Strong shot peening can greatly increase the residual compressive stress near the surface of automobile gear, promote the transformation of residual austenite into martensite, improve the bending fatigue strength of automobile gear, but also increase the roughness of automobile gear surface. Particle peening (≤ Φ 0.1 mm) technology can not only improve the residual compressive stress near the surface of automobile gear, but also effectively reduce the impact of shot peening on the surface roughness of automobile gear, which meets the requirements of automobile gear for surface roughness.
Nissan automatic transmission R & D center used high-pressure and strong composite shot peening technology to study the surface modification and strengthening of automobile transmission gears, which greatly improved the fatigue life of automobile gears. That is, first use high hardness and large pellets（ Φ 0.6 ~ 1.0 mm) shot peen under high pressure and high speed, and then use high hard shot with small diameter（ Φ 0.1 ~ 0.2 mm) to form composite residual compressive stress on the surface of automobile gear, reduce surface work hardening, improve surface roughness, improve surface hardness, significantly improve the residual compressive stress near the tooth surface, and then inhibit the expansion of fatigue crack of automobile gear. It not only ensures that there is a large residual compressive stress near the tooth root of automobile gear, but also does not change the tooth surface roughness. For the specific strength of strong shot peening, the test piece is generally used to measure the shot peening strength.
Figure 1 shows the distribution of residual compressive stress near the surface of automobile gear after different types of surface shot peening. The strengthening effect of composite shot peening is more obvious than particle shot peening and strong shot peening, and the maximum residual compressive stress on the surface can reach 1300 MPa.
As shown in Figure 2, the bending fatigue strength limit of various shot peening strengthening methods is made through the rotary bending fatigue test. After shot peening strengthening, the bending fatigue limit of automobile gears is greatly improved. After carburizing and quenching, the bending fatigue limit of automobile gears can be increased by more than twice by using surface composite shot peening.