The hottest SKF aviation rolling bearing technolog

2022-09-26
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SKF aviation rolling bearing technology

I. new materials for aero engine bearings

1 M50nil bearing steel

during the 25 years from 1955 to 1980, the rotating speed of aero-engine bearings increased steadily, and the DN value has reached nearly 2.5 million. In the 1990s, the high speed and high temperature of aeroengine put forward higher requirements for rolling bearings. However, the existing bearing steel, even the special high-temperature bearing steel, such as M50, and various derived steels of 14cr-4mo family, such as crb-7 and gb-42, can still work normally at a temperature significantly higher than the current engine bearing. However, there is another important constraint, which is the fracture susceptibility of hardened steel bearing rings under ultra-high speed conditions. This kind of failure has little or no precursor

in order to find a bearing steel with better performance and fracture resistance than the existing M50 bearing steel, SKF MRC bearing company carried out a series of research work with the support of the U.S. air force, and finally chose m50nil

m50nil has good microstructure and fatigue strength compared with other high temperature quenched bearing materials, except that the fracture toughness is improved. One of the reasons is that there is no large particle carbide in m50il, so this steel is not sensitive to fatigue cracks caused by carbides

although the exploration of m50nil raw materials is easier than M50, and the rolling and forging of materials are more convenient, in order to obtain the desired ideal harden layer, core microstructure and certain material properties, quenching and heat treatment processes must be accurately controlled. SKF has made great efforts and invested a lot of money to study the m50nil treatment method. MRC technicians believe that this material can generate residual compressive stress near the raceway surface through heat treatment. Under the condition of high DN value, this stress area can offset the effect of circumferential stress, so as to improve the service life of bearings. Using SKF phase interaction control process, higher compressive stress can be obtained, and the hardening depth is three times higher than that of traditional process. SKF has used the traditional method to conduct overheating treatment test on m50nil, and the fracture toughness value of the material is 275~350mpa-m1/2. When the bearing speed reaches dn=3 million, it has good crack arrest characteristics. To improve the bearing speed and/or produce larger surface defects, the fracture toughness value must be close to 700mpa-m1/2. In order to improve the core toughness, SKF has developed a process that enables m50nil after heat treatment to obtain a specific core toughness without losing surface properties. SKF researchers found that this process can also improve the residual compressive stress, thereby further improving the performance and reliability of bearings. This process includes cooling the workpiece from the austenitizing temperature to a temperature between the core and the temperature at which the surface martensite begins to form, then heating the workpiece to a higher temperature and tempering the core before the surface layer (hardened layer) cools and transforms. By selecting the appropriate tempering cycle of the core, the core can be heat treated to the required toughness and strength without much influence on the surface properties. It has been proved that the core hardness should be controlled at 30~45hrc according to the selected core tempering temperature. This core toughness can only be obtained in cbs600 and prowear53 before. Now the heat treatment process of KSF makes m50nil with good high temperature resistance also have song's core toughness. In addition, SKF researchers also found that this surface hardened steel also has some characteristics of other cup experiments, which use a cylindrical punch to press the clamped disc-shaped sample into the specified die. First, surface treatment. Surface treatment such as nitrided iron (FCN) has a beneficial effect on m50nil, which can produce high-pressure stress zone (> 1000MPa) on the surface without carbide microstructure. It is expected that this treatment method can improve corrosion resistance, wear resistance and surface induced fatigue resistance. Second, weldability. Due to the low carbon content of m50nil, this material can be used to reduce the cost when it is necessary to connect the bearing with flanges or other similar components or materials to manufacture unit bearings and composite structural components

at present, the bearings made of m50nil are being tested or applied on 12 different aircraft engines in the world, and SKF company is in the leading position in the world

2. Ceramic materials

the gas turbine engine, which provides the aircraft with the convenient handling force of moving doors and covers, is extremely efficient and can make the aircraft speed reach more than Mach 3. The working conditions of the engine spindle bearing are very high. It is expected that the spindle speed will exceed 30000 rpm, and the maximum limit temperature of the bearing is about 800~900 ℃. It can be seen from the research that the use of superalloy materials at the working temperature above 650 ℃ has little hope for a long life, while ceramic materials bring hope for the working temperature of bearings to be significantly higher than 650 ℃

Through research, SKF has selected a group of high-performance ceramic materials that can meet the working requirements of ultra-high temperature bearings. Under high temperature conditions above 1100 ℃, one of these ceramic materials has the best performance, which is hot pressed silicon nitride or isobaric silicon nitride (Si3N4) developed in the past decade. Silicon nitride is an ideal material because it has good high temperature strength and hardness, as well as a favorable strength/weight relationship. When fully lubricated, it also has excellent rolling fatigue resistance. In 1984, SKF conducted a long-term test of this material with solid lubricant at a high temperature above 500 ℃ in the United States

however, silicon nitride also has disadvantages, including low tensile strength, poor crack arrest toughness and extremely low coefficient of thermal expansion. Therefore, a lot of research work needs to be done to manufacture and apply ceramic bearings

at present, SKF researchers are evaluating the applicability of some other ceramic materials such as silicon carbide (SIC), titanium carbide (TIC) and silicon aluminum oxide (Sialon) as ball and ferrule materials. SKF has used silicon carbide in bearing tests of 40000 rpm. Silicon carbide as a high-temperature bearing has good beneficial performance, but most of them are low-end products. Its thermal conductivity, thermal diffusivity and oxidation resistance, as well as the high purity of the material (there is almost no influence caused by impurities). One of its disadvantages is the high elastic modulus, which is about 50% higher than hot pressed silicon nitride. This is considered to be a potential problem, because it has the risk of generating high Hertz contact stress. SKF researchers have tried to reduce this effect by adjusting the curvature ratio of the raceway, but doing so will lead to the intensification of friction heating

3. Solid lubricant

due to the high calculated working temperature of aircraft engines in the future, the existing synthetic lubricants are powerless to effectively lubricate the bearings under such working conditions. As we all know, when the temperature exceeds the allowable limit of liquid lubricants, solid compounds with high temperature resistance and stable performance such as oxides, sulfides and fluorides containing various chemical elements can be used

many ordinary solid lubricants become effective lubricants because their lattice is easy to be broken, such as graphite and molybdenum disulfide. In air, the temperature limit range of graphite lubrication depends on its oxidation degree. By adding oxidants or metal salts, the oxidation limit can be significantly increased, so as to improve the performance of the graphite film on the bearing surface

The test of

skf shows that silicon nitride is lubricated with graphite containing high-temperature additives, and a layer of friction resistant chemical film can be formed on its surface, which can reduce the friction coefficient of silicon nitride and minimize the rise and fall of friction temperature

SKF also studied some other solid lubricants that can withstand high temperatures above 550 ℃, and their thermal stability is better than that of graphite and molybdenum disulfide. At present, MRC bearing company is testing lead oxide (PBO), a eutectic calcium fluoride/barium fluoride (caf/baf) mixture and a cesium molybdenum compound. A solid lubricant with high temperature resistance and good lubrication performance is expected to come out in the near future

II New heat treatment process of bearing materials

at present, the standard material of aero-engine bearings is high-speed steel with good fatigue strength at temperatures up to 300 ℃. However, when the bearing operates at high or ultra-high speed, the material properties such as fracture toughness, hardness, crack generation and propagation resistance cannot meet the requirements. In order to improve the material structure, improve the material properties, and meet the requirements of aircraft engine technology development for bearings, SKF studied three different bearing materials, and explained the relationship between the material structure and properties through experiments. These three materials are a cast hot rolled steel (M2), a powder metallurgy material (asp23) and a high-speed steel derived material with a carbon content of 0.65 (weight percentage) and low alloy element content

1. Fracture toughness

in aircraft engines, the fracture toughness of bearing materials is particularly important because the bearing speed is very high, resulting in strong centrifugal force

in hardened high-speed steel, the fracture process includes the appearance of cracks caused by initial carbide cracks, or the generation of cohesion in the elastic zone at the end of the crack caused by martensitic matrix structure. SKF research on these three materials shows that with the improvement of material toughness, the hardness of the material decreases, and the quenching temperature decreases. This is mainly due to the low martensite content and the improvement of the elasticity of the half life matrix structure. Most carbides in steel are hardened by low temperature, which has an adverse effect on fracture toughness. In the case of low hardness, due to the reduction of material yield strength, the difference between fracture plants of low hardness steel depends on the specific distribution of carbides. In the case of high hardness, the volume of carbides decreases, and the gap and elasticity between carbides are about the same. At this time, the fracture toughness no longer mainly depends on the distribution of carbides, and the elasticity of matrix structure is also the smallest. Therefore, the fracture toughness values of these three materials are concentrated in 750hv and higher hardness. In the case of low hardness and large elastic zone, the fracture toughness can be improved by predicting that the domestic waste paper import volume will decrease by 30% and 38% year-on-year respectively in 2019 and 2020, increasing the carbide diameter or reducing the ratio of carbide under the condition of constant hardness. Moreover, when the hardness increases, the change rate of fracture toughness decreases. When the elastic zone is small and the gap between carbides is large, the matrix structure has the greatest influence on the fracture toughness

in a word, the fracture toughness of high-speed steel mainly depends on the elasticity of matrix structure, which is largely affected by martensite and its carbon content. SKF researchers also pointed out that of course, the influence of other factors such as initial carbides, residual stress, residual austenite and carbides precipitated from martensite in tempering on fracture toughness cannot be excluded

2. Fatigue crack development process

SKF researchers divide the crack development process into the following stages:

in the first stage, the crack begins to expand. Although the critical stress intensity of high-speed steel can be changed by heat treatment, its absolute value does not change much, ranging from 3 to 5MPa/m2

second stage, crack propagation. The crack growth rate of high-speed steel is also similar, for example, at 10mpa/m2, it is about 5 × Mm/cycle. However, using steel with high toughness can slow down the speed of crack propagation

in the third stage, the crack expands rapidly. When the crack reaches this stage, the bearing will fail immediately

3. Surface treatment process

according to the research and analysis of the relationship between fracture toughness and others and the development process of material cracks, it is not difficult for us

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