Basic requirements for smelting quality of bearing steel

The service life and reliability of rolling bearings are closely related to the quality of the steel used for bearing. Due to the characteristics of bearing steel, the quality of smelting is much stricter than that of general industrial steel, such as the chemical composition, purity, organization and uniformity of steel.

First, strict chemical composition requirements

Generally, the steel for bearing is mainly high carbon chromium bearing steel, that is, hypereutectoid steel containing about 1% of carbon, adding about 1.5% of chromium, and containing a small amount of manganese and silicon. Chromium can improve heat treatment performance, improve hardenability, uniformity of structure, tempering stability, and improve the rust resistance and grinding performance of steel. However, when the chromium content exceeds 1.65%, the retained austenite in the steel is increased after quenching, the hardness and dimensional stability are lowered, the carbide non-uniformity is increased, and the impact toughness and fatigue strength of the steel are lowered. For this reason, the chromium content in high carbon chromium bearing steel is generally controlled below 1,65%. Only by strictly controlling the chemical composition of the bearing steel can the microstructure and hardness satisfying the bearing performance be obtained through the heat treatment process.

Second, high precision size requirements

Steel for rolling bearings requires high dimensional accuracy of the steel because most of the bearing parts are pressure molded. In order to save materials and increase labor productivity, most of the bearing rings are forged, the steel balls are cold-rolled or hot-rolled, and the small-sized rollers are also cold-formed. If the dimensional accuracy of the steel is not high, the size and weight of the blank cannot be accurately calculated, and the product quality of the bearing parts cannot be guaranteed, and the damage of the equipment and the mold is easily caused.

Third, the strict requirements of purity

The purity of steel refers to the amount of non-metallic inclusions contained in the steel. The higher the purity, the less non-metallic inclusions in the steel. Harmful inclusions such as oxides and silicates in bearing steel are the main causes of early fatigue spalling of bearings and significant reduction in bearing life. In particular, brittle inclusions are the most harmful, because they are easily peeled off from the metal substrate during processing, which seriously affects the surface quality of the bearing parts after finishing. Therefore, in order to improve the service life and reliability of the bearing, it is necessary to reduce the content of inclusions in the bearing steel.

Fourth, strict low-magnification and microscopic (high-power) organizational requirements

The low-magnification structure of the bearing steel refers to general looseness, center looseness and flatness, and the microscopic (high-power) structure includes annealed structure of steel, carbide mesh, ribbon and liquid analysis. Carbide liquid is hard and brittle, and its hazard is the same as brittle inclusions. The reticulated carbide reduces the impact toughness of the steel and makes it unevenly organized, and is easily deformed and cracked during quenching. Banded carbides affect annealing and quenched tempered structures and contact fatigue strength. The advantages and disadvantages of low and high-magnification organizations have a great influence on the performance and service life of rolling bearings. Therefore, there are strict requirements for low and high-fold organization in bearing material standards.

5. Particularly strict surface defects and internal defect requirements

For bearing steel, surface defects include cracks, slag inclusions, burrs, crusting, oxide scales, etc. Internal defects include shrinkage cavities, bubbles, white spots, severe porosity and segregation. These defects have a great influence on the processing of the bearing, the performance and the life of the bearing, and it is clearly stated in the bearing material standard that these defects are not allowed.

Six, particularly strict carbide non-uniformity requirements

In the bearing steel, if the serious carbide distribution is uneven, the unevenness of the structure and hardness is easily caused during the heat treatment process, and the structural heterogeneity of the steel has a great influence on the contact fatigue strength. In addition, severe carbide inhomogeneity can easily cause cracks in bearing parts during quenching and cooling. Carbide inhomogeneity can also lead to reduced bearing life. Therefore, in the bearing material standard, the steel of different specifications is clear. special requirement.

Seven, particularly strict surface decarburization layer requirements

In the bearing material standard, there is strict regulation on the decarburization layer of the steel surface. If the surface decarburization layer is beyond the scope of the standard and is not removed in the process before the heat treatment, it is in the heat treatment quenching process. It is easy to produce quenching cracks and cause parts to be scrapped.

Eight, other requirements

In the bearing steel material standard, there are strict requirements on the smelting method of bearing steel, oxygen content, annealing hardness, fracture, residual elements, spark inspection, delivery status, and marking.