In order to make the material factors that affect the bearing life at its best, you need to control before quenching steel original organization, technical measures can be taken: high temperature (1050 ° C) austenitic quick cooling to 630 ° C isothermal normalizing pseudoeutectoid Analysis of fine pearlite or cooled to 420 ° C isothermal treatment, bainite. Wrought heat can be rapid thermal annealing, fine granular pearlite, to ensure that the carbides are small and evenly distributed. The original organization of this state quenching austenite, dissolved in austenite carbide, undissolved carbides will gather into a fine granular.
When the original tissue steel must when the quenched martensite carbon content (i.e., the carbon content of austenite after quenching), the amount of residual austenite, and the amount of undissolved carbides vary depending on the quenching temperature and hold time , with the increased quenching temperature (time constant), the steel the undissolved carbides reduce the number of (quenched martensite increased carbon content), increased number of retained austenite, hardness first with the increase of quenching temperature increases, reached a peak and then decreases as the temperature rises. When the quenching temperature is constant, the with the austenitizing extension of time, the number of undissolved carbides reduce the increase in the number of retained austenite, the hardness increased, a long time, this trend slowed. When fine carbides in the original tissue, due to carbides, easily dissolved in austenite, and therefore the hardness after quenching peak towards lower temperature and shorter austenitizing time.
In summary, GCrl5 steel after quenching undissolved carbides at about 7% to about 9% of retained austenite (hidden martensite the average carbon content of about 0.55%) is the best organization. Moreover, when the original organization carbide small, evenly distributed in the composition of the reliable control of the level of microstructure, is conducive to high mechanical performance, which has a high life. Should be noted that the original organization has a fine dispersion of carbides, hardening heat insulation, undissolved carbides grew up together, to coarsening. Therefore, with this original organization bearing parts quenching time not too long, rapid heating, quenching austenite process will be given a higher overall mechanical properties.
Bearing parts after quenching and tempering larger surface residual compressive stress leads to heated in the hardening carburizing or nitriding atmosphere, surface carburizing or nitriding for a short time. Since this steel quenching austenite carbon content is not high, far below the phase diagram showing the equilibrium concentration, and therefore can be sucked carbon (or nitrogen). When austenitic a high content of carbon or nitrogen, Ms lower the quenching surface than the inner and center martensitic transformation occurs, resulting in a larger residual stress. GCrl5 steel carburizing atmosphere and non-carburizing atmosphere heating quenching and low temperature tempering treatment, contact fatigue test can be seen, the the surface carburizing life than not carburizing increased 1.5 fold. The reason is that the surface of the carburized parts having a larger residual stress.
The main material factors and the degree of control that affect the service life of carbon chromium steel bearing parts:
(1) carbide steel in the original organization before quenching requires small dispersion. Can be high-temperature austenitic of 630 ° C or 420 ° C high temperature, Wrought heat rapid thermal annealing process.
(2) For GCr15 steel after quenching, asking for the average carbon content of about 0.55% hidden martensite, about 9% Ar and 7% was uniform, the round state undissolved carbides in the microstructure. Quenching can be used to control this microstructure heating temperature and time.
(3) Part quenching and tempering requirements of surface residues have a greater compressive stress, which helps the fatigue resistance. The surface of the short-term carburizing or nitriding treatment process may be used in quenching, so that the surface remains larger compressive stress.
(4) manufacturing steel bearing parts are required to have high purity, is mainly to reduce the content of O2, N2, P, oxides, and phosphides. ESR can be used to vacuum smelting technology measures the the material oxygen content ≤ 15PPM appropriate.
