Effects of Different Tempering Process on Microstructure and Mechanical Properties of 15CrNiMo Carburized Steel

The effects of 15CrNiMo carburized steel carburizing post-heat treatment process on the microstructure and properties were studied. The direct tempering process and the second quenching process were analyzed and compared. The direct tempering process was carried out low-temperature tempering treatment directly on the samples after carburizing and quenching, while the secondary quenching process was the heat treatment process in which the samples were subjected to high-temperature tempering and then quenching + low-temperature tempering after carburizing and quenching. The samples and impact fracture were observed by scanning electron microscope. At the same time, the surface hardness of the two samples was tested. The results show that there is little difference between the structure of the carburized layer and the transition layer between the two processes. The core of the direct tempering process is bainite with good impact toughness,while the core of the secondary quenching process has dual-phase structure, including another bainite structure, which has a negative impact on the impact toughness. In the secondary quenching process, the surface hardness of the sample decreases due to carbon diffusion. The direct tempering process shows better performance. The surface hardness is 59.5 HRC and impact energy is 192 J, which is 6.5 HRC and 32 J higher than that of the test steel in the secondary quenching process.