Important Properties of cast Iron
(1) Machinability. Cast iron has broad range of machinability, i.e., from very good to the most unmachinability. Annealed permanent mould iron has got the highest machinability as the carbon in such a cast iron is in the state of finely divided and dispersed graphite flakes and not in the combined state as carbide, and moreover it is free from burned-in sand at the surface. Ductile cast iron has also got very high machinability. Machinability of various types of cast irons in decreasing order are as follows :
Pearlite ferrite irons.
Pearlite irons.
Motteled iron with pearlitic and massive cementite white iron.
Due to the massive carbide structure white iron is particularly very difficult to machine.
(2) Mechanical properties. Cast iron tensile strength varies from 1,350 to 5,350 kg/cm2. Indefinite static load just short of the tensile strength can be sustained by grey iron, without distortion or breakage. Grey iron has low ductility and breaks with perceptible distortion. With static loading the the ultimate strength of cast iron in tension is less than that in compression, the impact strength of most cast irons is low> The damping capacity, or the ability to absorb vibrations, is high. Addition of heat treatment, chilling and alloy reduce the macinability. The white iron or chilled-iron casting is widely used for machinery parts to resist wear. High-alloy cast iron of the chromium, nickel and silicon type is specially resistant to sulphur and acid corrosion. Cast iron is most widely used in engineering and allied industries as it is available in the forms of soft, weak, hard, brittle, and strong irons and has wide range of useful properties.
(3) Weldability. The weldabiltiy of most cast irons are very low. Forge and submerged-melt welding cannot be used fro cast irons. Gas and arc welding can be employed with special rods sections have more than 6mm thickness provided casting is heated red hot before welding and then colled slowly to the room temp. Bronze welding is used mostly for grey & white irons before malleabilizing and without preheat, provided temperatures obtained are 810C to 800C.
(4) Corrosion Resistance. Cast irons with high content of chromium and silicon are quite resistant to acids. Formation of rust is very slow in cast irons in comparison to alloy steels. High nickel austenitic irons are resistant to acids, to stress corrosion in hot to alkalies if stresses are low. High silicon ( 11 to 17% Si) cast irons are remarkably good fro withstanding all acids except hot HCL.
(5) High Temperature Usefulness. Grey cast irons are useful upto 340C for pressure vessels. Lower carbon, lower silicon ad more chromium contents in cast irons increases their permissible temperatures.
