Abstract: Effect of post-subzero treatment and post-low temperature tempering on surface hardness, CHD and core hardness of 20Cr2Ni4A steel gear when the hardness was low after carburizing and quenching and low temperature tempering was studied. The results show that the stabilization of retained austenite in the 20Cr2Ni4A steel after carburizing, quenching and low temperature tempering is not obvious, thus subzero treatment can still improve the hardness of the workpiece. A secondary hardening phenomenon after low temperture tempering appears when the 20Cr2Ni4A steel contains more retained austenite, thus increasing the low temperature tempering temperature can also increase the surface hardness. Therefore, the post-subzero and post-low temperature tempering treatment process can be used to improve the hardness, instead of the conventional rework process of re-high temperature tempering, carburizing and quenching and low temperature tempering. The post-subzero treatment temperature shall be determined according to the Mf point. For the process of carburizing, high temperature tempering, re-heating and quenching and low temperature tempering, the Ms and Mf cannot be calculated according to the traditional method, however, it can be estimated by the retained austenite content.

Key words: carburizing and quenching, hardness, retained austenite, Mf, subzero treament, second hardening