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Effect of vacuum carburizing and quenching process on gear distortion

Gears are important parts in transmission machinery and are required to have comprehensive properties such as wear resistance, resistance to surface contact fatigue and resistance to bending fatigue. At present, vacuum carburizing is the mainstream process method in the manufacture of high-parameter hardened gears, but the outstanding technical problem is distortion. Gear distortion not only affects the gear’s load-bearing capacity and performance, but also increases the subsequent grinding amount and increases production costs. Therefore, studying the causes and influencing factors of gear distortion and proposing measures to reduce distortion have become important topics for improving product quality, extending service life and improving economic benefits.

Effect of pretreatment on distortion
The uniformity and stability of the preheat treatment structure of the gear blank have a great influence on the final heat treatment distortion of the gear. Different original structures have different specific volumes, and the distortion generated during vacuum heat treatment is also different. The structure after quenching and tempering is tempered troostite, in which cementite is granular, while the structure after normalizing is ferrite and pearlite, and cementite is lamellar. The quenching distortion of workpieces with spherical pearlite or quenched and tempered structures is less than that of workpieces with lamellar pearlite as the original structure. From the perspective of reducing the final heat treatment distortion of the gear, the fine and uniform structure obtained by pretreatment can reduce the final heat treatment distortion and improve the final heat treatment quality.

Effect of preheating on distortion
Preheating can reduce the temperature difference between the inside and outside of the workpiece, reduce the thermal stress inside the workpiece, and thus reduce the distortion. Since the distorted specimen in this test is small, the effect of preheating on distortion is not significant.

Effect of vacuum carburizing temperature on distortion
Vacuum carburizing temperature affects the austenite grain size. The higher the carburizing temperature, the coarser the austenite grains and the greater the gear distortion. In addition, the rigidity of the gear decreases with increasing temperature, that is, the distortion increases with increasing temperature. Therefore, the carburizing temperature should be reduced as much as possible to reduce distortion.

Effect of carbon potential on distortion
Carbon potential directly affects the surface carbon concentration. The higher the carbon potential, the higher the surface carbon concentration. The increase in carbon content increases the specific volume of martensite and significantly increases the organizational stress of quenched steel. If undesirable carbides are formed on the surface after carburizing, the distortion will be further increased. Therefore, the carbon potential during carburizing must be controlled to prevent the surface carbon concentration from being too high and to keep the carbon concentration of the carburized layer within the reasonable range of technical requirements.

The effect of vacuum quenching temperature on distortion
The higher the vacuum quenching temperature, the greater the thermal stress and structural stress generated, and the greater the distortion. In order to reduce distortion, the quenching temperature should be lowered as much as possible, but the quenching temperature should not be too low, otherwise there will be too much ferrite in the core and the required core hardness will not be achieved, which is not good for the performance of the parts.