| Bearing is the fundamental structural component of modern industrial equipment and the critical component of national major technical equipment.In recent years,with the rapid development of aerospace technology,the bearing’s DN（product of diameter and speed）value is increasing,and the service temperature is getting higher and higher.In order to meet the requirements of higher contact fatigue performance of the new generation of aviation-bearing steel under harsh conditions such as high temperature,high speed and high load,a new type of high Cr-Co-Mo high-temperature stainless bearing steel has been developed.Thermal processing and heat treatment process are the premise of obtaining the ideal recrystallization structure and improving the bearing steel’s comprehensive properties during preparation.The key to improving bearing steel’s service life is to refine austenite grains and martensite laths,and obtain fine dispersed precipitates and stable retained austenite by microstructure control.In this paper,the constitutive equation,hot processing map and dynamic recrystallization model are established by testing the true stress-strain curve of the test steel in the predetermined deformation range to provide a reference for the hot processing of aerospace precision bearings.In addition,the microstructure and mechanical properties of the test steel were studied by the new heat treatment process of cyclic quenchingcritical annealing and compared with the conventional quenching and tempering heat treatment.The strengthening and toughening mechanism of the test steel after cyclic quenching necessary annealing was revealed,which provided a reference for the hot processing and heat treatment of the new high Cr-Co-Mo bearing steel.The main conclusions are as follows:1.The flow stress of the tested steel decreases with the increase of deformation temperature or the decrease of strain rate during hot deformation,and the peak stress appears in the early stage of the true stress-strain curve.Based on the hot deformation curve,the Arrhenius-type constitutive equation and dynamic model（DMM）processing map of the test steel were established.The interaction between hot deformation parameters and carbides has a great influence on the recrystallization process.Combined with the microstructure observation,the optimum process parameters for the test steel forming were obtained:temperature range 1000～1070°C,strain rate range0.01～0.003 s-1.2.Dynamic recrystallization（DRX）occurs only when the strain reaches the critical strain（）,and the test steel is more prone to DRX at high temperatures and low strain rates.The simulated effective stress and DRX volume fraction of the tested steel are close to the measured values,indicating that the established DRX kinetic model can be used to predict the DRX behavior of the tested steel during hot deformation.3.Compared with conventional quenching and tempering,the test steel’s austenite grains and martensite laths are refined after cyclic quenching,and the unstable retained austenite is transformed into martensite to improve the hardness of the test steel.After critical annealing treatment,the test steel can generate a certain amount of reversed austenite.The stability of the reversed austenite is reduced by increasing the critical annealing temperature.4.After the new heat treatment process（cyclic quenching 620/600℃two-step critical annealing）,the test steel obtained excellent strength and toughness matching.The yield strength increased from 884.67 MPa to 1334.26 MPa,and the elongation increased from 12.46%to 16.56%.The test steel’s microstructure refinement and the reversed austenite’s TRIP effect are the main reasons for improving strength and toughness. |
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