- Gray cast iron solidifies according to the Fe-C equilibrium diagram, resulting in a microstructure of graphite flakes surrounded by ferrite or pearlite. It is cheap, easy to cast, and has good mechanical properties in compression but is brittle. - Ductile cast iron contains spheroidal graphite particles from additions of Mg and Ce, giving it greater strength and ductility than gray cast iron. It has applications in components like crankshafts and gears. - White cast iron solidifies as iron carbide, making it very hard but brittle. It finds use in applications requiring high abrasion resistance.

1. Cast Irons
Physical Metallurgy
EBB222

2.  Cast iron - Ferrous alloys containing carbon above 2.14 wt%.
 (3 – 4.5 wt%) C + (1-3 wt.%) Si.
 Cheap.
 Low melting (liquid at temperatures between approximately
1150 °C and 1300 °C)
 Advantages - can be can produce complex parts quickly
and easily through sand casting
 Disadvantage - BRITTLE so can’t be used for high stress or
shock loading
Overview of cast iron

3. Liquid
Austenite
a + Fe3C
d
g+ L
a + g
L + Fe3C
723˚C
910˚C
0% 0.8% ~2% ~4.3%
a
g + Fe3C
Cast Iron
Carbon
Steel
Iron Carbon Phase Diagram

4.  Pig iron, scrap steel, limestone and carbon (coke)
 Cupola
 Electric arc furnace
 Electric induction furnace
 Usually sand cast, but can be gravity die cast in
reusable graphite moulds
Production of cast iron

5. Types of cast iron
 Grey cast iron - carbon as graphite
 White cast iron – carbides
 Ductile cast iron
 nodular, spheroidal graphite
 Malleable cast iron
 Compacted graphite cast iron
 CG or Vermicular Iron

6. Chemical composition of cast iron

7. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
The iron-carbon phase diagram showing the relationship
between the stable iron-graphite equilibria (solid lines) and
the metastable iron-cementite reactions (dashed lines).

8. Factors influencing structure of
cast iron
 The rate of solidification
- slow rates of solidification in a sand mould allow for graphite
formation solidify as gray cast iron.
- rapid solidification will tend to give white cast irons structures
- metal chills are sometimes inserted into parts of sand
moulds in those areas where a high surface hardness is
required.
 Carbon content
- higher carbon content of the iron, the greater will be the
tendency for it to solidify as gray cast iron
 The presence of other elements
- some elements promote the formation of graphite in an iron
structure.
- silicon and nickel have strong graphitising tendencies.

9. Factors influencing structure of
cast iron – cont’d
 The effect of heat treatment
- the prolonged heating of a white iron will cause
graphitization to occur.
- This phenomenon is used as the basis for the production of
malleable irons
 The rate of cooling on the structure of cast iron.
- high rate of cooling during solidification tends to prevent the
decomposition of cementite.
- slow cooling, would become graphitic

10. Composition and types of cast iron

11. Carbon and silicon composition ranges for various cast irons and silicon-
containing steels

12. Effect of composition
• A CE over 4.3 (hypereutectic) leads to carbide or
graphite solidifying first & promotes gray cast iron
• A CE less than 4.3 (hypoeutectic) leads to austenite
solidifying first & promotes white cast iron
Carbon Equivalent, C.E. = % C + 1/3 % Si

13. Cooling rate and type of cast iron
The effect of cooling rate and composition on the structure of cast iron.
Moltted cast iron is a mixture of the white and the grey cast iron structure

14. Effect of alloying elements in cast iron
 Silicon
- To increase the amount of under-cooling required for the formation of
cementite and promote the formation of graphite during solidification.
- Influence fluidity.
- Graphitizer agent.
- Cooling rate control to decomposed carbide eutectic.
- promotes the precipitation of secondary graphite on the primary
graphite during the eutectoid transformation, which results in large
areas of ferrite (commonly called “free ferrite”) around the graphite
particles.
Sulfur
- residual impurity from the extraction process.
- stabilising the cementite and preventing the formation of flake
graphite thus harden the cast iron
- high sulfur tend to reduce fluidity and it also causes embrittlement
due to the formation of iron sulphide (FeS) at the grain boundaries.

15. Effect of alloying elements in cast iron
 Manganese
- addition of this element in small quantities is essential as it combines
with any residual sulphur present to form manganese sulphid MnS.
- Unlike ferrous sulphide, manganese sulphide is insoluble in the molten
iron and floats to the top of the melt to join the slag.
- thus by removing the sulphur, the manganese indirectly softens the cast
iron and also removes a source of embrittlement.
 Phosphorus
- residual impurity from the extraction process.
- it is present in cast iron as iron phosphide (Fe3P).
- this phosphide forms a eutectic with ferrite in gray cast irons, and with
ferrite and cementite in white cast irons.
- causes embrittlement in the cast iron and the amount present must be
kept to a minimum in castings.

16.  2.5 – 4wt.% C, 1 – 3 wt.% Si.
 fractured surface, gray appearance – gives the
name of gray cast iron.
 solidifies according to the equilibrium eutectic Fe-C.
 Si stabilises the Fe-C system, rather than Fe-Fe3C.
Gray Cast Iron

17.  so microstructure based on Fe + C (graphite) , where graphite in the
form of flakes surrounded by an α-ferrite or pearlite matrix.
 slow cooling : at liquidus temperatures primary γ dendrites form;
as temperature is decreased the eutectic temperature is reached
(γ +graphite) eutectic form; as temperature decreases in γ + graphite,
carbon is rejected from the γ which goes to the graphite. As continue to
cool to the eutectoid temperature α + graphite formed.
**gives a microstructure of graphite flakes in a ferrite matrix ferritic
gray iron
 faster cooling takes places through eutectoid temperature,
final microstructure pearlite + graphite pearlitic gray iron
Gray Cast Iron

18. Ferritic gray iron
Graphite
α- ferrite
Microstructure of gray cast iron

19. Pearlitic gray cast iron
(Fe 3.4wt%C 2.5wt%Si 0.01wt%P)
Graphite
Pearlite

20.  among least expensive metallic materials
 high fluidity – easy to cast, especially complex shapes
 low shrinkage during casting
 good mechanical properties in compression
 BUT brittle due to shape of graphite flakes
 excellent machinability
 excellent thermal conductivity
 excellent bearing properties
Properties of gray cast iron

21.  excellent damping properties
 excellent wear resistance
Properties of gray cast iron – con’td
(b)
(a)
(c)
Comparison of the relative vibrational damping capacities of (a) steel (b) ductile
(c) gray cast iron.

22.  base structure for machines and heavy equipment, damping plates for pianos,
engine blocks, engine cylinders, flywheels, piston rings, brake discs and
drums gears.
Application gray cast iron
Brake discs
Engine blocks

23.  Also known as spheroidal graphite (SG), and nodular
graphite iron
 addition of Mg (~0.1%) /Ce (~0.2-0.4%) added to Fe-C in
the gray iron composition range
 low levels of minor elements such as S and P
 greater strength and ductility than gray cast iron of similar
composition
 graphite forms as spheres rather than flakes improved
toughness
 microstructure : spheroidal graphite particles in ferrite or
pearlite matrix
Ductile cast iron

24. Fe, C 3.2, Si 2.5, Mg 0.05 (wt%)
α- ferrite
Nodular
graphite
Microstructure of ductile cast iron
Ductile cast iron with a matrix that is
predominantly ferrite. Original
magnification: 250×
Ferritic ductile cast iron

25. Pearlitic ductile or nodular iron
(Fe 3.4wt%C 2.5wt%Si 0.01wt%P 0.03wt%Mg)
Removing the graphite flakes improves the tensile
strength, ductility and toughness.

26.  high ductility
 high strength
 high modulus elasticity
 excellent castability
 excellent machinability
 wear resistance
 poor corrosion
Properties of ductile cast iron
Crankshaft

27.  poor weldability BUT can be welded with nickel and
iron electrodes
 excellent machinability is fair to excellent
 excellent castability
 good fatigue strength
Properties of ductile cast iron- cont’d

28. Crankshaft
Differential housing
 valves, pump bodies, gears, crankshafts, pistons, rolls for
rolling mills, tubes and door hinges, automotive engine
crankshafts, heavy duty gears, military and railroad vehicles.
Applications of ductile cast iron

29. Applications of ductile cast irons

30.  1.8 – 3.6 wt.% C, < 1 wt. % Si.
 A fracture surface of this alloy has a white appearance, and
thus it is termed white cast iron.
 solidification occur according to Fe-Fe3C metasable phase
diagram.
 microstructure based on Fe + Fe3C.
 Phase formation : primary γ dendrites γ + Fe3C (eutectic)
at eutectic temperature, as temperature decrease below
eutectic temperature, additional Fe3C form as C is rejected
from γ during cooling, below eutectoid temperature pearlite
formed
 Final microstructure consists of Fe3C in pearlite matrix.
White cast iron

31. Microstructure of white cast iron
Fe3C
pearlite

32. Fe-2.8wt%C-1.8wt%Si (White cast iron)
The cementite makes white cast iron
very hard and abrasion resistant.

33.  hard BUT brittle and almost impossible to machine.
 excellent wear resistance
 high compressive stress
Properties of white cast iron

34.  rollers in rolling mills, connecting rods, transmission gears, pipe
fittings, flanges. brake shoes, shot blasting nozzles, mill liners,
crushers, pump impellers and other abrasion resistant parts.
Application of white cast iron

35.  produced by heat treatment of white cast irons at temperature
between 800 ° C and 1470 ° C for a prolonged time period
(10-30 hr), in a neutral atmosphere (to prevent oxidation)
 causes decomposition of the cementite, forming graphite,
Fe3C Fe + C
which exists in the form of clusters or rosettes surrounded by a
ferrite or pearlite matrix, depending on cooling rate
Malleable cast Iron

36. Microstructure of malleable cast Iron

37. Microstructure of malleable cast Iron
Graphite
Ferrite
Ferritic Malleable Cast Iron
Pearlitic Malleable Cast Iron
Graphite
Pearlite

38.  Good ductility and machinability
 Good shock resistance properties
 Ferritic malleable cast irons are more ductile and less strong
and hard, than pearlitic malleable cast irons.
Properties of malleable cast iron

39.  parts of power train of vehicles, bearing caps, steering gear
housings, agricultural equipment, railroad equipment,
connecting rods, transmission gears, and differential cases
for the automotive industry, and also flanges, pipe fittings,
and valve parts, railroad, marine, and other heavy-duty
services.
Application of malleable cast iron
Connecting rod Transmission gears

40.  Si (1.7 - 3.0) wt%, C (3.1 – 4.0 wt.%).
 Microstructure consist of graphite in the form of worm-like (or
vermicular).
 The matrix phase either pearlite and/or ferrite depend on heat
treatment.
 Microstructure is intermediate between gray iron and ductile
(nodular) iron.
 Magnesium and/or cerium is also added in lower concentrations
than for ductile iron.
 compositions of magnesium, cerium, and other additives must be
controlled to produce a microstructure that consists of the worm-like
graphite particles, while at the same time limiting the degree of
graphite nodularity, and preventing the formation of graphite flakes.
Compacted graphite (CG) cast iron

41. Microstructure of CG cast iron
graphite
α- ferrite

42.  Higher thermal conductivity
 Better resistance to thermal shock (i.e., fracture
resulting from rapid temperature changes)
 Lower oxidation at elevated temperatures
Properties of CG cast iron

43.  diesel engine blocks, exhaust manifolds, gearbox housings,
brake discs for high-speed trains, and flywheels
Application of CG cast iron

44. Typical properties of cast iron

45. Types of cast iron Graphite Ductility Description
White No No Fast cooling rates
Grey Flake No Slow cooling rates
Malleable Anneal : flake
to nodule
Yes white iron + annealing
heat treatment
Ductile/Nodular nodular Yes additions of minor
elements made, so
nodules of graphite
form instead of flakes
CG cast iron Worm like Yes additions of minor
elements made, worm
like form instead of
flakes
Summary of cast iron

46. Gf - flake graphite
Gr - graphite rosettes
Gn -graphite nodules
P - pearlite
α - ferrite.
Microstructure of cast iron

47. Ductile cast iron
Summary of cast iron