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

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

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

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

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

White Cast Iron Fe3C Pearlite

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

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

Microstructure of gray cast iron Separate graphite flakes form X500 X100

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.

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.

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.

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.

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

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

Silicon promotes graphitization

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

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 (nodules)

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.

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.

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

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

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

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

Applications of ductile cast irons

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.