Shot peening is one of the effective ways to improve the contact fatigue strength of bevel gears. Because shot peening enhances the surface compressive stress and significantly improves its fatigue performance, it is more effective for workpieces bearing high cycle fatigue load. The residual compressive stress formed by shot peening can counteract part of the external load. During shot peening, small spherical steel shot strikes the surface of the workpiece to form a compressive stress. The impact of each shot will cause a certain plastic deformation of the metal, and eventually a permanent compressive stress state is formed because the surface cannot be completely recovered. As a surface strengthening process, shot peening can form residual compressive stress on the surface, which is equivalent to 55%~60% of the tensile strength limit of the material, and the workpiece surface is just the place where cracks are easy to initiate.
For carburized and quenched bevel gears, the pressure stress formed can reach 1177 ～ 1725MPa, which can greatly improve the fatigue performance. The depth of compressive stress layer is a function of shot peening strength (or shot peening energy), which increases with the increase of shot size or shot speed. It can be seen from the comparative verification data of residual stress test that after the surface strengthening process, the residual compressive stress on the tooth root surface is increased from 824MPa to 1252MPa, and the contact fatigue performance is greatly improved.
1) After carburizing, quenching and tempering, because of the existence of internal oxidation soft layer, the bevel gear is easy to produce stress concentration during shot peening, which becomes the starting point of fatigue crack source. It is suggested to select low Si materials on the premise of ensuring the hardenability of the materials, try to control the content of Cr, Mn, Ti, V and other alloy elements, purify the carburizing feed gas (methanol or natural gas), reduce the content of oxidizing components, such as water, sulfur and other impurities, and reduce the internal oxidation degree of the products after heat treatment.
2) Strengthening shot peening can improve the bending fatigue strength and contact fatigue strength of bevel gears. It is an important way to improve the anti seizure ability and increase the service life of bevel gears.
3) Control the purity and segregation of inclusions from the source of materials, cooperate with the pressure quenching process to reduce the deformation of heat treatment, improve the consistency of the contact zone, and improve the working conditions.