Core Tips: Common quality defects of KOYO imported
bearing parts after heat treatment include: quenching microstructure overheating, underheating, quenching cracks, insufficient hardness, heat treatment variants, surface decarburization, soft spots.
Common quality defects of KOYO imported bearing parts after heat treatment include: quenching microstructure superheat, underheating, quenching crack, insufficient hardness, heat treatment variant, surface decarburization, soft point.
Overheat
Overheating of the microstructure after quenching can be observed from the rough mouth of KOYO imported bearing parts.
However, in order to correctly judge the degree of overheating, it is necessary to observe the microstructure. 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 heating of the quenching heating temperature or the heating temperature is too long; or it may be due to the serious banding of the original structure, forming a local martensite needle-like coarse in the low carbon zone between the two zones. Cause local overheating. The retained austenite increases in the superheated structure, and the dimensional stability decreases. Due to the overheating of the quenched structure, the crystal of the steel is coarse, which leads to a decrease in the toughness of the part and a decrease in the impact resistance. The life of the KOYO bearing is also reduced. Excessive heat can even cause quenching cracks.
2. Underheat
Low quenching temperature or poor cooling will produce a tortite structure exceeding the standard in the microstructure, called underheated structure, which will reduce the hardness and sharply reduce the wear resistance, affecting the life of KOYO
bearings.
3. Quenching crack
The crack formed by KOYO imported bearing parts due to internal stress during quenching and cooling is called quenching crack. The causes of such cracks are: the heating temperature is too high due to quenching or the cooling is too fast. The thermal stress and the mass of the metal change the tissue stress greater than the fracture strength of the steel; the original defects of the working surface (such as micro cracks or scratches on the surface) or internal defects of the steel (such as slag inclusions, severe non-metallic inclusions) , white spots, shrinkage holes, etc.) stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient firepower or timely tempering after quenching; excessive cold stress caused by previous processes, forged folding, deep Turning tool marks, sharp edges and corners of oil hooks.
4. Heat treatment deformation
KOYO 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 laws can make the deformation of KOYO imported bearing parts (such as ellipse of the sleeve, large size increase, etc.) in a controllable range, which is conducive to the production. Of course, mechanical collisions during heat treatment can also cause deformation of the part, but this deformation can be reduced and avoided with improved operation.
