Annealing hot treatment process

Annealing and heating treatment process overheating: the microstructure overheating after quenching can be observed from the rough mouth of bearing parts of idler accessories. But to determine exactly how hot it is, you need to look at the microstructure.

If there is acicular martensite in the quenched microstructure of GCr15 steel, it is quenched superheated microstructure. The reason may be that the quenching heating temperature is too high or the heating holding time is too long. It may also be due to the severe carbides in the original tissue and the formation of local martensite acicular in the low carbon zone between the two zones, resulting in local overheating. The residual austenite in overheated tissues increased and the dimensional stability decreased. Because the quenched structure is overheated, the steel crystal is thick, will cause the toughness of the parts to decline, the impact resistance to reduce, the life of the bearing is also reduced. Severe overheating can cause quenching cracks.

Underheat: if the quenching temperature is too low or the cooling is not good, the tortolitic structure exceeding the standard will be produced in the microstructure, which is called underheat structure. It will reduce the hardness and wear resistance, and affect the bearing life of idler accessories.

Quenching crack: the crack formed by internal stress in the cooling process of roller bearing is called quenching crack. The reasons for this kind of crack are as follows: because the quenching heating temperature is too high or the cooling is too rapid, the microstructure stress when the thermal stress and metal mass volume change is greater than the fracture strength of steel; The original defects on the working surface (such as fine cracks or scratches on the surface) or defects in the steel (such as slag inclusion, serious non-metallic inclusions, white spots, shrinkage residual, etc.) form stress concentration during quenching; Severe surface decarburization and carbide segregation; Insufficient tempering after quenching or not tempering in time; The cold drawing stress caused by the previous process is too large, forging folding, deep turning knife marks, sharp edges and corners of oil groove, etc.

In short, the cause of quenching crack may be one or more of the above factors, and the existence of internal stress is the main reason for the formation of quenching crack. The quenching crack is deep and long, the fracture is straight, and the broken section has no oxidation color. It is usually longitudinal flat crack or ring crack on the bearing ring; The shapes on the bearing steel balls are s-shaped, t-shaped or ring-shaped. Quenching crack is characterized by no decarburization on both sides of crack, which is different from forging crack and material crack.

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