Bearing heat treatment is directly related to the subsequent processing quality, which ultimately affects the use performance and life of the parts. At the same time, bearing heat treatment is a major energy consumer and a major polluter in bearing manufacturing. The heat treatment equipment of the bearing directly affects the heat treatment quality of the bearing, as well as energy consumption and pollution. In recent years, with the advancement of science and technology, the development trend of heat treatment technology is mainly reflected in the following aspects:
(1) Wastewater, waste gas, waste salt, waste oil and electromagnetic radiation formed by the heat treatment of heat treatment will cause environmental pollution. To solve the environmental pollution problem of heat treatment, the implementation of clean heat treatment (or green heat treatment) is one of the development directions of heat treatment technology. This places high demands on the heat treatment atmosphere, quenching oil and cleaning equipment.
(2) Precision heat treatment precision heat treatment is to ensure the stability of the optimized process, to achieve a small dispersion of product quality (or zero), heat treatment distortion to zero, reduce grinding retention to improve production efficiency, and save materials. To achieve precision heat treatment, it must have good furnace temperature uniformity, temperature control accuracy, and good cooling and stability of the quenching agent. The precise heat treatment of the bearing can take the two steps of integral quenching and induction hardening.
(3) Energy-saving heat treatment Select new insulation materials to improve the energy efficiency of heat treatment equipment; optimize the heat treatment process, increase the process output, and fully utilize the equipment's capabilities. At this stage, all bearing manufacturers are doing experiments in this area, such as making full use of waste heat and waste heat. Some manufacturers have used forging waste heat to perform spheroidizing annealing of bearing parts. The process with low energy consumption and short cycle replaces long cycle and high energy consumption. The process of the lower bainite quenching process replaces the long cycle and high energy carburizing process with a short bainite quenching process to a certain extent and within a certain range.
(4) Less oxidation-free heat treatment is heated by a protective atmosphere heating instead of an oxidizing atmosphere to control the carbon potential and nitrogen potential in a controlled atmosphere. The properties of the parts after heat treatment are improved, and heat treatment defects such as oxidation, decarburization and cracking are greatly reduced. The finishing allowance after heat treatment is reduced, which improves material utilization and machining efficiency. Vacuum heating, gas quenching, vacuum or low pressure carburizing, nitriding, nitrocarburizing and boronizing can significantly improve the quality of the workpiece, reduce distortion and improve life.
Second, bearing heat treatment equipment
At present, the annealing equipment in China is the coexistence of oxidation furnace and protective atmosphere furnace. The oxidation furnace is more than the protective atmosphere furnace; the periodic furnace and the continuous furnace coexist, and the cycle furnace is more than the continuous furnace. In the United States, Britain, Japan and other countries, as early as the 1960s and 1970s, push-rod and roller-type isothermal spheroidizing annealing furnaces were fully promoted, shortening the annealing cycle, saving energy, and improving annealing quality. With the development of bearing parts processing technology, as well as the use of precision forging and precision enthalpy (cold enthalpy) processes, the processing precision of part blanks is getting higher and higher, which brings about the need for annealing of protective atmosphere. The bearing industry should rapidly promote spheroidal annealing of protective atmosphere to reduce oxidative decarburization after annealing, improve processing efficiency, save energy and reduce costs.
2. Conventional martensitic quenching tooling equipment
At this stage, the conventional martensite quenching tooling equipment in China's bearing industry is uneven. From the protective medium, it can be divided into an oxidation furnace without a protective medium, a controlled atmosphere furnace and a vacuum furnace. The non-protective medium oxidation furnace accounts for about 25% of the bearing heat treatment equipment, and the non-oxidation furnace with protective medium accounts for about 50% of the bearing heat treatment equipment. The controllable atmosphere furnace accounts for about 20% of the bearing heat treatment equipment, and the vacuum furnace accounts for the bearing heat treatment. About 5% of the equipment. From the production cycle can be divided into cycle furnace and continuous furnace.
In recent decades, the development of martensitic quenching tooling equipment has focused on the following aspects:
(1) Automated production line At present, the automatic production line adopted in China can be divided into: according to its structure (or the movement mode of the workpiece in the furnace): there are muffle belt furnace, no muffle belt furnace, casting chain furnace, roller hearth furnace , drum furnace mesh belt furnace and rotary hearth furnace series production line, feeding, pre-cleaning, protective atmosphere (or controlled atmosphere) heating, quenching, post-cleaning (sometimes also carried out secondary cryogenic roller hearth furnace for secondary deep cooling ) and tempering are done automatically. The automation degree and control precision of the production line are high, and the quality of the workpiece after processing is uniform. The whole heat treatment production line can be used as part of the automatic bearing production line. Different heat treatment production lines are suitable for heat treatment of bearing parts of different types and sizes according to their structural characteristics. For example, mesh belt furnace is suitable for small and medium-sized bearing rings; rotary hearth furnace is suitable for small batch bearing rings; roller bottom furnace is equipped with automatic lifting Quenching device for bearing parts with large wall thickness; drum furnace for rolling elements and small ferrules.
(2) Multi-purpose furnace can be carburized or carbonitrided under controlled atmosphere, and quenched under protective atmosphere to complete the non-oxidative quenching process of the workpiece. It is a periodic furnace, which is mainly suitable for heat treatment of small batch and multi-variety bearing parts.
(3) Induction heating quenching and tempering equipment induction heating has the advantages of fast heating speed, energy saving and small deformation, and the processed workpiece has some properties that are not conventionally heated. At present, China's bearing heat treatment applications are mainly concentrated on slewing bearings and car wheel bearings, but there is basically no application on GCr15 steel.
(4) The vacuum furnace can reduce or avoid the oxidation of the workpiece under vacuum condition. The high-pressure gas quenching can control the cooling and deformation of the workpiece, avoiding the environmental pollution caused by oil quenching and achieving clean heat treatment. In addition, after the vacuum heat treatment, the microstructure of the workpiece is finer and uniform, the surface is consistent with the core structure, and the hardness is uniform, which is beneficial to the improvement of the bearing fatigue life. The vacuum furnace is mainly used in the heat treatment of special steel (heating temperature above 1 000 Â°C) and quenching of some high-precision GCr15 steel bearing parts. It is widely used in the field of bearing heat treatment in China. The development trend of vacuum furnaces is to replace oil quenching with ultra-high pressure gas quenching, and to apply vacuum carburizing furnaces to solve the application of new steels in the bearing field.
(5) Quenching cooling medium and equipment quenching medium can be divided into three categories: oil-based, water-based and gas quenching media. Oil-based quenching media are the most commonly used quenching media. Ordinary quenching oil is N32 or N22 system-wide loss oil. In order to improve its cooling performance, anti-aging performance, brightening performance and high temperature performance, the addition of refrigerant, cleaning agent, brightener and antioxidant respectively forms a fast oil. Series of quenching oils such as fast bright oil and high temperature graded isothermal oil for quenching of bearing parts of different sizes and requirements; in addition, low volatility vacuum quenching oil. A disadvantage of oil-based quenching media is that air fumes are generated during the quenching process, causing air pollution and causing water contamination during subsequent cleaning processes.
The water-based quenching medium is an aqueous solution composed of an organic polymer, an anticorrosive agent, and other additives. Different cooling characteristics can be obtained by changing the type and concentration of the organic polymer to suit the quenching cooling requirements of different bearing parts. During the quenching and cooling process, organic matter adheres to the surface of the part, which can reduce the risk of quenching and cracking of the part, and does not produce soot, easy to clean, and free of pollution, which is the development direction of the quenching medium.
Gas quenching uses an inert gas as a medium (commonly used nitrogen) to spray compressed gas through a specially designed nozzle onto the surface of the workpiece to achieve quenching and cooling of the workpiece. The cooling characteristics and deformation can be controlled by adjusting the pressure of the gas and the structure of the nozzle. Gas quenching is cleaner and less costly than water-based quenching. The key technology is the structural design of the nozzle. China's gas quenching is still in its infancy in the field of bearings, and there is still a certain gap in the structural design technology of nozzles.
Quenching and cooling equipment is another major factor affecting the quenching effect of the workpiece in addition to the quenching medium. At present, there are few control parameters for quenching and cooling equipment in China, and generally only the oil temperature and oil circulation are controlled. Foreign countries apply multi-parameter control to commonly used quenching oil tanks, such as oil temperature, oil cooling characteristics, oil circulation and agitation direction, and speed, workpiece oil feeding method, etc., in order to obtain the best quenching structure and performance, Reduce distortion to a minimum. Some bearing manufacturers in China are also trying multi-parameter control, especially by controlling the oil inlet mode of the tapered bearing to solve the change of the angle during the quenching process of the bearing parts.
The multi-station automatic quenching press has been applied in China, but the degree of automation cannot keep up with foreign countries.
(6) Cleaning of cleaning equipment is usually an essential process after oil quenching. The cleaning equipment used in the continuous production line generally completes the processes of hot soap water soaking, water spray and hot air drying, and has a water-water separation device. In the advanced cleaning equipment, a secondary cooling device (with refrigeration equipment and temperature between 5 and 10 Â°C) is added before drying to reduce the content of retained austenite and improve the dimensional stability. At present, the cleaning equipment in our country is mainly cleaned with high-pressure cleaning liquid. This kind of cleaning is ineffective for non-through holes and small oil grooves, which often causes tempering fumes and pollution, and sometimes causes fire.
Vacuum cleaning is a cleaning device developed in recent years. The advantages of this type of cleaning equipment are: good cleaning effect, especially for the cleaning of complex parts, the surface of the workpiece is bright after cleaning; safe and clean, the cleaning liquid is clean water, no harmful chlorides are added Petroleum solvent; high degree of automation, and can replace low temperature tempering with cleaning heating, saving tempering costs. In recent years, ultrasonic cleaning and eddy current cleaning have also been applied well in heat treatment.
3. Bainite austempering equipment
According to the characteristics of bearing processing, there are two main types of equipment used: automatic production line and cycle furnace.
(1) Bearings with small variety and large batch size for automatic production lines are suitable for automatic production. For example, automatic production lines are used for the production of railway bearings. The automatic production line is mainly composed of a protective atmosphere heating furnace and an isothermal quenching tank. The equipment used in the austempering part can be divided into three types according to the movement of the workpiece: a rotary bottom type, a push type and a conveyor belt type. The rotary-bottom and push-plate isothermal quenching equipment developed by Ai Xielin Company puts the rotary bottom or push-pull mechanism in the isothermal salt bath, the workpiece enters and exits according to a certain cycle, and the memory control sequence is discharged. The action is controlled by the PLC program. . This type of equipment is highly automated, but with a large investment.
(2) Periodic austempering equipment Rolling mill bearings Because of the variety and small batch size, small and medium-sized rolling mill bearing manufacturers often use box furnace or pit furnace + quenching cooling tank + isothermal tank + cleaning tank, also use box furnace + more Isothermal tanks. The configuration has less investment and strong applicability, but the labor intensity is large and the safety is poor. At present, in the promotion of bainite austempering, the use of bainite austempering instead of carburizing steel has been successfully applied in rolling mill bearings with low load.
4. Protection atmosphere and control
With the improvement of the surface quality requirements after quenching the workpiece, the protection (controllable) atmosphere heating is becoming more and more popular, and the heat treatment including annealing is heated by a protective (or controllable) atmosphere. In the 1970s, an endothermic atmosphere was mainly used. The endothermic atmosphere is a mixed gas of a raw material gas and an air partially reacted under the action of a catalyst to form a protective atmosphere containing (CO) 18% to 23%, (H2) 37% to 42%, and a balance of N2. The atmosphere is still used in the field of bearing heat treatment in China. The nitrogen-based propane atmosphere is the most widely used in the heat treatment quenching line of the bearing field in China. The nitrogen-nitrogen atmosphere component is similar to the endothermic atmosphere, and the nitrogen-based atmosphere represented by nitrogen-methanol is more applied on the mesh belt furnace. In the 1990s, there was a gas production method in which air and hydrocarbons were directly introduced into a furnace above 800 Â° C, that is, a direct atmosphere. Through research, it is found that the atmosphere containing high CH4 component, although its gas reaction does not reach the equilibrium similar to the endothermic atmosphere, its carbon transport capacity is controlled by the content of CO and H2 in the atmosphere. The carbon potential control of the probe combined with the CO2 analyzer is achievable. The main advantage of the direct atmosphere is that it saves a lot of raw material gas consumption. According to statistics, whether the atmosphere is used in a periodic atmosphere furnace or a continuous atmosphere furnace, the cost of raw material gas consumption can reach about 70%. Today, there are more than 300 sets of atmosphere furnaces in the world that use this atmosphere for various heat treatments such as carburizing, carbonitriding and protective atmosphere quenching. The direct atmosphere is widely used in the field of carburizing in China's bearing field.
The practical application in recent years shows that the service life of the oxygen probe is not well determined, and the difference between the carbon potential measured by the oxygen probe and the actual value is also unpredictable. Therefore, the controllable atmosphere is sometimes uncontrollable due to the failure of the oxygen probe, causing defects such as decarburization and oxidation. At present, China often uses periodic calibration to verify its measurement accuracy, which is not conducive to the full automation of the furnace, and sometimes even affects normal production.
In view of the above reasons, a dual measurement system has been developed abroad, one with a standard oxygen probe for normal control of the carbon potential, and another independent measurement system for detecting the operation of the oxygen probe, ie the two systems measure the atmosphere separately. Carbon potential, when the result shows a large deviation, it will alarm. The second measuring system working element can be a CO2 infrared analyzer or a miniature oxygen probe (Î»-probe). To date, many atmosphere furnaces have been installed with this dual measurement system.
Looking at the whole, domestic bearing heat treatment technology and equipment are still far away from the developed countries, especially in the annealing equipment. Bearing heat treatment technology and equipment also have a certain distance from other industries, improving bearing quality is related to the revitalization of China's equipment manufacturing industry.
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