Conventional heat treatment process of the bearing

1. High carbon chromium bearing steel annealing isothermal spheroidizing annealing of high carbon chromium bearing steel spheroidizing annealing is to obtain ferrite matrix uniformly distributed on the fine carbide particles, small, uniform, round, for cold working and final quenching tempering after tissue preparation. Traditional spheroidizing annealing process is slightly higher than the temperature of Ac1 ( Such as GCr15 is 780 ~ 810 ℃) Slow cooling after heat preservation with furnace ( 25℃/小时) Below 650 ℃ from air cooling. The process of heat treatment time is long, More than 20 h) , and the carbide particles are not uniform, after annealing effect after cold working and the microstructure and mechanical properties of the final quenching tempering. Later, according to characteristics of supercooled austenite transformation, development of isothermal spheroidizing annealing process: in the cold heat, quick to Ar1 below a certain temperature range, 690~720℃） Isothermal, completed in the process of isothermal austenite to ferrite and carbide, shift can be directly released after the completion of air cooling. This process has the advantage of saving heat treatment time ( The whole process about 12 ~ 18 h) , after processing the carbide tiny evenly in the organizations. Another time saving process is repeated spheroidizing annealing: cooling after it was first heated to 810 ℃ to 650 ℃, cooling after heated to 790 ℃ to 650 ℃ from air cooling. Although the process can save some time, but more complex operation. 2. High carbon chromium bearing steel martensite quenching tempering martensite conventional quenching tempering microstructure and properties of organization: martensite, residual austenite, not soluble ( Residues) Carbide formed martensite organization forms: under the metallurgical microscope ( Magnification is generally lower than 1000 times) Martensite can be divided into plate strip martensite and lamellar martensite two kinds of typical organization, usually for strip and flake of martensite after quenching hybrid organization or between the two forms - in the middle - A handful of martensite - - Bearing industry on the so-called cryptocrystalline martensite, crystalline martensite under electron microscopy at high magnification, the substructure can be divided into dislocation tangles and twin. Their specific organization form mainly depends on the carbon content of austenite matrix the higher the temperature, the original organization is unstable, the austenitic matrix of carbon content is higher, the organization of retained austenite after quenching, the more the more lamellar martensite, the greater the size, the structure of the greater the proportion of twin, and easy hardening microscopic cracks. The matrix carbon content less than 0. 3%, the martensite mainly dislocation substructure of lath martensite; The matrix carbon content is higher than 0. 6%, mixed with the twin martensite is dislocation substructure of lamellar martensite; The matrix carbon content is 0. 75%, appear with visible ridge surface of large plate martensite, and the growth of lamellar martensite when collisions with microcrack. , the increase of the austenitizing temperature quenching hardness, toughness, but the austenitizing temperature is too high caused by excessive residual austenite after quenching hardness decrease. Conventional martensite after quenching organization content of retained austenite in general is 6 ~ 15%. Residual austenite as soft metastable phase, under certain conditions, Such as tempering, the use of natural aging or parts) in the process of , the instability occurred is decomposed into martensite and bainite. Decomposition is the aftermath of the components of the hardness, toughness, size change and affect the size precision of parts even work properly. Bearing parts of high dimension accuracy requirement, generally want to residual austenite as little as possible, such as supplement water or cryogenic treatment after quenching, using high temperature tempering, etc. Residual austenite can improve the toughness and crack propagation resistance, certain conditions, the workpiece surface residual austenite can also reduce contact stress concentration, improve the contact fatigue life of bearings, this case on the technology and material composition take certain measures to keep a certain amount of residual austenite and improve its stability, such as joining austenitic stabilizing element Si, Mn, stabilizing treatment and so on. Conventional martensite quenching tempering process after heating to 830 ~ 860 ℃ heat preservation, quenching in oil, low temperature tempering GCr15 steel bearing: 150 ~ 180 ℃ GCr15SiMn steel bearing parts: 170 ~ 190 ℃, 50 - - 78 ℃ cold treatment of martensite step quenching residual austenite stability to obtain high dimensional stability and high toughness. Tempered martensite quenching distortion and dimensional stability of parts of shape and size, the uniformity of the original organization, when the state of rough machining before quenching, quenching temperature and heating speed, workpiece put way, into the oil way, the characteristics of quenching medium and circulation mode, such as the temperature of the medium influence the deformation of the parts. Measures to control deformation: the rotational hardening, die quenching and control parts into the oil way of steam to boil membrane phase transition temperature is too high, the big cold speed of thermal stress in the low yield point of austenite deformation caused by distortion of spare parts. Deformation is oiled uneven between individual parts or components, especially with the new oil is easier to appear this kind of situation. In the Ms point of cooling rate on the deformation, the decisive under Ms points and using the low temperature cold speed can reduce the deformation. Because of different cooling ways, the diameter of the ring will have different degree of 'enlargement', and the increase of medium temperature, the ring size diameter increases levels tend to be more consistent, namely 'horn' shape deformation is reduced, at the same time, the ring of the elliptical deformation decreased; Inner ring for stiffness is bigger, the deformation is less than the outer ring.