Study On The Microstructure, Toughness And Fatigue Properties Of Co-Mo Stainless Bearing Steel

In the aerospace field,the bearings face a harsh service environment,however,the higher Co and Mo elements in the bearing steel greatly affect the performance of the steel.Therefore,it is very important to study the influence of Co and Mo on the strength and toughness of bearing steel and the performance of low Co and Mo steel for the development and application of new air bearing steel.This paper studied the microstructure and toughness of steels after the contents of Co and Mo based on 12Co-5Mo steel.The influence of Co and Mo on the strength and toughness of low carbon Cr-Co-Mo-Ni stainless steel bearing is discussed.The heat treatment process and torsional fatigue properties of 12Co-5Mo steel with higher Co and Mo were studied and the notched.tensile properties of 12Co-5Mo steel,5Co-2Mo-3Ni steel and 8Co-3Mo steel with promising application were studied.We have made high temperature torsion performance of 12Co-5Mo steel.We observed the structure of the test steel,the substructure,the second phase and the fracture morphology by optical microscope,SEM,EDS,XRD,TEM and other means of analysis.Quenched organization and tempering organization of test steels ware lath martensite and retained austenite.The difference between Creq and Nieq become bigger result from the influence of Co and Mo element which cause retained austenite reduced in organization of test steels and produced 8-ferrite.Furthermore,the carbide changed from granular M6C with 1.17%to acicular M2C with 0.15%.The effect of carbides on the strength of test steels is relatively large.The toughness of the test steel is mainly affected by ?-ferrite,If the content of ?-ferrite more than 6.3%,it would cause brittleness emerge in test steels.But brittleness not reduced when contents of 8-ferrite less than 1.1%.Hardness,impact energy and strengthness of the steels had not change much with different solution temperature.However,Fracture toughness raise a little bit as the solution temperature risesed.Carbide was the major factor of influenced toughness of test steels.When carbide content reached 2.5%,fracture toughness would obviously reduce.The best mechanical properties of steel would achieved when the solution temperature selected 107? and cold treatment process maintain for two hours under-90?.Smooth stretch fracture of test steels was toughness fracture,notched stretch fracture of it was cleavage fracture.As the stress concentration factor of the sample increases,fracture cleavage platform increased and crack propagation accelerated.Furthermore,the velue of NSR which ratio of the strength of notched stretch samples and Smooth stretch samples of 12Co-5Mo steel and 5Co-2Mo-3Ni steel would be larger.Than their reinforcement effect of the notch became more and more.At a critical point Kt of 8Co-3Mo,NSR reaches the maximum and reinforcement effect of the notch of the steel was strongest.This paper studied the process of the high temperature torsion fracture of test steels,samples began fracture under torsion stress,than Shear-like dimple formed after crack extended with fast rate.When the crack arrives near the center of the extension,nearly instantaneous fracture zone formed.Lastly,samples fractured at the center and equiaxial dimple formed.With the increase of test temperature,the content and spacing of carbides in steel did not change,but the size increased slightly.With the stress rised,the fracture type of 12Co-5Mo steel is in accordance with the order of NF+TS—NF+LS—LS.The failure modes of steel are mainly caused by surface failure(S)and near surface failure(SI).The type of defects is mainly composed of oxide inclusions.The value of ?Kth was 1.46?1.83 MPa·m1/2 under under type ? loads,The value of ?Kth was 3.66?4.57 MPa·m1/2.At the beginning of the crack formation,the crack satisfies the condition of steady state expansion if ?K more than 1.46 MPa·m1/2.When no large cracks existed,fiber formed after ?K increased to 3.66 MPa·m1/2.The load process of 12Co-5Mo steel during fracture was:type ? load-type ? load—type ? load-type ? + ? load.When the large crack appeared,fiber zone would not appear,that load process was type ? load—type ?+ ?load.Under 350MPa stress,if inclusion size less than 5um,the fatigue life of the steel would more than 107 cycles.