2022 September the Fifth Week VISTA Technical Knowledge: Effect of Material and Heat Treatment on Grinding Crack of Bearing Rings

Put forward measures and effectively control the generation of grinding cracks on bearing rings

Abstract: Grcinding cracking of bearing rings is alnays the knottiest problem to the bearing manufacturers. On the basis of our examples ,this prper has expounded the mechanism of grinding cracking ,analyged effecf of mate2 rial and heat treating of the bearing on grinding cracking and indicated precautionary measures. The grinding cracking of the bearing rings has thus been efficiently overcome.

Key Words: bearing ring ;srinding crack ;material ,heat treatment.

1. Mechanism of grinding cracks

The causes of grinding cracks are complex and multifaceted. From the mass production practice of our company, the most important and direct causes are grinding heat and residual internal stress. When grinding parts, the grinding surface is affected by the grinding resultant forces such as the friction force and cutting force of the grinding wheel, which causes plastic deformation of the part surface and generates a lot of grinding heat. This grinding heat will make the local instantaneous temperature in the grinding area reach more than 1000 ℃. If such a large amount of grinding heat can not be taken away by the coolant in time, the structure of the grinding surface layer will change, that is, a surface deterioration layer will be formed.

When the surface temperature of the workpiece caused by grinding heat is lower than the phase change point (745 ℃) of the bearing steel, the surface modified layer is a high-temperature tempering structure (fine point troostite or sorbite), which is dark black after cold pickling, and the hardness at this place decreases, which forms grinding burns.

When the surface temperature of the workpiece is higher than the transformation point, the surface structure there will be transformed from the original tempered martensite to austenite, and then secondary quenching will occur due to the quenching effect of the coolant to obtain the quenched martensite. When the martensitic transformation occurs and the structural stress is greater than or equal to the breaking strength of the steel, grinding cracks will be formed.

2 Effects of materials

Among the numerous grinding crack ferrules of our company, 90% of ferrules are made of GCr15SiMn steel. Therefore, grinding cracks are obviously related to materials. According to the chemical composition of GCr15 and GCr15SiMn, GCr15SiMn contains silicon (0.15% ～ 0.35%) and manganese (0.90% ～ 1.20%). The addition of Si and Mn can improve the hardenability of steel. Therefore, medium and large bearings are generally made of GCr15SiMn steel. As Mn will increase the overheating sensitivity of steel, GCr15SiMn steel is easy to overheat during heat treatment and the grain is coarse, thus inducing quenching cracks. At the same time, during the grinding process, a large amount of grinding heat generated on the grinding surface will also make the surface grains coarse, causing grinding cracks. According to this material property, we adopt the process of "low temperature+long time" in the heat treatment of GCr15SiMn steel to control the grain growth of the steel during heating; In addition, measures such as uniform grinding, timely dressing of the grinding wheel and increasing the coolant flow are taken during the grinding process to reduce the generation of grinding heat and avoid grinding burns and cracks.

3 Effect of internal stress

Grinding cracks have a very important relationship with the stress state of the part itself, mainly including heat treatment stress and grinding stress.

3.1 Effect of heat treatment stress

The heat treatment of bearing parts is mainly to improve the hardness and wear resistance of the bearing by changing the internal structure of the steel, and ultimately improve the service life of the bearing. The heat treatment of bearing parts in China generally adopts overall quenching and low temperature tempering, which can obtain a mixed structure containing more than 80% martensite, 5%~15% residual austenite and 5%~10% residual carbide, thus obtaining good mechanical properties.

However, it can be seen from the structure of bearing ring that the thickness of each section of the same ring is not uniform, which inevitably leads to the uneven stress state of each part of the ring after heat treatment. Indeed, we checked the ferrules with grinding cracks, and the cracks were all at the stress concentration parts, such as the rib, oil groove, and some parts of the end face. Therefore, we adopt the method of increasing the tempering temperature and appropriately reducing the hardness of the ferrule to reduce the internal stress of heat treatment, so as to reduce the impact on the grinding cracks of the ferrule. (If possible, the traditional martensite quenching can be replaced by bainite isothermal quenching, with better effect). At the same time, when grinding these parts, the feed shall be uniform, and the feed rate shall not be too large, so as to avoid the superposition of heat treatment stress and grinding stress, resulting in grinding cracks.

3.2 Effect of grinding stress

According to the formation mechanism of grinding cracks, the grinding cracks are formed at a certain stage after rough grinding. This is because the grinding amount of rough grinding is large, accounting for 3/4 of the total grinding amount, and the grinding stress at this time reaches the maximum value. When the grinding stress exceeds the strength limit of the material, it will be released in some ways, such as microcracks. If the grinding stress is eliminated or reduced before microcracks occur, the possibility of microcracks can be greatly reduced. For this reason, we stipulate that a stabilizing treatment (also known as additional tempering) shall be added to medium and large rings after rough grinding, and this process must be carried out in time after rough grinding. In addition, in each commodity workshop, a pickling room is set up, and multiple magnetic particle flaw detectors are equipped to strengthen the pickling and flaw detection of rough grinding rings, so as to completely eliminate grinding burns and grinding cracks in the bud.

4 Conclusion

4.1 From the company's mass production practice, the most important reasons for grinding cracks are grinding heat and residual internal stress.

4.2 The bearing ring made of GCr15SiMn material is easier to produce grinding cracks than that made of GCr15 material. Measures shall be taken in heat treatment and grinding.

4.3 Measures to reduce the internal stress of heat treatment and grinding can effectively control the generation of grinding cracks on bearing rings.

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