Bearing steel quality requirements and defects

1. Quality requirements for bearing steel. Rolling bearings should work at high speed and for a long time under complex stress states such as tensile, compression, bending, shearing, alternating, and high stress values. Therefore, in the production process, there are many quality control inspection items for bearing steel, and the control scope is narrow. The production process is strict and complicated. It requires certain tooling equipment, inspection methods and a certain level of villain technology. In order to ensure good bearing performance and high life, the quality requirements for bearing steel are as follows:

(1) Chemical composition: Chemical composition is the most essential factor of bearing steel. The physical, chemical, mechanical properties and metallographic structure of steel are determined by chemical composition. Changing the chemical composition changes the basic properties of steel. Therefore, the chemical composition of the bearing steel must meet the allowable range of the standard.

(2) Internal quality: can be divided into macro quality and micro quality.

Macro quality: It is required that there are no defects such as white spots, shrinkage holes, slag inclusions, civilian metals, cracks, over-burning, and subcutaneous bubbles in the bearing steel. Internal segregation and loose control of the bearing steel are required to be within a certain range. In short, the inside of the bearing steel is dense, and it is not allowed to have a visible defect to cut the base of the steel.

Microscopic quality: It is required that the internal structure of the bearing steel should be uniform and the purity should be high.

The internal structure of the bearing steel refers to a carbide ribbon, a carbide network, a carbide liquid deposition, and an annealed structure. Carbide is one of the main components of bearing steel, and it exists objectively. How to make the distribution of carbides uniform, scattered and small is one of the important topics to improve the quality of bearing steel. In recent years, major manufacturers of bearing steel have adopted high-temperature diffusion treatment, controlled rolling new technology and continuous annealing furnace equipment. Even so, the distribution of carbides is still not as good. Therefore, in the standard, their allowable range and control level are specified.

The purity of the bearing steel refers to the degree of contamination of the steel by non-metallic inclusions. Non-metallic inclusions are the kind of material of the bearing steel matrix, which destroys the continuity of the matrix and is one of the main causes of early fatigue and spalling of the bearing. Therefore, it is required that the amount of non-metallic inclusions in the bearing steel is as small as possible. In order to limit and control the presence of non-metallic inclusions in steel, they are strictly controlled in the standard. In addition to electric furnace smelting and electric slag remelting, the manufacturer also adopts electric furnace smelting and refining, vacuum deoxidation, argon blowing treatment, and furnace powder spraying treatment, in an effort to reduce the oxygen content in the steel to less than 20 ppm.

(3) Surface quality: The forming method of bearing parts, currently forging, turning and cold punching. According to different molding methods, there are different requirements for the surface quality of steel. In general, the bearing steel surface must not have cracks, folds, cracks, crusting and slag inclusions. For the cold drawn steel used for cold punching, in addition to the above defects, the surface should be clean and free of defects such as rust and burrs. There must be no serious decarburization on the surface of the bearing steel. According to the different requirements of the molding process of the bearing parts, in the standard, there are different restrictions on the depth of the decarburization layer of different types of steel.

(4) Dimensional allowable tolerance: According to the molding process of the bearing parts and the production process of steel, the dimensional tolerances of various varieties and specifications of the bearing steel are specified in the standard. The dimensional tolerance of forged steel is generally in accordance with GB908-72 standard, the dimensional tolerance of hot rolled steel is according to GB702-86, the cold drawn steel is according to GB905-82 standard, and the cold drawn steel wire is according to YB245-64 standard.