Thermal Stability Of The Plasma-Nitrided Layer On Fe-Based Superalloy GB2132

The nitrided layer of Fe-based superalloy GH2132 is mainly composed of the supersaturated solid solution of nitrogen in austenite,the expanded austenite(?_N),with high hardness and good corrosion resistance.Thermal stability of the nitrided layer was investigated by heating the nitrided samples at different temperature and with different time.The evolution of microstructure and phase constituents were analyzed by X-ray diffraction(XRD),optical microscopy(OPM),scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS).Surface hardness,tribological properties and corrosion behaviour of heated and unheated nitrided layers were evaluated.The following conclusions are reached:(1)When heating the nitrided sample below the nitriding temperature(200-400 ?),with different heating time,the ?_N phase does not decompose,which exhibits good thermal stability.The diffusion of nitrogen towards to matrix and the surface would occur without long-range diffusion of other main alloy elements(Cr,Ni and Fe).(2)When heating the nitrided samples above the nitriding temperature(500-700 ?),with different heating time,the ?_N phase would decompose.And the transformation of ?_N?? N(less expansion)+CrN would occur.The amount of CrN would increase with raising heating temperature and increasing heating time.Precipitation of CrN formed on the grain boundary and proceeded by discontinuous precipitation mechanism to form cellular structure of CrN and y.And the degree of lattice expansion of the ?_N phase would gradually decreased.(3)The surface hardness and wear-friction properties of the nitrided samples heated at different temperatures does not decrease compared to the nitrided sample unheated.(4)The corrosion resistance of the nitrided samples heated at different temperatures decreased slightly compared with the nitrided sample unheated.