Causes and preventive measures of grinding cracks in carburized and quenched gears of rolling mill

Gear is an important basic part of mechanical products. Gear grinding is a finishing process of gear processing. Gear grinding can not only correct various errors caused by gear pre-processing, but also significantly improve the overall accuracy of gearbox. With the increasing demand for gear precision, high efficiency and high precision control is the goal of enterprise production. In grinding, 60% ~ 95% of the heat in grinding heat is transferred directly to the workpiece instantaneously. These heat accumulate on the surface to form local high temperature, which will cause thermal damage when exceeding the critical value. During heat treatment, the sensitivity of grinding cracks will be increased if there are coarse network or strip massive carbides in the structure, high content of residual austenite, high surface carbon content, large fluctuation and large distortion, or unreasonable selection of grinding wheel type, grinding wheel hardness and particle size, feed rate, stroke speed and cooling conditions during grinding.

In the process of gear manufacturing, many factors cause defects after tooth surface grinding, such as grinding burns, grinding cracks, grinding steps, etc. among them, grinding cracks are generated after serious grinding burns, which will seriously affect the manufacturing accuracy, product quality and service life of gears. In order to find out the causes of defects, the author analyzes two typical cases of grinding cracks in the production of an enterprise in order to avoid the recurrence of such accidents.

Improper grinding process of carburized quenched gear (excessive feed during rough grinding) will lead to the formation of secondary quenched white bright layer in tensile stress state on the outermost layer. The white bright layer is composed of quenched cryptocrystalline martensite under rapid heating and cooling, with high hardness and brittleness. At the same time, the hardness of grinding tempering layer on the subsurface layer decreases by about 10hrc, Finally, the carburized layer forms the typical hardness distribution characteristics of high low high grinding quenching burn. The microstructure and stress state of grinding burn layer are easy to induce grinding cracks and cause early failure of workpiece. When the forming gear grinding machine is used for gear grinding, the grinding crack is generally below the indexing circle. It is recommended to conduct inspection after grinding. In case of grinding quenching burn, stress relief tempering shall be carried out immediately to avoid further grinding cracks.

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