Heat treatment of hottest bearing materials

Heat treatment of bearing materials

the heat treatment quality control of bearing parts is the strictest in the whole machinery industry. Bearing heat treatment has made great progress in the past 20 years, mainly in the following aspects: Research on the basic theory of heat treatment; Research on heat treatment process and application technology; Development of new heat treatment equipment and related technologies

1 annealing of high carbon chromium bearing steel

spheroidizing annealing of high carbon chromium bearing steel is to obtain the microstructure with fine, uniform and round carbide particles uniformly distributed on the ferrite matrix, and to prepare for the future cold processing and final quenching and tempering. The traditional spheroidizing annealing process is to heat preservation at a temperature slightly higher than AC1 (such as 780~810 ℃ for GCr15), and then slowly cool down with the furnace (25 ℃/h) to below 650 ℃ for air cooling. The heat treatment time of this process is long (more than 20h) [1], and the particles of carbide after annealing are uneven, which affects the future cold working and final quenching and tempering structure and properties. Then, according to the transformation characteristics of supercooled austenite, isothermal spheroidizing annealing process is developed: after heating, it is quickly cooled to a certain temperature range below AR1 (690~720 ℃), isothermal, and the transformation of austenite to ferrite and carbide is completed in the isothermal process. After the transformation is completed, it can be directly out of the furnace for air cooling. 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 repeated spheroidizing annealing: heat it to 810 ℃ for the first time, then cool it to 650 ℃, heat it to 790 ℃ and then cool it to 650 ℃ for air cooling. Although the process can save a certain amount of time, the process operation is more complicated

2 martensitic quenching and tempering of high carbon chromium bearing steel

2.1 microstructure and properties of conventional martensitic quenching and tempering

in recent 20 years, the development of martensitic quenching and tempering process of conventional high carbon chromium bearing steel is mainly divided into two aspects: one is to carry out the influence of quenching and tempering process parameters on Microstructure and properties, such as microstructure transformation during quenching and tempering, decomposition of residual austenite, toughness and fatigue properties after quenching and tempering [2~10]; On the other hand, the technological properties of quenching and tempering, such as the influence of quenching conditions on size and deformation, dimensional stability, etc. [11~13]

the microstructure of conventional martensite after quenching is composed of martensite, retained austenite and insoluble (residual) carbides. Among them, the microstructure of martensite can be divided into two types: under the metallographic microscope (the magnification is generally less than 1000 times), martensite can be divided into two typical structures: strip martensite and sheet martensite. Generally, after quenching, it is a mixed structure of lath and sheet martensite, or jujube martensite in the middle of the two (so-called cryptocrystalline martensite and crystalline martensite in the bearing industry); Under high-power electron microscope, its substructure can be divided into dislocation entanglement and twins. The specific microstructure mainly depends on the carbon content of the matrix. The higher the austenitic temperature is, the more unstable the original structure is, the higher the carbon content of the austenitic matrix is, the more retained austenite in the quenched structure is, the more lamellar martensite is, the larger the size is, the greater the proportion of twins in the substructure is, and quenching microcracks are easy to form. Generally, when the matrix carbon content is less than 0.3%, martensite is mainly lath martensite with dislocation substructure; When the matrix carbon content is higher than 0.6%, martensite is a sheet martensite with mixed substructure of dislocation and twins; When the carbon content of the matrix is 0.75%, large-scale martensite with obvious mid ridge surface appears, and there are microcracks at the impact of lamellar martensite when it grows [8]. At the same time, with the increase of austenitizing temperature, the hardness increases and the toughness decreases after quenching. However, if the austenitizing temperature is too high, the hardness decreases due to too much residual austenite after quenching

the content of residual austenite in the structure after conventional martensite quenching is generally 6~15%, and the residual austenite is a soft metastable phase. Under certain conditions (such as tempering, natural aging or the use of parts), its instability will decompose into martensite or bainite. The consequence of decomposition is that the hardness of the parts is increased, the toughness is reduced, and the size changes, which affects the dimensional accuracy of the parts and even the normal work. For bearing parts with high dimensional accuracy requirements, it is generally expected that the less retained austenite the better, such as supplementary water cooling or cryogenic treatment after quenching, tempering at a higher temperature, etc. [12~14]. However, retained austenite can improve toughness and crack propagation resistance. Under certain conditions, retained austenite on the surface of the workpiece can also reduce contact stress concentration and improve the contact fatigue life of the bearing. In this case, certain measures should be taken in terms of process and material composition to retain a certain amount of retained austenite and improve its stability, such as adding austenite stabilizing elements Si and Mn, and carrying out stabilization treatment [15,16]

2.2 conventional martensite quenching and tempering process

conventional martensite quenching and tempering of high carbon chromium bearing steel is: the bearing parts are heated to 830~860 ℃ for heat preservation, 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. With the increase of tempering temperature and holding time, the hardness decreases and the strength and toughness increase. Suitable tempering process can be selected according to the working requirements of parts: GCr15 steel bearing parts: 150~180 ℃; GCr15SiMn steel bearing parts: 170~190 ℃. For parts with special requirements, higher temperature tempering is used to improve the service temperature of the bearing, or -50~-78 ℃ cold treatment is carried out between quenching and tempering to improve the dimensional stability of the bearing, or martensite step quenching is carried out to stabilize the residual 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

g. lowisch et al. [3, 8] studied the mechanical properties of bearing steel 100Cr6 quenched after twice austenitizing: first, austenitize at 1050 ℃ and quickly cool to 550 ℃ and then air cool after holding, to obtain uniform fine lamellar pearlite, and then austenitize at 850 ℃ and oil quenching. The size of martensite and carbide in the quenched structure is fine, and the carbon content and residual austenite content of martensite matrix are high, Through tempering at higher temperature, austenite decomposes, and a large number of fine carbides precipitate from martensite, which reduces quenching stress and improves hardness, strength and toughness and bearing capacity. Under the action of contact stress, its performance needs further research, but it can be inferred that its contact fatigue performance should be better than conventional quenching

jiuyu Sakai et al. [7] studied the microstructure and mechanical properties of SUJ2 bearing steel after cyclic heat treatment: heat it to 1000 ℃ for 0.5h to make the spherical carbide solid solution, and then pre cool it to 850 ℃ for oil quenching. Then repeat the thermal cycle from rapid heating to 750 ℃ and oil cooling to room temperature after holding for 1min for 1~10 times, and finally quickly heating to 680 ℃ and holding for 5min. At this time, the microstructure is ultra-fine ferrite plus fine carbide (ferrite grain size is less than 2 m, carbide is less than 0.2 m), and superplasticity appears at 710 ℃ (fracture elongation can be up to 500%), which can be used for warm processing and forming of bearing parts. Finally, heat it to 800 ℃ for heat preservation, oil quenching and tempering at 160 ℃. After this treatment, the contact fatigue life L10 is greatly increased compared with the conventional treatment, and its failure mode is changed from the early failure mode of the conventional treatment to the wear failure mode

bearing steel is austenitized at 820 ℃ and then subjected to short-term graded isothermal air cooling at 250 ℃, followed by tempering at 180 ℃, which can make the carbon concentration distribution in martensite after quenching more uniform, and the impact toughness is twice as high as that of conventional quenching and tempering. Therefore, В.В.БЁЛОЗЕРОВ 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 deformation and dimensional stability of martensite quenching and tempering

during martensite quenching and tempering, due to uneven cooling of all parts of the part, thermal stress and structural stress inevitably occur, resulting in the deformation of the part. The deformation of quenched and tempered parts (including size change and shape change) is affected by many factors, which is a very complex problem. For example, the shape and size of the part, the uniformity of the original structure, the rough machining state before quenching (the large amount of feed during turning, the residual stress of machining, etc.), the heating speed and temperature during quenching, the placement mode of the workpiece, the oil feeding mode, the characteristics and circulation mode of the quenching medium, and the temperature of the medium all affect the deformation of the part. A lot of research has been carried out at home and abroad, and many measures to control deformation have been put forward, such as rotary quenching, die quenching, and controlling the oil feeding mode of parts [11,13, 18]. Beck et al. Showed that when the transition temperature from vapor film stage to boiling stage is too high, large cooling rate will produce large thermal stress, which will deform austenite with low yield point and cause distortion of parts. If l is not replaced in time, it will affect the test results. Bben et al. Believe that the deformation is caused by the uneven immersion of oil between single parts or parts, especially when new oil is used. Tensi et al. Believe that the cooling rate at MS point plays a decisive role in deformation, and low cooling rate at and below MS point can reduce deformation. Volk's active identification and shutdown disposal muth 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 due to different cooling methods, the diameter of the ferrule will increase to varying degrees, and with the increase of the medium temperature, the diameter increase degree of the large and small ends of the ferrule tends to be the same, that is, the horn shaped deformation decreases, and at the same time, the elliptical deformation of the ferrule (VDP and V in a single radial plane, according to the composite polyurethane A-class plate with double-sided mortar and the detection report provided by some manufacturers, DP) decreases; The deformation of inner ring is smaller than that of outer ring due to its high rigidity

the dimensional stability of parts after martensite quenching and tempering is mainly affected by three different transformations [12,14]: carbon migrates from the martensitic lattice to form carbides, residual austenite decomposes and forms Fe3C, and the three transformations are superimposed. Between 50 ℃ and 120 ℃, the volume of the parts is reduced due to the precipitation of - carbides. Generally, the parts have completed this transformation after tempering at 150 ℃, and its influence on the dimensional stability of the parts in the future use can be ignored; Between 100~250 ℃, residual austenite decomposes and transforms into martensite or bainite, which will be accompanied by volume increase; The series production lines of belt furnace and muffle free belt furnace, casting chain furnace, roller hearth furnace and roller furnace are automatically completed from feeding, pre cleaning, heating under protective atmosphere (or controllable atmosphere), quenching, post cleaning (sometimes secondary cryogenic) and tempering. The automation degree and control precision are high, and the quality of the processed workpiece is uniform. Some production lines are also equipped with detection equipment to detect and control the deformation or quality of the processed workpiece. The whole heat treatment production line can be used as a part of the automatic bearing production line. Different heat treatment production lines are suitable for the heat treatment of different types and sizes of bearing parts according to their structural characteristics. For example, the belt furnace is suitable for small and medium-sized bearing rings; Roller hearth furnace is equipped with automatic lifting quenching device, which is suitable for bearing parts with large size; Roller furnace is suitable for rolling elements and small ferrules

(2) multi purpose furnace

the multi-purpose furnace combines controllable atmosphere heating and quenching under protective atmosphere to complete the oxygen free of the workpiece