PURPOSE: To provide superior toughness to spheroidal graphite cast iron contg. a specified amount of Mn by heating and holding the cast iron and quenching and holding it to convert the matrix structure into bainite.

CONSTITUTION: Spheroidal graphite cast iron contg. 0.4W0.8wt% Mn is heated and held at 850W950°C for 1W4hr. It is then quenched to 250W450°C and held at the temp. for 2.5W6.0hr to convert the matrix structure into bainite. Other components of the cast iron are regulated to about 2.5W4.0% C, about 1.8W3.5% Si, ≤ about 0.10% P, ≤ about 0.02% S and about 0.02W0.10% Mg.