The purpose of the austempering treatment of ductile iron is to transform the matrix structure of iron castings into a strong lower bainite structure . The heat treatment of iron castings can only change the matrix structure, but not the shape and distribution of graphite. The mechanical properties are changed by the matrix structure. Caused by changes. The graphite flakes of ordinary gray cast iron ( including inoculated cast iron ) have a great influence on the mechanical properties ( strength, ductility ) , and the mechanical properties of gray cast iron are not significantly improved by heat treatment. It should also be noted that the thermal conductivity of cast iron is worse than that of steel, and the presence of graphite results in higher notch sensitivity than steel. Therefore, the cooling rate ( especially quenching ) in the heat treatment of cast iron should be strictly controlled.