Lecture10.Phase Diagram Fe-Fe3C

METALLURGY I(RM-1420)

Dosen:Fahmi Mubarok, ST., MSc.Metallurgy LaboratoryMechanical EngineeringITS- Surabaya2008

Crystal Structures of Iron

Fe – Fe3C Phase Diagram

Steels

Cast Iron

http://www.its.ac.id/personal/material.php?id=fahmi

LECTURE XLECTURE X

Fahmi Mubarok

X 2Metallurgy Lab. Mech. Eng. Dept. ITS Surabaya

Review (Concept of solubility)

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

(a) The three forms of water – gas, liquid, and solid – are each a phase.

(b) Water and alcohol have unlimited solubility.

(c) Salt and water have limited solubility.

(d) Oil and water have virtually no solubility.

Illustration of phases and solubility:

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Crystal Structures of ironCrystal Structures of iron

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Fe - FeFe - Fe33C C Phase Phase DiagramDiagram

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Fe-Fe3C Phase Diagram

• Region– Pure Iron < 0.008% wt C– Steel 0.008 < % wt C < 2.14– Cast Isron 2.14 < %wt C < 6.70

• Phases:– α-Ferrite (α)– Austenite ()– δ-Ferrite (δ)– Cemenite (Fe3C)

• Critical temperature:

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Four Solid Phases

• α-Ferrite (α)– Solid solution of a carbon in α-Iron– BCC structure– Carbon only slightly soluble in matrix

• Maximum solubility of 0.022 % wt C at 727oC to about 0.008 wt% C in room temperature

• Austenite ()– Solid solution of a carbon in δ-Iron– FCC structure can accomodate more carbon than ferrite

• Maximum solubility of 2.14 % wt C at 1147oC, then decreased to 0.8 wt% C at 727oC.

• The difference in C solid solubility between and α is the basis of hardening in many steel.

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Four Solid Phases

• δ-Ferrite (δ)– Solid solution of a carbon in δ-Iron– BCC structure– NO technological importance cause only stable at high

temperature.• Maximum solubility of ferrite being 0.09 % wt C at 1493oC

• Cementite (Fe3C)– Intermetallic Fe-C compound– Fe3C : 6.7 wt% C + 93.3 wt% Fe– Forms when solubility limit of carbon in α-ferrite is exceeded

below 727oC– Orthorombic crystal structure : very hard and brittle.

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Three Invariant ReactionsThree Invariant Reactions

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Eutectoid steel (Pearlitic steel)

• Microstructure: pearlite- Lamellar eutectoid product

alternates plates of α + Fe3C- Two phases grow simultaneously

• Lever rule

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Formation of Pearlite

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Hypoeutectoid steel

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Hypoeutectoid steel ->Lever Rule

3

αt(total ferrite)

Fe C (Cementite)

6.70 0.38 100%6.70 0.022

= 95%1 95% 5%

W x

W

(proeutectoid ferrite)

γ(that will form pearlite)

0.76 0.38 100%(0.76 0.022)

52%1 52% 48%

W x

W

1

2

3a

3b Example: Calculating composition of steel with 0.38 wt%CT = 730oC T=725oC3b3a

The fraction of eutectoid ferrite thus are:

αe ααt 95% 52% 43%

W W W

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Hypoeutectoid steel composition (0.38 wt% C)

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Hypereutectoid steel

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Hypereutectoid steel -> lever rule

Exercise 10a. Determine the following composition of 1.4 wt%C at a temperature near eutectoid line :

a. The fraction of pealite and proeutectoid cementiteb. The fraction of total ferrite and cementite phasesc. The fraction of eutectoid cementite

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Hypereutectoid steel composition (1.4 wt% C)

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Plain Carbon Steels

1. Satisfactory where strength and other requirement are not too severe

2. Used successfully at room temperatures and in atmospheres that are not highly corrosive

3. Can be produced in a great range of strengths at a relatively low costLimitation

1. Cannot be strengthened beyond about 100.000 psi without significant loss in toughness (impact resistance) and ductility

2. Large section cannot be made with a martensitic structure throughout

3. Rapid quench rates are necessary for full hardening in medium-carbon plain carbon steels to produce a martensitic structure. This rapid quenching leads to shape distortion and cracking of heat-treated steel

4. Show a marked softening with increasing tempering temperature5. Poor impact resistance at low temperatures6. Poor corrosion resistance for many engineering environments7. Oxidezed readily at elevated temperatures

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Elements in Plain Carbon Steels1.Sulfur (<0.05 %)

• Sulfur combines with iron to form iron sulfide (FeS), which usually occurs as a grain boundary precipitation

• FeS is hard and has a low melting point, it can cause cracking during hot working of steel (hot-short)

2.Manganese (0.03 % -1.0 %)• The fuction of manganese in counteracting the negative effects

of sulfur• Manganese combines with the sulfur persent in the steels to

produce manganese sulfide (MnS), thus no FeS will form.3.Phosphorus (< 0.04 %)

• This small quantity tends to dissolve in ferrite, increasing the strength and hardness slightly

• In large quantities, phosphorus reduces ductility, thereby increasing the tendency of the steel to crack when cold worked (cold-short)

4.Silicon (from 0.05%-0.30%)• Silicon dissolves in ferrite, increasing the strength of the steel

without greatly decreasing the ductility• Silicon is used as a deoxidizer, and forms SiO2 or silicate

inclusions

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Alloying SteelsPlain Carbon Steels

Plain-carbon steels properties are not always adequate for all engineering

applications of steel

Alloy Steels1. Alloy steels have been developed which, although they cost more, are

more economical for many uses2. In some applications, alloy steels are the only materials that are able to

meet engineering requirements3. The principal element that are added to make alloy steels are nickel,

chromium, molybdenum, manganese, silicon, and vanadium4. Other elements sometimes added are cobalt, cooper, and lead

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Effect of carbon content

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Hardness and Strength

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Toughness and Ductility

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Cast Iron

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White Cast Iron

• Chemical composition:– Carbon 1.8-3.6 %– Silicon 0.5-1.9 %– Manganese 0.25-0.80 %– Sulfur 0.06-0.20 %– Phosphorus 0.06-0.18 %

• Solidification rate fast enough• Carbon combined with iron cementite (hard, brittle)• Microstructure pearlite in a white interdendritic network of

cementite• Shows a “white” crystalline fractured surface

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White Cast Iron

• High compressive strength and excellent wear resistance but extremely brittle and difficult to machine

• Used where:– resistance to wear is most important– The service does not require ductility

• White cast iron Malleable cast iron (malleabilization)• Mechanical properties:

– Hardness brinell 375 – 600 BHN– Tensile strength 20.000 – 70.000 psi– Compressive strength 200.000 – 250.000 psi– Modulus of elasticity 24 – 28 milion psi

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White Cast Iron

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Summary

• ! Pearlite• ! Peritectic reaction• ! Phase• ! Phase diagram• ! Phase equilibrium• ! Primary phase• ! Proeutectoid cementite• ! Proeutectoid ferrite• ! Solidus line• ! Solubility limit• ! Solvus line• ! System• ! Terminal solid solution• ! Tie line• ! Liquidus line• ! Metastable

• ! Austenite• ! Cementite• ! Component• ! Congruent transformation• ! Equilibrium• ! Eutectic phase• ! Eutectic reaction• ! Eutectic structure• ! Eutectoid reaction• ! Ferrite• ! Hypereutectoid alloy• ! Hypoeutectoid alloy• ! Intermediate solid solution• ! Intermetallic compound• ! Invariant point• ! Isomorphous• ! Lever rule

Make sure you understand language and concepts:

METALLURGY I(RM-1420)

Dosen:Fahmi Mubarok, ST., MSc.Metallurgy LaboratoryMechanical EngineeringITS- Surabaya2008

MINGGU XI-XIIIMINGGU XI-XIII

NON EQUILIBRIUMTRANSFORMATION

-Isothermal transformation diagram- Coling tranformation diagram

- Formation of martensite

Lecture10.Phase Diagram Fe-Fe3C

Documents

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