1. Heat treatment deformation

During heat treatment, bearing parts have thermal stress and structural stress. This internal stress can be superimposed or partially offset each other, and is complex and changeable, because it can change with the changes in heating temperature, heating speed, cooling method, cooling speed, part shape and size, so heat treatment deformation is inevitable. Understanding and mastering its changing rules can put the deformation of bearing parts (such as the ellipse of the ring, the size expansion, etc.) within a controllable range, which is conducive to production. Of course, mechanical collisions during heat treatment will also cause deformation of parts, but this deformation can be reduced and avoided by improving operations.

2. Overheating

Overheating of the microstructure after quenching can be observed from the rough mouth of the bearing parts. However, to accurately judge the degree of overheating, the microstructure must be observed. If coarse needle-shaped martensite appears in the quenched structure of GCr15 steel, it is a quenched overheated structure. The cause of the formation may be the overall overheating caused by the quenching heating temperature being too high or the heating and holding time being too long; it may also be due to the serious banded carbides in the original structure, forming local martensite needles in the low-carbon zone between the two bands, causing local overheating. The residual austenite in the overheated structure increases and the dimensional stability decreases. Due to the overheating of the quenched structure, the crystals of the steel are coarse, which will lead to the decrease of the toughness of the parts, the decrease of the impact resistance, and the life of the bearing.

3. Underheating

If the quenching temperature is too low or the cooling is poor, the troostite structure exceeding the standard will be produced in the microstructure, which is called underheating structure. It reduces the hardness and wear resistance sharply, affecting the life of the bearing.

4. Soft spot

The phenomenon of insufficient local hardness on the surface of the bearing parts caused by insufficient heating, poor cooling, improper quenching operation, etc. is called quenching soft spot. Like surface decarburization, it can cause a serious decrease in surface wear resistance and fatigue strength.

5. Surface decarburization

During the heat treatment process, if the bearing parts are heated in an oxidizing medium, oxidation will occur on the surface, reducing the mass fraction of carbon on the surface of the parts, causing surface decarburization. If the depth of the surface decarburization layer exceeds the final processing allowance, the parts will be scrapped. The depth of the surface decarburization layer can be determined by metallographic method and microhardness method in metallographic inspection.

6. Quenching cracks

The cracks formed by internal stress in the quenching cooling process of bearing parts are called quenching cracks. The causes of this crack are: due to the quenching heating temperature being too high or the cooling being too rapid, the thermal stress and the organizational stress during the change of metal mass volume are greater than the fracture strength of the steel; the original defects of the working surface (such as surface fine cracks or scratches) or the internal defects of the steel (such as slag inclusions, serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) form stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient tempering or untimely tempering of parts after quenching; excessive cold stamping stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and corners of oil grooves, etc.

In short, the cause of quenching cracks may be one or more of the above factors, and the existence of internal stress is the main cause of quenching cracks.