With the high speed and light weight of the main engine, the working conditions of the bearing are more demanding, and the performance requirements of the bearing are higher and higher, such as smaller volume, lighter weight, larger carrying capacity, higher life and reliability. Sex and so on. Among them, the life and reliability of domestic bearings have become more and more prominent problems in recent years. The development of new heat treatment technologies and the improvement of heat treatment quality have always been the subject of concern for bearing manufacturers and related enterprises and institutions at home and abroad. This paper reviews the progress of heat treatment technology in recent years in the coming year, in order to draw lessons from relevant personnel in the bearing industry in China.
1. Annealing of high carbon chromium bearing steel
The ideal annealed structure of high carbon chromium bearing steel is a structure in which fine, small, uniform and round carbide particles are distributed on the ferrite matrix, and is prepared for later cold working and final quenching and tempering. At present, except for a few enterprises that use periodic equipment, a single-channel pusher type isothermal annealing furnace without a protective atmosphere is commonly used. The annealed microstructure and hardness control are relatively mature and reliable, and the annealed structure can be easily controlled to the 2nd to 3rd or fine point structure in the JB1255 standard. The problem is that the energy consumption is high and the oxidative decarburization is severe after annealing. In recent years, from the perspective of energy saving, the isothermal annealing furnace for oil-electric composite heating isothermal annealing furnace and double-chamber head-to-tail juxtaposition (horizontal or upper and lower) has been developed. The energy-saving effect is remarkable and should be promoted vigorously. At the same time, with the precision forming of the blank The emergence of processes and equipment began to use nitrogen-based protective atmosphere isothermal annealing furnace to reduce oxidative decarburization during annealing, reducing raw material consumption and machining costs.
2. Martensite quenching and tempering of high carbon chromium bearing steel
The development of conventional high carbon chromium bearing steel martensite quenching and tempering process is mainly divided into three aspects: First, the basic research on the influence of quenching and tempering process parameters on microstructure and properties, such as the structural transformation during quenching and tempering, Decomposition of retained austenite, toughness and fatigue properties after quenching and tempering; second, research on the performance of quenching and tempering, such as the influence of quenching conditions on size and deformation, dimensional stability, etc.; third, ban oxidation or protection The atmosphere is heated to promote controlled atmosphere heating.
2.1 Organization and performance
The microstructure after conventional martensite quenching is composed of martensite, retained austenite and undissolved (residual) carbide. After the quenching of the bearing steel, the martensite matrix has a carbon content of about 0.55%. The microstructure is generally a mixed structure of lath and flake martensite, or an intermediate form between the two - jujube nucleus martensite The so-called cryptocrystalline martensite and crystalline martensite in the bearing industry; its substructure is mainly dislocation entanglement and a small amount of twins. As the quenching temperature increases or the holding time prolongs, the morphology of the tissue gradually changes from cryptocrystalline to crystalline to fine needle-like. Generally, the normal structure after quenching is a mixture of cryptocrystals + crystals + fine needle-shaped martensite. Once a large number of distinct acicular martensites are present, the tissue is unqualified and should be avoided.
A great deal of research has been done at home and abroad on the effect of quenching and tempering on performance. The Luoyang Bearing Research Institute carried out the research on the heat treatment process of GCr15 steel in the 1980s. The results show that when the quenching heating is 835~865°C and the tempering is 150~180°C, the better comprehensive mechanical properties and contact fatigue life can be obtained. At 845°C quenching, the crushing load is the highest and the fatigue life is the longest. The tempering temperature is increased and the holding time is prolonged, the hardness is lowered, and the strength and toughness are improved. For parts with special requirements or use higher temperature tempering to increase the temperature of the bearing, or between -40~-78 °C between quenching and tempering to improve the dimensional stability of the bearing, or martensite Graded quenching to stabilize retained austenite results in high dimensional stability and high toughness. After quenching and heating, the bearing steel is subjected to short-time grading isothermal air cooling at 250 °C, followed by 180 °C tempering, or isothermal martensitic transformation (martensitic austempering) to obtain carbon concentration in the martensite after quenching. More uniform, increasing the amount of stable retained austenite, and impact toughness is doubled compared to conventional quenching and tempering.
At present, foreign bearings generally adopt the so-called individualized design, that is, the condition of the bearing before the supply of the bearing is investigated, the targeted design is carried out for the working condition, and the targeted requirements for the quality of the heat treatment are also put forward, which has the longest bearing life. . Domestic heat treatment requirements for bearing parts are uniformly controlled according to JB1255, which is relatively rough and non-personalized. The requirements for the bearing condition should be refined, and the hardness of the inner and outer rings and the rolling elements should be matched.
2.2 Development trend of conventional martensite quenching
At present, the conventional martensite quenching of bearing parts mostly uses continuous quenching equipment such as a chain furnace and a mesh belt furnace, and the indexes such as microstructure and hardness after quenching are easily controlled within a desired range. For such quenching process, the future development direction has the following two aspects:
2.3.1 Control of quenching deformation
The quenching heating equipment basically adopts a protective atmosphere or a controlled atmosphere, which can ensure no decarburization, or carbon or carburization as needed, so that the processing allowance after heat treatment can be greatly compressed. However, the compressibility of the machining allowance is often limited by the quenching deformation. At present, quenching deformation (especially distortion) becomes the main factor controlling the machining allowance; and for the ferrule of the sealed dustproof bearing, the quenching distortion will affect the pressing of the dust cover, thereby affecting the sealing performance. Therefore, reducing quenching distortion or achieving zero distortion will be a major problem to be solved in conventional martensitic quenching. Because of the many factors affecting the quenching distortion, the mechanism of deformation is more complicated. Therefore, each manufacturer should explore some effective measures to control distortion from the production practice according to various factors such as its own equipment and product characteristics. Such as controlling the placement of the workpiece, the oil input method, the quenching oil and the oil temperature, stirring, etc., to achieve less, no distortion quenching.
2.3.2 Residual stress and control and evaluation of retained austenite
In China's current thermal inspection standards, there are no limits on the residual stress and retained austenite. A large number of studies have shown that residual stress affects the contact fatigue properties, toughness and grinding cracks of the part. Appropriate residual compressive stress can improve contact fatigue life, prevent grinding and installation cracks; retained austenite reduces dimensional stability. The degree of influence is related to the stability, quantity and location of the retained austenite itself. However, an appropriate amount of retained austenite can improve fracture toughness and contact fatigue properties. A number of foreign famous bearing companies have included residual stress and retained austenite in heat treatment control indicators. Therefore, further study on the effects of residual stress and retained austenite on the properties after heat treatment and its mechanism, and study the effects of quenching and tempering process on residual stress and retained austenite, and then propose residual stress according to the bearing condition. The control index of retained austenite, etc., will be one of the main directions of heat treatment research in China's bearing industry.
3. Bainite austempering
Bainite austempering is a hot spot in the domestic bearing industry research in recent years. Since the 1980s, Luoyang Bearing Research Institute has cooperated with Chongqing Bearing Factory to start the application research of bainite austempering on railway bearings, and then carried out bainite austempering on rolling mill bearings with Shahe Rolling Mill Bearing Factory. The application research has achieved good results, and the recommended technical requirements for austempering of bainite are introduced in JB1255-1991. At the same time, the bearing industry has also begun the promotion and application of bainite austempering. With the help of the national “Eighth Five-Year†key enterprise technology development project “rail passenger car bearingâ€, the relevant units have carried out systematic research on the microstructure and performance of bainite austempering, and successfully applied to the production of quasi-high-speed railway bearings. In the revision of JB1255 in 2001, the technical content of bainite austempering was officially included in the standard formal provisions. The bainite quenching process has been widely used in rolling mills, locomotives, and iron passengers.
The outstanding characteristics of bainite structure are impact toughness, fracture toughness, wear resistance, dimensional stability, and surface residual stress is compressive stress. Therefore, it is suitable for assembling bearings with large interference and poor service conditions, such as railways, rolling mills, cranes and other bearings that are subjected to large impact loads, mine transportation machinery or mine loading and unloading systems with poor lubrication conditions, and bearings for coal mines. The high-carbon chromium bearing steel BL austempering process has been successfully applied in railway and rolling mill bearings, and achieved good results.
In the production of railway and rolling mill bearings, due to the large size and heavy weight of the ferrule, the martensite structure is brittle during oil quenching. In order to obtain high hardness after quenching, strong cooling measures are often taken, resulting in quenching microcracks; due to martensite After quenching, the surface is tensile stress. The superposition of grinding stress during grinding increases the overall stress level, which is easy to form grinding cracks and cause batch waste. When bainite is quenched, the bainite structure is much better than the M structure, and the surface is formed with compressive stress of up to -400 ~ -500MPa, which greatly reduces the tendency of quenching cracks [16]; The compressive stress counteracts part of the grinding stress, which reduces the overall stress level and greatly reduces the grinding crack.
SKF Company applies high-carbon chromium bearing steel bainite austempering process mainly to railway bearings, rolling mill bearings and bearings used under special working conditions, and has developed steel grades suitable for bainite quenching (SKF24, SKF25, 100Mo7). ). When quenching, a longer isothermal time is used, and after quenching, a full lower bainite structure is obtained. Recently, SKF has developed a new steel grade of 775V, and through a special austempering to obtain a more uniform lower bainite, the hardness after quenching increases while the toughness is 60% higher than the conventional austempering, and the wear resistance is improved. The treated ferrule wall thickness exceeds 100 mm.
The performance of the martensite/bainite composite structure after partial isothermal treatment is still controversial, such as the content of BL is the best. Even if there is an optimum content, how to control in the actual production, and the composite structure needs an additional temper after isothermal, increasing the production cost. In addition, as far as bainite austempering is concerned, although its process, organization and properties have been systematically studied, while promoting this process, attention should be paid to the limitations of the process. Not all bearing parts are suitable for shellfish. Austempering of austenite. The development of bainite austempering steel should also be carried out to further improve the performance of bainite after isothermal quenching; the development of heat treatment equipment for replacing nitrate salt isothermal, reduce environmental pollution and the like.
4. Special heat treatment
The high carbon chromium bearing steel is generally hardened by the whole, and the residual stress after quenching is the surface tensile stress state, which is easy to cause quenching crack and reduce the bearing performance. A special type of heat treatment is to increase the carbon and nitrogen content of the surface layer by carburizing, nitriding or carbonitriding of high carbon chromium bearing steel, reducing the Ms point of the surface layer, and forming a surface after the surface is changed during the quenching process. Compressive stress, improved wear resistance and rolling contact fatigue performance [17,18]. On the other hand, by a certain method, a certain amount of stable retained austenite is retained in the heat-treated bearing parts, and the edge effect of the indentation is reduced by the deformable retained austenite, so that the surface fatigue source originating from the indentation edge is obtained. It is not easy to form and expand, thus improving the contact fatigue life of the bearing under pollution conditions. Generally, the above object can be attained by controlling the carbon (nitrogen) potential of the atmosphere during quenching heating. NSK's NSJ2 steel [19] and KOYO's SH technology [20] were developed based on this theory.
Another special type of heat treatment is the use of high toughness carburized steel with a high carbon content (0.4%) and special carburizing or carbonitriding heat treatment. Firstly, the composition of the carburized steel is adjusted: the matrix carbon content is increased to improve the strength of the matrix while ensuring the toughness, and the Si and Mn contents are increased to improve the stability of the retained austenite, and Mo is added to refine the carbide. Carbonitride. Secondly, the carburizing or carbonitriding process is strictly controlled to obtain more retained austenite (about 30% to 35%) and a large amount of fine carbides and carbonitrides on the surface after the parts are processed. On the one hand, a large number of fine carbides and carbonitrides can ensure the hardness and wear resistance of the surface to make the indentation difficult to form; on the other hand, even if the indentation is formed, more stable retained austenite can reduce the edge effect. To prevent the formation and expansion of fatigue sources. Based on this theory, NSK and KOYO have developed TF series technology (HTF, STF, NTF) and KE technology respectively, which greatly improves the life of bearings under contaminated lubrication conditions. For example, the tapered roller bearings produced by NSK using HTF technology have a fatigue life of 10 times that of ordinary bearings under contaminated lubrication conditions [21]. Companies such as NSK use special heat treatment techniques in a variety of newly developed bearing products.
In recent years, Luoyang Bearing Research Institute has cooperated with relevant units to carry out research on special heat treatment process of high carbon chromium bearing steel, and also carried out research on special heat treatment process of medium carbon alloy steel. Preliminary results indicate that the contact fatigue life can be significantly achieved by special heat treatment. This process will have significant promotion value in the bearing industry and will become a hot technology for the research and application of the bearing industry in China.
5. Surface modification technology
5.1 ion implantation
The US Naval Laboratory has conducted research on ion implantation of bearing parts since 1979, and countries such as the United Kingdom, Denmark, and Portugal have been working similarly to the US Naval Laboratory since 1989. The results show that the injection of chromium ions can significantly improve the corrosion resistance of M50 steel, and the contact fatigue resistance is also improved. In addition, boron ions are added to improve the wear resistance of the instrument bearing; nitrogen plasma source is applied to bearing steel 52100. After ion implantation (PSII), a thin layer of nitride is formed on the surface to improve the corrosion resistance of the bearing steel and replace it with expensive stainless steel. Nitrogen and boron ion implantation on the SUS440C stainless steel ball bearing can reduce the micro-swing of the ball bearing. Vibration wear and dust emission of the bearing, in addition, (Ti + N) or (Ta + N) plasma immersion ion implantation (PSIII) on stainless steel can significantly improve its microhardness, wear resistance and life.
Since the 1980s, China's bearing industry has applied ion implantation on bearings, and has successfully applied it to aviation and aerospace, and achieved good results.
5.2 Surface coating
Surface coating techniques include: physical vapor deposition (PVD), chemical vapor deposition (CVD), radio frequency sputtering (RF), ion spray coating (PSC), electroless plating, and the like. Compared with CVD, PVD has a low temperature during processing, and does not need to be heat treated after plating. It is widely used in the surface treatment of bearing parts. Steel bearing parts such as 100Cr6 and 440C can improve the wear resistance, contact fatigue resistance and surface friction coefficient of bearing parts after PVD, CVD or RF plating of TiC, TiN, TiAlN hard film or MoS2 type soft film.
In recent years, SKF has developed two coating technologies: one is to use PVD to coat the surface of the bearing ring and the rolling element with diamond (Diamond-Like Carbon, DLC) with extremely high hardness. The surface hardness is higher than that of hardened bearing steel. 40%~80%, the friction coefficient is similar to PTFE or MoS2, it has self-lubricating characteristics, and it has good bonding with the substrate, no peeling, bearing life and wear resistance are greatly improved, and it can still work normally under the condition of oil cut. It is called “No Wear bearingâ€; the second is to use PSC to spray a layer of 100μm thick alumina on the outer circumference of the outer ring of the bearing, so that the bearing insulation capacity is up to 1000V, and the bearing has higher insulation by increasing the thickness of the alumina. ability. The coated alumina is firmly bonded to the substrate and improves the corrosion resistance of the bearing. The post-plated bearing (INSOCOATTM bearing) can be mounted like a normal bearing.
6. Conclusion
Throughout the development of bearing heat treatment technology at home and abroad, there is still a big gap between China's bearing industry and foreign developed countries' heat treatment technology, which seriously restricts the quality of China's bearings, especially the improvement of life and reliability. The entire bearing industry should pay attention to the research of heat treatment basic theory and new technology, and vigorously promote the application of research results in actual production to improve the heat treatment level in China as soon as possible.
1. Annealing of high carbon chromium bearing steel
The ideal annealed structure of high carbon chromium bearing steel is a structure in which fine, small, uniform and round carbide particles are distributed on the ferrite matrix, and is prepared for later cold working and final quenching and tempering. At present, except for a few enterprises that use periodic equipment, a single-channel pusher type isothermal annealing furnace without a protective atmosphere is commonly used. The annealed microstructure and hardness control are relatively mature and reliable, and the annealed structure can be easily controlled to the 2nd to 3rd or fine point structure in the JB1255 standard. The problem is that the energy consumption is high and the oxidative decarburization is severe after annealing. In recent years, from the perspective of energy saving, the isothermal annealing furnace for oil-electric composite heating isothermal annealing furnace and double-chamber head-to-tail juxtaposition (horizontal or upper and lower) has been developed. The energy-saving effect is remarkable and should be promoted vigorously. At the same time, with the precision forming of the blank The emergence of processes and equipment began to use nitrogen-based protective atmosphere isothermal annealing furnace to reduce oxidative decarburization during annealing, reducing raw material consumption and machining costs.
2. Martensite quenching and tempering of high carbon chromium bearing steel
The development of conventional high carbon chromium bearing steel martensite quenching and tempering process is mainly divided into three aspects: First, the basic research on the influence of quenching and tempering process parameters on microstructure and properties, such as the structural transformation during quenching and tempering, Decomposition of retained austenite, toughness and fatigue properties after quenching and tempering; second, research on the performance of quenching and tempering, such as the influence of quenching conditions on size and deformation, dimensional stability, etc.; third, ban oxidation or protection The atmosphere is heated to promote controlled atmosphere heating.
2.1 Organization and performance
The microstructure after conventional martensite quenching is composed of martensite, retained austenite and undissolved (residual) carbide. After the quenching of the bearing steel, the martensite matrix has a carbon content of about 0.55%. The microstructure is generally a mixed structure of lath and flake martensite, or an intermediate form between the two - jujube nucleus martensite The so-called cryptocrystalline martensite and crystalline martensite in the bearing industry; its substructure is mainly dislocation entanglement and a small amount of twins. As the quenching temperature increases or the holding time prolongs, the morphology of the tissue gradually changes from cryptocrystalline to crystalline to fine needle-like. Generally, the normal structure after quenching is a mixture of cryptocrystals + crystals + fine needle-shaped martensite. Once a large number of distinct acicular martensites are present, the tissue is unqualified and should be avoided.
A great deal of research has been done at home and abroad on the effect of quenching and tempering on performance. The Luoyang Bearing Research Institute carried out the research on the heat treatment process of GCr15 steel in the 1980s. The results show that when the quenching heating is 835~865°C and the tempering is 150~180°C, the better comprehensive mechanical properties and contact fatigue life can be obtained. At 845°C quenching, the crushing load is the highest and the fatigue life is the longest. The tempering temperature is increased and the holding time is prolonged, the hardness is lowered, and the strength and toughness are improved. For parts with special requirements or use higher temperature tempering to increase the temperature of the bearing, or between -40~-78 °C between quenching and tempering to improve the dimensional stability of the bearing, or martensite Graded quenching to stabilize retained austenite results in high dimensional stability and high toughness. After quenching and heating, the bearing steel is subjected to short-time grading isothermal air cooling at 250 °C, followed by 180 °C tempering, or isothermal martensitic transformation (martensitic austempering) to obtain carbon concentration in the martensite after quenching. More uniform, increasing the amount of stable retained austenite, and impact toughness is doubled compared to conventional quenching and tempering.
At present, foreign bearings generally adopt the so-called individualized design, that is, the condition of the bearing before the supply of the bearing is investigated, the targeted design is carried out for the working condition, and the targeted requirements for the quality of the heat treatment are also put forward, which has the longest bearing life. . Domestic heat treatment requirements for bearing parts are uniformly controlled according to JB1255, which is relatively rough and non-personalized. The requirements for the bearing condition should be refined, and the hardness of the inner and outer rings and the rolling elements should be matched.
2.2 Development trend of conventional martensite quenching
At present, the conventional martensite quenching of bearing parts mostly uses continuous quenching equipment such as a chain furnace and a mesh belt furnace, and the indexes such as microstructure and hardness after quenching are easily controlled within a desired range. For such quenching process, the future development direction has the following two aspects:
2.3.1 Control of quenching deformation
The quenching heating equipment basically adopts a protective atmosphere or a controlled atmosphere, which can ensure no decarburization, or carbon or carburization as needed, so that the processing allowance after heat treatment can be greatly compressed. However, the compressibility of the machining allowance is often limited by the quenching deformation. At present, quenching deformation (especially distortion) becomes the main factor controlling the machining allowance; and for the ferrule of the sealed dustproof bearing, the quenching distortion will affect the pressing of the dust cover, thereby affecting the sealing performance. Therefore, reducing quenching distortion or achieving zero distortion will be a major problem to be solved in conventional martensitic quenching. Because of the many factors affecting the quenching distortion, the mechanism of deformation is more complicated. Therefore, each manufacturer should explore some effective measures to control distortion from the production practice according to various factors such as its own equipment and product characteristics. Such as controlling the placement of the workpiece, the oil input method, the quenching oil and the oil temperature, stirring, etc., to achieve less, no distortion quenching.
2.3.2 Residual stress and control and evaluation of retained austenite
In China's current thermal inspection standards, there are no limits on the residual stress and retained austenite. A large number of studies have shown that residual stress affects the contact fatigue properties, toughness and grinding cracks of the part. Appropriate residual compressive stress can improve contact fatigue life, prevent grinding and installation cracks; retained austenite reduces dimensional stability. The degree of influence is related to the stability, quantity and location of the retained austenite itself. However, an appropriate amount of retained austenite can improve fracture toughness and contact fatigue properties. A number of foreign famous bearing companies have included residual stress and retained austenite in heat treatment control indicators. Therefore, further study on the effects of residual stress and retained austenite on the properties after heat treatment and its mechanism, and study the effects of quenching and tempering process on residual stress and retained austenite, and then propose residual stress according to the bearing condition. The control index of retained austenite, etc., will be one of the main directions of heat treatment research in China's bearing industry.
3. Bainite austempering
Bainite austempering is a hot spot in the domestic bearing industry research in recent years. Since the 1980s, Luoyang Bearing Research Institute has cooperated with Chongqing Bearing Factory to start the application research of bainite austempering on railway bearings, and then carried out bainite austempering on rolling mill bearings with Shahe Rolling Mill Bearing Factory. The application research has achieved good results, and the recommended technical requirements for austempering of bainite are introduced in JB1255-1991. At the same time, the bearing industry has also begun the promotion and application of bainite austempering. With the help of the national “Eighth Five-Year†key enterprise technology development project “rail passenger car bearingâ€, the relevant units have carried out systematic research on the microstructure and performance of bainite austempering, and successfully applied to the production of quasi-high-speed railway bearings. In the revision of JB1255 in 2001, the technical content of bainite austempering was officially included in the standard formal provisions. The bainite quenching process has been widely used in rolling mills, locomotives, and iron passengers.
The outstanding characteristics of bainite structure are impact toughness, fracture toughness, wear resistance, dimensional stability, and surface residual stress is compressive stress. Therefore, it is suitable for assembling bearings with large interference and poor service conditions, such as railways, rolling mills, cranes and other bearings that are subjected to large impact loads, mine transportation machinery or mine loading and unloading systems with poor lubrication conditions, and bearings for coal mines. The high-carbon chromium bearing steel BL austempering process has been successfully applied in railway and rolling mill bearings, and achieved good results.
In the production of railway and rolling mill bearings, due to the large size and heavy weight of the ferrule, the martensite structure is brittle during oil quenching. In order to obtain high hardness after quenching, strong cooling measures are often taken, resulting in quenching microcracks; due to martensite After quenching, the surface is tensile stress. The superposition of grinding stress during grinding increases the overall stress level, which is easy to form grinding cracks and cause batch waste. When bainite is quenched, the bainite structure is much better than the M structure, and the surface is formed with compressive stress of up to -400 ~ -500MPa, which greatly reduces the tendency of quenching cracks [16]; The compressive stress counteracts part of the grinding stress, which reduces the overall stress level and greatly reduces the grinding crack.
SKF Company applies high-carbon chromium bearing steel bainite austempering process mainly to railway bearings, rolling mill bearings and bearings used under special working conditions, and has developed steel grades suitable for bainite quenching (SKF24, SKF25, 100Mo7). ). When quenching, a longer isothermal time is used, and after quenching, a full lower bainite structure is obtained. Recently, SKF has developed a new steel grade of 775V, and through a special austempering to obtain a more uniform lower bainite, the hardness after quenching increases while the toughness is 60% higher than the conventional austempering, and the wear resistance is improved. The treated ferrule wall thickness exceeds 100 mm.
The performance of the martensite/bainite composite structure after partial isothermal treatment is still controversial, such as the content of BL is the best. Even if there is an optimum content, how to control in the actual production, and the composite structure needs an additional temper after isothermal, increasing the production cost. In addition, as far as bainite austempering is concerned, although its process, organization and properties have been systematically studied, while promoting this process, attention should be paid to the limitations of the process. Not all bearing parts are suitable for shellfish. Austempering of austenite. The development of bainite austempering steel should also be carried out to further improve the performance of bainite after isothermal quenching; the development of heat treatment equipment for replacing nitrate salt isothermal, reduce environmental pollution and the like.
4. Special heat treatment
The high carbon chromium bearing steel is generally hardened by the whole, and the residual stress after quenching is the surface tensile stress state, which is easy to cause quenching crack and reduce the bearing performance. A special type of heat treatment is to increase the carbon and nitrogen content of the surface layer by carburizing, nitriding or carbonitriding of high carbon chromium bearing steel, reducing the Ms point of the surface layer, and forming a surface after the surface is changed during the quenching process. Compressive stress, improved wear resistance and rolling contact fatigue performance [17,18]. On the other hand, by a certain method, a certain amount of stable retained austenite is retained in the heat-treated bearing parts, and the edge effect of the indentation is reduced by the deformable retained austenite, so that the surface fatigue source originating from the indentation edge is obtained. It is not easy to form and expand, thus improving the contact fatigue life of the bearing under pollution conditions. Generally, the above object can be attained by controlling the carbon (nitrogen) potential of the atmosphere during quenching heating. NSK's NSJ2 steel [19] and KOYO's SH technology [20] were developed based on this theory.
Another special type of heat treatment is the use of high toughness carburized steel with a high carbon content (0.4%) and special carburizing or carbonitriding heat treatment. Firstly, the composition of the carburized steel is adjusted: the matrix carbon content is increased to improve the strength of the matrix while ensuring the toughness, and the Si and Mn contents are increased to improve the stability of the retained austenite, and Mo is added to refine the carbide. Carbonitride. Secondly, the carburizing or carbonitriding process is strictly controlled to obtain more retained austenite (about 30% to 35%) and a large amount of fine carbides and carbonitrides on the surface after the parts are processed. On the one hand, a large number of fine carbides and carbonitrides can ensure the hardness and wear resistance of the surface to make the indentation difficult to form; on the other hand, even if the indentation is formed, more stable retained austenite can reduce the edge effect. To prevent the formation and expansion of fatigue sources. Based on this theory, NSK and KOYO have developed TF series technology (HTF, STF, NTF) and KE technology respectively, which greatly improves the life of bearings under contaminated lubrication conditions. For example, the tapered roller bearings produced by NSK using HTF technology have a fatigue life of 10 times that of ordinary bearings under contaminated lubrication conditions [21]. Companies such as NSK use special heat treatment techniques in a variety of newly developed bearing products.
In recent years, Luoyang Bearing Research Institute has cooperated with relevant units to carry out research on special heat treatment process of high carbon chromium bearing steel, and also carried out research on special heat treatment process of medium carbon alloy steel. Preliminary results indicate that the contact fatigue life can be significantly achieved by special heat treatment. This process will have significant promotion value in the bearing industry and will become a hot technology for the research and application of the bearing industry in China.
5. Surface modification technology
5.1 ion implantation
The US Naval Laboratory has conducted research on ion implantation of bearing parts since 1979, and countries such as the United Kingdom, Denmark, and Portugal have been working similarly to the US Naval Laboratory since 1989. The results show that the injection of chromium ions can significantly improve the corrosion resistance of M50 steel, and the contact fatigue resistance is also improved. In addition, boron ions are added to improve the wear resistance of the instrument bearing; nitrogen plasma source is applied to bearing steel 52100. After ion implantation (PSII), a thin layer of nitride is formed on the surface to improve the corrosion resistance of the bearing steel and replace it with expensive stainless steel. Nitrogen and boron ion implantation on the SUS440C stainless steel ball bearing can reduce the micro-swing of the ball bearing. Vibration wear and dust emission of the bearing, in addition, (Ti + N) or (Ta + N) plasma immersion ion implantation (PSIII) on stainless steel can significantly improve its microhardness, wear resistance and life.
Since the 1980s, China's bearing industry has applied ion implantation on bearings, and has successfully applied it to aviation and aerospace, and achieved good results.
5.2 Surface coating
Surface coating techniques include: physical vapor deposition (PVD), chemical vapor deposition (CVD), radio frequency sputtering (RF), ion spray coating (PSC), electroless plating, and the like. Compared with CVD, PVD has a low temperature during processing, and does not need to be heat treated after plating. It is widely used in the surface treatment of bearing parts. Steel bearing parts such as 100Cr6 and 440C can improve the wear resistance, contact fatigue resistance and surface friction coefficient of bearing parts after PVD, CVD or RF plating of TiC, TiN, TiAlN hard film or MoS2 type soft film.
In recent years, SKF has developed two coating technologies: one is to use PVD to coat the surface of the bearing ring and the rolling element with diamond (Diamond-Like Carbon, DLC) with extremely high hardness. The surface hardness is higher than that of hardened bearing steel. 40%~80%, the friction coefficient is similar to PTFE or MoS2, it has self-lubricating characteristics, and it has good bonding with the substrate, no peeling, bearing life and wear resistance are greatly improved, and it can still work normally under the condition of oil cut. It is called “No Wear bearingâ€; the second is to use PSC to spray a layer of 100μm thick alumina on the outer circumference of the outer ring of the bearing, so that the bearing insulation capacity is up to 1000V, and the bearing has higher insulation by increasing the thickness of the alumina. ability. The coated alumina is firmly bonded to the substrate and improves the corrosion resistance of the bearing. The post-plated bearing (INSOCOATTM bearing) can be mounted like a normal bearing.
6. Conclusion
Throughout the development of bearing heat treatment technology at home and abroad, there is still a big gap between China's bearing industry and foreign developed countries' heat treatment technology, which seriously restricts the quality of China's bearings, especially the improvement of life and reliability. The entire bearing industry should pay attention to the research of heat treatment basic theory and new technology, and vigorously promote the application of research results in actual production to improve the heat treatment level in China as soon as possible.
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