Chapter 11 -

MME-292

Arman HussainLecturerDGCE, BUET

Metallic Materials Sessional

Lecture 05

Cast Iron

September 7, 2014

Chapter 11 - 2

Chapter 11: Applications and Processing of Metal Alloys

ISSUES TO ADDRESS...

• How are Cast Iron classified and what are their common applications?

• What are some of the common fabrication techniques for metals?

• What heat treatment procedures are used to improve the mechanical properties of Cast Irons?

Chapter 11 - 3

Adapted from Fig. 9.24, Callister & Rethwisch 8e. (Fig. 9.24 adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

Adapted from Fig. 11.1, Callister & Rethwisch 8e.

Classification of Metal AlloysMetal Alloys

Steels

Ferrous Nonferrous

Cast Irons

<1.4wt%C 3-4.5wt%CSteels

<1.4 wt% CCast IronsCast Irons3-4.5 wt% C3-4.5 wt% C

Fe3C

cementite

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

austenite

+L

+Fe3Cferrite

+Fe3C

+

L+Fe3C

(Fe) Co , wt% C

Eutectic:

Eutectoid:0.76

4.30

727ºC

1148ºC

T(ºC) microstructure: ferrite/Pearlitegraphite/cementite

Chapter 11 - 4

Cast Irons

• Ferrous alloys with > 2.1 wt% C– more commonly 3 - 4.5 wt% C

• Low melting – relatively easy to cast, amenable to cast

• Generally brittle→ casting is the most favorable process of fabrication

• Cementite decomposes to ferrite + graphite

Fe3C 3 Fe () + C (graphite)

– generally a slow process

Chapter 11 - 5

Fe-C True Equilibrium Diagram

Graphite formation promoted by

• Si > 1 wt%

• slow cooling

Adapted from Fig. 11.2, Callister & Rethwisch 8e. [Fig. 11.2 adapted from Binary Alloy Phase Diagrams, 2nd ed.,Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.]

1600

1400

1200

1000

800

600

4000 1 2 3 4 90

L

+L

+ Graphite

Liquid +Graphite

(Fe) C, wt% C

0.65

740ºC

T(ºC)

+ Graphite

100

1153ºCAustenite 4.2 wt% C

Chapter 11 -

Types of Cast Iron

Gray iron

Ductile iron

White iron

Malleable iron (Heat treated white iron)

6

Chapter 11 - 7

Gray iron

• C content: 2.5-4.0%• Si Content 1.0-3.0%• graphite flakes• Fracture surface is gray• weak & brittle in tension• stronger in compression• excellent vibrational dampening• wear resistant• Least expensive• Application:

– Base of structure of machines and heavy equipments

Adapted from Fig. 11.3(a) & (b), Callister & Rethwisch 8e.

Chapter 11 -

Ductile iron

• Add Mg and/or Ce• graphite as nodules not

flakes• Greatly influence

mechanical properties• Matrix: pearlite or ferrite

depending on heat treatment

• As-cast piece is pearlite• stronger but less ductile

8

Chapter 11 -

Contd.

• Heat treatment for several hours at about 7000C will yield a ferrite matrix

• Stronger and much more ductile

• Applications: valves, pump bodies, crank-shaft, gears etc.

9

Chapter 11 -10

White iron

• < 1 wt% Si (low Si)• Rapid cooling rate• C remains as cementite instead of

forming graphite• Fracture surface has a white

apperance• Only suface layer is chilled, graphite

flake is formed in the interior• pearlite + cementite• very hard and brittle thus unmachinable

Application: hard and wear resisting surface without much ductility

i.e. rollers in rolling machine, bearings etc.

Adapted from Fig. 11.3(c) & (d), Callister & Rethwisch 8e.

Chapter 11 -11

Chapter 11 -

Malleable iron

12

• heat treat white iron at 800-900ºC for prolonged time in neutral atm.

• Decomposition of cementite

• graphite in rosettes surrounded by ferrite or pearlite matrix

• reasonably strong and ductile

Applications: Connecting rods, transmission gears, and differential

cases for the automotive industry, and also flanges, pipe fittings, and valve parts

for railroad, marine, and other heavy-duty services.

Chapter 11 -13

Chapter 11 -

Effect of Si

14

• • Si is a graphitizer

• Favors solidification to the stable iron-graphite system

• C is precipitated at the form of flake

• Introducing Mg or Ce promote spheroidal graphite precipitation instead of flake

Chapter 11 -

Effect of S in cast iron

• Commercial gray irons contains: 0.06 to 0.12 % S

• Effect of Si and S on the form of C are reversed

• S content ↑ → Cementite formation ↑ → promote white iron formation

• S reacts with Fe and form FeS → low M.P. compound → increase possibility of cracking

• S↓ es fluidity → increases blow holes

• Mn is used for desulphurization

• MnS emains as dispersed inclusions

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Chapter 11 -

Next Class

Structure properties

relationship

16