When the contact stress of automobile gear is too large, the plastic flow of tooth surface metal due to yield is called tooth surface plastic deformation. It generally occurs on the soft tooth surface. When the hardness of the tooth surface is low, due to short-term overload or large friction coefficient of the gear tooth surface, there are more non martensitic soft tissues in the tooth surface tissue, which will make the metal on the gear tooth surface flow and produce plastic deformation. The plastic deformation of gear teeth can be divided into thermoplastic deformation and cold plastic deformation. Usually, the violent temperature rise caused by abnormal lubrication reduces the yield limit of the material and causes thermoplastic deformation. Plastic materials with low strength produce cold plastic deformation under overload.
Improving the hardness of the tooth surface, reducing the contact stress and improving the lubrication are helpful to reduce or prevent the plastic deformation of the tooth surface.
In order to make the gear teeth have a certain ability to resist the above failure, when selecting the gear material, the tooth surface should have sufficient hardness and wear resistance to resist the wear, pitting, gluing and plastic deformation of the tooth surface, and should have sufficient bending strength to resist the fracture of the tooth root. Therefore, the basic requirements for the gear material are: the tooth surface should be hard and the tooth center should be tough. At the same time, the requirements of processing and heat treatment technology and economy should also be considered when selecting materials.