overheat The overheating of the microstructure after qu […]
The overheating of the microstructure after quenching can be observed from the roughness of the bearing parts of the roller accessories. But to accurately judge the degree of overheating, it is necessary to observe the microstructure. If thick acicular martensite appears in the quenched structure of GCr15 steel, it is a quenched superheated structure. The cause of formation may be the overall overheating caused by the excessively high quenching heating temperature or the heating and holding time for too long; it may also be caused by the serious band-like carbides of the original structure, and the formation of local martensite needle-like coarseness in the low carbon zone between the two belts. Local overheating caused by. The retained austenite in the superheated structure increases, and the dimensional stability decreases. Due to the overheating of the quenched structure and the coarse steel crystals, 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 may even cause quenching cracks.
Low quenching temperature or poor cooling will produce troostite structure in the microstructure that exceeds the standard, called underheated structure, which reduces the hardness and the wear resistance sharply, which affects the life of the roller accessories.
The cracks formed by internal stress during the quenching and cooling process of roller bearing parts are called quenching cracks. The reasons for this kind of cracks are: because the quenching heating temperature is too high or the cooling is too rapid, the thermal stress and the structural stress of the metal mass volume change are greater than the fracture strength of the steel; the original defects of the working surface (such as surface micro-cracks or scratches) Marks) or internal defects of the steel (such as slag inclusions, serious non-metallic inclusions, white spots, shrinkage residuals, etc.) that form stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient tempering of parts after quenching Or not tempered in time; excessive cold punch stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and corners of the oil groove, 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 ring-shaped. The structural feature of quenching cracks is that there is no decarburization on both sides of the cracks, which is obviously different from forging cracks and material cracks.
Heat treatment deformation
During the heat treatment of NACHI bearing parts, there are thermal stress and structural stress. This internal stress can be superimposed or partially offset by each other, which is complicated and changeable, because it can change with the heating temperature, heating speed, cooling method, cooling speed, and parts. The shape and size change, so heat treatment deformation is inevitable. Recognizing and mastering its changing law can make the deformation of bearing parts (such as the ellipse of the ring, the size expansion, etc.) be placed in a controllable range, which is beneficial to the production. Of course, mechanical collisions in the heat treatment process will also cause deformation of the parts, but this deformation can be reduced and avoided by improved operation.
During the heat treatment process of roller accessories bearing parts, if they are heated in an oxidizing medium, oxidation will occur on the surface to reduce the mass fraction of carbon on the surface of the parts, resulting in surface decarburization. If the depth of the surface decarburization layer exceeds the final machining allowance, the part will be scrapped. The depth of surface decarburization layer can be measured by metallographic method and microhardness method in metallographic inspection. The surface layer microhardness distribution curve measurement method shall prevail, which can be used as the arbitration criterion.
Due to insufficient heating, poor cooling, improper quenching operation, etc., the phenomenon of insufficient local hardness on the surface of roller bearing parts is called quenching soft point. Like surface decarburization, it can cause a serious decrease in surface wear resistance and fatigue strength.