Influence of heat treatments on microstructure and toughness of austempered ductile iron

High silicon content in nodular cast iron leads to a large amount of retained austenite (Y) during isothermal transformation in the bainitic zone by inhibiting the precipitation of carbides. The presence of Y phase results in high toughness, with an optimum fracture toughness K_Ic of about 85 MN m^−3/2 for a 0·2% proof stress of 1000 MN m⁻² being observed at 30% retained austenite. The presence of martensite after transformation for short times and of coarse carbides after transformation for long times greatly reduces the toughness. For lower bainite having austenite volume fractions less than 30%, optimum fracture toughness is obtained when the fracture is predominantly transgranular ductile. For austenite volume fractions greater than 30% in the upper bainite region, Y→α′ (martensite) transformation induced plasticity occurs, leading to superior toughness compared with conventional cast irons. In this case the fracture, although predominantly ductile, also contains some intergranular zones due to the transformation of austenite to martensite in the plastic zone ahead of the crack, and the strain hardening exponent for the tensile tests increases with strain, also indicating strain induced transformation of austenite to martensite.