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Grinding For gears with high precision requirements, grinding is required after heat treatment to eliminate heat treatment deformation. Grinding changes the stress condition of the gear surface for two reasons: (1) The heat generated by the grinding heat and the quenching of the coolant will introduce new stresses. (2) When the grinding heat is too high, the surface will be tempered, the tempered martensite structure will be further decomposed, and the specific volume will become smaller, resulting in tensile residual stress on the surface. Therefore, incorrect grinding operations during gear grinding may result in residual tensile stress on the surface.
The surface residual compressive stress of the gear (Group 1) after the carburizing and quenching of the gear was 225 MPa, while the surface residual compressive stress of the normal cold to room temperature gear (Group 2) was 345 MPa, which was significantly larger than the former. This is mainly because the gear that is tempered when it is not cooled to room temperature during quenching has a large amount of retained austenite on the surface, and the volume expansion is small when the surface phase changes, so that the residual compressive stress is small. Therefore, after the car gear is carburized and quenched, it must be cooled to room temperature and then tempered to ensure a large residual compressive stress on the gear surface. In addition, if the gear is subjected to a cryogenic treatment after normal carburizing and quenching, part of the retained austenite may be further transformed into martensite, thereby obtaining a larger surface residual compressive stress [1]. If the gear does not temper after normal carburizing and quenching (Group 3), the surface residual compressive stress is 415 MPa, which is higher than the stress value after tempering. This is because martensite decomposes during tempering, and carbides are precipitated, which reduces the carbon content and squareness of martensite [2], so that the specific volume is also small, which eventually leads to a reduction in surface residual compressive stress, but Fire improves toughness and strength.
Effect of Grinding on Residual Stress on Gear Surfaces Grinding is actually a machining process that is equivalent to a grinding wheel with many small tools embedded. This gear-processed deformation layer produced by grinding is more limited to the surface than the general cutting, and is accompanied by a large heat generation. In order to investigate the effects of grinding and grinding conditions, the same batch of carburized and quenched gears were taken and tested in several groups. Sample 1, Sample 2, and Sample 3 were cut from gears without grinding gears, surface grinding depths of 0.10 mm, and grinding depths of 0.15 mm, respectively. In order to further confirm the influence of the grinding process, the sample 4 was taken from the grinding cracked gear and the residual stress measurement results.
The most superficial compressive stress after carburizing heat treatment is most significant. Properly performing the grinding operation does not significantly create new stresses, but the amount of grinding directly affects the degree of retention of the surface high pressure stress layer. Improper grinding operations can strongly change the stress distribution of the surface layer. From the measurement results of the grinding and cracking tooth surface, the compressive stress increases on the outermost surface due to the influence of the grinding wheel and the heat, and the secondary surface layer is tempered due to the action of heat. This leads to the generation of tensile stress, which occurs after the tensile strength. The generation of these grinding tensile stresses adversely affects the performance of the gears.
Conclusion There is a large residual stress on the final surface of the gear. The main factors affecting are the carburizing heat treatment process, grinding process and enhanced shot peening. The carburizing and quenching cooling method has a great influence on the residual stress of the automobile gear surface. It must be ensured that the gear is quenched and then completely cooled to room temperature and then tempered, otherwise the residual compressive stress on the gear surface will be reduced. After the carburized and quenched gears are tempered, the surface residual compressive stress decreases, but the overall mechanical properties of the gears increase. After the carburizing heat treatment, the high-pressure stress is at the top surface. The correct grinding process does not change the stress distribution, but the grinding depth directly affects the retention of the high-pressure stress layer. The incorrect grinding operation will strongly change the stress after heat treatment. Distribution, the generation of tensile stress not only easily leads to grinding cracks, but also seriously affects the performance of the gear.