Microstructure And Properties Of Cr-Co-Mo-Ni Bearing Steel Under Thermal And Dynamic Interaction

In this paper, a kind of high cleanliness Cr-Co-Mo-Ni bearing steel is regarded as the research steel. After the conventional heat treatment, the mechanical properties at room temperature state, the high temperature instantaneous tensile properties, the high-temperature creep properties under different creep stress conditions at350℃and500℃, the rotating bending fatigue tests before and after the carburizing at room temperature and the changes of the microstructure under each state’s tests of test steel were analyzed.After the heat treatment, the grain size of test steel is about20μm. The matrix structure are the martensite lath, the residual austenite and a small amount of M6C carbides. The width of martensite lath is about180nm and the density of dislocation is about6.9×1011cm-2. The average size of M6C type of carbide is0.3μm. At the room temperature, the tensile strength is1825MPa, the yield strength is1411MPa, the percentage elongation and reduction of area ratios are17.5%and62%, The hardness is51.5HRC and the impact power is72J. The test steel has excellent microstructure and mechanical properties.After high temperature instantaneous tensile at the temperature of200℃,350℃and500℃, the matrix structure have not changed significantly. With the increasing of stretching temperature, its robust performance shows a downward trend and the grains occur varying degrees of deformation. The tensile fracture is dark gray and it is ductile fracture.The test steels creep after different stresses and different time under the temperature of350℃and500℃. And the creep rupture life are respectively above and below1000h. After creep, the width of lath martensite is varying degrees changed, the dislocation density has declined and the original M6C carbides coarsen at different degrees. After creep at350℃, the M23C6type carbides are freshly precipitated and the size is about0.5μm. Numerous fine dispersed Laves phase precipitate on the matrix, which are in the average size of20nm or less and the area ratio about20%to32%. The creep mechanism of test steel is dislocation slip mechanism and the precipitation pore space is an important mechanism of nucleation of creep. The rotating bending fatigue strength before and after carburizing of test steel are566MPa and935MPa. After carburizing, the surface microhardness is960HV. The microhardness of carburizing specimen heart and the not carburizing sample are about560HV. After carburizing, the sample surface showed compressive stress. Before and after carburizing, the fatigue fracture modes are respectively main to surface of the substrate from cracking and inclusions from cracking. And the ratio are79%and74%respectively. The inclusions are spherical or irregular lumps in the presence of oxides such as Al2O3and MgO and the critical size of inclusions is17μm. The fracture type of specimens before and after carburizing are dimple-type.