The influence of bearing ring on surface quality during heat treatment is as follows:
1. Effect on residual stress on ferrule surface
The distribution of surface residual stress of bearing ring after quenching is greatly affected by cooling rate and quenching medium. For GCr15 bearing, after heating to 840C and quenching in oil, its axial stress and shear stress σ The distribution characteristics of T along the section are generally consistent and similar in size. The tensile stress is near the inner surface and the outer surface, while the compressive stress is at the center of the section. The increase of residual stress is approximately proportional to the thickness of ferrule. The residual stress of water quenching is 2 ~ 4 times larger than that of oil quenching. If the material and quenching process of ferrule are different, the distribution law of surface stress is different, or even the opposite.
2. Effect of on quenching crack of ferrule
There are two kinds of quenching cracks after quenching of bearing ring: deep crack and surface crack. Deep cracks are caused by stress related to temperature gradient; Surface cracks are related to surface decarburization. Other process factors that cause the above two kinds of quenching cracks are mainly due to the reduction of the brittle fracture strength of newly formed martensite due to the high quenching temperature. Increasing the quenching temperature will reduce the number of quenching cracks. If stabilization treatment is required, the ferrule should be cooled slowly to 60 ° C at least before strong cooling. On the contrary, removing the ferrule from the hot oil and cleaning it immediately will induce cracks; Even the mixture of a small amount of water in the quenching oil will significantly increase the risk of cracks; If the ferrule is re quenched without sufficient intermediate annealing or removal of decarburized layer, the possibility of crack generation will also be increased. The surface quenching crack caused by decarburization is largely related to the knife mark height causing stress concentration on the ferrule surface after machining. The greater the knife mark height before quenching, the longer the crack after quenching.
Whether turning or grinding, reducing the surface tool marks will reduce the depth of surface cracks. In grinding, even if the tool mark (wear mark) is relatively small, the possibility of quenching crack is much greater than that of cutting. This is because the radius at the bottom of the wear mark is relatively small and the stress concentration is large during quenching.
3. Effect on surface oxidation and decarburization
In heat treatment, oxidation and decarburization of ferrule surface are inevitable. The thickness of these oxidation and decarburization layers is called heat treatment metamorphic layer. The heat treatment metamorphic layer should be removed in the subsequent grinding process, which increases the grinding allowance, thus increasing more metal waste and grinding consumption. Therefore, the thickness of metamorphic layer should be minimized in the heat treatment process, which can be easily achieved by adopting the protective atmosphere heat treatment method.