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Grinding crack is a very fine surface crack, which is mostly perpendicular to the grinding direction, sometimes also in the form of a mesh. The depth is 0.03 to 0.15 Inrno3. The characteristics of the grinding crack and the inspection of grinding cracks and other cracks (such as quenching cracks). Compared with the crack of the raw material, the grinding crack is short and dense, and the depth is shallow. The surface around the crack is often accompanied by a burnt-annealed surface characterized by a dark or yellow surface, that is, a burn. There are three common forms of grinding cracks: vertical grinding direction, parallel direction and mesh cracking.
The grinding prevention process for grinding cracks As described above, the occurrence of grinding cracks is caused by the grinding heat. First, the rational use of cutting fluid: most of the current grinding uses water-based synthetic cutting fluid, should choose cooling, lubrication performance, stable performance of the cutting fluid (2 to 4 emulsified oil, N Royal CO3, NaNq, each 0.2 plus water mixed). The cutting liquid can be added by a pouring method, a high pressure injection method, a grinding wheel internal injection method, or the like.
Second, the choice of grinding wheel: to choose the grinding wheel particle size reasonably, such as wA40-60 K is better; to remove the surface area of ​​the grinding wheel in time, reduce the grinding depth, increase the number of feeding, slow down the table speed, take (11-2 Division) Min. The third is to repair the grinding wheel in time: otherwise, after the passivation of the grinding wheel, the cutting effect will be reduced, which will increase the frictional extrusion between the grinding wheel and the workpiece, so that the grinding force and the grinding heat will increase significantly, which will easily cause the surface of the workpiece to burn and crack. Trimming the grinding wheel to ensure the sharpness of the abrasive grain will reduce the grinding crack. The fourth is the coarse grinding: the coarse grinding uses the grinding wheel with coarser grain size to facilitate the strong grinding and improve the efficiency, and then use the fine grinding wheel. Fine grinding (sharp depth).
In the heat treatment process, the carburizing temperature of 20CrMnTi is generally 900-930 ° C. Because the carburizing temperature is too high, the time is long and the grains are coarse. In order to refine the grains and eliminate the network cementite, the heat treatment 1 after carburizing is divided into two quenchings, the first quenching temperature is 880 ° C, and the second quenching temperature is 870 ° C. After quenching, the martensite structure is an expanded state, and there is stress. To reduce and eliminate this stress, it should be tempered immediately. The tempering temperature is 220 °C, and the tempering time must be above 4 h to fully eliminate the residual stress. In this way, the grinding crack can be reduced from 20 to 30 at the time of one quenching to 5.
Tempering is an effective means to eliminate the quenching stress. The tempering is insufficient, the quenching stress is not sufficiently eliminated, and the microstructure after quenching is unstable. At this time, the workpiece can be subjected to secondary tempering or artificial aging. When time permits, natural aging can be performed for one to two months, and the grinding is performed after stress is removed, and good results are obtained. Pre-machining the gear surface after carburizing and quenching To further reduce the grinding crack, a surface pre-machining method can be used before grinding. That is, in order to remove the black scale on the surface of the carburized hardened gear, it can be sanded on a lathe and then ground, which has a significant effect on reducing cracks.