Common quality issues in bearing components following heat treatment

Common quality issues in bearing components following heat treatment

Common quality defects in bearing components following heat treatment include: overheating or underheating of the quenched microstructure, quenching cracks, insufficient hardness, heat treatment distortion, surface decarburisation and soft spots.

1. Overheating

Overheating of the quenched microstructure can be observed on the rough-machined surface of bearing components. However, to accurately determine the extent of overheating, the microstructure must be examined. If coarse needle-like martensite appears in the quenched microstructure of GCr15 steel, this indicates overheating during quenching. This may be caused by generalised overheating resulting from an excessively high quenching temperature or an excessively long holding time; alternatively, it may be due to localised overheating caused by the formation of coarse, needle-like martensite in the low-carbon zones between bands of banded carbides in the as-forged microstructure. Overheated microstructures contain an increased amount of retained austenite, resulting in reduced dimensional stability. As the quenched microstructure is overheated, the steel’s grain size becomes coarse, leading to a reduction in the component’s toughness and impact resistance, as well as a shortened bearing life. Severe overheating may even result in quenching cracks.

2. Under-hardening

If the quenching temperature is too low or cooling is inadequate, a proportion of troostite in the microstructure exceeding the standard specification will result. This is known as under-hardening, which reduces hardness, causes a sharp decline in wear resistance, and affects the service life of the bearing.

3. Quenching Cracks

Cracks formed in bearing components during the quenching and cooling process due to internal stresses are known as quenching cracks. The causes of such cracks include: when the quenching temperature is too high or cooling is too rapid, the thermal stresses and microstructural stresses resulting from changes in the volume of the metal exceed the steel’s fracture strength; pre-existing defects on the working surface (such as fine surface cracks or scratches) or internal defects in the steel (such as slag inclusions, severe non-metallic inclusions, white spots, or shrinkage cavities) that cause severe stress concentrations during quenching; severe surface decarburisation and carbide segregation; insufficient tempering or failure to temper the components in a timely manner after quenching; Excessive cold-forming stresses caused by preceding processes, forging folds, deep turning marks, or sharp edges on oil grooves. In summary, quenching cracks may result from one or more of the above factors, with the presence of internal stresses being the primary cause. Quenching cracks are deep and elongated, with straight fracture surfaces and no signs of oxidation. On bearing rings, they often appear as straight longitudinal cracks or annular cracks; on bearing balls, they may take the form of S-shaped, T-shaped or annular cracks. The microstructural characteristic of quenching cracks is the absence of decarburisation on either side of the crack, which clearly distinguishes them from forging cracks and material cracks.

4. Heat Treatment Deformation

During heat treatment, bearing components are subject to thermal stresses and microstructural stresses. These internal stresses may either combine or partially offset one another, resulting in a complex and variable pattern, as they vary with changes in heating temperature, heating rate, cooling method, cooling rate, and the shape and size of the component. Consequently, heat treatment deformation is inevitable. Understanding and mastering the patterns of these variations allows the deformation of bearing components (such as the ovalisation of rings and dimensional expansion) to be kept within controllable limits, thereby facilitating the production process. Of course, mechanical impacts during the heat treatment process can also cause deformation of the components, but such deformation can be reduced or avoided by improving operational procedures.

5. Surface Decarburisation

During the heat treatment of bearing components, if heating takes place in an oxidising atmosphere, oxidation occurs on the surface, reducing the mass fraction of carbon and resulting in surface decarburisation. If the depth of the decarburised layer exceeds the final machining allowance, the component will be scrapped. The depth of the decarburised layer can be determined during metallographic examination using metallographic analysis and microhardness testing. The measurement method based on the microhardness distribution curve of the surface layer serves as the definitive criterion and may be used as arbitration evidence.

6. Soft Spots

The phenomenon of insufficient local surface hardness in bearing components, caused by insufficient heating, poor cooling or improper quenching operations, is referred to as a quenching soft spot. Like surface decarburisation, this can lead to a severe reduction in surface wear resistance and fatigue strength.