| The family of maraging stainless steel is attractive for many industrial sectors, such as medical device,aerospace engineering and nuclear reactor due to its high strength,high toughness,good weldability,good machinability and with strong wear resistance and high corrosion resistance together.Recently with its use in the metal belt CVT in automotive industry,the application of the maraging stainless steel is becoming more and more wide.The materials used in metal belt CVT need good ductility combined with high wear resistance.So,to improve the wear resistance of the maraging stainless steel has important engineering and ecomomical values. Regarding to this background,the present work was focused on plasma nitriding process of maraging stainless steel to improve its wear resistance.The precent alloy melted is a 00Cr12Ni9Mo4Cu2Ti maraging stainless steel. After solution treatment and cold rolling,the steel then plasma nitrided at various temperature and time.By means of optical microscopy,microhardness testing,X-ray diffractometry(XRD),Scanning electron microscopy(SEM),Energy dispersive spectrometer(EDS),wear resistance measurement and salt spray corrosion test,the microstructure,microhardness,wear resistance and corrosion resistance properties of the nitrided layers treated at various nitriding processes were investigated.The results showed that after plasma nitriding process,the specimens can form a nitriding layer with thickness of 40～100μm.When the specimens were treated at 400℃,the microstructure was dominated by nitrogen supersaturated martensite,αN(expanded martensite).While the nitriding temperature was up to 500℃,the nitrided layer consisted ofαN,γ'-Fe4N andεphase.CrN phase began to form at this temperature.When treated at a higher temperature about 600℃,CrN andεphase became dominant,γ'-Fe4N slowly decreased andαN was decomposed at the same time.With increase of nitriding time,the amounts ofεphase and CrN phase increased slightly.The surface hardness and wear resistance were increased after plasma nitriding process and the highest surface hardness of 1350HV50g was obtained at 500℃,4h. The microhardness changed with nitriding temperature and time in a parabolic curve. Referring to the XRD and EDS datas,it is considered that the increase of microhardness was related to the formation of the hard phases ofαN,γ',εand CrN, while the decrease in microhardness was related to the formation of the high temperature brittle phases and the microstructure coarsening.The change of wear resistance with nitriding temperature and time exhibits a similar way of hardness.Compared with the untreated specimens,the corrosion resistance declined after plasma nitriding process.With the increase of nitriding temperature and time,the corrosion resistance decreased monotonously.This could be attributed to the chrome depletion in the matrix due to the formation of chrome-rich precipitates.The study also found that cracks often formed in the nitrided layers in the present maraging stainless steel.These cracks appeared always at the grain boundaries of the martensite,and there was no cracking in retained austenite phase.The presence of local residual stresses in the nitrided layers is considered to be the causes of cracks.
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