What are the basic requirements for bearing materials?
The most common material for INA bearings is bearing steel. Bearings have strict requirements on their internal structure, uniformity of chemical composition, non-metallic inclusions, carbide particle size and distribution, and degree of decarburization. If these parts have defects, they will It will significantly shorten the service life of the bearing and reduce the applicability of the material. The requirements for bearing materials can be analyzed from the following three perspectives.
1. Purity of bearing steel
Fatigue spalling of bearing parts is related to non-metallic inclusions in steel, and the influence of oxides is particularly large. This is because when the rolling bearing is rotating, the contact surface of the bearing parts is under the action of the pulse load, and the non-metallic inclusions have continuous local damage to the steel matrix structure, causing internal stress concentration, plastic deformation around the non-metallic inclusions, and the formation of micro crack.
In addition, the amount of gas (oxygen, nitrogen, hydrogen) in the bearing steel is also an important indicator of purity. The oxygen dissolved in the steel precipitates with the decrease of temperature and forms oxide inclusions with aluminum, calcium, silicon, manganese and other elements. Nitrogen forms very dispersed aluminum nitride inclusions and coarser titanium nitride and titanium carbonitride in the steel. Inclusions, when the aggregated hydrogen atoms combine to form hydrogen molecules, a great pressure is generated. Once the strength limit of the steel is exceeded, internal cracks will occur and white spots will be formed. Reducing the gas content is an important part of improving the purity of bearing steel.
2. Uniformity of bearing steel
The uniformity of bearing steel refers to the uniformity of chemical composition and the uniformity of carbides. Steel ingot structure, ingot weight, casting temperature, ingot casting method, etc. affect the distribution of chemical components in steel. The heating process of steel ingots and billets before hot working, the termination temperature of steel hot working and subsequent cooling methods, and the spheroidizing annealing process affect the uniformity of carbides. According to the shape distribution of the carbide inhomogeneity in the bearing steel on the microstructure and the reason for its formation, it can be divided into liquefied carbides, banded carbides and network carbides. The harmfulness of liquefied carbides is equivalent to that of steel. In the inclusions, the band-shaped carbide rating of 3 to 4 can reduce the fatigue life of the steel by 30%. An increase of the network carbide by one level can reduce the life of the bearing by 1/3, and the size of the carbide particles directly or indirectly affects the life of the bearing. The rating levels of liquefied carbides, banded carbides, and networked carbides are indicators to measure the uniformity of carbides.
3. The dimensional accuracy and surface quality of steel
Improving the dimensional accuracy of steel can not only increase the utilization rate of bearing materials, but also reduce unnecessary cutting. Some steel mills use short-stress line and high-rigidity rolling mills, and the deviation of steel cross-section dimensions is greatly reduced. For round steel with a diameter of less than 30mm, the diameter deviation can reach about 0.3mm.
Due to the influence of materials and heat treatment processes, factors such as low material strength, poor purity, unreasonable orientation of the fiber structure of metal materials, low hardness, inappropriate forging ratio, large residual stress and other factors can easily damage the bearing. The impact of foreign steel quality on bearing life accounted for about 50% in the 1950s and 1960s, and it was reduced to 1/3 in the 1980s. In my country, currently affected by technology and equipment, the quality of bearing steel affects about 60% of bearing life. Therefore, for bearing manufacturers, selecting qualified and high-quality bearing materials is one of the key factors to improve bearing life.