The quality of the heat treatment is directly related to the subsequent processing quality, which ultimately affects the performance and life of the parts. At the same time, the heat treatment is a major energy consumer and a major polluter in the machinery industry. In recent years, with the advancement of science and technology and its application in heat treatment, the development of heat treatment technology is mainly reflected in the following aspects:
(1) The waste water, waste gas, waste salt, dust, noise and electromagnetic radiation formed by the heat treatment of the 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 in developed countries. In order to reduce the emission of SO2, CO, CO2, dust and cinder, coal has been basically eliminated as a fuel, and the use of heavy oil is becoming less and less. Most of the light oil is used, and natural gas is still the most ideal fuel. The waste heat utilization of the combustion furnace has reached a high degree. 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; the use of gas carburizing, carbonitriding The vacuum heat treatment technology replaces the salt bath treatment to reduce the pollution of waste water and CN-toxic substances to the water source; the water-soluble synthetic quenching oil is used to replace part of the quenching oil, and the biodegradable vegetable oil is used to replace part of the mineral oil to reduce oil pollution.
(2) Precision heat treatment precision heat treatment has two meanings: on the one hand, according to the use requirements, materials, structural dimensions, using physical metallurgy knowledge and advanced computer simulation and detection technology, optimize process parameters to achieve the required performance. Or maximize the potential of the material; on the other hand, it is sufficient to ensure the stability of the optimized process, to achieve a small dispersion of product quality (or zero) and heat treatment distortion to zero.
(3) Energy-saving heat treatment Scientific production and energy management are the most promising factors for effective energy utilization. It is a scientific management choice to establish a professional heat treatment plant to ensure full-load production and full use of equipment capabilities. In the heat treatment energy structure, priority is given to primary energy; waste heat and waste heat are fully utilized; and processes with low energy consumption and short cycle are used to replace processes with long cycle and high energy consumption.
(4); less oxidation-free heat treatment is heated by a protective atmosphere instead of an oxidizing atmosphere to control the carbon potential and nitrogen potential in a controlled atmosphere, the performance of the parts after heat treatment is improved, and heat treatment defects such as decarburization and cracking are greatly reduced. The amount of 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, reduce distortion and improve life.
The quality control of heat treatment 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 theory of heat treatment; research on heat treatment process and application technology; development of new heat treatment equipment and related technologies. 1. The spheroidizing annealing of the annealed high carbon chromium bearing steel of high carbon chromium bearing steel is to obtain the microstructure of fine, small, uniform and round carbide particles evenly distributed on the ferrite matrix, for the subsequent cold working and final Quenching and tempering for tissue preparation. The conventional spheroidizing annealing process is performed at a temperature slightly higher than Ac1 (such as GCr15 of 780-810 ° C) and then cooled slowly (25 ° C / h) to below 650 ° C. The process has a long heat treatment time (above 20h) [1], and the particles of the carbide after annealing are not uniform, which affects the subsequent cold working and the final quenching and tempering structure and properties. Then, according to the transformation characteristics of supercooled austenite, the isothermal spheroidizing annealing process is developed: after heating, it is cooled to a temperature range below Ar1 (690~720 °C) for isothermal, and the austenite orientation is completed in the isothermal process. The transformation of ferrite and carbide can be directly cooled out after the completion of the transformation. The advantage of this process is that it saves 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 repeated spheroidizing annealing: after the first heating to 810 ° C, it is cooled to 650 ° C, then heated to 790 ° C and then cooled to 650 ° C to be air cooled. 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 Microstructure and properties of conventional martensite quenching and tempering In the past 20 years, the development of martensitic quenching and tempering process of conventional high carbon chromium bearing steel has two main aspects: one is to carry out quenching and tempering process parameters to the organization. And performance effects, such as microstructure transformation during quenching and tempering, decomposition of retained austenite, toughness and fatigue properties after quenching and tempering [2~10]; on the other hand, the process performance of quenching and tempering, such as The influence of quenching conditions on size and deformation, dimensional stability, etc. [11~13]. The microstructure after conventional martensite quenching is composed of martensite, retained austenite and undissolved (residual) carbide. Among them, the microstructure of martensite can be divided into two categories: under the metallographic microscope (magnification is generally less than 1000 times), martensite can be divided into two types: lath martensite and flaky martensite. Typical organization, after quenching, is a mixed structure of slats and flake martensite, or intermediate form between the two - jujube nucleus martensite (the so-called cryptocrystalline martensite, crystallized in the bearing industry) Martensite; under high power electron microscope, its substructure can be divided into dislocation entanglement and twinning. The specific histological morphology mainly depends on the carbon content of the matrix. The higher the austenite temperature, the more unstable the original microstructure, the higher the carbon content of the austenite matrix, and the more retained austenite in the microstructure after quenching, the sheet The more martensite, the larger the size, the larger the proportion of twins in the substructure, and the formation of quenching microcracks. Generally, when the carbon content of the matrix is ​​less than 0.3%, the martensite is mainly lath martensite mainly composed of dislocation substructures; when the carbon content of the matrix is ​​higher than 0.6%, martensite is a dislocation and twin mixed substructure. The flaky martensite; when the carbon content of the matrix is ​​0.75%, large martensite with a pronounced mid-ridge surface appears, and the lamellar martensite has microcracks when it collides with each other [8]. 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 microstructure after quenching of conventional martensite is generally 6-15%, and the retained austenite is a soft metastable phase under certain conditions (such as tempering, natural aging or the use of parts). Medium), its instability is decomposed into martensite or bainite. The consequence of the decomposition is that the hardness of the part is increased, the toughness is lowered, and the dimensional change affects the dimensional accuracy of the part and even normal operation. For bearing parts with high dimensional accuracy requirements, it is generally desirable to have less retained austenite, such as replenishing water cooling or cryogenic treatment after quenching, and using higher temperature tempering [12~14]. However, retained austenite can improve the toughness and crack propagation resistance. Under certain conditions, the retained austenite on the surface of the workpiece can also reduce the contact stress concentration and improve the contact fatigue life of the bearing. In this case, the composition of the process and materials. 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 [15,16].
2.2 Conventional martensite quenching and tempering process Conventional high carbon chromium bearing steel martensite quenching and tempering: after heating the bearing parts to 830~860 °C, quenching in oil, followed by low temperature tempering. The mechanical properties after quenching and tempering are largely dependent on the tempering temperature and time, in addition to the original structure and quenching process before quenching. As the tempering temperature increases and the holding time increases, the hardness decreases, and the strength and toughness increase. According to the working requirements of the parts, the appropriate tempering process can be selected: GCr15 steel bearing parts: 150~180°C; GCr15SiMn steel bearing parts: 170~190°C. 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. Many scholars have studied the transformation in the heating process [2,7~9,17], such as the formation of austenite, the recrystallization of austenite, the distribution of residual carbides and the use of aspherical structures as the original tissue. Wait. G.Lowisch et al [3,8] studied the mechanical properties of bearing steel 100Cr6 after austenitizing twice: first, austenitizing at 1050 °C and cooling to 550 °C for air cooling, to obtain uniform Fine-grained pearlite, followed by secondary austenitizing and quenching at 850 °C. The size of martensite and carbide in the microstructure after quenching is fine, the carbon content of martensite matrix and residual austenite content are higher. The austenite is decomposed by tempering at a higher temperature, a large amount of fine carbides are precipitated in the martensite, the quenching stress is lowered, and the hardness, toughness and bearing capacity of the bearing are improved. Under the effect of contact stress, its performance needs further research, but it can be speculated that its contact fatigue performance should be better than conventional quenching. Jiu Jiuyu et al [7] studied the microstructure and mechanical properties of SUJ2 bearing steel after cyclic heat treatment: firstly heated to 1000 ° C for 0.5 h to make the spheroidal carbide solid solution, and then pre-cooled to 850 ° C quenching. Then repeat 1~10 times of thermal cycle from rapid heating to 750 °C, heat preservation for 1 min, oil cooling to room temperature, and finally rapidly heat to 680 ° C for 5 min oil cooling. At this time, the microstructure is ultra-fine ferrite plus fine carbide (ferrite grain size is less than 2μm, carbide is less than 0.2μm), superplasticity occurs at 710 °C (break elongation can reach 500%), and can be utilized. This property of the material performs the warm forming of the bearing parts. Finally, the heat was heated to 800 ° C to quench the 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 form is changed from the conventional failure type to the wear failure type. The bearing steel is austenitized at 820 °C and then subjected to short-time grading isothermal air cooling at 250 °C, followed by 180 °C tempering, which can make the carbon concentration distribution in the quenched martensite more uniform, and the impact toughness is more than the conventional quenching and tempering. Doubled. Therefore, Ð’.Ð’.БÐЛОЗЕРОВ et al. proposed that the carbon concentration uniformity of martensite can be used as a supplementary quality standard for heat-treated parts [6].
2.3; Martensite quenching and tempering deformation and dimensional stability During the martensite quenching and tempering process, due to uneven cooling of various parts of the part, thermal stress and structural stress inevitably occur, resulting in deformation of the part. The deformation of the part after quenching and tempering (including dimensional changes and shape changes) is affected by many factors and is a rather complicated problem. Such as the shape and size of the part, the uniformity of the original structure, the roughing state before quenching (the amount of feed during turning, the residual stress of machining, etc.), the heating speed and temperature during quenching, the way the workpiece is placed, The oil inlet mode, the characteristics of the quenching medium, the circulation mode, and the temperature of the medium all affect the deformation of the part. A lot of researches have been carried out at home and abroad, and many measures to control deformation have been proposed, such as rotary quenching, die hardening, and oil control methods for controlling parts [11, 13, 18]. Studies by Beck et al. have shown that when the transition temperature from the vapor film stage to the boiling period is too high, a large thermal stress causes a large thermal stress to deform the austenite at a low yield point and cause distortion of the part. Lübben et al. believe that deformation is caused by uneven oil immersion between individual parts or parts, especially when new oils are used. Tensi et al. believe that the cooling rate at the Ms point plays a decisive role in the deformation, and a low cooling rate at the Ms point and below can reduce the deformation. Volkmuth et al. [13] systematically studied the quenching deformation of the inner and outer rings of tapered roller bearings by quenching media (including oil and salt bath). The results show that the diameter of the ferrule will increase to a certain extent due to different cooling methods, and as the temperature of the slab increases, the diameter of the ferrule end tends to be uniform, that is, the "horn" deformation is reduced. At the same time, the elliptical deformation of the ferrule (the diameter variation Vdp, VDp in a single radial plane) is reduced; the inner ring is less deformed than the outer ring due to the greater rigidity. The dimensional stability of parts after martensite quenching and tempering is mainly affected by three different transformations [12,14]: carbon migrates from martensite lattice to form ε-carbide, retained austenite decomposition and forms Fe3C, The three transitions are superimposed on each other. Between 50~120 °C, due to the precipitation of ε-carbide, the volume of the part is reduced. The general part has completed this transformation after tempering at 150 °C, which can affect the dimensional stability of the part in the future use. Neglecting between 100 and 250 ° C, the residual austenite decomposition, transformation to martensite or bainite, will be accompanied by volume increase; above 200 ° C, ε-carbide conversion to cementite, resulting in volume reduction. Studies have also shown that retained austenite can also decompose under external load or at lower temperatures (even at room temperature), resulting in dimensional changes in the part. Therefore, in actual use, the tempering temperature of all bearing parts should be higher than the use temperature of 50 ° C. For parts with higher dimensional stability, the content of retained austenite should be reduced as much as possible, and a higher tempering temperature should be used. .
Pipe Fitting Eccentric Reducer,Seamless Concentric Reducer,Black Steel Pipe Fittings
Jimeng Highstrength Flang-Tubes Group Co., Ltd. , http://www.jmpipefitting.com