Common quality defects of bearing parts after heat treatment include: quenching microstructure overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots, etc.
Overheat
Overheating of the microstructure after quenching can be observed from the rough mouth of the bearing part. 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 coarse crystal of the steel causes the toughness of the part to decrease, the impact resistance is lowered, and the life of the bearing is also lowered. Excessive heat can even cause quenching cracks.
2. Underheat
If the quenching temperature is low or the cooling is poor, the tortite structure exceeding the standard is formed in the microstructure, which is called the underheated structure, which lowers the hardness and sharply reduces the wear resistance, which affects the bearing life.
3. Quenching crack
The crack formed by the internal stress of the bearing part during the quenching and cooling process is called quenching crack. 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 annular crack on the bearing ring; the shape on the 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.
4. Heat treatment deformation
When the bearing parts are heat treated, there are thermal stresses and tissue stresses. These internal stresses can overlap or partially cancel each other. It is complex and variable because it can follow the heating temperature, heating speed, cooling method, cooling speed, and part shape. And the change in size changes, so heat treatment deformation is inevitable. Knowing and mastering the law of change can make the deformation of the bearing parts (such as the ellipse 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.
5. Surface decarburization
During the heat treatment process of the 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.
6. Soft point
The phenomenon that the surface hardness of the bearing parts is insufficient due to insufficient heating, poor cooling, improper quenching operation, etc. is called quenching soft point. It can cause a serious drop in surface wear resistance and fatigue strength like surface decarburization.
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