Effect of Pre-nitriding on Double Glow Plasma Chromizing of Carbon Tool Steel

For lowering the temperature of double glow plasma chromizing and enhancing the velocity of chromizing at relatively low temperature,the ion-nitriding at 550 ℃ was performed firstly,and then low temperature of 560 ℃×4 h double glow plasma chromizing was conducted.Microstructure and hardness of the chromizing layer were studied.The results show that the chromizing layer formed with different processes consist of sediment layer and diffused layer.The sediment depth is 4-5 μm and micro-structure is compact and combined with basis closely.The chromium content and micro-hardness increase with the increase of pre-nitriding times and have a gradient distribution.The depth of diffused layer of the sample without nitriding is about 20 μm,the main phases of chromizing layer are Fe,Fe-Cr,Cr7C3,Cr23C6 and the micro-hardness is about 700 HV.The depth of diffused layer after pre-nitriding is about 25-30 μm,the phases of chromizing layer is Fe-Cr,Cr7C3,Cr23C6,CrN,and the micro-hardness is 915-1 250 HV which increases 45% compared with that without pre-nitriding.