The purpose of the austempering treatment of ductile iron is to transform the matrix structure of the cast iron into a strong lower bainite structure, the strength limit can exceed 1100MPa, and the impact toughness AK≥32J. The treatment process is: heating the ductile iron castings to a temperature of 830-870℃ to keep the matrix austenitized, and then put them in molten salt at 280-350℃ to keep the austenite partly transformed into lower bainite. The original spheroidal graphite is not change. Obtain high-strength ductile iron.
The above heat treatment of cast iron shows that the heat treatment of iron castings can only change the matrix structure, but not the shape and distribution of graphite. The change in mechanical properties is caused by the change in the matrix structure. 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 causes higher notch sensitivity than steel. Therefore, the cooling rate (especially quenching) in the heat treatment of cast iron should be strictly controlled.