Defects of gear heat treatment and technical improvement scheme

Gear heat treatment deformation is affected by many aspects, such as part shape, raw materials, forging and preliminary heat treatment, workpiece residual stress before cold and hot processing and heat treatment, and heat treatment process methods. Therefore, gear heat treatment deformation control should be solved from the comprehensive control and adjustment of the above factors. In the cold and hot working coordination, people’s requirement for heat treatment is that the deformation trend and deformation range are stable. Therefore, the carburizing temperature, heating speed and carburizing time should be controlled in strict accordance with the requirements of the best process, especially the clamping mode and quantity of workpieces and the variety of quenching oil. The oil temperature and mixing speed should not be changed at will.

In order to reduce the deformation of heat treatment, it is noted that most workpiece deformation is generated in the process of quenching and cooling, so the shape limiting quenching measure of mechanical clamping is adopted during quenching. This kind of process is characterized by high one-time qualified rate, but it can only be operated in a single piece, complex process and large equipment investment. At present, new progress has been made in quenching and cooling technology at home and abroad, and a special technical field has been gradually formed. Instead of conventional quenching medium, spray cooling is used to realize the computer control of ideal cooling rate at specified location and time.

Due to limited conditions, most factories adopt special processes for parts of the whole furnace to reduce deformation, such as reducing carburizing temperature; Rapid carburizing is used to reduce the duration of high temperature, reduce the quenching temperature under the condition of ensuring the core strength, reduce the oil stirring speed, etc.

Some thin-walled spline holes are easy to have a bell mouth at the end. A compensation sleeve can be added at the end to reduce the cooling speed and improve the deformation. For medium-sized or thin parts, martensite step quenching can greatly reduce the dispersion of deformation. In addition, there is a pretreatment process of sub temperature quenching, which is suitable for processing parts with large stress and finally dominated by thermal stress deformation. The method is that the rough machined parts are sub temperature quenched in the dual-phase zone of 790 ~ 800 ℃, the hardness is below 28hrc after tempering, then finish machining, and then conventional carburizing heat treatment. For the parts whose spline hole deformation has exceeded the tolerance, some units have also taken rescue measures such as electrolytic machining modification after heat or pushing the knife to squeeze the hole after heat (the inner hole needs anti-seepage).

In short, obvious deformation occurs in the heat treatment process, especially in the quenching and cooling process, but the size and law of deformation mainly depend on the material, design characteristics and specific process. Therefore, the increase of deformation caused by this process should be eliminated as far as possible. Qualified material, precision forging, standardized pretreatment and effective cold and hot processing cooperation are the links that can not be ignored to reduce and stabilize heat treatment deformation.

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