Common defects in heat treatment of bearing parts
Common quality defects of TIMKEN bearing parts after heat treatment are: quenching microstructure overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots, etc.
Overheating Overheating of the microstructure after quenching can be observed from the roughness of the bearing parts. 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 may be the overheating caused by the quenching heating temperature being too high or the heating and holding time too long, or it may be caused by the severe 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. 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.
Underheating, 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 bearing.
Quenching cracks, cracks formed by internal stress in the quenching and cooling process of 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 fast, 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 on the working surface or the internal defects of the steel are quenching Stress concentration, severe surface decarburization and carbide segregation, insufficient tempering or not timely tempering of parts after quenching, excessive cold punching stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and corners of the oil groove Wait.
Quenching cracks may be caused by one or more of the above factors, and the existence of internal stress is the main reason for 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, and the shape on the bearing steel ball is S-shaped, T-shaped or ring-shaped. The structural feature of quenching crack is that there is no decarburization on both sides of the crack, which is obviously different from forging crack and material crack.
Heat treatment deformation, bearing parts have thermal stress and structural stress during heat treatment. This internal stress can be superimposed or partially offset by each other, which is complex and changeable, because it can vary with heating temperature, heating rate, cooling method, and cooling rate. , The shape and size of parts change, so heat treatment deformation is inevitable. Recognizing and mastering its changing law can make the deformation of bearing parts 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.