common problem

Overheating Overheating of the microstructure after quenching can be observed from the rough mouth of the bearing parts. But to accurately judge the degree of its overheating must observe the microstructure. If coarse acicular martensite appears in the quenched structure of GCr15 steel, it is a quenched superheated structure. The reason for the formation may be the overall overheating caused by the quenching heating temperature is too high or the heating and holding time is too long; it may also be due to serious banded carbides in the original structure, forming local martensitic needle-like thick in the low-carbon area between the two bands, localized overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases. Due to the overheating of the quenched structure and the coarse crystals of the steel, the toughness of the parts will be reduced, the impact resistance will be reduced, and the life of the bearing will also be reduced. Severe overheating can even cause quenching cracks. If the underheated quenching temperature is too low or the cooling is poor, a tortenite structure exceeding the standard will be produced in the microstructure, which is called underheated structure. High quenching cracks or too rapid cooling, the thermal stress and the structural stress of the metal mass and volume change are greater than the fracture strength of the steel; the original defects of the working surface (such as surface microcracks or scratches) or the internal defects of the steel (such as slag inclusions) , serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) form stress concentration during quenching; severe surface decarburization and carbide segregation; parts are insufficiently tempered or not tempered in time after quenching; cold caused by previous processes Excessive punching stress, forging folding, deep turning tool marks, sharp edges and corners of oil grooves, etc. In short, the cause of quenching cracks may be one or more of the above factors, and the existence of internal stress is the main reason for the formation of quenching cracks. The quenching crack is deep and slender, the fracture is straight, and the fracture surface has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring; the shape on the bearing steel ball is S-shaped, T-shaped or annular. The organizational characteristics of quenching cracks are that there is no decarburization on both sides of the cracks, which is obviously different from forging cracks and material cracks. Heat treatment deformation NACHI bearing parts have thermal stress and tissue stress during heat treatment. This internal stress can be superimposed or partially offset, which is complex and changeable, because it can vary with heating temperature, heating speed, cooling method, cooling speed. , The shape and size of the parts change, so heat treatment deformation is inevitable. Knowing and mastering its changing law can make the deformation of bearing parts (such as the ellipse of the ferrule, the size increase, etc.) in a controllable range, which is beneficial to the production. Of course, the mechanical impact during the heat treatment will also deform the part, but this deformation can be reduced and avoided with improved operations. During the heat treatment process of surface decarburized bearing parts, if they are heated in an oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the surface of the parts, resulting in surface decarburization. The depth of the surface decarburization layer exceeds the allowance of final machining and the part is scrapped. Determination of the depth of the surface decarburization layer can be used in metallographic examination metallographic method and microhardness method. The measurement method of the microhardness distribution curve of the surface layer shall prevail, which can be used as the arbitration criterion. Insufficient soft spot heating, poor cooling, improper quenching operation and other reasons, the phenomenon of insufficient local hardness on the surface of roller bearing parts is called quenching soft spot. Like surface decarburization, it can cause a serious decrease in surface wear resistance and fatigue strength.