Cast Iron.ppt (Size: 2.73 MB / Downloads: 54)
What is cast iron?
Alloys of iron and carbon with more than 2.11% carbon are called cast irons.
Family of ferrous alloys
• Cast into desired shape – not worked
• 2-4% C and 1-3% Si
• Instability of Fe3C:
– Cementite / graphite flakes / graphite
White cast iron
• Fe3C + pearlite
• Hard, brittle
• Shows a “white” crystalline
• Excellent wear resistance
• High compressive strength
Has excellent wear resistance
But is very brittle
Finds use as
balls for grinding mills,
liners for cement mixers and
rolls for paper manufacture
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.
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
• 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.
Advantages/disadvantagesof 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.
Factors that influence the formation of graphite
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
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.
Malleable cast iron
• White cast iron (typical composition 2.5%C and
1%Si)+ prolonged heat treatment at 900-950oC
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