1. What is cast iron?
Alloys of iron and carbon with more than 2.11% carbon are called cast irons.

2. Cast iron Family of ferrous alloys
• Cast into desired shape – not worked
• 2-4% C and 1-3% Si
• Instability of Fe3C:
– Cementite / graphite flakes / graphite
nodules

4. Classification of cast iron
Type of cast iron
Graphite
Ductility
White No
Fast cooling rates
Gray
Flake
Slow cooling rates
Malleable
Spherical aggregates
Yes
White iron + annealing heat treatment
Nodular
Additions made so that nodules of graphite form instead of flakes

5. Iron rich end of the Fe-C phase diagram
Gamma+Fe3c  
Iron rich end of the Fe-C phase diagram

6. White cast iron • Fe3C + pearlite • Hard, brittle
• Shows a “white” crystalline
fractured surface
• Excellent wear resistance
• High compressive stress

7. White Cast Iron
Fe3C
Pearlite

8. White cast iron (Contd.)
Has excellent wear resistance
But is very brittle
Finds use as
balls for grinding mills,
liners for cement mixers and
rolls for paper manufacture

9. Gray cast iron During slow solidification carbon in
Fe separates or graphitizes to form
separate graphite flakes

10. Microstructure of gray
cast iron
Separate graphite
flakes form
X500
X100

11. Ferritic vs.Pearlitic gray iron
If all the carbon is in the form of graphite, we have ferritic gray iron, where the graphite flakes are embedded in a matrix of ferrite
If only a part of the carbon is in the form of graphite, we have the pearlitic gray iron, with pearlite as the matrix.

12. General characteristics/advantages
of gray cast iron
• Cheaper than steel, as temperature to be
attained for making it is several hundred
degrees lower than for casting steel. Also
control of impurities is not critical here, as in
steel making.
• It has excellent fluidity, even large complex
shapes can be cast advantageously.
• Excellent machinability, as chip formation is
promoted by the graphite flakes. In addition the
flakes serve as a lubricant for the cutting tool.

13. General characteristics/advantages of gray cast iron
The wear resistance of gray iron is very good, as graphite flakes act as lubricant.
The damping capacity (ability to damp vibrations) of gray iron is superior to that of steel
Can be alloyed to improve properties, e.g. Nihard iron with 4%Ni and 1.5%Cr has excellent wear resistance.
Graphite flakes are sharp at their tips and act like internal cracks or stress raisers. For this reason gray iron is brittle and shows only about 0.5% elongation in tension.

14. Great at dampening!
Relative ability of ferrous metals to dampen vibrations. The energy absorbed per cycle, or specific damping capacity of these can differ by more than 10 times.

15. Factors that promote the formation of graphite
Slow cooling
Thick cross sections or castings in sand moulds tend to have graphite, as the cooling rate is slow.
Chill castings (in metal moulds) and thin cross sections tend to have cementite.
This effect can also be seen in the fracture appearance across the cross section varying from white at the surface to gray inside. The transition region has the “mottled” appearance

16. Factors that promote the formation of graphite
Alloying elements influence graphitisation:
Silicon has a very high effect.
Effect of alloying elements other than Si is described in terms of
Si equivalent = %Si+3(%C)+0.3(%Ni)+0.3(%Cu)+0.5(%Al)-0.25(%Mn)-0.35(%Mo)-1.2(%Cr)
Increasing carbon in the melt tends to promote graphite formation

17. Silicon promotes graphitization

18. Stress-strain curves in tension and compression for Class 20 and Class 40 cast irons

19. Uses of gray iron
The good damping capacity and the high compressive strength make it suitable as a base for erection of machinery.
Ease of machining, good wear resistance and damping capacity are utilized in applications such as locomotive and internal combustion engine cylinder blocks and heads
Ease of casting and low cost make it suitable for flywheels and counterweights for lifts
Niresist with 20%Ni and 2%Cr has excellent corrosion resistance and heat resisting properties and is used for handling alkalis at high temperatures.

20. Malleable cast iron • White cast iron (typical composition 2.5%C and
1%Si)+ prolonged heat treatment at oC
followed by very slow cooling
• During this treatment cementite decomposes to
the more stable form (graphite). The free carbon
precipitates in the form of spheroidal particles
(nodules)

21. Temper graphite in malleable iron (Fe-2.9%C-1.5%Si-0.53%Mn-0.06%P-0.22%S-0.08%Ni-
0.1%Cu-0.09%Cr-0.003%Bi)
The casting was annealed
at 950 °C, held 10 h, furnace cooled to 720 °C, held 16 h, and air cooled.

22. Malleable cast iron-Contd.
Has a tensile strength up to 700 MPa, with an elongation of 10-15%
They are more expensive than gray irons, because of the heat treatment involved.
They are used in applications such as automobile crankshafts, chain links and brackets.

24. Ductile/nodular/spheroidal graphite(SG) cast iron
• Small quantities of Mg (modifier)
added to the melt to produce this iron
The basic composition of the melt is 3-4%C and
2.5%Si
The fairly high Si equivalent produces
graphitisation during solidification.
The modifier has the effect of making the growth
rate of graphite same in all directions, so that a
spherical shape results

25. Contrasting gray and nodular/ductile
cast iron
Separate graphite
flakes form
Mg added to molten iron – helps spherodise graphite
Low levels of minor elements such as S and P
X100
X100
X500

26. Gray vs Nodular cast iron Ferritic Pearlitic Gray – graphite as flakes
Brittle
Nodular – graphite as nodules
Ductile

27. Nodular iron (Contd.)
Nodular iron is a major engineering material, as it combines the advantages of steel with the processing economies of iron
Tensile strength ranges from 400 to 700 MPa, with elongation in the range 10-18%
Agricultural components, industrial fan hubs, coke oven doors, crankshafts and gears are some of the applications

28. Applications of ductile cast irons

29. Great at dampening!
Relative ability of ferrous metals to dampen vibrations. The energy absorbed per cycle, or specific damping capacity of these can differ by more than 10 times.