Property development in S.G iron by heat treatment

Abstract

Cast iron is an alloy of iron containing more than 2% carbon as an alloying element. It has almost no ductility and must be formed by casting. ductile iron structure is developed from the melt of cast iron. The presence of Si in higher amount promote the graphitizarion inhibiting carbon to form carbides with carbide forming elements present the carbon forms into spheres when Ce, Mg, are added to the melt of iron with very low sulphur content having this special microstructure containing graphite in nodular form gives ductile irin thus the ductility and toughness superior to that of any cast iron and steel structure finding numerous success in industrial application however heat treatment is a valuable and versatile too for extending both the consistency and range of properties of ductile iron casting beyond the limits of those produced in as-cast condiotion. Thus to fully utilize the potentioal of ductile iron castings, the designer should be aware of wide range of heat treatment available for ductile irin and its response to this heat treatment. Although ductile iron and steel are superficially similar metallurgically, the high carbon and silicon level in ductile iron results in important differences in their respionses to heat treatment. The high carbon leels increase hardenibility, permitting heavier sections to be heat treated with lower requirements for expensive alloying or severe quenching media also may cause, quench cracking due to the formation of high C martensite. This undesireable phenomena maje the control of composition, austenitising temperature and quenching conditions more critical in ductile iron. Since the formation of martensite is accompanied byu internal stresses, tempering is necessary in order to relieve the internal stresses, decreases the amount of retained austenite and reduces the probability of cracking. Austempering is a critical heat treating process in which austenite transforms isothermally to lower bainite rather than martensite and thus objectively reduces distortion and cracks. It is possible to achieve much larger ranges of tensile strength, ductility with toughness by adopting austempering, heat treatment process of ductile iron.