Process Study On Quenching & Tempering And Gas Nitriding Of Steel 1Cr12Ni3MoVN

The investigation of quenching & tempering as well as gas nitriding for 1Cr12Ni3MoVN steel was conducted due to the issues,such as,large contact load,wear failure and low service life for 1Cr12Ni3MoVN steel components served in gas turbine.The relationship between the microstructure and properties,the microstructural evolution and kinetics as well as the mechanical properties of the modified layer were studied according to a series of experiments and analysis of microstructure and properties.The research results are as follows:The austenitizing temperature is in the range of 940-1250? according to thermodynamic calculation.The specimen with sorbite microstructure was obtained after quenched at 1050? and tempered at 650? based on the optical microscope.The hardness,tensile strength,yield strength,elongation and reduction of area are HV0.1325,994 MPa,844MPa,20%,64%,respectively,according to microhardness test and tensile experiment.The tempered specimens were nitrided for 4-32 h at 580 ? with the ammonia dissociation rate of 65%.The layer growth as the duration conforms to parabolic law and the thickness of the hardened layer is 133-335 ?m based on the optical microscope.The surface hardness decreases with the nitriding duration increasing.The surface hardness of the specimens treated at 580 ? with the ammonia dissociation rate of 65% for 4 h and 32 h are HV0.11217 and HV0.11003,respectively.The ratio of decrease is 17.58%.The hardness of the location about 50-100 ?m below the top surface is low when the specimens were nitrided at 560? with with ammonia dissociation rate of 64% for 28 h and kept constant temperature with ammonia dissociation rate of 100% for 4h.However,the hardness of the location over 100 ?m below the top surface are not affected completely.The thickness of the compound layer is 5-10 ?m in the present work.Vein-like and network structure appeared in the grain boundary at the surface layer with the nitriding time increasing.The diffusion treatment for 4 h after the nitriding significantly reduces the thickness of the vein-like structure and compound layer.Correspondingly,the thickness of the diffusion layer increase slightly.The nitrides precipitated on the surface are composed of ?-Fe2-3N,?'-Fe4N and slight of CrN for the specimens nitrided at 580 ? for 4-32 h according to the results of SEM and XRD.The proportion of ?-Fe2-3N phase in the compound layer increases first and then decreases.The maximum value was obtained when nitrided for 8 h.The ?'-Fe4N is main phase in the compound layer after the treatment of nitriding for 28 h and diffusion for 4 h.The results of the tribological experiment for nitrided specimens demonstrate that the main wear mechanism is adhesive and oxidation wear.More severe adhesive wear occurred when the nitriding temperature increased or the diffusion treatment was added(increase of ammonia dissociation rate).The main cause of oxidation wear is due to the incorporation of oxygen from the environment during the wear tests.According to the requirements of design,the process parameters for 1Cr12Ni3MoVN steel and parts is quenching at 1050? for 2h,then tempering at 650? for 3h,next,gas nitriding at 580? for 28h(with ammonia dissociation rate of 75%)and finally diffusion treatment at 580? for 4h(with ammonia dissociation rate of 100%).