Overheating of the microstructure after quenching can be observed on the rough mouth of FAG imported bearing parts. However, the exact microstructure must be observed to determine the degree of overheating. If coarse needle-shaped martensite appears in the quenched structure of GCr15 steel, it is quenched and superheated. The formation may be caused by excessive overheating of the quenching heating temperature or excessive heating and holding time; or it may be due to the serious banding of the original structure, forming a local martensite needle-like coarseness in the low carbon zone between the two zones. Local overheating caused. The retained austenite increases in the superheated structure, and the dimensional stability decreases. Due to the overheating of the quenched structure, the coarseness of the steel crystals will result in a decrease in the toughness of the parts, a decrease in the impact resistance, and a decrease in the life of the FAG bearings. Excessive heat can even cause quenching cracks
If the quenching temperature is low or the cooling is poor, the tortite structure exceeding the standard is produced in the microstructure, which is called the underheated structure, which reduces the hardness and the wear resistance sharply decreases, which affects the bearing life of the FAG imported bearing.
The cracks formed by FAG imported bearing parts due to internal stress during quenching and cooling are called quenching cracks. The causes of such cracks are: due to excessive heating temperature of quenching or too rapid cooling, the microstructure stress when the thermal stress and metal mass volume change is greater than the fracture strength of the steel; the original defects of the working surface (such as surface micro cracks or scratches) Trace) or steel internal defects (such as slag inclusions, severe non-metallic inclusions, white spots, shrinkage residuals, etc.) form stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient tempering after quenching of parts Or not tempered in time; the cold punching stress caused by the previous process is too large, forging and folding, deep turning tool marks, sharp edges and corners of the oil groove. 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 cracks. The quenching crack is deep and slender, the fracture is straight, and the fractured surface has no oxidation color. It is often a longitudinal straight crack or ring crack on the FAG imported bearing ring; the shape of the FAG imported bearing steel ball is S-shaped, T-shaped or ring-shaped. The microstructure of the quenching crack is that there is no decarburization on both sides of the crack, and the difference is obvious with forging cracks and material cracks.
FAG imported bearing parts have thermal stress and tissue stress during heat treatment. These internal stresses can be superimposed or partially canceled. It is complex and variable because it can follow heating temperature, heating speed, cooling method, cooling rate, The shape and size of the part change, so heat treatment deformation is inevitable. Knowing and mastering the changing rules can make the deformation of FAG imported bearing parts (such as the oval of the ferrule, the size of the ferrule, etc.) in a controllable range, which is conducive to the production. Of course, mechanical collisions during heat treatment can also deform the part, but this deformation can be reduced and avoided with improved handling.
In the heat treatment process of FAG imported bearing parts, if it is heated in an oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the surface of the part, resulting in surface decarburization. The depth of the surface decarburization layer exceeds the final processing allowance and the part is scrapped. Determining the depth of the surface decarburization layer The metallographic method and the microhardness method can be used in the metallographic examination. The surface layer microhardness distribution curve measurement method shall prevail and the arbitration criterion may be used.
The phenomenon that the surface hardness of FAG imported bearing parts is insufficient due to insufficient heating, poor cooling, improper quenching operation, etc. is called quenching soft point. It decarburizes the surface and can cause a serious drop in surface wear resistance and fatigue strength.
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