NSK bearing parts common quality defects in the heat treatment process are: to quench the overheated microstructure, less heat, quenching cracks, insufficient hardness, heat distortion, surface decarburization, the soft spot.

1. Overheating

 FAG Bearings overall parts rough mouth can be observed in the microstructure after quenching overheating. But to determine exactly the degree of overheating must observe the microstructure. GCr15 Steel Quenching coarse acicular martensite was quenched overheating organization. Formation may be due to the quenching temperature is too high or the heating and heat preservation time is too long resulting in a comprehensive overheating; may also be banded carbide due to the original organization is serious, the formation of local coarse martensite needle between the two with a low carbon zone caused by local overheating. The overheating organization of retained austenite increased, decreased dimensional stability. Due to overheating of the quenched structure of the crystal thick steel parts toughness decrease, impact resistance reduced bearing life is also reduced. Overheating of serious and even cause quenching cracks.

2 subcooled

 Low quenching temperature or cooling the bad will be in the microstructure's tissues more than the standard requirements of care, known as the Organization of the subcooled, it is the decrease in hardness, wear resistance dramatically reduce the impact of NTN Bearing life.

(3) quenching cracks

SKF bearing parts in the quenching process due to internal stress crack formation that quenching cracks. The reason for this crack: the quenching temperature is too high or cooling too fast, the stress of heat stress and metal quality volume change anti-breaking strength greater than steel; the original defect of the working surface (such as surface micro-cracks or scratches) or steel internal defects (such as slag, a serious non-metallic inclusions, white spots, shrinkage and residual, etc.) in quenching the formation of stress concentration; serious surface decarburization and carbide segregation; parts after quenching and tempering or timely tempering; cold punch stress caused in the previous process is too large, forged folded, deep turning marks, sharp edges and corners of the oil groove technical requirements. In short, the cause quenching cracks may be due to one or more of the above factors, the presence of internal stress is the main reason for the formation of quenching cracks. Quenching cracks are deep and slender, the fracture straight fracture surface oxide color. It bearing rings are often vertical straight crack or ring cracking; the shape of the S-shaped, T-shaped or ring of the bearing on the ball. The organizational features of quenching cracks are cracks on both sides of decarburization phenomenon, the significant difference between forging cracks and material crack.

(4) heat treatment distortion

NACHI bearing parts during heat treatment, the existence of thermal stress and stress within this stress can stack with each other or partially offset, is complex, because it along with the heating temperature, heating rate, cooling method, cooling rate, parts changes in shape and size changes, so the heat treatment distortion is inevitable. Understanding and mastery of its variation can make the deformation of the bearing parts (such as ring oval, size up, etc.) placed in a controlled range conducive to the production. Mechanical collision course, during the heat treatment can also cause parts distortion, the deformation can be used to improve operations to reduce and avoid.

(5) surface decarburization

INA Bearing parts in the process of heat treatment, if it is heated in the oxidizing medium, surface oxidation occurs so that parts of the surface carbon content decreased, resulting in surface decarburization. Surface decarburized layer depth over the last processing allowance will be scrapped parts. Determination of surface decarburization depth metallographic method available in the metallographic examination and microhardness method. Surface Microhardness distribution curve measurement of law prevail, do arbitration Criterion.

6 soft spot

IKO bearings parts surface local hardness is not enough due to lack of heating, cooling adverse quenching improper operation caused by the phenomenon known as hardening the soft spot. It is the same as the surface decarburization can cause a serious decline of the surface wear resistance and fatigue strength.


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