Effect of a novel salt bath nitriding process on corrosion resistance of K55 steel

K55 pipeline steel was treated by a novel salt bath nitriding process,the cross-sectional morphology,microhardness distribution along the depth direction,and sulfur corrosion resistance of the nitriding layer were studied.The K55 steel nitrided in salt bath at 560℃ for 2 h consists of three layers including loose layer,nitriding layer,and diffusion layer of substrate.The thickness is 12.48μm for the nitriding layer and 1.54μm for the external thin loose oxidation layer.The total penetration depth of nitriding layer is about 30μm.The microhardness of K55 steel is improved remarkably after nitriding and up to 695 HV at the surface,but decreased sharply with increasing penetration depth.The corrosion resistance of K55 steel to sulfur is improved markedly after nitriding.Salt bath nitriding can be used to reduce the corrosion defects of pipelines before their application in oil wells.