The objective of this work is to study the effect of different heat treatment processes on the hardness of AISI 8620 carburizing gear teeth. In this study, theoretical and experimental heat treatments were performed on the main gear, followed by hardness testing of the as-received and heat-treated gear teeth, and microstructural analysis of the as-received teeth. The theoretical part of the study describes the main gear transmission, its properties and characteristics, and explains the heat treatments used on this gear in the experimental part of the study. Additionally, steels used for surface heat treatments are mentioned, and the effect of carburizing the surface of the sample is shown. The hardness of materials and the most common hardness measurement methods are explained. In the experimental part, heat treatments of the material were carried out, and after each treatment, the treated samples were subjected to hardness testing. The hardness results showed that the application of heat treatment processes had a significant effect on the hardness of AISI 8620 gear teeth. The results indicate that the heat treatment of tempering in oil or water was carried out to obtain a hardness of approximately 60 HRC in the teeth, considering that this region, after cementation, presents between 0.8 and 0.9 %C.