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Phase Transformation

This document discusses the iron-carbon equilibrium system and the microstructure and properties of steels and cast irons. It describes the phases in the Fe-C diagram including ferrite, austenite, cementite, and graphite. The transformations that occur during cooling, such as pearlite formation and eutectic/eutectoid reactions, influence the microstructure and properties. Properties depend on carbon content and cooling rate, with pearlite and graphite structures forming at different carbon levels. Cast irons offer advantages over steel like lower cost and machinability but have lower strength.

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Nikhil Swaraj
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181 views26 pages

Phase Transformation

This document discusses the iron-carbon equilibrium system and the microstructure and properties of steels and cast irons. It describes the phases in the Fe-C diagram including ferrite, austenite, cementite, and graphite. The transformations that occur during cooling, such as pearlite formation and eutectic/eutectoid reactions, influence the microstructure and properties. Properties depend on carbon content and cooling rate, with pearlite and graphite structures forming at different carbon levels. Cast irons offer advantages over steel like lower cost and machinability but have lower strength.

Uploaded by

Nikhil Swaraj
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Iron-carbon equilibrium

system

Fe-C system
Depending on the form of C
Fe3C iron-carbide (cementite)
metastable
Possible phases
delta iron
austenite
ferrite
iron-carbide

C graphite
stable
delta iron
austenite
ferrite
graphite

Possible structural constituents:


Primary, secondary, tertiary,
eutectic, eutectoid iron-carbide
austenite, ferrite,
ledeburite (eutectic)
perlite (eutectoid)

Primary, secondary, tertiary,


eutectic, eutectoid graphite
austenite, ferrite, graphitic eutectic,
and graphitic eutectoid
2

Iron-Carbon diagram
Metastable system (Fe-Fe3C)

Microstructure
and mechanical
properties

Austenite
-Interstitional solid
solution
(C is solved in face
centered cubic lattice of
Fe)
-Limited solubility
max. solubility:
2,06% C at 1147 C
6

Transformations in solid state


Limited solubility of C in
austenite
Iron-carbide segregation

Transformations in solid state


Allotrophic
transformation
of austenite into ferrite

Transformations in solid state


Limited solubility of C in
ferrite
Iron-carbide segregation

Effect of C content

C%=0,10,8

10

Effect of C content

0,45 % C
Microstructure
ferrite + pearlite

ferrite
pearlite
11

Pearlite: transformation of austenite to ferrite


and cementite at 723oC; C=0,8%

12

Hypereutectoid steel C 1,3 %


Microstructure
pearlite+ secondary
cementite

pearlite
Secondary cementite
(net)

13

Ledeburite (eutectic)
At 1147 C
Phases of ledeburite:
austenite
iron-carbide

14

Ledeburite
During cooling to
room temperature:
austenite transforms
to pearlite
Hard, rigid, wear
resistante
Pearlite formed
from austenite

Iron-carbide

15

Hypoeutectic cast irons


(white cast iron)
Microstructure:
pearlite + ledeburite +
secondary cementite
pearlite

ledeburite

16

Hypereutectic
(white) cast irons

Primary cementite

ledeburite

17

Hypoeutectic graphitic cast iron


- graphitic eutectic
- secondary graphite
- graphitic eutectoid

ferrite

Structure at room temperature:

ferrite - graphite

graphite
18

Contains Si, Mn, P, S


Effect of wall thickness of the cast part
(cooling rate)
Solidification and transformation
stable - metastable
19

Cast irons may often be used in place of steel at considerable cost


savings. The design and production advantages of cast iron
include:
Low tooling and production cost
Good machinability
Ability to cast into complex shapes
Excellent wear resistance and high hardness (particularly
white cats irons)
High inherent damping capabilities
The properties of the cast iron are affected by the following
factors:
Chemical composition of the iron
Rate of cooling of the casting in the mold (which depends
on the section thickness in the casting)
Type of graphite formed (if any)
20

Advantages:
Graphite acts a s a chip breaker and a tool lubricant.
Very high damping capacity.
Good dry bearing qualities due to graphite.
After formation of protective scales, it resists corrosion in many common
engineering environments.
Disadvantages:
Brittle (low impact strength) which severely limits use for critical applications.
Graphite acts as a void and reduces strength. Maximum recommended design
stress is 1/4 of the ultimate tensile strength. Maximum fatigue loading limit is 1/3 of
fatigue strength.
Changes in section size will cause variations in machining characteristics due to
variation in microstructure.
21
Higher strength gray cast irons are more expensive to produce.

Ductile Cast Iron


Nodular Cast Iron
Malleable Cast Iron
22

pearlite

ledeburite
23

pearlite

graphite
24

Carbon, wt. %

Maurer diagram

25

Silicon, wt. %

Carbon + Silicon wt. %

Greiner - Klingenstein diagram

Wall thickness, mm
26

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