The strength of automobile gear is mainly the bending fatigue strength and tooth surface contact fatigue strength of gear. Figure 1 shows the bending, fracture and damage of automobile gear. The main reason is that the crack at the root of automobile gear is caused by repeated concentrated stress and gradually expands to cause failure. The bending fatigue crack is generated from the grain boundary oxide layer on the surface of automobile gear and extends to the depth of hardened layer along the austenite crystal boundary below the surface, resulting in the damage of crystal grain boundary.
The grain boundary oxide layer on the surface of the material is mainly composed of Si, Mn, Cr and other alloy elements that can improve the quenching performance. The edge of the grain boundary is easy to produce local incomplete quenching field, forming an incomplete carburizing abnormal layer composed of Troostite and bainite. Figure 2 shows the grain boundary oxidation structure of 20CrMoH sample after carburizing and quenching. The black whisker extending from the surface to the inside is the oxide of Si, Mn and Cr.
To improve the bending fatigue strength of automobile gears, it is usually used to increase the R angle of tooth root and the design of high-pressure force angle. The surface treatment technologies such as carburizing quenching or carbonitriding heat treatment and shot peening are used for heat treatment. For example, the method to improve the abnormal layer of incomplete carburization can generally be to increase the quenching speed, but pay attention to avoid large tooth surface deformation; Or reduce the content of Si, Mn, Cr and other elements while increasing Ni, Mo and other alloy elements conducive to improving quenching performance.