1. Forging folding crack. In the forging process, the surface is easy to form fold cracks due to uneven cutting, burrs, flares and other reasons. It is characterized by large cracks, irregular shape, easy to appear on the surface of forgings.
2. Overheating forging, material forging heating temperature is too high, holding time is too long will produce overheating, serious grain boundary oxidation or even melting. Microscopically, not only the surface metal grain boundary oxidation cracking, but also the metal internal composition analysis is serious, grain boundary also began to melt, severe form of sharp cave. Overheated material forged in this defect state, by heavy hammer forging, punching, rolling, expansion, defects will further tear, forming larger defects, forging seriously overheated surface morphology, such as orange peel, distributed with small cracks and thick oxide skin.
Two, quenching crack.
In the quenching process of forgings, because the quenching temperature is too high or the cooling speed is too fast, when the internal stress is greater than the fracture strength of the material, the quenching crack will appear.
Three, grinding crack.
In the grinding process of bearing materials, due to the large feed of grinding wheel, runout of grinding wheel shaft, insufficient supply of cutting fluid, pure grinding grain of grinding wheel, grinding cracks are easy to occur in parts. In addition, the quenching temperature is too high in the heat treatment process, resulting in overheating of the parts, coarse grains, large residual Austenite volume, reticular and coarse grains.
4. Defects of raw materials.
The defects of raw materials include material cracks, shrinkage tube residue, white spots, decarbonization, inclusions, microscopic pores, steel plate lamination, etc. Common material cracks, such as the crack detection of the material, along the steel rolling direction distribution, a strip, some more, most of the surface cracks formed when rolling.