Investigation of the impact of various heat treatment techniques on the mechanical properties of AISI 8620 and AISI 5115 steel gear wheels

In this study, the mechanical impacts on helical gear wheels that are made of AISI 8620 (DIN 21NiCrMo2 / 1.6523) and AISI 5115 (DIN 16MnCr5 / 1.7131) steels with modules 1.5 and 2 have been investigated after the application of surface hardening heat treatment using several techniques including low-pressure carburizing, gas carburizing, and plasma nitriding. Hardness distribution profile, surface roughness, the alteration on microstructure, dimensional changes, and lifetime value of the gear wheel samples have been examined comparing the values from before and after the heat treatment process. The alterations on the microstructure of the samples has been examined using optical microscope and Clemex Vision Lite programs, whereas hardness distribution profile has been examined using Emcotest Duroscan 20 microhardness device, and surface profiles has been examined using Mitutoyo SJ-301 profilometer. In order to measure the dimensional changes on the gear wheels, 0.001 mm sensitive micrometer has been used. Solidworks Premium 2015 SP 2.0 CAD program has been leveraged for the fatigue analysis of the gear wheels. The detailed examination revealed that the low-pressure carburizing is more effective compared to gas carburizing in reaching the desired surface hardness and effective thickness of the hard layer values in a shorter time span of the application process. After the application process of the plasma nitriding, the thickness of the white layer and hardness value of material interior parts values has been observed to be in the desired intervals for both types of steel gear wheels, however the obtained total thickness of diffusion layer of the gear wheels made of AISI 8620 steel (module 1.5) could not reach the minimum value required. The maximum dimensional increase of the helical gear wheels after the heat treatment has been observed with the gas carburizing application, as a result of the long processing interval at high temperatures. It was concluded that heat treatment alters the surface roughness values of the gear wheels, and for the desired level of surface quality, surface-finish treatments is needed after the entire heat treatment applications have been completed. A correlation between the types of heat treatment and the resulting surface hardness and depth of hard layer values has been determined, and this differentiation is proven to have an impact on the lifetime of the gear wheels.