1、 Development trend of bearing heat treatment technology
Bearing heat treatment is directly related to the subsequent processing quality, which ultimately affects the Bearing Heat service performance and service life of parts. At the same time, bearing heat treatment is a large energy consumption and pollution in bearing manufacturing.
The bearing heat treatment equipment directly affects the bearing heat treatment quality, energy Bearing Heat 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 Bearing Heat treatment production will pollute the environment.
Solving the environmental pollution problem of heat treatment and Bearing Heat implementing clean heat treatment (or green environmental protection 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, realize small (or zero) product quality dispersion and zero heat treatment Bearing Heat distortion, reduce grinding allowance, improve production efficiency and save materials.
To realize precision heat treatment, it is necessary to have good furnace temperature uniformity, temperature control accuracy, and good cooling Bearing Heat 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 selects new thermal insulation materials to improve the energy utilization rate of heat treatment equipment; Optimize the heat treatment process, improve the process output and give full play to the capacity of the equipment.
At present, all bearing manufacturers are doing this test, 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;
The lower bainite quenching process replaces the carburizing process with long cycle and high energy consumption to a certain extent and within a certain range.
(4) Less and no oxidation heat treatment is from using protective atmosphere heating instead of oxidation atmosphere heating to controllable atmosphere heating with accurate control of carbon potential and nitrogen potential.
After heat treatment, the performance of parts is improved, heat treatment defects such as oxidation, decarburization and cracks are greatly reduced, finishing allowance after heat treatment is reduced, and material utilization and machining efficiency are improved.
Vacuum heating and gas quenching, vacuum or low pressure carburizing, nitriding, nitrocarburizing and boronizing can significantly improve the quality of the workpiece, reduce distortion and improve the service life.
1. Annealing equipment
At present, China’s annealing equipment is the coexistence of oxidation furnace and protective atmosphere furnace, and oxidation furnace is more than protective atmosphere furnace; Cycle furnace and continuous furnace coexist, and cycle 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 roll bottom isothermal spheroidizing annealing furnaces to shorten the annealing cycle, save energy and improve the annealing quality. With the development of bearing parts processing technology and the adoption of precision forging and precision rolling (cold rolling)
technology, the processing accuracy of parts is getting higher and higher, which brings the demand for protective atmosphere annealing. Protective atmosphere spheroidizing annealing should be rapidly promoted in the bearing industry to reduce oxidative decarburization after annealing, improve processing efficiency, save materials, save energy and reduce cost.
2. Tooling 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.
Oxidation furnace without protective medium accounts for about 25% of bearing heat treatment equipment, oxidation furnace without protective medium accounts for about 50% of bearing heat treatment equipment, controllable atmosphere furnace accounts for about 20% of bearing heat treatment equipment,
and vacuum furnace accounts for about 5% of bearing heat treatment equipment. In terms of production cycle, it 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:
(1) Automatic 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, quenching under protective atmosphere (or controllable atmosphere) After cleaning (sometimes secondary cryogenic cooling is carried out in roller hearth furnace) and tempering are completed automatically.
The production line has high degree of automation and control precision, and the quality of workpiece after treatment is uniform. The whole heat treatment production line can be used as a part of 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, such as 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; Drum furnace is suitable for rolling elements and small ferrules.
(2) The multi-purpose furnace can carburize or carbonitriding in a controlled atmosphere and quench in a protective atmosphere to complete the non oxidation quenching process of the workpiece. It is a cycle 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. The treated workpiece has some properties that conventional heating does not have. At present, the application of bearing heat treatment in China is mainly concentrated in turntable bearing and car hub bearing, but it is basically not applied in GCr15 steel.
(4) Heating in the vacuum state of the vacuum furnace can reduce or avoid the oxidation of the workpiece. Combined with high-pressure gas quenching, it can control the cooling and deformation of the workpiece, avoid the 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 is consistent with the core structure, and the hardness is uniform, which is conducive to the improvement of bearing fatigue life.
Vacuum furnace is mainly used in heat treatment of special steel (heating temperature above 1000 ℃) and quenching of bearing parts made of GCr15 steel with high requirements. It is widely used in the field of bearing heat treatment in China.
The development trend of vacuum furnace is ultra-high pressure gas quenching instead of oil quenching, and the application of vacuum carburizing furnace to solve the application of new steel in the field of bearing.
(5) Quenching cooling medium and equipment quenching medium can be divided into three categories: oil-based, water-based and gas quenching medium. Oil based quenchants are commonly used. The ordinary quenching oil is N32 or N22 full system consumption oil.
In order to improve its cooling performance, anti-aging property, brightness and high temperature performance, it adds refrigerant, cleaning agent, brightener and antioxidant separately. It has formed series of quenching oil, such as fast oil, quick bright oil and high temperature isothermal oil, to be used for quenching of bearing parts with different sizes and requirements.
In addition, there is low volatile vacuum quenching oil. The disadvantage of oil-based quenching medium is that oil fume is generated in the quenching process, resulting in air pollution and water pollution in the subsequent cleaning process.
Water based quenching medium is an aqueous solution composed of organic polymers, corrosion inhibitors and other additives. Different cooling characteristics can be obtained by changing the type and concentration of organic polymers to meet the quenching and 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 cracking of parts, without oil fume, convenient cleaning and no pollution. It is the development direction of quenching medium.
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. 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 cheaper than water-based quenching. The key technology is the structural design of nozzle. Gas quenching in China is still in its infancy in the field of bearing, and there is still a certain gap in the structural design technology of nozzle.
Quenching cooling equipment is another major factor affecting the quenching effect of workpiece in addition to quenching medium. At present, there are few control parameters of quenching and cooling equipment in China.
Generally, it only controls the oil temperature and oil circulation. 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 and oil feeding mode of workpiece, so as to obtain the quenching structure and properties and reduce the deformation to a small extent.
Some bearing manufacturers in China are also trying multi parameter control, especially through the control of workpiece oil feeding mode of conical bearing to solve the change of angle in the quenching process of bearing parts.
(6) 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 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, the cleaning equipment in China is mainly cleaned with high-pressure cleaning fluid.
This cleaning is ineffective for non holes and small oil grooves, often resulting in more tempering 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
The cleaning liquid is clean water, and chloride and petroleum solvents harmful to the environment are not added; High degree of automation, and cleaning and heating can be used to replace low-temperature tempering, so as to save tempering cost. 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) The bearings with few varieties and large batch of automatic production lines 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 and push plate 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, the memory control sequence is discharged, and the action is controlled by PLC program. This kind of equipment has a high degree of automation, but the investment is large.
(2) Due to the large variety and small batch of rolling mill bearings of periodic isothermal quenching equipment, medium and small rolling mill bearing manufacturers mostly use box furnace or well furnace + quenching cooling tank + isothermal tank + cleaning tank, and some also use box furnace + multiple isothermal tanks.
The configuration has less investment and strong applicability, but it has high labor intensity and poor applicability. At present, when bainite isothermal quenching is vigorously promoted, bainite isothermal quenching 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 mixture of feed gas and air, which partially reacts under the action of catalyst to form a mixture containing? (CO）18％～23％、? The protective atmosphere of (H2) 37% ~ 42% and residual N2 is also used in the field of bearing heat treatment in China.
Nitrogen based propane atmosphere is widely used in the heat treatment quenching line in the bearing field in China. The composition of nitrogen methanol atmosphere is similar to that 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 ℃, that is, direct generation atmosphere. It is found that although the gas reaction in this atmosphere with high CH4 content can not reach the balance degree similar to the endothermic atmosphere, the carbon transmission capacity is still controlled by the content of CO and H2 in the atmosphere.
It is possible to control the carbon potential with 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. Direct generation atmosphere is widely used in the field of bearing 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.
In view of the above reasons, a dual measurement system has been developed abroad, in which one with 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.
When the results show great deviation, it will alarm. The working element of the second measurement system can be a CO2 infrared analyzer or a micro oxygen probe（ λ— Probe). So far, many atmosphere furnaces have been installed with this dual measurement system.
On the whole, there is still a big gap between domestic bearing heat treatment technology and equipment compared with developed countries, especially in 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.