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TRODUCTION

THE type of 17-4 precipitation hardening (PH) stainless steel (AISI 630) is widely used as a structural material for chemical and nuclear power plants, such as light water reactors (LWRs) and pressurized water reactors (PWRs), due to its high strength, high fracture toughness, good weldability, and ease of machinability.[1–4] However, its wider application is restricted by its poor tribological properties, which have necessitated the development of advanced surface engineering technologies to address the problem.[5–7] There have been several surface modification methods to improve the properties of 17-4PH stainless steel.[8–16] Fortunately, it has been demonstrated that low-temperature plasma nitriding is an effective means of increasing surface hardness and improving wear resistance of stainless steels.[17–20] In particular, low-temperature plasma nitriding can improve the wear resistance of popular stainless steels without loss of corrosion resistance, by producing a metastable phase of supersaturated nitrogen atoms (nitrogen concentration is 20 to 30 at. pct), JUN WANG, Associate Professor, and HONGYUAN FAN, Professor, are with the School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, P.R. China. Contact e-mail: [email protected] YUANHUA LIN, Professor, and DEZHI ZENG, Associate Professor, are with the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, P.R. China. JING YAN, Engineer, is with the Southwest Oil and Gasfield Company Research Institute of Natural Gas, Chengdu 610023, P.R. China. Manuscript submitted April 27, 2012. Article published online December 15, 2012. 414—VOLUME 44B, APRIL 2013

which has high hardness and good corrosion resistance, in the modified surface.[21–25] Previous reports claimed that the nitriding treatment must be carried out at low temperature of 623 K to 753 K (350 C to 480 C) to prevent CrN precipitation and segregation of this kind of stainless steel, which will greatly reduce the corrosion resistance.[26–28] Recently, Manova et al.[12] reported the formation of expanded martensite in nitrogen ion implanted 17-4PH steel at temperatures ranging from 653 K to 673 K (380 C to 400 C); a transition to CrN and the disappearance of expanded martensite took place at 693 K (420 C). In addition to the formation of expanded martensite, Sun and Bell[13] and Dong et al.[14] indicated that the existence of an expanded austenite layer was detected by X-ray diffraction (XRD) and transmission electron microscopy (TEM). However, Kochman´ski and Nowacki[15,16] did not find the expanded austenite but Fe0.0324N and expanded alpha-Fe during activated gas nitriding of 17-4PH steel at temperatures ranging from 743 K to 843 K (470 C to 570 C). Liu and Yan[27] conducted 17-4PH stainless steel plasma nitrocarburized at 733 K (460 C) for improving its mechanical properties without compromising its desirable corrosion resistance, and they found the phases compositions in the nitrocarburized layer are