1) Heat treatment deformation
When bearing parts undergo heat treatment, thermal stress and tissue stress are created, resulting in internal stress. This stress can be either superimposed or partially offset, making it complex and variable. Heat treatment deformation is inevitable and can vary with heating temperature, heating rate, cooling method, cooling rate, part shape, and size.
By understanding and mastering the law of change, deformation of the bearing parts (such as the ellipse of the ferrule or the size of the ferrule) can be controlled within a certain range, making it beneficial to production. Mechanical collisions during heat treatment can also cause deformation of the part, but this deformation can be minimized and prevented through improved operation.
Evidence of microstructure overheating after quenching can be observed from the rough mouth of the bearing part. However, the precise microstructure must be examined to determine the extent of overheating.
If coarse needle-shaped martensite is present in the quenched structure of GCr15 steel, it has been quenched and superheated. This formation may be caused by excessive heating during the quenching heating process, excessive heating and holding time, or severe original structure of the banded carbide. This may result in localized martensite needle-like coarseness in the low carbon zone between the two zones, leading to local overheating.
The superheated structure results in an increase in retained austenite, leading to decreased dimensional stability. Overheating of the quenched structure causes the steel’s coarse crystals, which lowers the toughness of the part and reduces impact resistance, and shortens the life of the bearing.
A low quenching temperature or poor cooling can result in the formation of a tortite structure in the microstructure that exceeds the standard, known as an underheated structure. This can reduce the material’s hardness and significantly decrease its wear resistance, ultimately impacting the bearing’s lifespan.
4) The soft point
The phenomenon of inadequate surface hardness in bearing parts resulting from insufficient heating, poor cooling, improper quenching operation, and other factors is known as quenching soft spot.
Surface decarburization can significantly reduce surface wear resistance and fatigue strength.
5) Surface decarburization
During the heat treatment process of bearing parts, if they are heated in an oxidizing environment, the surface may become oxidized, reducing the mass fraction of carbon on the part’s surface. This can result in surface decarburization, which can lead to the part being scrapped if the depth of the decarburization layer exceeds the allowable limit.
To determine the depth of the surface decarburization layer, the metallographic method and the microhardness method can be used in the metallographic examination.
6) Quenching crack
A crack that occurs in the bearing part during the quenching and cooling process due to internal stress is called a quenching crack. Several factors contribute to the formation of these cracks, including:
- The quenching heating temperature being too high or the cooling process being too rapid, causing the microstructure stress to exceed the fracture strength of the steel due to thermal stress and changes in the metal mass volume.
- Defects in the working surface or internal steel, such as surface cracks, scratches, slag inclusions, severe non-metallic inclusions, white spots, or shrinkage residuals, which create stress concentration during quenching.
- Severe surface decarburization and carbide segregation.
- Inadequate tempering or quenching of parts after quenching.
- Excessive cold punching stress resulting from previous processes such as forging and folding, deep turning tool marks, sharp edges, and corners of oil grooves.
In summary, the formation of quenching cracks may result from one or more of the above factors, with internal stress being the primary cause.