In addition to surface hardening to improve the hardness and strength of the surface layer, in order to improve the bending fatigue performance of the gear root and extend the contact fatigue life of the tooth surface, shot peening, rolling and laser impact strengthening are often used to introduce residual compressive stress after surface hardening treatment to improve the residual stress state in the surface layer and further improve the service life of the gear.
Shot peening has a history of more than 60 years. It was originally to solve the problem of gear root fracture, and gradually developed into a key modification process to improve the service performance of gear, such as contact fatigue, bending fatigue, stress corrosion cracking and wear. At present, shot peening has been changed from conventional shot peening to fine shot peening in foreign countries, and the new technology of shot peening successively with different shot diameters and shot peening intensities on the tooth root and tooth surface  and the new technology of shot peening twice with high strength and then low strength  have been developed, thus further improving the service performance of gears. In China, shot peening is still the traditional shot peening technology, lacking of strict control of fine shot peening and research and development of new shot peening technology. The key to control the surface integrity of gear peening is the state control of residual stress distribution in the surface layer and the optimization of surface roughness. To avoid the two improper peening strengthening methods of “under spraying” and “over spraying” . “Under spraying” can not achieve the expected strengthening effect due to the low shot peening strength, “over spraying” will cause “too much” due to the introduction of excessive strength into surface microcracks or excessive roughness.
The rolling strengthening of gears is to introduce residual compressive stress on the surface by rolling. It mainly uses round roller or ball to roll the surface of gear. There is no systematic research on the root of gear, which may be limited by the hard movement of the root roller or ball. There are few researches on this in China, so it is necessary to develop rolling strengthening equipment and related technology to improve the service performance of gears.
In recent years, laser shock strengthening has been applied in industry due to its high pulse energy and high transmission efficiency. It was initially applied in the engine compressor blades of the aviation industry, and in recent years, it has been widely applied in the gears of the automobile industry. Compared with shot peening, the surface residual compressive stress is larger, which can improve the fatigue performance and prolong the service life of gears. The recent development trend in foreign countries is to develop a multi pulse laser emission device, so as to prepare a laser similar to the operating table searchlight to irradiate parts. On the one hand, it can improve the production efficiency of laser shock strengthening, on the other hand, it can increase the surface coverage of laser shock strengthening. At present, the research of laser shock strengthening in China is still in the primary stage, which is reflected in that on the one hand, the basic research is not systematic and sufficient, and on the other hand, it does not have the corresponding R & D ability of high-performance and stable laser shock strengthening equipment.