fallsem2015-16_cp1812_04-aug-2015_rm01_unit-ii_mee-203_part-2

1 In their simplest form, steels are alloys of Iron (Fe) and Carbon (C).

The Fe-C phase diagram is a fairly complex one, but we will only consider the steel part of

the diagram, up to

d 7% C b

The IronIron Carbide (FeFe3C) Phase Diagram

around 7% Carbon.

Phases present

-ferrite, -ferrite, -ferrite,

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Fe3C (iron carbide or cementite)

Fe-C liquid solution

-ferrite - solid solution of C in BCC Fe Stable form of iron at room temperature.

The maximum solubility of C is 0.022 wt%

T f t FCC t it t 912 C

Phases in FeFe3C Phase Diagram

Fe3C (iron carbide or cementite) This intermetallic compound is

metastable, it remains as a compound

indefinitely at room T, but decomposes Transforms to FCC -austenite at 912 C

-austenite - solid solution of C in FCC Fe The maximum solubility of C is 2.14 wt %.

Transforms to BCC -ferrite at 1395 C

Is not stable below the eutectic

temperature (727 C) unless cooled

(very slowly, within several years) into -

Fe and C (graphite) at 650 - 700 C

Fe-C liquid solution

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rapidly

-ferrite - solid solution of C in BCC Fe The same structure as -ferrite

Stable only at high T, above 1394 C

Melts at 1538 C2

-ferrite austenite

2 Pure iron when heated experiences two changes in crystal structure before it melts.

At room temperature the stable form, ferrite ( iron) has a BCC crystal structure. Ferrite experiences a polymorphic transformation to FCC austenite ( iron) at 912 C (1674 F).

At 1394C (2541F) austenite reverts back to BCC phase ferrite and melts at 1538C (2800F)

Changes in Crystal Structure

At 1394 C (2541 F) austenite reverts back to BCC phase ferrite and melts at 1538 C (2800 F). Iron carbide (cementite or Fe3C) an

intermediate compound is formed

at 6.7 wt% C.

Typically, all steels and cast irons have

carbon contents less than 6.7 wt% C.


Carbon is an interstitial impurity in iron

and forms a solid solution with the

, , phases.

3

C is an interstitial impurity in Fe. It forms a solid solution with , , phases of iron

Maximum solubility in BCC -ferrite is limited (max. 0.022 wt% at 727 C) which can be

explained by the shape and size of the BCC interstitial positions, which make it difficult to

A few comments on FeFe3C system

p y p p ,

accommodate the carbon atoms. BCC has relatively small interstitial positions. Even though

present in relatively low concentrations, carbon significantly influences the mechanical

properties of ferrite

Maximum solubility in FCC austenite is 2.14 wt% at 1147 C - FCC has larger interstitial

positions


Mechanical properties: Cementite is very hard and brittle - can strengthen steels.

Mechanical properties also depend on the microstructure, that is, how ferrite and cementite

are mixed.

Magnetic properties: -ferrite is magnetic below 768 C, austenite is non-magnetic4

3 Three types of ferrous alloys:

Iron:

less than 0.008 wt % C in ferrite at room T

Classification - Types of ferrous alloys

Steels:

0.008 - 2.14 wt % C (usually < 1 wt % );

-ferrite + Fe3C at room T

Cast iron:

2.14 - 6.7 wt % (usually < 4.5 wt %)


In binary phase diagrams, a horizontal line always indicates an invariant reaction.

Three invariant reactions are present in IronIron Carbide (FeFe3C) Phase Diagram.

1. Peritectic reaction

Invariant Reactions in FeFe3C System

2. Eutectic reaction

1493 C

1150 C


3. Eutectoid reaction

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727 C

4Peritectic, involves the following phase transformation.

L(0.53% C) + (BCC Ferrite of 0.1% C) (FCC Austenite of 0.18% C)

Peritectic reaction - FeFe3C System

MP-

B

( ) ( ) ( )

The maximum solubility of carbon in BCC -iron is 0.1% (point M)whereas in FCC -iron, it is greater. The presence of carboninfluences the allotropic changes. As carbon is increased or added

to the iron, the temperature increases from 1400C to 1493C at

0 1% b

N


0.1% carbon.

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Consider the portion NMPB in Peritectic Reaction

On cooling, the portion NM represents the beginning of the crystal

structure change from BCC iron to FCC iron for alloyscontaining less than 0 1% carbon

Peritectic reaction - FeFe3C System

MP-

B

containing less than 0.1% carbon.

Line MP represents the beginning of crystal structure change by

means of peritectic reaction for the alloys between 0.1 & 0.18%

Carbon.

Line NP represents the end of crystal structure change for alloys

containing less than 0.18% C.

N


Portion PB represents the end of crystal structure by means of

peritectic reaction for the alloys between 0.18- 0.5% carbon. Here

the reaction takes place isothermally (i.e.) at constant temperature.

At the peritectic reaction point, liquid of 0.53% C combines with ferrite of 0.1% C to form FCC austenite of 0.18% C

8

5 Eutectic Point is given by point E (refer fig.2) exists at 4.3% Carbon

and at the temperature of 1147C.

Horizontal line represents the eutectic temperature line and

whenever an alloy crosses the line must undergo the eutectic

Eutectic reaction - FeFe3C System

L

y g

reaction

Any liquid that is present when this line is reached must solidify

now into very fine intimate mixture of two phases namely austenite

() and cementite (Fe3C). The eutectic mixture has been given with the name LEDEBURITE

and the equation is given as


(4.3% C) (FCC)

0.18% C

6.67%C

The eutectic mixture is not usually seen in the microscope because the austenite is not stable at

room temperatures and must undergo another reaction during cooling

An alloy of eutectoid composition (0.76 wt% C) as it is cooled from a temperature within the phase

region, say, 800 Cthat is, beginning at point a and moving down the vertical line xx`.

Initially, the alloy is composed entirely of the austenite

phase having a composition of 0.76 wt% C (Figure a).

Development of Microstructure - FeFe3C System

p g p ( g )

As the alloy is cooled, no changes will occur until the

eutectoid temperature (727 C) is reached.

Upon crossing this temperature to point b, the austenite

transforms to and Fe3C) . This microstructure, represented schematically in

point b, is called pearlite (alternating layers or lamellae


of and Fe3C.

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6 Formation of pearlite structure

Nucleating at grain boundary, growth by diffusion of C to achieve the compositions

of and Fe3C (with structural changes)

Pearliteupper-critical-

temperature line

of and Fe3C (with structural changes) lamellae much thick ( relative layer thickness

is approximately 8 to 1)


Redistribution of carbon

by diffusion Austenite 0.76 wt% C;

Ferrite - 0.022 wt% C

Cementite - 6.70 wt% C

Composition C0 to the left of the eutectoid, between 0.022 and 0.76 wt% C; is termed a hypoeutectoid

(less than eutectoid) alloy.

At about 875C, point c, the microstructure will consist

entirely of grains of the phase (Fig c)

Hypoeutectoid Alloys

y g p ( g ) Cooling to point d, at about 775C, both phase and

phase coexist (Fig d). Cooling from point d to e, just above the eutectoid but

still in the + region, will produce an increased fraction of the phase and a microstructure similar to fig e,the particles will have grown larger.


As the temperature is lowered just below the eutectoid, to

point f, all the phase that was present at temperature Te

(and having the eutectoid composition) will transform

to pearlite,

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7 In the austenite range, it is a uniform interstitial solid solution. Upon slow cooling, nothing happens until

the line MO is crossed at point d. This MO line is known as the upper-critical-temperature line on the

hypoeutectoid side.


At d, ferrite must begin to form at the austenite grain

boundaries. Since ferrite can dissolve very little carbon,

in those areas that are changing to ferrite the carbon

must come out of solution before the atoms rearrange

themselves to BCC

The carbon which comes out of solution is dissolved in

the remaining austenite, so that, as cooling progresses

and the amount of ferrite increases, the remaining


and the amount of ferrite increases, the remaining

austenite becomes richer in carbon.

Its carbon content is gradually moving down and to the

right along the MO line. Finally, the line NO is reached at

point f.

The ferrite phase will be present both in the pearlite and also as the phase that formed while

cooling through the and phase region. The ferrite that is present in the pearlite is

ll d t t id f it


called eutectoid ferrite,

whereas the other, that formed above Te, is

termed proeutectoid ferrite (meaning pre- or

before eutectoid)


8 Compositions to the right of eutectoid (0.76 - 2.14 wt % C) hypereutectoid (more than

eutectoid -Greek) alloys.

+ Fe3C + Fe3CH t t id ll t i t t id tit

Hypereutectoid Alloys

Hypereutectoid alloys contain proeutectoid cementite

(formed above the eutectoid temperature) plus pearlite

that contain eutectoid ferrite and cementite.


In the austenite range, this alloy consists of a uniform FCC solid solution with each grain containing 1.0

percent carbon dissolved interstitially.

Hypereutectoid Alloys

Upon slow cooling, nothing happens until the line OP iscrossed at point h. This line is called upper-critical-temperature line on the hypereutectoid side.p yp

The OP line shows the maximum amount of carbon that canbe dissolved in austenite as a function of temperature.

Above the OP line, austenite is an unsaturated solidsolution.

At h, the austenite is saturated in carbon. As thetemperature is decreased, the carbon content of theaustenite, that is, the maximum amount of carbon that canbe dissolved in austenite, moves down along OP line


towards point O. Therefore, as the temperature decreases from h to i, the

excess carbon above the amount required to saturateaustenite is precipitated as cementite primarily along thegrain boundaries.

Finally, the eutectoid line is reached at i. This line is calledthe lower-critical-temperature line on the hypereutectoid side

9Development of Microstructure - FeFe3C System

Eutectoid steel Hypoeutectoid steel Hypereutectoid steel


+Fe3C

Pearlite

yp

+Fe3C

Pearlite +proeutectoid ferrite

yp

+Fe3C

Pearlite +proeutectoid cementite

How to calculate the relative amounts of proeutectoid phase ( or Fe3C) and pearlite?

Application of the lever rule with tie line, that extends from the eutectoid composition (0.76 wt% C)

to ( + Fe3C) boundary (0.022 wt% C) for hypoeutectoid alloys and


to ( + Fe3C) Fe3C boundary (6.7 wt% C) for hypereutectoid alloys.

Fraction of phase is determined

by application of the lever rule

across the entire ( + Fe3C) phase.


10

Example for hypoeutectoid alloy with composition

Fraction of pearlite:


740022.0

0220760022.0 '0

'0 ==+=

CCUT

TWP

'0C

Fraction of proeutectoid ferrite :

74.0022.076.0 +UT

74.076.0

022.076.076.0 '0

'0

'

CCUT

UW ==+=


Example for hypereutectoid alloy with composition

Fraction of pearlite:


94570.6

76070670.6 '1

'1 CC

XVXWP

==+=

'1C

Fraction of proeutectoid cementite:

94.576.070.6XV +

94.576.0

76.070.676.0 '1

'1

3

==+=

CCXV

VW CFe


11

Determination of relative amount of ferrite, cementite and pearlite




12



The microstructural development of ironcarbon alloys it has been assumed that, upon

cooling, conditions of metastable equilibrium have been continuously maintained; that is,

sufficient time has been allowed at each new temperature for any necessary adjustment in

phase compositions and relative amounts as predicted from the FeFe3C phase diagram

Influence of other Alloying Elements - Teutectoid changes

phase compositions and relative amounts as predicted from the Fe Fe3C phase diagram.

These cooling rates are impractically slow and

really unnecessary; in fact, on many occasions

nonequilibrium conditions are desirable. Two

nonequilibrium effects of practical importance are

1. the occurrence of phase changes or

transformations at temperatures other than those


Fig 1: The dependence of eutectoidtemperature on alloy concentration for

several alloying elements in steel

transformations at temperatures other than those

predicted by phase boundary lines on the phase

diagram, and

2. the existence at room temperature of non-

equilibrium phases that do not appear on the

phase diagram.

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Additions of other alloying elements (Cr, Ni,Ti, etc.) bring about rather dramatic changes in

the binary ironiron carbide phase diagram, Fig 1. The extent of these alterations of the

positions of phase boundaries and the shapes of the phase fields depends on the particular

alloying element and its concentration

Influence of other Alloying Elements - Ceutectoid changes

alloying element and its concentration.

One of the important changes is the shift in position of

the eutectoid with respect to temperature and to carbon

concentration. Fig 1 and 2, which plot the eutectoid

temperature and eutectoid composition (in wt% C) as a

function of concentration for several other alloying

elements. Thus, other alloy additions alter not only the


Fig 2: The dependence of eutectoid

composition (wt% C) on alloy concentration

for several alloying elements in steel.

temperature of the eutectoid reaction but also the relative

fractions of pearlite and the proeutectoid phase that form.

Steels are normally alloyed for other reasons, however-

usually either to improve their corrosion resistance or to

render them amenable to heat treatment

Cementite (Fe3C)

Contains 6.67% wt of Carbon

Hard, Brittle Interstitial compound

Definitions of structures

Tensile strength 5000 psi approx. and has high compressive strength

Crystal structure is orthorhombic

Austenite () Interstitial solid solution of carbon

Has FCC crystal structure can accommodate more carbon than ferrite

Max. solubility of carbon in this phase is 2% at 1148 C and lowers to 0.8% at 723 C


Max. solubility of carbon in this phase is 2% at 1148 C and lowers to 0.8% at 723 C

Tensile strength 1,50,000 psi; Elongation 2% in 2

Hardness 40 HRC

Not normally stable at room temperatures

26

14

- Ferrite Interstitial solid solution of carbon in BCC crystal lattice

As indicated in the Iron- Iron carbide equilibrium diagram, carbon is only slightly soluble

i F it d h th l bilit f 0 025% t 723 C

Definitions of structures

in -Ferrite and has the solubility of 0.025% at 723 C

Softest structure that appears on the diagram

Average Props : TS 40000 psi, Hardness 90BHN

Pearlite ( + Fe3C) Eutectoid mixture containing 0.8% Carbon and is formed at 723 C on very slow cooling

Microstructure has very fine plate like / lamellar structure


y p

Average Props : TS 120000 psi, Hardness 20HRC

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fallsem2015-16_cp1812_04-aug-2015_rm01_unit-ii_mee-203_part-2

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