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This post was last edited by chwj01 on 2015-6-25 at 15:03. Six common quality defects in bearings after heat treatment Bearing, as a precision component in mechanical equipment, has a direct impact on the performance of such equipment; therefore, when using bearings, it is essential to meet certain performance requirements. To improve the performance of bearings, many manufacturers employ heat treatment processes. However, after such treatment, bearings often develop six common quality defects, which are shared below by China Standard Parts Network. 1. Overheating Overheating of the microstructure after quenching can be observed at the rough surfaces of the bearing parts. However, to accurately determine the degree of overheating, it is necessary to examine the microstructure. If coarse acicular martensite appears in the quenched structure of GCr15 steel, it is a overheated quenched structure. The cause may be overall overheating resulting from an excessively high quenching heating temperature or too long heating and holding time; it could also be local overheating caused by severe band-like carbides in the original microstructure, which leads to the formation of large, needle-shaped martensite particles in the low-carbon regions between these bands. The amount of retained austenite in the overheated tissue increases, resulting in a decrease in dimensional stability. Due to overheating of the quenched structure, the crystals in the steel become large, which leads to a decrease in the toughness of the parts, a reduction in their impact resistance, and a shorter lifespan for the bearings. Severe overheating can even cause quenching cracks. 2. Under-quenching: If the quenching temperature is too low or cooling is inadequate, a troostite structure that exceeds the standard specifications will form in the microstructure; this is known as under-quenched structure. It results in a decrease in hardness and a sharp reduction in wear resistance, thereby affecting the lifespan of bearings. 3. Quenching cracks Cracks that form in bearing parts during the quenching and cooling process due to internal stresses are known as quenching cracks. The causes of such cracks include: excessive quenching heating temperature or too rapid cooling, resulting in thermal stress and structural stress arising from changes in the metal’s mass and volume, which exceed the steel’s fracture resistance; 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 impurities, white spots, and residual shrinkage pores) that cause stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient or delayed tempering after quenching; as well as excessive cold shock stress from previous processing steps, forging folds, deep turning marks, and sharp edges from oil grooves. In summary, the causes of quenching cracks may be one or more of the factors mentioned above, with the presence of internal stress being the main reason for their formation. The quenching cracks are deep and elongated; the fracture surface is straight, with no oxidation color on the broken area. On the bearing rings, it is usually a longitudinal straight crack or a circumferential crack; on the bearing steel balls, its shape can be S-shaped, T-shaped, or ring-shaped. The microstructural characteristic of quenching cracks is the absence of decarburization on both sides of the cracks, which clearly distinguishes them from forging cracks and material cracks. 4. Thermal treatment deformation During heat treatment, bearing components are subject to thermal stresses and structural stresses. These internal stresses can either add up to each other or partially cancel each other out; the situation is complex and variable, as these stresses can change depending on factors such as the heating temperature, heating rate, cooling method, cooling rate, as well as the shape and size of the components. Therefore, thermal treatment deformation is inevitable. Understanding and mastering its variation patterns allows the deformation of bearing components (such as the elliptical shape of the rings or increases in size) to be kept within controlled limits, which facilitates the production process. Of course, mechanical impacts during the heat treatment process can also cause deformation in the parts, but this deformation can be reduced or avoided by improving the processing methods. 5. Surface decarburization: During the heat treatment of bearing components, if heating takes place in an oxidizing medium, oxidation occurs on the surface, resulting in a decrease in the carbon content in that surface layer and thus surface decarburization. If the depth of the surface decarburized layer exceeds the remaining amount after the final machining, the part will become unusable. The determination of the depth of the surface decarburized layer in metallographic inspection can be carried out using metallographic methods and microhardness testing. The measurement method based on the microhardness distribution curve of the surface layer can be used as an adjudicative criterion. 6. Soft spots: The phenomenon of insufficient local hardness on the surface of bearing parts, caused by inadequate heating, poor cooling, or improper quenching procedures, is known as quenching soft spots. It can cause a severe decline in surface wear resistance and fatigue strength, similar to surface decarburization. The original article is available at http://www.luosi99.com; please retain this source when reproducing it