The non main load-bearing gear is often machined directly after quenching and tempering of 45 steel. The recently developed low carbon bainite martensitic steel does not need heat treatment to make gears. In the mechanical industry, 20CrMnTi, 20CrMnTi h42crmo, 39ni crmo3, 40Cr Mo A17, sae4140 and aisi5130 are often used to make important bearing gears, while in the aerospace industry, high-quality high-strength alloy steels are used, such as m50ni L and 9310 steels with high-purity inclusion fine control, which are often used to make bearing gears. Induction hardening, carburizing, nitriding or carbonitriding, as well as boronizing or other elements are often used to increase the hardness and strength of the surface layer, so as to increase the fatigue resistance of the surface layer and reduce the surface wear. The proper depth and hardness gradient of the hardened layer are the key to heat treatment hardening of gears. It is very easy to form a hard layer with eggshell like shape on a super hard surface and a larger hardening gradient or a shallower hardening layer depth, which will lead to early peeling off of the surface layer after the gear is loaded. Therefore, enough attention should be paid to it and proper hardening process should be adopted to avoid such situation. Lower than the specified hardness and smaller hardness gradient or deeper hardening layer, it is easy to form wear on the surface or hard to achieve the expected hardening effect. In recent years, the development of deep carburizing and deep nitriding technology provides a good new idea and mature technology for the formation of suitable hardening layer depth and hardness gradient.
The surface integrity control of gear heat treatment hardening is to control the structure, hardness gradient, hardening depth and residual stress of the case. Improper hardening control of gear heat treatment can easily lead to defects such as black structure, decarburization, deep white bright layer and micro cracks on the surface, and also can easily lead to residual compressive stress which should be formed into residual tensile stress.