Bearing parts after heat treatment common quality defects : quenching microstructure overheating due to heat, quenching cracks , hardness is not enough , heat deformation , surface removal, soft points.
    1 . Overheating     From the mouth of the bearing parts coarse microstructure was observed after quenching overheating. But to determine the exact extent of overheating must observe the microstructure . If there is coarse acicular martensite steel GCr15 hardened tissue , the tissue was quenched overheating . Formation may be due to overheating overall quenching temperature is too high or too long resulting heat insulation ; may also be due to the original organization banded carbide serious , low carbon zone between the two with the formation of local martensite needles thick, caused by local overheating . Tissue overheating retained austenite increases, the size stability. Since the quenched structure overheated steel coarse crystals , the toughness of the parts will lead to decline , reducing the impact resistance, the bearing life is reduced . Overheating can cause serious or even quenching cracks .
    2 . Less heat     Quenching temperature is low or poor cooling will occur in the microstructure trust's tissue than standard , called less heat tissue, which decrease in hardness , wear resistance drastically reduced , affecting bearing life.
    3 . Quenching cracks     Bearing parts in the quenching process due to stress cracks formed within said quench cracks. There are reasons for this crack : As the quenching temperature is too high or too fast cooling , thermal stress and the stress changes in the volume of metal mass anti- breaking strength greater than steel ; previous work surface defects (such as cracks or surface micro- zoned marks ) or steel internal defects ( such as slag, serious non-metallic inclusions , white point , residual shrinkage , etc. ) formed during quenching stress concentration ; severe surface decarburization and carbide segregation ; insufficient tempering quenching or timely tempering ; cold- stress caused by the previous process is too large, forged folded , turning deep marks, sharp edges and other oil groove . In short , the quenching cracks may cause one or more of the above factors , there is the main reason for the formation of stress cracks quenching . Quenching cracks deep and slender, straight fracture , the fracture surface without oxidation colors. It is often in the longitudinal direction of the bearing ring or annular flat crack crack ; bearing ball shape on the S-shaped , T -shaped or ring . Organizational characteristics quenching cracks are cracks on both sides of decarburization phenomenon , significant differences with forging cracks and material cracks.
4 heat deformation     Bearing parts during heat treatment, the presence of thermal stress and stress , this stress can be superimposed on each other or partially offset , is complex because it along with the heating temperature , heating rate , cooling , cooling rate, part shape and changes in the size of the change, so the heat deformation is inevitable. Understanding and mastering its variation can deform the bearing parts ( such as elliptical rings , size up big , etc.) placed controllable range , were conducive to the production . Of course, the mechanical impact during the heat treatment also causes deformation of parts , but this deformation can be reduced to improve the operation and to avoid.
    5. Surface decarbonization     Bearing part during the heat treatment , if it is heated in an oxidizing medium, oxidation of the surface of the parts occurs in the surface carbon content reduced , causing surface decarburization . Depth over the last remaining amount of surface decarburization process will make parts scrapped . Determination of surface decarburization depth available in metallographic examination and microhardness metallographic method method. To the surface layer microhardness distribution curve measurements prevail , do arbitration criterion .
    6 soft spots     Due to insufficient heating , cooling bearing parts of the surface hardness of the local poor , improper quenching operation causes the phenomenon known as quenching soft enough points . It is the same as the surface decarbonization can cause a serious decline in surface wear resistance and fatigue strength.
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