Research On The Microstructure And Mechanical Behavior Of Carburized And Nitrided Cr-Mo Bearing Steel

After surface strengthening treatment,the medium and low carbon bearing steel is widely used in aerospace and other fields because of high surface hardness,good core toughness,high wear resistance and fatigue resist ance,and having the ability to resist impact load.It was studied the microstructure,strength and toughness of medium and low carbon Cr-Mo bearing steel,as well as the microstructure and hardness distribution characteristics of the carburized layer afte r nitriding and carburizing in this paper.It was also analyzed the microstructure characteristics of high strength and toughness in the core and surface and the interaction law of alloy elements.On this basis,the wear resistance and fatigue behavior of medium carbon Cr-Mo bearing steel were studied,In the meanwhile,the wear resistance and anti-fatigue property of surface strengthening layer was analyzed.Sorbite is the microstructure of medium carbon Cr-Mo steel after quenching and tempering.And the tensile strength,yield strength and impact energy of this steel are 1518 MPa,1261 MPa and 99 J.The microstructure of low carbon Cr-Mo steel is lath martensite after quenching,cold treatment and tempering.The ten sile strength,yield strength are 1435 MPa and 1263 MPa.While,the impact energy is only 43 J.It is found that the toughness of medium carbon chromium molybdenum steel is higher than that of low carbon chromium molybdenum steel.In medium carbon chromium molybdenum steel,the fine and uniform nano carbides can improve the strength of steel and also can maintain high toughness.After nitriding,the nitriding layer can be divided into compound layer and transition layer in this steel.The thickness of ion nitrogen layer is only 390?m.And the residual compressive stress is a s high as 610MPa at 300?m away from the surface of nitrided layer.After carburizing,the thickness of carburized layer of low carbon Cr-Mo steel is 2.3 mm.And the hardness is 849 HV at 0.5 mm away from the surface layer.The wear mechanisms of medium carbon Cr-Mo steel after nitriding are adhesive wear and fatigue wear.The wear mechanism is mainly adhesive wear when the loading force F is 90 N and the wear time T is 15 min.When the applied load F increases from 60 N to 150 N,the wear mechanism change s from adhesive wear to fatigue wear at a sliding speed V of 0.028 m·s^-1 and a wear time T is 15 min.When the wear time T increases from 1 min to 20 min,the formation of fatigue cracks is induced by the pulse structure of nitride layer at a loading force F of 90 N and the sliding speed V of 0.028 m·s^-1,and the wear mechanism changes from adhesive wear to fatigue wear.The wear mechanisms of low carbon Cr-Mo steel are adhesive wear,abrasive wear and fatigue wear.When the applied load F increasing from 60 N to 180N,crack nucleation is induced by M₂C carbide which rich in Mo element.Then,the fatigue crack is formed.It is showed that the bending fatigue strengt h of medium carbon chromium molybdenum steel is 860 MPa.While,the bending fatigue strength is 974 MPa through surface nitriding of this steel.When loading the stress lower than 980 MPa,the fatigue crack is mainly generated from the internal non-metallic inclusions of medium carbon chromium molybdenum steel.The fatigue fracture is mainly cause d by the surface roughness when the loading stress increasing higher than 980 MPa.The fatigue life decreases from 1.96?10⁵ to 1.58?10⁵ when the size of non-metallic inclusions ranging in 5.30 mm to 5.90 mm and the local stress increasing from 859.35MPa to 977.75 MPa.After nitriding,the fatigue cracks initiate in the surface compound layer and the inner non-metallic inclusions.The cracks propagate along the compound layer,the diffusi on layer and the matrix,finally,the cracks break down in the sample matrix.The propagation rate of crack is fast because of the lower toughness of low carbon Cr-Mo steel.And the rotating bending fatigue strength is 715MPa.After carburizing,the thick carburized layer makes the propagation rate of fatigue crack slow down.therefore,the rotating bending fatigue strength increases to 1067 MPa.