Causes and countermeasures of bearing quenching cracks

- Jul 16, 2019-


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1. The main cause of quenching cracks in bearing parts

The brittle cracking of the bearing parts during the quenching process due to the internal stress generated is greater than the breaking strength of the material, called quenching crack. Internal stresses include tissue transformation stress and quenching and cooling thermal stress, which are the essential factors causing quenching cracks. Combined with the actual production of bearing parts, the causes of quenching cracks are more complicated, including material metallurgy defects (severe non-metallic inclusions, shrinkage residuals, hairline, severe carbide segregation, etc.), part structural defects (part wall thickness) Defects, oil grooves and sharp edges, etc.), defects between the pre-quenching process (forging, cold forming stress, deep tooling marks, etc.) and poor quenching process (quenching temperature is too high, poor cooling, surface Decarburization, quenching and rework process is not appropriate).

2. Morphological characteristics and analysis methods of quenching cracks in bearing parts

The quenching crack shape of the bearing parts is very irregular, some in the transverse direction, some in the longitudinal direction, some in the surface of the part in the shape of "S" or "Y", and some are cracked mesh. The depth of the quenching cracks is also different, but the depth is much larger than the grinding burn crack. Observing the fracture of the quenching crack, it can be found that there are oil, water and tempering colors on the fracture surface, and the uncontaminated fracture surface is clean and fine porcelain. Observed by a metallographic microscope, the crack spreads in a tearing manner, and the tail is tapered, generally distributed along the grain boundary. The main difference between quenching crack and forging crack and raw material crack is that there is no decarburization on both sides of the crack.