Bearing steel is the most common material, bearing internal structure, uniformity of chemical composition, non-metallic inclusions contained, carbide particle size and distribution state, degree of decarbonization and other requirements are very strict, otherwise, these defects will significantly shorten the service life of bearing and material applicability.

1. The fatigue spalling of bearing parts is related to non-metallic inclusions in steel, especially oxides. This is because when the rolling bearing rotates, the contact surface of bearing parts is under the action of pulse load, the non-metallic inclusion causes the continuous local destruction of the steel matrix structure, resulting in the concentration of internal stress, plastic deformation around the non-metallic inclusion, and the formation of micro-cracks.

In addition, the content of gas (oxygen, nitrogen and hydrogen) in bearing steel is also an important indicator to measure purity. Dissolved oxygen precipitation decrease with temperature in the steel and aluminum, calcium, silicon, manganese and other elements to form the oxide inclusion, nitrogen in the form very diffuse aluminum nitride inclusions in steel and the thick titanium and carbon nitride, titanium nitride inclusions after the gathering of hydrogen atoms into molecular hydrogen produced tremendous pressure, once more than the ultimate strength of steel can cause inside crack, form a white dot. Reducing gas content is an important part of improving the purity of bearing steel.

In recent years, most of the main bearing steel manufacturers in China have built and put into operation ultra-high power electric arc furnace, using advanced eccentric furnace bottom steel extraction technology, secondary refining, so that the oxygen content in steel down to about 10×10-6, individual minimum oxygen content down to (4~3)×10-6. The secondary refining ratio also reached 100%. Therefore, the amount of inclusions in steel is well controlled.

2. The uniformity of bearing steel

The uniformity of bearing steel refers to the uniformity of chemical composition and carbide. Ingot structure, ingot weight, casting temperature and casting method affect the distribution of chemical components in steel. The heating process of steel ingot and billet before hot working, the end temperature of hot working and subsequent cooling method of steel, and spheroidizing annealing process affect the uniformity of carbide. According to the microstructure shape of carbide inhomogeneity in bearing steel. It can be divided into liquid phase carbides, strip carbides and network carbides. The harm of liquid phase carbides is similar to the inclusions in steel, and the fatigue life of steel can be reduced by 30% when the grade of strip carbides reaches grade 3-4. The life of bearing can be reduced by 1/3 when the network carbide increases by l grade, and the size of carbide particles directly or indirectly affects the life of bearing. The grades of liquidated carbides, banded carbides and reticulated carbides are the indexes to measure the uniformity of carbides.

3, steel dimensional accuracy and surface quality

Improving steel dimensional accuracy not only improves bearing material utilization rate, but also reduces unnecessary cutting. Some steel mills use short stress line high rigidity rolling mill, steel section size deviation greatly reduced, diameter of less than 30mm round steel, diameter deviation can reach about 0.3mm.

Bearing life and reliability are related to bearing structural design, materials used, hot and cold processing technology and equipment, installation and service conditions, among which materials and heat treatment is one of the main factors affecting bearing life and feasibility. Due to the influence of materials and heat treatment process, such factors as low material strength, poor purity, unreasonable orientation of metal material fiber structure, low hardness, improper forging ratio, large residual stress and so on can easily cause bearing damage. The influence of foreign steel quality on bearing life was about 50% in 50~60’s, and reduced to 1/3 in 80’s. China is currently affected by the process and equipment, bearing steel quality on bearing life of about 60% of the impact, so for bearing production enterprises, the selection of qualified, high-quality bearing materials is also one of the key factors to improve bearing life.

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