Quality issues with bearing components following heat treatment
The company mainly produces three core series of products: cylindrical roller bearings, self-aligning roller bearings, and thrust self-aligning roller bearings. The product can be adapted to multiple industrial fields such as metallurgical equipment, mining machinery, heavy machinery, engineering equipment, etc., and can meet the operational and load-bearing requirements of equipment under different working conditions.
SZ Bearings
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 surfaces 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 excessively high quenching temperatures or prolonged holding times; alternatively, it may be due to localised overheating caused by coarse, needle-like martensite forming in the low-carbon zones between bands of banded carbides in the original microstructure. In overheated microstructures, the proportion 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 troostite than specified in the standard 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 bearing life.
3. Quenching Cracks
Cracks formed in 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 microstructural stresses due to volume changes in the metal exceeding 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 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 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 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 and avoided through improved operational practices.
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 carbon content and resulting in surface decarburisation. If the depth of the decarburised layer exceeds the machining allowance for the final finish, the component will 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 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.