Abstract: Using orthogonal test, different processes of carburizing and quenching treatment were carried out on small and medium modulus gear made of 18CrNiMo7-6 steel, and the surface hardness, effective hardening layer depth, and microstructure after heat treatment were analyzed. The results show that the heat treatment processes with a strong carburizing potential of 1.10% and a diffusion carbon potential of 0.65%-0.85%, quenching time of 1.5 h and tempering time of 4 h, as well as quenching time of 1.0 or 1.5 h and tempering time of 6 h, can all achieve a surface hardness of over 59 HRC for the 18CrNiMo7-6 steel gear, and the effective hardening layer depth and microstructure meet the technical requirements. However, the microstructure observation finds that under the process of diffusion carbon potential of 0.85%, quenching time of 1.5 h, and tempering time of 4 h, both the carbides and retained austenite are at critical values, and there is a risk of exceeding the standard during mass production. After comprehensive consideration, the optimum carburizing and quenching process for the tested 18CrNiMo7-6 steel gear is determined as follows: strong carburizing potential of 1.10%, tempering time of 6 h, as well as diffusion carbon potential (0.65%-0.85%) matched with quenching time (1.0-1.5 h) by contour method.

Key words: 18CrNiMo7-6 steel, gear, carburizing quenching, hardness, microstructure