Gear carburizing and quenching change

　　XNUMX. Explore the gear quenching process

　　Let's discuss the residual stress of the surface layer after carburizing and quenching in the gear machining process.Regardless of carburizing quenching or non-carburizing normal overall quenching, the surface layer is in direct contact with the cooling medium to cool faster than the core, so the surface thermal stress direction is generally negative compressive stress, while the structure stress direction may increase after surface carburization The surface carbon content is reduced, and the martensite starting temperature is lowered, so that the martensite transformation occurs in the core before the surface, which affects the direction of surface stress.

　　XNUMX. Negative pressure stress of gear

　　The infiltration of carbon or nitrogen atoms in the material of the gear causes density changes on the surface of the part, and also causes obvious surface residual compressive stress. When superimposed with the negative thermal stress, the surface of the part presents a negative compressive stress.From the results of residual stress testing,Gear processingAfter carburizing and quenching treatment, the surface layer residual compressive stress is about 200-400MPa, the highest peak of compressive stress appears in the subsurface layer about 0.03-0.05 mm, which can reach more than 500MPa, and the compressive stress below the subsurface layer decreases. The final residual stress is thermal stress, The superposition of tissue stress and additional stress.Analyze the mechanism of internal stress in principle and qualitatively analyze the internal stress generated by some heat treatment. Although the strength of internal stress cannot be obtained quantitatively, from the direction of heat treatment deformation, it can be preliminarily judged whether the internal stress is mainly thermal stress or tissue stress What is the role of? To narrow down the scope of the test in a targeted manner, and quickly find the cause and solution of heat treatment deformation under specific conditions.

　　XNUMX. Gear processing temperature

　　Gear carburized and hardened parts are mainly subjected to thermal stress in the carburizing stage. Too fast heating is likely to cause thermal stress with a large temperature difference between inside and outside. At this stage, preheating or slow heating or stepped heating can be used to reduce thermal deformation.When the carburizing temperature is high, the material strength decreases. The additional stress generated by the weight of the workpiece and the supporting load may be close to or exceed the material strength. If the production cost efficiency is considered, lower temperature carburizing should be used to reduce thermal deformation.The quenching stage is usually the key stage of heat treatment deformation, which will produce thermal stress and structural stress at the same time. Therefore, the pre-cooling method can be used at this stage, that is, when the workpiece is at a higher temperature or the material strength is lower, the cooling rate is slower at the beginning , To avoid large thermal stress; Then when the temperature drops to a certain temperature or the strength of the workpiece increases, the cooling rate increases again and quickly passes through the pearlite bainite transformation zone; finally the workpiece starts to transform at the martensite starting temperature Ms to martensite In the transition end temperature Mf range, the cooling rate needs to be slowed down to reduce the generation of tissue stress.

　　Fourth, the gear summary

　　The principle of thermal deformation is well explained in theory, but in practice it is difficult to precisely control it.However, both experimental research and production practice have shown that uneven cooling is one of the main causes of heat treatment warpage, and rapid cooling will inevitably lead to an increase in thermal deformation. This is the main research at present, but the thermal deformation is extremely important when the amount of thermal deformation increases. Whether the difference is also increasing, there are not many research reports.