After carburizing, the carburized parts are slowly cooled to room temperature, and then quenched at a temperature higher than a in the center to refine the core grain and eliminate the network carbide in the carburized layer. This temperature is too high for the carburized layer, so it is necessary to carry out another quenching which is higher than the a temperature of the carburized layer but lower than that of the carburized layer to improve the microstructure of the carburized layer. The heating temperature of alloy carburizing steel for twice quenching.