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The common quality defects of Koyo imported bearing parts after heat treatment are: quenching microstructure overheating, undercooling, quenching crack, insufficient hardness, heat treatment deformation, surface decarburization and soft spots.
1. Overheating
The microstructure overheating after quenching can be observed from the rough mouth of bearing parts imported from Koyo.
However, in order to correctly judge the degree of overheating, the microstructure must be observed. If coarse acicular Common Quality martensite appears in the quenched structure of GCr15 steel, it is quenched superheated structure.
The cause may be the overall overheating caused by too high quenching heating temperature or too long heating temperature time; It may also be due to the serious banded carbide in the original structure and
the formation of local martensite acicular coarseness in the low-carbon region between the two Common Quality bands, resulting in local overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases.
Due to the overheating of Common Quality quenching structure and the coarseness of steel crystal, the toughness and resistance of parts will be reduced, and the service life of Koyo imported bearings will also be reduced. Serious overheating may even cause quenching cracks.
2. Under heating
Low quenching temperature or poor cooling will produce troostite structure exceeding the standard in the microstructure, which is called underheated structure. It will reduce the hardness and sharply reduce the wear Common Quality resistance, and affect the service life of Koyo imported bearings.
The crack formed by internal stress during quenching and Common Quality cooling of Koyo imported bearing parts is called quenching crack. The reasons for this kind of crack are: the quenching heating temperature is too high or the cooling is too urgent. When the thermal stress and metal mass volume change,
the microstructure stress is greater than the fracture strength of steel; The original defects on the working surface (such as micro cracks or scratches on the surface) or internal defects of steel (such as slag inclusion, Common Quality serious non-metallic inclusions, white spots, shrinkage cavity residue, etc.)
form stress concentration during quenching; Severe surface decarburization and carbide segregation; Insufficient firepower or timely tempering of parts after quenching; Excessive cold punching stress, forging folding, deep turning knife marks, sharp edges and corners of oil hook caused by the previous process.
4. Heat treatment deformation
During heat treatment, Koyo imported bearing parts have Common Quality thermal stress and structural stress. This internal stress can be superimposed or partially offset each other, which is complex and changeable. Because it can change with the changes of heating temperature, heating speed, cooling method, cooling speed,
part shape and size, heat treatment deformation is inevitable. Knowing and mastering the change law can make the deformation of Koyo imported bearing parts (such as the ellipse of the sleeve, size increase, etc.) in a controllable range, which is conducive to the production.
Of course, mechanical collision during heat treatment will also cause deformation of parts, but this deformation can be reduced and avoided by improved operation.
5. Surface decarburization
During the heat treatment of Koyo imported bearing parts, if they are heated in oxidizing medium, the surface will undergo oxidation, reducing the mass fraction of carbon on the surface of the parts and causing surface decarburization. If the depth of surface decarburization layer exceeds the allowance of post-processing,
the parts will be scrapped. The depth of surface decarburization layer can be measured by metallographic method and microhardness method. Based on the published curve measurement method of surface microhardness, it can be used as an arbitration criterion.
6. Soft spot
The surface of Koyo imported bearing parts is locally hard due to insufficient heating, poor cooling, improper quenching operation and other reasons