Nano- and Microhardness Distribution in the Carburized Case of Nb-Microalloyed Gear Steel

The characteristics of in-depth hardness distribution in the high-temperature carburized case of Nb-microalloyed gear steel before and after cryogenic treatment were studied by nano-indentation method and Vickers hardness method. The results showed that there was no significant difference in Vickers hardness of the carburized case before and after cryogenic treatment, while the nano-hardness of the carburized case was increased after cryogenic treatment. The change of nano-hardness could be related to the decrease in retained austenite content. The retained austenite content in the surface of the carburized case was 26%, which was reduced to 18% after cryogenic treatment at - 196 degrees C, with the nano-hardness increased from 11.8 to 12.6 GPa. Nano-indentation can be used to reliably measure the properties of the high-temperature carburized case with fine retained austenite for Nb-microalloyed gear steel.