1. overheat
Rough opening from the bearing parts can be observed microstructure after quenching overheating. But to determine the exact extent of overheating must observe the microstructure. If there is coarse acicular martensite in quenching GCr15 steels, for the quenching overheating organization. Causes may be general overheating quenching temperature is too high or too long due to heat insulation; it may be due to the original organization banded carbide severe in the low-carbon area between the two zones of coarse acicular martensite formation of local, causing localized overheating. Overheating tissues retained austenite increased dimensional stability decreased. Since quenching overheating, steel coarse crystals, the toughness of the parts will lead to decline, reducing the impact resistance, the bearing life is reduced. Even cause severe overheating quenching cracks.
2. Less heat
Quenching temperature is low or poor cooling will occur in the microstructure prop's body tissue than standard, known as the less heat tissue, which decrease in hardness, wear resistance drastically reduced, affecting bearing life.
3. quenching cracks
Bearing parts in the quenching process due to stress cracks formed within said quench cracks. There are reasons for this crack: As the quenching temperature is too high or too fast cooling, thermal stress and the stress metal mass volume change is greater than the resistance to fracture strength steel; original work surface defects (such as surface micro-cracks or draw marks) or steel internal defects (such as slag, serious non-metallic inclusions, white point, residual shrinkage, etc.) is formed on the stress concentration quenching; severe surface decarbonization and carbide segregation; inadequate parts after quenching and tempering or timely tempering; cold-stress caused by the previous process is too large, forged folded, turning deep marks, oil ditch sharp edges and the like. In short, the cause quenching cracks may be one or more of the above factors, the presence of internal stress is the main reason for the formation of quenching cracks. Quenching cracks deep and slender, straight fracture, broken section without oxidation colors. It is often in the bearing rings flat longitudinal cracks or ring cracking; shape on the ball bearings are S-shaped, T-shaped or ring. Organizational characteristics quenching cracks are cracks on both sides of decarburization phenomenon, a clear distinction with the forging cracks and the material cracks.
4. The heat deformation
Bearing parts during the heat treatment, there is thermal stress and stress, this stress can be superimposed or partially offset each other, is complex, because it along with the heating temperature, heating rate, cooling, cooling speed, the part shape and change the size of the change, so the heat deformation is inevitable. Understanding and mastering its variation can deform the bearing parts (such as elliptical rings, size up big, etc.) in a controlled range, there will be conducive to the production. Of course, mechanical collision during heat treatment also make parts deformation, but the deformation can be used to improve operations and reduce avoidable.
5. The surface decarbonization
Bearing parts during the heat treatment, if it is heated in an oxidizing medium, surface oxidation occurs the surface of the parts to reduce the carbon content, resulting in surface decarburization. Over the final machining allowance depth of surface decarburization layer will make parts scrapped. Determination of the depth of surface decarburization layer metallographic examination method available metallographic and microhardness law. In the surface layer microhardness distribution curve measurement prevail, do arbitration criterion.
6. soft spot
Due to insufficient heating, cooling bearing part surface hardness of local poor, improper operation causes hardening enough phenomenon called quenching soft spot. It is the same as the surface decarbonization can cause a serious decline in surface wear resistance and fatigue strength.
