NSK bearings components after heat treatment by the common quality defects are: quenching microstructure overheating, owe hot, quenching cracks, hardness is not enough, heat treatment deformation, surface decarburization, soft points, etc.

1. Overheat

From FAG bearing parts rough mouth can be observed in hardened microstructure overheating. But to be exact judge its overheating degree must observe the microstructure. If in GCr15 steel quenching organization acicular martensites appear coarse, the quenching overheating organization. Formation reasons may be quench heating temperature is too high or heat insulation for too long a time of comprehensive cause overheating; May also be a primitive organization ribbon carbide serious, in between the two with low carbon area forming local martensite needle bulky, cause local overheating. Overheating remained in the organization of the austenite increase, the size stability decline. Because quenching organization overheating, steel crystals bulky, can lead to parts of the toughness was decreased, impact resistance is reduced, bearing life also reduced. Serious overheating can have even quenching cracks.

2. Owe hot

Quenching temperature is low or cooling bad will in the microstructure of the prescribed standards produced more than "austenite organization, called owe hot organization, it dropped hardness, wear resistance sharply reduced, influence NTN bearing life.

3. The quenching cracking

SKF bearing parts in the quenching process for internal stress by the formation of the crack says quenching cracks. The reason cause the crack: due to quench heating temperature is too high or cooling too quickly, thermal stress and metal quality volume change the organization when stress is more than steel resistance of the fracture strength; The original work surface defects (such as surface imperceptible crack or scratch) or steel internal defects (such as slag, serious non-metallic inclusions, white, shrinkage residual, etc) in the hardened form stress concentration; The surface of the serious decarburization and carbide segregation; Parts after quenching insufficient or not seasonable tempering; The cold front process caused by blunt stress, forging fold, deep turning point, oil groove sharp edges, etc. In short, cause the quenching cracking reason may be the above factors of one or more, the existence of the internal stress is the main cause of quenching cracks formation. Quenching cracking deep and slender, fracture with broken section without oxidation color. It is often the front-runners in bearing the longitudinal crack or flat ring crack; In the shape of the steel ball bearings on a S form, T shape or ring type. Quenching cracking organization characteristics of cracking is no on both sides of the decarburization phenomenon, the obvious difference and forging crack and material crack.

4. Heat treatment deformation

NACHI bearing parts in heat treatment, there are thermal stress and organization stress, the internal stress can stack with each other or partly offset by, is complicated and changeable, because it can with heating temperature, heating speed, the method of cooling, cooling speed, parts shape and size change and change, so the heat treatment deformation is unavoidable. Know and grasp the changing law of it can make the bearing parts of deformation (such as rings of elliptic, size up big, etc) in the controllable scope, be helpful for production. Of course in the heat treatment process of mechanical collision would also make parts deformation, but this kind of deformation is can use to improve operations to reduce or avoid.

5. The decarburization surface

INA bearing parts in heat treatment process, if it was in oxidizing medium heat, surface oxidation happens the parts in the quality of the surface carbon scores decline, resulting in the decarburization surface. The depth of the decarburization layer surface more than the last processing will make allowance for scrap parts. The surface of the decarburization layer depth in determination of metallographic composition metallographic method and available in the micro hardness method. Surface layer microhardness distribution curve to measure method shall prevail, to do the arbitration criterion.

6. Soft PM

Due to insufficient heating, cooling bad, quenching improper operation causes such as IKO bearing parts of partial surface hardness is not enough phenomenon known as quenching soft points. It is like surface as decarburization can cause fatigue strength and wear resistance of the surface of the serious decline.



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