Bearing heat treatment method one
The quality of heat treatment is directly related to the quality of subsequent processing and ultimately affects the performance and life of parts. At the same time, heat treatment is a major energy consumer and a major polluter in the machinery industry. In recent years, with the progress of science and technology and its application in heat treatment, the development of heat treatment technology is mainly reflected in the following aspects:
(1); Waste water, waste gas, waste salt, dust, noise and electromagnetic radiation formed by the production of clean heat treatment will pollute the environment. Solving the environmental pollution of heat treatment and implementing clean heat treatment (or green heat treatment) is one of the development directions of heat treatment technology in developed countries. In order to reduce the emission of SO2, CO, CO2, dust and coal slag, the use of coal as fuel has been basically eliminated, and the use of heavy oil is becoming less and less. Most people switch to light oil.
Natural gas is still the most ideal fuel. The waste heat utilization of the combustion furnace has reached a very high level. The optimization of the burner structure and the strict control of the air-fuel ratio ensure that the NOX and CO are reduced to a minimum under the premise of reasonable combustion; gas carburizing and carbonitriding are used And vacuum heat treatment technology replaces salt bath treatment to reduce the pollution of waste salt and CN-containing toxic substances to water sources; uses water-soluble synthetic quenching oil to replace part of quenching oil, and uses biodegradable vegetable oil to replace part of mineral oil to reduce oil pollution.
(2); Precision heat treatment Precision heat treatment has two meanings: on the one hand, it is based on the use requirements, materials, and structural dimensions of the parts, using physical metallurgy knowledge and advanced computer simulation and testing technology to optimize process parameters to achieve the required performance Or maximize the potential of the material; on the other hand, fully guarantee the stability of the optimized process, and achieve a small (or zero) product quality dispersion and zero heat treatment distortion.
(3); Scientific production and energy management of energy-saving heat treatment are the most potential factors for the effective use of energy. It is the choice of scientific management to establish a professional heat treatment plant to ensure full-load production and give full play to equipment capabilities. In terms of heat treatment energy structure, priority is given to primary energy; waste heat and waste heat are fully utilized; processes with low energy consumption and short cycles are used instead of processes with long cycles and high energy consumption.
(4); Less non-oxidizing heat treatment by using protective atmosphere heating instead of oxidizing atmosphere heating to controllable atmosphere heating with precise control of carbon potential and nitrogen potential, the performance of parts after heat treatment is improved, and heat treatment defects such as decarburization and cracks 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 quality, reduce distortion and increase life.
The heat treatment quality control of bearing parts is the most stringent in the entire machinery industry. Bearing heat treatment has made great progress in the past 20 years, mainly in the following aspects: research on basic heat treatment theory; research on heat treatment technology and application technology; development of new heat treatment equipment and related technologies. 1; Annealing of high-carbon chromium bearing steel The spheroidizing annealing of high-carbon chromium bearing steel is to obtain a structure with fine, small, uniform and round carbide particles uniformly distributed on the ferrite matrix, for the subsequent cold working and final Quench and temper for organization preparation. The traditional spheroidizing annealing process is to keep the temperature slightly higher than Ac1 (for example, GCr15 is 780~810℃) and then slowly cool with the furnace (25℃/h) to below 650℃.
This process has a long heat treatment time (above 20h) , and the carbide particles are not uniform after annealing, which affects the subsequent cold working and final quenching and tempering structure and performance. Afterwards, according to the transformation characteristics of undercooled austenite, an isothermal spheroidizing annealing process was developed: after heating, it was quickly cooled to a temperature range below Ar1 (690~720℃) for isothermal, and the austenite orientation was completed in the isothermal process.
The transformation of ferrite and carbide can be directly discharged and air cooled after the transformation is completed. The advantage of this process is to save heat treatment time (the whole process is about 12~18h), and the carbides in the treated structure are fine and uniform. Another time-saving process is to repeat the spheroidizing annealing: heating to 810°C for the first time and then cooling to 650°C, then heating to 790°C and then cooling to 650°C and then air cooling. Although this process can save a certain amount of time, the process operation is more complicated.
2; Martensite quenching and tempering of high carbon chromium bearing steel
2.1 The structure and performance of conventional martensite quenching and tempering In the past 20 years, the development of conventional high-carbon chromium bearing steel martensite quenching and tempering process is mainly divided into two aspects: on the one hand, the development of quenching and tempering process parameters affects the structure And the influence of performance, such as the structure transformation during quenching and tempering, the decomposition of retained austenite, the toughness and fatigue properties after quenching and tempering, etc. [2~10]; the other hand is the process performance of quenching and tempering, such as The effect of quenching conditions on size and deformation, dimensional stability, etc. [11-13].
The structure of conventional martensite after quenching is composed of martensite, retained austenite and undissolved (residual) carbides. Among them, the structure of martensite can be divided into two categories: under a metallographic microscope (magnification is generally less than 1000 times), martensite can be divided into lath martensite and lamellar martensite Typical structure, generally after quenching, is a mixed structure of lath and flaky martensite, or an intermediate form between the two-jujube-shaped martensite (the so-called cryptographic martensite in the bearing industry, crystalline Martensite); Under high-power electron microscopy, its substructure can be divided into dislocation entanglement and twinning. The specific structure mainly depends on the carbon content of the matrix.
The higher the austenite temperature, the more unstable the original structure, and the higher the carbon content of the austenite matrix. The retained austenite in the quenched structure will be ?? Xianxianjiao, Jiaobao, Jiaoqiang, Jiangtang, the wall, strange new time Уmouyang, Jiangxi, Yizi, weft, and halo ⒘ Salt beer, R paradox, Kangtuo, at 3%, Ma’s The body is mainly lath martensite with dislocation substructure; when the matrix carbon content is higher than 0.6%, the martensite is lamellar martensite with a mixed substructure of dislocations and twins; the matrix carbon content is 0.75% At the time, large flaky martensite with obvious ridges appeared, and the flaky martensite had microcracks where it collided with each other when it grew.
At the same time, as the austenitizing temperature increases, the hardness after quenching increases and the toughness decreases. However, if the austenitizing temperature is too high, the hardness decreases due to excessive retained austenite after quenching. The content of retained austenite in the structure after conventional martensite quenching is generally 6~15%. Retained austenite is a soft metastable phase. Under certain conditions (such as tempering, natural aging or the use of parts) In), its instability occurs and decomposes into martensite or bainite.
The consequence of decomposition is that the hardness of the parts is increased, the toughness is decreased, and the size changes, which affect the dimensional accuracy of the parts and even normal operation. For bearing parts that require high dimensional accuracy, it is generally hoped that the less retained austenite, the better, such as supplementary water cooling or cryogenic treatment after quenching, and higher temperature tempering [12-14].
However, retained austenite can improve toughness and crack propagation resistance. Under certain conditions, the retained austenite on the surface of the workpiece can also reduce contact stress concentration and increase the contact fatigue life of the bearing. In this case, the process and material composition Take certain measures to retain a certain amount of retained austenite and improve its stability, such as adding austenite stabilizing elements Si, Mn, and stabilizing treatment, etc.
2.2 Conventional Martensite Quenching and Tempering Process Conventional high-carbon chromium bearing steel martensite quenching and tempering is as follows: After the bearing parts are heated to 830~860°C, they are quenched in oil and then tempered at low temperature. The mechanical properties after quenching and tempering are not only related to the original structure and quenching process before quenching, but also largely depend on the tempering temperature and time.
As the tempering temperature increases and the holding time increases, the hardness decreases, and the strength and toughness increase. The appropriate tempering process can be selected according to the working requirements of the parts: GCr15 steel bearing parts: 150~180℃; GCr15SiMn steel bearing parts: 170~190℃. For parts with special requirements, use higher temperature tempering to increase the service temperature of the bearing, or perform a cold treatment at -50~-78℃ between quenching and tempering to improve the dimensional stability of the bearing, or use martensite Step quenching to stabilize retained austenite to obtain high dimensional stability and high toughness.
Many scholars have studied the transformation during heating [2,7~9,17], such as the formation of austenite, the recrystallization of austenite, the distribution of residual carbides and the use of non-spheroidized structure as the original structure Wait. G. Lowisch et al. [3, 8] studied the mechanical properties of bearing steel 100Cr6 quenched after twice austenitization: First, austenitize at 1050°C and quickly cool to 550°C and then air-cooled to obtain a uniform The flake pearlite is then subjected to secondary austenitization at 850°C and quenched.
The size of martensite and carbides in the quenched structure is small, and the carbon content and retained austenite content of the martensite matrix are relatively high. , The austenite is decomposed by tempering at a higher temperature, and a large number of fine carbides are precipitated in the martensite, which reduces the quenching stress and improves the hardness, toughness and bearing capacity of the bearing.
Under the action of contact stress, its performance needs to be further studied, but it can be inferred that its contact fatigue performance should be better than conventional quenching. Sakai Jiuyu et al.  studied the microstructure and mechanical properties of SUJ2 bearing steel after cyclic heat treatment: first heating to 1000°C for 0.5h to make spherical carbide solid solution, and then pre-cooling to 850°C to quench the oil.
Then repeat the heat cycle from rapid heating to 750°C for 1 minute and then oil cooling to room temperature for 1 to 10 times, and finally rapid heating to 680°C for 5 minutes oil cooling. At this time, the structure is ultrafine ferrite plus fine carbides (ferrite grain size less than 2μm, carbides less than 0.2μm), superplasticity appears at 710℃ (elongation at break can reach 500%), which can be used This feature of the material is used for warm processing and forming of bearing parts. Finally, it is heated to 800°C to keep the quenching oil and tempered at 160°C. After this treatment, the contact fatigue life L10 is greatly improved compared with the conventional treatment, and the failure mode is changed from the early failure type of the conventional treatment to the wear failure type.
After being austenitized at 820℃, the bearing steel is subjected to short-time hierarchical isothermal air cooling at 250℃, followed by 180℃ tempering, which can make the carbon concentration distribution in the martensite after quenching more uniform, and the impact toughness is better than conventional quenching and tempering Doubled. Therefore, В.В.БЁЛОЗЕРОВ et al. proposed that the uniformity of carbon concentration of martensite can be used as a supplementary quality standard for heat-treated parts.