Low temperature salt bath nitriding of 304austenitic stainless steel

A 304 austenitic stainless steel was nitrided in salt bath at 430 ℃ for different time,and the effects of nitriding time on microstructure and properties of the nitrided layers were investigated.The phase components,thickness,microhardness and microstructure of the nitrided layers were examined by X-ray diffraction(XRD),optical microscopy,hardness test and scanning electron microscopy(SEM),respectively.The results show that thickness and hardness of the nitride layers increase with increasing nitriding time.The nitrided layer only contains a single S phase after nitriding for 1 h,and CrN appears in the nitride layer after nitriding for 8 h.With prolonging treating time over 16 h,the nitrided layer is composed of S phase and CrN.Corrosion resistance of the nitrided layers was evaluated by potentiodynamic polarization tests,and the results indicate that the nitriding treatment improves the pitting corrosion resistance of stainless steel.When nitriding time is 4 h,the sample exnibits better corrosion resistance than untreated sample.With prolonging nitriding time over 8 h,the corrosion property of the nitrided sample deteriorates.