Development trend of bearing heat treatment technology and treatment equipment

1. Development trend of bearing heat treatment technology

Bearing heat treatment is directly related to the subsequent processing quality, which ultimately affects the service performance and life of parts. At the same time, bearing heat treatment is a major energy consumer and polluter in bearing manufacturing. The bearing heat treatment equipment directly affects the bearing heat treatment quality, energy consumption and pollution. In recent years, with the progress of science and technology, the development trend of heat treatment technology is mainly reflected in the following aspects:

(1) Clean heat treatment

Waste water, waste gas, waste salt, waste oil and electromagnetic radiation from heat treatment production will pollute the environment. To solve the environmental pollution problem of heat treatment and implement clean heat treatment (or green heat treatment) is one of the development directions of heat treatment technology. This puts forward high requirements for the atmosphere of heat treatment, quenching oil and cleaning equipment.

(2) Precision heat treatment

Precision heat treatment is to fully ensure the stability of the optimized process, achieve small (or zero) product quality dispersion and zero heat treatment distortion, reduce grinding allowance, improve production efficiency and save materials. To realize precision heat treatment, it is necessary to have good uniformity of furnace temperature, accuracy of temperature control, and good cooling and stability of quenchant. There are two ways to realize the precision heat treatment of bearings: integral quenching and induction quenching.

(3) Energy saving heat treatment

Select new insulation materials to improve the energy utilization rate of heat treatment equipment; Optimize the heat treatment process, increase the process output and give full play to the capacity of the equipment. At present, all bearing manufacturers are doing tests in this regard, such as making full use of waste heat and waste heat. Some manufacturers have used forging waste heat for spheroidizing annealing of bearing parts; Adopt the process with low energy consumption and short cycle to replace the process with long cycle and high energy consumption; To a certain extent, the lower bainite quenching process replaces the carburizing process with a shorter bainite quenching process with a long cycle and high energy consumption.

(4) Less oxidation heat treatment

The performance of the parts after heat treatment is improved, the defects such as oxidation, decarburization and cracks are greatly reduced, the finishing allowance after heat treatment is reduced, and the utilization rate of materials and machining efficiency are improved. Vacuum heating and gas quenching, vacuum or low pressure carburizing, nitriding, nitrocarburizing and boronizing can obviously improve the quality of the workpiece, reduce distortion and increase service life.

2. Bearing heat treatment equipment

(1). Annealing equipment

At present, the annealing equipment in China is the coexistence of oxidation furnace and protective atmosphere furnace, and the oxidation furnace is more than the protective atmosphere furnace; Periodic furnace and continuous furnace coexist, and periodic furnace is more than continuous furnace. As early as the 1960s and 1970s, the United States, Britain, Japan and other countries have comprehensively promoted push rod and roller hearth isothermal annealing furnaces to shorten annealing cycle, save energy and improve annealing quality. With the development of machining technology of bearing parts and the adoption of precision forging and precision rolling (cold rolling) processes, the machining accuracy of workpiece is getting higher and higher, which brings the demand for protective atmosphere annealing. The bearing industry should rapidly promote protective atmosphere spheroidizing annealing to reduce oxidation and decarburization after annealing, improve processing efficiency, save materials and energy, and reduce costs.

(2). tooling and equipment for conventional martensite quenching

At present, the tooling equipment for conventional martensite quenching in China's bearing industry is uneven. From the perspective of protective medium, it can be divided into oxidation furnace without protective medium, controllable atmosphere furnace and vacuum furnace. The oxidation furnace without protective medium accounts for about 25% of the bearing heat treatment equipment, the less 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 about 5% of the bearing heat treatment equipment. The production cycle can be divided into periodic furnace and continuous furnace.

In recent decades, the development of tooling equipment for martensite quenching mainly focuses on the following aspects:

a. Automated production line

At present, the automatic production lines adopted in China can be divided into: muffle mesh belt furnace, muffle free mesh belt furnace, chain casting furnace, roller hearth furnace, roller hearth furnace, mesh belt furnace and rotary hearth furnace series production lines according to their structure (or the movement mode of workpieces in the furnace). Feeding, pre cleaning, heating under protective atmosphere (or controllable atmosphere), quenching, post cleaning (sometimes secondary cryogenic roller hearth furnace for secondary cryogenic) and tempering are automatically completed. The production line has a high degree of automation and control precision, and the quality of the workpiece after treatment is uniform. The whole heat treatment production line can be used as a part of the bearing automatic production line. Different heat treatment production lines are suitable for heat treatment of different types and sizes of bearing parts 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 of bearing rings; Roller hearth furnace is equipped with automatic lifting quenching device, which is suitable for bearing parts with large wall thickness; Roller furnace is suitable for rolling elements and small ferrules.

b. Multipurpose furnace

It can be carburized or carbonitriding in a controlled atmosphere, and quenched in a protective atmosphere to complete the non oxidation quenching process of the workpiece. It is a kind of periodic furnace, which is mainly applicable to the heat treatment of small batch and multi variety bearing parts.

c. Induction heating quenching and tempering equipment

Induction heating has the advantages of fast heating speed, energy saving and small deformation. The processed workpiece has some properties that conventional heating does not have. At present, the application of bearing heat treatment in China is mainly focused on the rotary table bearing and car hub bearing, but it is not applied to GCr15 steel.

d. Vacuum furnace

Heating in vacuum state can reduce or avoid oxidation of workpiece. Combined with high-pressure gas quenching, the cooling and deformation of workpiece can be controlled to avoid environmental pollution caused by oil quenching and realize clean heat treatment. In addition, after vacuum heat treatment, the microstructure of the workpiece is more fine and uniform, the surface and heart structure are consistent, and the hardness is uniform, which is conducive to the improvement of bearing fatigue life. Vacuum furnaces are mainly used for heat treatment of special steels (heating temperature above 1000 ℃) and quenching of bearing parts made of GCr15 steel with high requirements. They are widely used in the field of bearing heat treatment in China. The development trend of vacuum furnace is to replace oil quenching with ultra-high pressure gas quenching, and to apply vacuum carburizing furnace to solve the application of new steel in bearing field.

e. Quenching cooling medium and equipment

Quenching media can be divided into three categories: oil-based, water-based and gas quenching media. Oil based quenchants are the most commonly used quenchants. Ordinary quenching oil is N32 or N22 full system loss oil. In order to improve its cooling performance, anti-aging performance, brightening performance and high-temperature performance, a series of quenching oils, such as quick oil, quick brightening oil and high-temperature graded isothermal oil, have been formed to be used for the quenching of bearing parts with different sizes and requirements by adding a refrigerant, a cleaning agent, a brightener and an antioxidant; In addition, there is low volatile vacuum quenching oil. The disadvantage of oil-based quenching medium is that it produces oil fume during quenching, which causes air pollution and water pollution during subsequent cleaning.

Water based quenchants are aqueous solutions composed of organic polymers, corrosion inhibitors and other additives. By changing the type and concentration of organic polymer, different cooling characteristics can be obtained to meet the quenching cooling requirements of different bearing parts. In the process of quenching and cooling, organic matter is attached to the surface of parts, which can reduce the risk of quenching and cracking of parts, and do not produce oil smoke. It is convenient to clean and pollution-free. It is the development direction of quenching media.

Gas quenching uses inert gas as the medium (commonly used nitrogen) to spray compressed gas onto the workpiece surface through a specially designed nozzle to realize quenching and cooling of the workpiece. Cooling characteristics and deformation can be controlled by adjusting gas pressure and nozzle structure. Gas quenching is cleaner and cheaper than water-based quenching. The key technology is the structure design of the nozzle. Gas quenching in China 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 cooling equipment is another major factor affecting the quenching effect of workpiece besides quenching medium. At present, there are few control parameters for quenching and cooling equipment in China. Generally, only oil temperature and oil circulation are controlled. In foreign countries, the commonly used quenching oil tank is controlled by multiple parameters, such as oil temperature, oil cooling characteristics, oil circulation and mixing direction, as well as speed, workpiece oil feeding mode, etc., in order to obtain the best quenching structure and performance, and reduce the deformation to the minimum. Some bearing manufacturers in China are also trying multi parameter control, especially through the control of oil feeding mode of conical bearings to solve the angle change during the quenching process of bearing parts.

Multi position automatic quenching press has been applied in China, but its automation level can not catch up with that of foreign countries.

f. Cleaning equipment

Cleaning is usually a necessary process after oil quenching. The cleaning equipment used in the continuous production line generally completes the processes of hot soap water immersion, clean water spraying and hot air drying, and is equipped with an oil-water separation device. In the advanced cleaning equipment, a secondary cooling device (equipped with refrigeration equipment and adjustable temperature between 5 ~ 10 ℃) is added before drying to reduce the content of residual austenite and improve the dimensional stability. At present, high-pressure cleaning fluid is mainly used for cleaning equipment in China. This kind of cleaning has no effect on through holes and small oil grooves, often resulting in a lot of backfire oil smoke, large pollution, and sometimes fire.

Vacuum cleaning is a cleaning equipment developed in recent years. The advantages of this kind of cleaning equipment are: good cleaning effect, especially for parts with complex structure, and bright workpiece surface after cleaning; It is safe and clean, and the cleaning solution is clean water, without adding chlorides and petroleum solvents harmful to the environment; It has a high degree of automation, and can use cleaning and heating instead of low-temperature tempering to save tempering costs. In recent years, ultrasonic cleaning and eddy current cleaning have also achieved good results in heat treatment.

3. Bainite isothermal quenching equipment

According to the characteristics of bearing processing, the equipment used mainly includes two categories: automatic production line and periodic furnace.

(1) Automatic production line

Bearings with small varieties and large quantities are suitable for automatic production. For example, automatic production lines are often used in the production of railway bearings. The automatic production line is mainly composed of protective atmosphere heating furnace and isothermal quenching tank. The equipment used in the isothermal quenching part can be divided into three types according to the action of transporting workpieces: bottom rotating type, push plate type and conveyor belt type. The rotary bottom type and push plate type isothermal quenching equipment developed by aixielin company places the rotary bottom or push plate mechanism in the isothermal salt bath, the workpiece enters and exits according to a certain beat, remembers the control sequence, and the action is controlled by the PLC program. This kind of equipment has a high degree of automation, but the investment is large.

(2) Periodic isothermal quenching equipment

Due to the variety and small batch of rolling mill bearings, medium and small rolling mill bearing manufacturers mostly adopt box type furnace or well type furnace + quenching cooling tank + isothermal tank + cleaning tank, and also adopt box type furnace + multiple isothermal tanks. The configuration has less investment and strong applicability, but it has high labor intensity and poor safety. At present, when bainite austempering is widely promoted, bainite austempering instead of carburized steel has been successfully used in rolling mill bearings with small load in China.

4. Protective atmosphere and control

With the improvement of surface quality requirements of workpiece after quenching, protective (controllable) atmosphere heating is becoming more and more popular. Protective (or controllable) atmosphere heating is used for heat treatment, including annealing. In the 1970s, endothermic atmosphere was mainly used. Endothermic atmosphere is a kind of atmosphere containing ϕ (CO） 18%～23%、 ϕ (H₂) 37% ～ 42%, residual N₂ protective atmosphere, which is also used in the field of bearing heat treatment in China. Nitrogen based propane atmosphere is the most widely used in the heat treatment quenching line in the bearing field in China. The components of nitrogen methanol atmosphere are similar to those of endothermic atmosphere. Nitrogen based atmosphere represented by nitrogen methanol is widely used in mesh belt furnace. In the 1990s, there was a method of directly introducing air and hydrocarbons into the furnace above 800 ℃, i.e. direct generation atmosphere. It is found that although the gas reaction in this atmosphere with high CH₄ content can not reach the balance level similar to the endothermic atmosphere, the carbon transmission capacity is still controlled by the content of c o and H₂ in the atmosphere. It is possible to control the carbon potential by using an oxygen probe combined with a CO₂ analyzer. The main advantage of direct generation atmosphere is that it saves a lot of feed gas consumption. According to statistics, whether this atmosphere is used in periodic atmosphere furnace or continuous atmosphere furnace, the feed gas consumption savings can reach about 70%. Today, more than 300 sets of atmosphere furnaces around the world use this atmosphere for various heat treatments such as carburizing, carbonitriding and protective atmosphere quenching. The direct atmosphere is mostly used in the field of carburization in China.

The practical application in recent years shows that the service life of the oxygen probe is uncertain, 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, resulting in defects such as decarburization and oxidation. At present, China often uses regular calibration of its measurement accuracy, which is not conducive to the full automation of the furnace, and sometimes even affects the normal production.

In view of the above reasons, a dual measurement system has been developed abroad, in which one with a standard oxygen probe is used to control the carbon potential normally, and the other independent measurement system is used to detect the working condition of the oxygen probe, that is, the two systems measure the carbon potential of the atmosphere respectively, and will alarm when there is a large deviation in the results. The working element of the second measuring system can be a CO2 infrared analyzer or a miniature oxygen probe（ λ— Probe). So far, many atmosphere furnaces have been installed with this dual measurement system.

On the whole, there is still a long way to go in terms of bearing heat treatment technology and equipment in China compared with developed countries, especially in terms of annealing equipment. Bearing heat treatment technology and equipment also have a certain distance compared with other industries. Improving bearing quality is related to the revitalization of China's equipment manufacturing industry.

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