1. Physical Metallurgy EBB222
Cast Irons

2. Overview of cast iron
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

3. Iron Carbon Phase Diagram
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

4. Production of cast iron
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

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
©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 Carbon Equivalent, C.E. = % C + 1/3 % Si
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

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. Gray Cast Iron 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.

17. Gray Cast Iron
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

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

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

20. Properties of gray cast iron
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

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

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

23. Ductile cast iron Also known as spheroidal graphite (SG), and nodular
graphite iron
addition of Mg (~0.1%) /Ce (~ %) 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

24. Microstructure of ductile cast iron
Nodular graphite
α- ferrite
Ductile cast iron with a matrix that is predominantly ferrite. Original magnification: 250×
Fe, C 3.2, Si 2.5, Mg 0.05 (wt%)
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. Properties of ductile cast iron
high ductility
high strength
high modulus elasticity
excellent castability
excellent machinability
wear resistance
poor corrosion
Crankshaft

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

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

29. Applications of ductile cast irons

30. White cast iron 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.

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

34. Application of white cast iron
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.

35. Malleable cast Iron
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

36. Microstructure of malleable cast Iron

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

38. Properties of malleable cast iron
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.

39. Application of malleable cast iron
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.
Transmission gears
Connecting rod

40. Compacted graphite (CG) cast iron
Si ( ) 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.

41. Microstructure of CG cast iron
graphite
α- ferrite

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

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

44. Typical properties of cast iron

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

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

47. Summary of cast iron
Ductile cast iron