Common quality defects of bearing parts after heat treatment
1. Overheating
Overheating of the microstructure after quenching can be observed from the rough mouth of the SFC bearing parts. But to accurately determine the degree of overheating, it is necessary to observe the microstructure. If coarse needle like martensite appears in the quenched structure of GCr15 steel, it is a quenched overheated structure. The possible cause may be excessive quenching heating temperature or prolonged heating and holding time leading to overall overheating; It may also be due to severe banded carbides in the original structure, resulting in local martensite needle like coarseness in the low-carbon zone between the two zones, causing local overheating. The residual austenite in the overheated tissue increases and the dimensional stability decreases. Due to the overheating of the quenched structure and the coarsening of the steel crystals, the toughness and impact resistance of the parts will decrease, and the life of the bearings will also be reduced. Severe overheating can even cause quenching cracks.
2. Underheating
If the quenching temperature is too low or the cooling is poor, it will produce a Trotskyite structure in the microstructure that exceeds the standard requirements, called undercooled structure. It will cause a decrease in hardness, a sharp decrease in wear resistance, and affect the service life of SFC bearings.
3. Quenching cracks
The cracks formed by internal stress during the quenching and cooling process of SFC bearing parts are called quenching cracks. The reasons for this type of crack include: due to excessive quenching heating temperature or rapid cooling, the structural stress caused by thermal stress and changes in metal mass volume is greater than the fracture strength of the steel; The original defects on the working surface (such as surface micro cracks or scratches) or internal defects in the steel (such as slag inclusions, severe non-metallic inclusions, white spots, residual shrinkage cavities, etc.) form stress concentration during quenching; Severe surface decarburization and carbide segregation; Insufficient or untimely tempering of parts after quenching; Excessive cold stamping stress, forging folding, deep turning tool marks, sharp oil grooves and corners caused by the previous process. In short, the causes of quenching cracks may be one or more of the above factors, and the presence of internal stress is the main reason for the formation of quenching cracks. The quenching crack is deep and slender, with a straight fracture surface and no oxidation color on the fracture surface. It often has longitudinal straight cracks or circular cracks on the bearing ring; The shape on the bearing steel ball can be S-shaped, T-shaped, or ring-shaped. The organizational characteristic of quenching cracks is the absence of decarburization on both sides of the crack, which is significantly different from forging cracks and material cracks.
4. Surface decarburization
During the heat treatment process of SFC bearing parts, if heated in an oxidizing medium, oxidation will occur on the surface, reducing the mass fraction of carbon on the surface of the parts and causing decarburization. If the depth of the decarburization layer on the surface exceeds the final processing allowance, the part will be scrapped. The determination of the depth of surface decarburization layer can be carried out using metallographic examination and microhardness method. The measurement method based on the microhardness distribution curve of the surface layer can be used as an arbitration criterion.
5. Heat treatment deformation
During heat treatment of SFC bearing components, there are thermal and structural stresses that can be superimposed or partially offset, making them complex and variable. This is because they can change with changes in heating temperature, heating rate, cooling method, cooling rate, component shape and size, making heat treatment deformation inevitable. Understanding and mastering its changing patterns can keep the deformation of bearing parts (such as the ellipse of the ring, size expansion, etc.) within a controllable range, which is beneficial for production. Of course, mechanical collisions during heat treatment can also cause deformation of parts, but this deformation can be reduced and avoided through improved operations.
6. Soft points
The phenomenon of insufficient local hardness on the surface of SFC bearing parts caused by insufficient heating, poor cooling, improper quenching operation, etc. is called quenching soft spot. It can cause a serious decrease in surface wear resistance and fatigue strength, just like surface decarburization.
