Gradient microstructure evolution and hardening mechanism of carburized steel under novel heat treatment

The microstructure evolution of 15MnNi4Mo steel after carburizing was studied, and the hardening mechanism of carburized layer under the influence of sub-temperature quenching was discussed. The results show that due to the high Ni content, the high-temperature tempering process of 15MnNi4Mo is slow, and the carburizing layer evolution behavior is mainly the decomposition of martensite and retained austenite, while the heart region is mainly the decomposition of M/A island. The sub-temperature quenching retains some of the carbides after tempering which increases the hardness (60HV) of the carburized layer by reducing the retained austenite, refining the substructures of martensite and accumulating dislocations around carbides. (C) 2020 Elsevier B.V. All rights reserved.