Defects in the heat treatment of imported FAG bearing components
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 edges of imported FAG 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. The cause may be general overheating resulting from an excessively high quenching temperature or an excessively long holding time; alternatively, it may be localised overheating caused by severe banded carbides in the original microstructure, leading to the formation of coarse, needle-like martensite in the low-carbon zones between the bands. In overheated microstructures, the amount of retained austenite increases, leading to a reduction in dimensional stability. As the quenched microstructure is overheated, the steel’s grain size becomes coarse, resulting in reduced toughness and impact resistance of the component, as well as a shorter bearing life. Severe overheating may even cause quenching cracks.
2. Under-hardening
If the quenching temperature is too low or cooling is inadequate, a structure containing more than the standard-specified amount of troostite will form in the microstructure; this is known as an under-hardened structure. It results in reduced hardness and a sharp decline in wear resistance, thereby affecting the service life of FAG imported bearings.
3. Quenching Cracks
Cracks formed in FAG imported bearing components during the quenching and cooling process due to internal stresses are referred to as quenching cracks. The causes of such cracks include: excessive quenching temperatures or overly rapid cooling, resulting in thermal stresses and structural stresses arising from volume changes in the metal that 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 inclusions, severe non-metallic inclusions, white spots, or shrinkage cavities) creating stress concentrations during quenching; severe surface decarburisation and carbide segregation; insufficient tempering or failure to temper promptly after quenching; excessive cold working 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 a combination 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 fractures; on bearing balls, they 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 can either combine or partially offset one another, creating a complex and variable situation, 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 FAG imported bearing components (such as the ovalisation of rings and dimensional expansion) to be kept within a controllable range, 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 and avoided through improved operational practices.
5. Surface Decarburisation
During the heat treatment of FAG imported bearing components, if heating takes place in an oxidising atmosphere, oxidation occurs on the surface, reducing the carbon content and resulting in surface decarburisation. If the depth of the decarburised layer exceeds the final machining allowance, the component must be scrapped. The depth of the decarburised layer can be determined during metallographic inspection using metallographic analysis and microhardness testing. The measurement method based on the microhardness distribution curve of the surface layer shall be taken as the standard and may serve as arbitration evidence.
6. Soft Spots
The phenomenon of insufficient local surface hardness in FAG imported 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.
