Abstract: In response to the problem that there are local soft points and surface hardness unqualified on the lockpin of the aux synchronizer after carburizing and quenching, the reasons are investigated and explored. The results show that the soft points are caused by the depth of the locally unqualified carburized layer after heat treatment, and the reason for the local unqualified of the depth of carburized layer is the residue of cutting fluid and cleaning fluid before heating, which is hindered during the carburizing process due to the distinctiveness of the process equipment.Process optimization and improvement were carried out based on the reasons, thus solving the soft points problem.