Plasma nitriding on welded joints of AISI 304 stainless steel J. Alphonsa a, ⁎, B.A. Padsala b , B.J. Chauhan b , G. Jhala a , P.A. Rayjada a , N. Chauhan a , S.N. Soman b , P.M. Raole a a Facilitation Centre For Industrial Plasma Technologies (FCIPT), IPR, Gandhinagar, Gujarat, India b Department of Metallurgical Engineering, M.S. University Baroda, Gujarat, India abstract article info Available online 2 June 2012 Keywords: Plasma nitriding Stainless steel Welded joints X-ray diffraction Plasma nitriding is widely used for the improvement of the mechanical/tribological properties of various steel components in many applications such as automobile parts, turbine parts, textile machinery parts, etc. How- ever, plasma nitriding on components with welding joints have not been intensively studied so far. In this study, we attempted to study the welded region of AISI 304 stainless steel after carrying out the plasma nitriding process. The plasma nitriding of stainless steel welded joints has been done with a H 2 :N 2 (4:1) gas mixture at 570 and 450 °C for different time durations. The parent and the welded regions were charac- terized using a microhardness tester, a scanning electron microscope (SEM) and an X-ray diffractometer (XRD) for surface hardness, case depth and phase formation respectively. The results show that the nitrided layer formed on the welded region consists of Fe 4 N and Fe 3 N, CrN and γ phases. Also, welded joints using the multiple pass technique gave better nitriding properties compared with that of the single pass technique. There is an improvement in the hardness by 3 times which could be correlated with structural changes and process parameters. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Austenitic stainless steels of AISI 304 are often used as construc- tion materials in chemical and food processing industries. Though, the corrosion resistance of these materials is excellent, their hardness and wear resistance are relatively low [1]. Plasma nitriding is a common surface hardening process in which nitrogen ions are intro- duced into steel or other iron based alloys at elevated temperatures (between 520 and 580 °C) to improve its hardness and wear resis- tance. It is more economical compared with conventional nitriding processes because it introduces faster nitrogen diffusion, which in turn allows for lower nitriding temperatures or shorter treatment times. It has been observed that though plasma nitriding of austenitic stainless steel at these temperatures showed an improvement of sur- face hardness and wear resistance, it was accompanied by reduction in corrosion resistance. This was due to chromium nitride precipita- tion and depletion of chromium in the substrate [2–6]. It is therefore, necessary to create a hard layer on the surface of austenitic stainless steels without compromising corrosion resistance. Plasma nitriding of stainless steels at temperatures of about 380 to 450 °C generally has no adverse effects on corrosion performance due to the formation of a hard nitrided layer composed of the γ N phase with an fcc struc- ture commonly known as expanded austenite [7]. The expanded austenite phase is also known as nitrogen supersaturated in solid solution. Many materials and joining processes are currently available for use in the industries. Welding is a reliable and efficient metal- joining process [8]. Stainless steel is routinely welded, but it must be welded under an inert gas atmosphere. The most reliable method for welding stainless steel is the tungsten inert gas (TIG) process, also known as gas tungsten arc welding (GTAW). TIG welding has the advantage of a small weld bead as it requires lower heat input, and filler metal is optional [9]. In welding, as the heat source interacts with the material, the severity of thermal excursions experienced by the material varies from region to region, resulting in three distinct regions in the weldment. These are the fusion zone (FZ), also known as the weld metal, the heat-affected zone (HAZ), and the un- affected parent metal (PM) [10]. The microstructure development in the FZ depends on the solidification behavior of the weld pool. The typical weld microstructure of stainless steel consists of austenite and residual ferrite. This is produced by primary ferrite solidification followed by secondary austenite solidification and ferrite transforma- tion to austenite during solid-state cooling [11]. Plasma nitriding of stainless steel with welded joints has not yet been reported according to the author's knowledge. In the present work, an attempt has been made to study the effect of plasma nitriding on the welded joints. The plasma nitriding process was done at two different temperatures depending on their applica- tions like wear resistance and corrosion resistance. Welding was done with multiple and single passes using low and high currents. A comparison is also made on the properties of the nitrided layer Surface & Coatings Technology 228 (2013) S306–S311 ⁎ Corresponding author at: Facilitation Centre for Industrial Plasma Technologies, Institute for Plasma Research, A-10/B, Sector 25, GIDC Electronic Estate, Gandhinagar 382044, Gujarat, India. Tel.: +91 79 23269029; fax: +91 79 23269001. E-mail addresses: alphonsa@ipr.res.in, alphonsai@yahoo.com (J. Alphonsa). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.113 Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat