Basic Requirements For Metallurgical Quality Of Bearing Steel

The service life and reliability of rolling bearings are closely related to the metallurgical quality of the bearing steel. Due to the characteristics of bearing steel, the requirements for metallurgical quality are much stricter than those of general industrial steel.

  1.Strict chemical composition requirements

 Generally, the steel used for rolling bearings is mainly high-carbon chromium bearing steel, which is hypereutectoid steel containing about 1% carbon, adding about 1.5% chromium, and containing a small amount of manganese and silicon elements. Only by strictly controlling the chemical composition of bearing steel can the structure and hardness meet the performance of the bearing through the heat treatment process.

  2.Higher dimensional accuracy

 For rolling bearing steel, the steel requires high dimensional accuracy because most bearing parts must be formed by pressure processing. In order to save materials and improve labor productivity, most bearing rings are forged, steel balls are cold forged or hot forged, and small-sized rollers are also cold forged. If the dimensional accuracy of the steel is not high, it will Unable to accurately calculate the cutting size and weight, the product quality of bearing parts cannot be guaranteed, and it is easy to cause equipment failure.

 Damage to equipment and molds.

    3.Exceptionally stringent purity requirements

 Purity in steel refers to the amount of non-metallic inclusions contained in the steel. The higher the purity, the lower the content of non-metallic inclusions in the steel. Harmful inclusions such as oxides, silicates, and point-like non-deformable inclusions in bearing steel are the main reasons for early fatigue spalling of bearings and significantly reducing bearing life. Moreover, brittle inclusions are easy to peel off from the metal matrix during the grinding process, seriously affecting the surface quality of bearing parts after finishing. In order to improve the service life and reliability of bearings, the content of non-metallic inclusions in bearing steel must be reduced.

    4.Strict low-magnification tissue and microscopic (high-magnification) tissue requirements

 The low-magnification structure of bearing steel refers to general porosity, central porosity and segregation. The microscopic (high-magnification) structure includes the annealed structure of bearing steel, carbide network, band and liquid segregation, etc. The quality of low and high magnification structures has a great impact on the performance and service life of rolling bearings. Therefore, there are strict requirements for low and high magnification structures in bearing steel material standards.

    5.Specially stringent requirements for surface defects and internal defects

For bearing steel, surface defects include surface cracks, surface slag inclusions, burrs, folds, scars, oxide scale, etc.; internal defects include shrinkage cavities, bubbles, white spots, overburning, severe porosity and segregation, and microscopic pores. wait. These defects have a great impact on bearing processing, bearing performance and service life. It is expressly stipulated in the bearing steel material standards that these defects are not allowed.

    6.Specially stringent carbide heterogeneity requirements

In bearing steel, if carbides are unevenly distributed, it is easy to cause uneven structure and hardness during heat treatment. In areas where carbides are less distributed, a martensite needle-like structure is formed and the hardness is low. Because in the “Technical Conditions for Heat Treatment of Rolling Bearing Parts” there are strict requirements for the structure, hardness and hardness uniformity after heat treatment, and the uneven carbide can easily cause cracks in bearing parts during quenching and cooling, and the uneven carbide It also leads to a reduction in bearing life.

    7.Specially stringent surface decarburization layer requirements

There are strict regulations on the surface decarburization layer of steel in the bearing steel material standards. If the surface decarburization layer exceeds the scope of the standard and is not completely removed during the processing before heat treatment, it will easily cause damage during the heat treatment and quenching process. Quenching cracks occur, causing parts to be scrapped.