Bearing coarse mouth can be observed in the microstructure after quenching overheating. But the exact judge its overheating degree must observe microstructure. If the coarse acicular martensite in quenching microstructure of GCr15 steel, is overheated tissue. Residual austenite increase overheated structure, size stability decreased. The quenching microstructure in the overheated, crystal thick steel, will lead to a decline in parts of the toughness, reduce the impact resistance performance, NSK bearings life also reduced.
Serious even can cause overheating quenching crack quenching temperature, low or poor cooling produces troostite tissue over standard in the microstructure, called owed hot organization, it makes the hardness decreases sharply, wear resistance, impact bearing life
Crack of bearing during quenching process due to internal stress formed by quenching crack. Causes of the cracks are: due to quenching heating temperature is too high or cooling too quickly, the thermal stress resistance to fracture strength and metal quality of volume change stress greater than steel; the original defects of working surface (such as surface fine crack or scratch) or internal defects of steel (such as slag, serious non-metallic inclusions white spots, etc.), shrinking hole remnant stress concentration is formed in the quenching; serious decarburization and carbide surface segregation; parts after quenching and tempering is inadequate or not timely tempering; previous process caused by cold stress is too large, forging folding, turning deep marks, sharp edges and corners, oil groove. In short, the causes of cracks caused by hardening may be one or more of the above factors, the existence of internal stress is the main reason for the formation of quenching crack. Quenching crack deep and long, straight fracture, fracture surface without oxide color. It is often the NSK bearing rings on the longitudinal crack straight or circular crack; in ball bearings on the shape of S shape, T shape or a ring. Quenching crack propagation characteristics of the organizations on both sides without decarburization phenomenon, the obvious difference and forging cracks and crack.
Bearing during the heat treatment, there are thermal stress and organizational stress, stress the overlapped or partially offset, is complex and changeable, because it can change with the change of heating temperature, heating rate, cooling method, cooling rate, the shape and size of the parts, so the heat treatment deformation is inevitable. Understand and grasp the regularity of deformation which can make the bearing (such as rings, the elliptic dimension expansion etc.) in the controllable range, is favorable to the production of. Of course, mechanical collision during heat treatment also makes the part deformation, the deformation can be reduced and avoided with improved operation.
NSK bearings in the process of heat treatment, if heating in oxidizing medium, surface oxidation occurs so that the quality of fractional parts of the surface carbon reduction, resulting in surface decarburization. The surface decarburized layer depth over the machining allowance will be scrapped parts. Determination of surface decarburized layer depth in the metallographic examination of available metallography and microhardness method. The microhardness distribution curve measurement method is accurate, can be used for arbitration criterion.
Due to insufficient heating, cooling, local hardness NSK bearing surface caused by improper quenching operation is not the phenomenon known as the quenching soft spot. Serious decline it to surface decarburization can cause surface wear resistance and fatigue strength.