alloy can be easily cast into various shapes but it has poor ductility. The fracture surface has silvery

white luster. This is why it is known as white cast iron. On the basis of its microstructure there are two

types of white cast iron. These are (i) hypoeutectic white cast iron (2.0 < % C < 4.3) and (ii) hypereutectic

white cast iron (4.3 < %C < 6.67). Cementite (Fe3C) is the hardest constituent in the Fe – Fe3C system. Its

hardness is of the order of 1000HV (Vickers Hardness Number). This is why it has excellent wear

resistance. Recall that the % carbide in eutectic is around 60%. Hard materials are known to be brittle.

There is no wonder that white cast iron has poor ductility. Let us look at the evolution of microstructure

in hypo & hyper eutectic white cast iron. This has been explained with the help of a set of sketches given

in Fig 8 – 9.

Fig 8 T°C

Wt% C

0.8 2.0 0.02 0

1539

1495

1394

727

910

L

+ L

+ L

+ Cm

+ Cm

L

+ Cm

Cm

time

T + L

4.3 6.67

L +Cm

3.0

L

Ledeburite

Pearlite

x

Transformed

Ledeburite

Fig. 7: Shows the structure of the eutectic Fe‐Fe3C

at room temperature. Note that volume fraction of

austenite has decreased and it has transformed

into pearlite. It is called transformed ledeburite.

Fig. 6: Shows a schematic representation of the

eutectic consisting of cementite as white matrix

and austenite as grey particles at 1145°C. It is

called ledeburite.

Cm

Cm

Pearlite


14

Figure 8 describes the evolution of the microstructure in a hypoeutectic cast iron. The vertical dotted

line marked x is the location of the alloy with respect to the Fe‐Fe3C phase diagram. The sketch on the

right gives its cooling curve. Solidification begins with nucleation and growth of austenite grains. As it

cools the amount of austenite increases at the cost the liquid. The composition of the austenite and the

liquid keep changing. At the eutectic temperature the composition of liquid is 4.3 whereas that of the

austenite is 2.0. If %C in the cast iron is 3.0, % austenite at this temperature =100 x (4.3‐3.0)/(4.3‐2.0) =

57%. The balance 43% is liquid. On cooling this it solidifies as a mixture of austenite and cementite. The

eutectic is known as ledeburite. The austenite that forms before eutectic transformation is called pro‐

eutectic austenite or primary austenite. On subsequent cooling, the volume fraction of cementite

increases at the cost of austenite. At the eutectoid temperature the primary austenite and the austenite

in the eutectic transform into pearlite. The final structure would consist of relatively large nodule of

pearlite (originating from primay austenite) and a fine dispersion of tiny nodules of pearlite (originating

from the austenite in the eutectic) in a matrix of cementite.

Figure 9 describes the evolution of the microstructure in a hyper‐eutectic white cast iron. The vertical

dotted line marked x is the location of the alloy with respect to the Fe‐Fe3C phase diagram. The sketch

on the right gives its cooling curve. Solidification begins with nucleation and growth of primary (pro‐

eutectic) grains of cementite. As it cools the amount of cementite increases at the cost the liquid. Note

that cementite has a fixed composition. However the composition of the liquid keeps changing. At the

eutectic temperature the composition of liquid is 4.3. If %C in the cast iron is 5.0, %liquid at this

temperature =100 x (6.67‐5.0)/(6.67‐4.3) = 70%. On further cooling it solidifies as a mixture of austenite

and cementite. The eutectic is known as ledeburite. The cementite that forms before eutectic

transformation is called pro‐eutectic cementite or primary cementite. On subsequent cooling, the

volume fraction of cementite increases at the cost of austenite within the eutectic. The cementite that

forms during this stage is called secondary cementite. At the eutectoid temperature the austenite in the

eutectic transforms into pearlite. The final structure consists of relatively large primary cementite and a

fine dispersion of tiny nodules of pearlite (originating from the austenite in the eutectic) in a matrix of

Fig 9 T°C

Wt% C

0.8 2.0 0.02 0

1539

1495

1394

727

910

L

+ L

+ L

+ Cm

+ Cm

L

+ Cm

Cm

time

T

4.3 6.67

L +Cm

5.0

Ledeburite

x

Transformed

Ledeburite

+ L

L Cm

Cm

Cm


15

cementite. The eutectic in cast iron at room temperature is known as transformed ledeburite. The

cementite present in iron carbon system has several distinct morphologies. In hyper‐eutectic white cast

iron has all of theses. Cementite precipitating from the liquid is called primary cementite, that

precipitating from austenite is called secondary cementite, and those precipitating from ferrite below

eutectoid temperature is called tertiary cementite. Apart from these cementite is also present as thin

plates within nodules of pearlite.

Summary:

In this lecture we have learned Fe‐Fe3C meta‐stable phase diagram. It gives a graphical representation of

the stability of different phases one may come across in such a system over a range of temperature (0°‐

1550°C) and compositions (0‐6.67%C). It has 3 invariant reactions. These are peritectic, eutectic and

eutectoid. Barring pure iron, cementite and the eutectic all alloys belonging to this system do not have a

definite melting point. We have learnt how to draw or guess the cooling curve of an alloy if its

composition were known. It helps in predicting the type of structure that evolves during solidification.

Iron carbon alloys having less than 2% carbon are known as steel whereas those having carbon more

than 2% but less than 6.67% are known as cast iron. At room temperature all iron carbon alloys are

made of ferrite and cementite. However the distribution and the amount of these may vary widely. On

the basis of the distribution of the two phases we can have 3 types of steel and two types of cast iron.

These are (i) terminal solid solution having only ferrite or at best extremely small amounts of tertiary

carbides (ii) hypo‐eutectoid steel (iii) hyper‐eutectoid steel (iv) hypo ‐ eutectic white cast iron and (v)

hyper‐ eutectic white cast iron. How the structure evolves in these during solidification has been

explained with illustrations. The iron cementite phase diagram at the end of this lecture gives an

overview of the structural features of each of the 5 types of alloys belonging to this system. However

while talking about the evolution of structure we do assume that cooling rate is slow. This indeed is a

major limitation of phase diagram. We have already seen the effect of non‐equilibrium cooling on

segregation during liquid to solid transformation. In Fe‐Fe3C system we do have a eutectoid

transformation where the effect of cooling rate is likely to be much more severe. We shall talk about it

in a subsequent lecture.

Exercise:

1. FCC is a more close packed structure yet solubility of carbon in austenite which is FCC is higher

than that in ferrite which is BCC. Why it is so?

2. Sketch the microstructure of 0.2% C steel. Calculate %Pearlite % cementite, % proeutectoid

ferrite and % total ferrite.

3. Estimate the ratio of the widths of ferrite and cementite plates in lamellar pearlite.

4. Sketch the temperarture – time diagram during the heating cycle of a 0.8% C steel. Use standard

Fe‐Fe3C phase diagram.

5. Estimate %Cm in Ledeburite just below eutectic and just above eutectoid temeratures. What is

its structure at room temperature?

Answer:


16

1. There are 2 types of interstitial sites octahedral & tetrahedral. In FCC the former is significantly

larger than the latter. Whereas in BCC these are nearly same. The total open space is shared by

more number of sites. Therfore interstitial gap in BCC is much smaller than that of FCC. This is

why carbon which occupies interstitial site has higher solubility in austenite (FCC).

2. The stucture would consist of proeutectoid ferrite and pearlite. Assume solubility of carbon in

ferrite is negligible. Refer to phase diagram to get % Pearlite in 0.2% carbon steel = .

.

100 25 Balance 75% is proeutectoid ferrite. % Cementite = .

.100 3% and total

ferrite = 97%.

3. Assume density of ferrite and cementite to be same (Note that in reality density of cementite is

a little higher than that of ferrite). % ferrite in pearlite = . .

.100 88. Balance 12% is

cementite. Therefore the ratio of the widths of the two = 12/88. This is approximately equal to

1:7.

4. 0.8 % carbon corresponds to an eutectoid steel. It would first transform into austenite and then

it starts melting when the temperature reaches its solidus temperature. It becomes totally liquid

when temperature goes beyond its liquidus. Shematic heating curve is as follows:

5. Assume % C in eutectic = 4.3. % Cm just below eutectic temperature = . .

. .100 49%and

just above eutectoid =. .

. .100 60%Between eutectic to eutectoid temperatures

proeutectoid cementite precipitates from austenite in eutectic. On subsequent cooling the

austenite transforms into pearlite. At room temperatutre what we have is transformed eutectic.

T

Time

liquid L+

+cm: Pearlite


17

Iron – Cementite Meta‐stable Binary Phase diagram

The above sketch is a little enlarged version of the phase diagram. Fe & Fe3C (also known as cementite)

are the two components. Nomenclature: P denotes Pearlite the product of eutectoid reaction in iron

carbon system. It is a mixture of ferrite and cementite. Ferrite = , = austenite, = high temperature

ferrite, LD = Ledebutite: the product of the eutectic reaction. Note that austenite can form either by

peritectic reaction between & L and also by direct precipitation from liquid when % C > 0.55. TLD =

transformed Ledeburite. Ledeburite is a eutectic mixture of austenite and cementite. At room

temperature austenite is not stable. It transforms into Pearlite. TLD therefore consists of tiny nodules of

pearlite in cementite matrix. It also describes the main structural features of five types of iron carbon

alloys. Low carbon steel consists of ferrite and tertiary cementite. Hypo‐eutectoid steel has ferrite, pro‐

eutectoid cememtite and pearlite. Hyper‐eutectoid steel has pro‐eutectoid cementite (secondary) and

pearlite. Hypo‐eutectic cast iron has pearlite, transformed ledeburite and pro‐eutectoid (secondary)

cementite. Hyper‐eutectic cast iron has primary cementite and transformed ledeburite. So far evolution

of microstructure that takes place during solidification of iron – carbon alloys has been explained with

the help of schematic microstructure. The following page gives a few typical microstructures of steel and

cast iron. These were prepared by some of my students and colleagues at I I T Kharagpur and CSIR‐NML

Jamshedpur.

T°C

Wt% C

0.8 2.00.02 0

1539

1495

1394

727

910

L

+ L

+ L

+ Cm + P

Cm+ LD

1550

4.3 6.67

L +Cm

+Cm

Cm + TLD

+ Cm + LD

P + TLD +Cm Cm + P

1146

727

Cm Fe

Hyper‐eutectic

white cast iron Hypo‐eutectic

white cast iron

Hyper‐eutectoid

steel

Hypo‐eutectoid

steel

+ Cm

0.18

0.55

0.08

L = + Cm

= + Cm

+ L =


18

Typical microstructures of steel and cast iron

Fig A1: Microstructure of 0.15% carbon steel consisting of ferrite (bright) and pearlite (dark). Lamellar feature pearlite is not visible. (Curtsey G Das CSIR‐NML Jamshedpur)

Fig A2: Microstructure of medium carbon steel consisting of ferrite (bright) and pearlite (dark). Amount of pearlite increases with % carbon. (Curtsey G Das CSIR‐NML Jamshedpur).

Fig A3: Microstructure of a near eutectoid steel having a little pro‐eutectoid cementite in a matrix of pearlite. In some of regions finer details of pearlitic (lamellar) structure is visible.

Fig A4: SEM image of 0.84% C steel having nearly 100% pearlite. Lamellar feature is clearly visible. The dark region is pro‐eutectoid cementite. (Curtsey G Das CSIR‐NML Jamshedpur)

Fig A5: Microstructure of hyper‐eutectic white cast iron showing primary carbides (cm). The rest of the region consists of a mixture of pearlite and cementite or transformed eutectic (ledeburite).

Fig A6: Microstructure of grey cast iron having graphite flakes in a matrix consisting of mostly ferrite and some amount of pearlite. (Curtsey G Das CSIR‐NML Jamshedpur)

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