Evaluation of an induction-assisted friction stir welding technique for super duplex stainless steels † Ana I. Álvarez, a Miguel García, a Gloria Pena, a * Jose Sotelo b and David Verdera b The microstructure changes and the mechanical properties of a friction stir welded (FSW) super duplex stainless steel (GX2CrNiMoN26-7-4) were analyzed. A PCBN tool was used to weld 5-mm-thick plates at a constant rotational speed of 300 rpm, at 100 and 200 mm/min, under two different conditions: conventional FSW and Induction-Assisted FSW (IA-FSW). Preheating technique allows welding with a reduction in forge forces close to 31% at the same welding speed, or doubling speed (200 mm/min) at the same axial force, obtaining sound welds. No sigma phase was detected in the welds, and the clear grain size reduction led to an increase in mean hardness value and the tensile strength of the stir zone. Ferrite percentage was found to be between 50 and 70% through the welds. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: SDSS; FSW; induction-assisted FSW; microstructure; mechanical properties Introduction The excellent resistance to stress corrosion of the super duplex stainless steels is due to their two-phase microstructure, containing well-balanced proportions of ferrite and austenite. This characteris- tic and their good mechanical properties make them suitable for service in highly corrosive conditions and where high strength is required: offshore oil and gas exploration and production and in heat exchangers in petrochemical and chemical processing. Joining of these materials represents a significant challenge since the melting and the solidification processes associated with conventional fusion welding can radically change the optimum phase balance. Also the balance of alloying elements can be disturbed, because of the metastable nature of the weld, leading to the precipitation of brittle phases (chromium nitrides, sigma phase) and secondary austenite, in the weld metal and in HAZ. All these transformations cause a loss of mechanical and corrosion properties. Consequently, the development and assessment of more suitable joining techniques for super duplex stainless steels is a key research issue for the future application of these materials. FSW is a relatively new solid-state joining process initially developed for Al alloys [1] and now well established for low melting temperature metals. [2,3] One of the main problems that limit the extension of FSW to high melting point materials is the limited tool life, [4,5] since tools have to withstand the temper- atures, loads and stresses involved with welding such materials. With the developing of appropriate tools (WRe, PCBN and the composite between them [6,7] ) for welding steels and other high temperature alloys, an increasing number of works has been published in the last years. However, tool wear or unpredictable tool breakage is not completely solved, and tool cost is still too high. These difficulties have led to various attempts to reduce the forces experienced by the tool. One of the ways that is being explored is the use of preheating techniques to soften the material to be welded. Softening the material would cause a decrease in tool wear and therefore an increase in tool life. [8,9] This approach is termed ‘hybrid’ or ‘assisted-FSW’ Although some references exist on the use of a TIG arc-welding torch, the most common systems are laser or induction preheating systems. [9] Nowadays, only a few papers are related to the use of FSW on duplex and superduplex stainless steels, but the maintenance of the duplex structure and a grain refinement in the stir zone is reported. [10–12] The aim of this study is to evaluate the applicability of the induction preheating technique on the FSW of a commercial cast super duplex stainless steel SDSS analyzing the microstructural changes and the mechanical properties of friction stir welded joints. Experimental procedure The base material used in this study is a 5-mm-thick plate of cast super duplex stainless steel GX2CrNiMoN26-7-4 (Nr 1.4469). The nominal composition of the grade is (wt%) C 0.03 max., Si 1.00 max., Mn 1.00 max., P 0.035 max., S 0.025 max., Cr 25.0–27.0, Mo 3.0–5.0, Ni 6.0–8.0, N 0.12–0.22 and Cu 1.3 max. The minimum tensile strength of this cast super duplex stainless steel in the * Correspondence to: G. Pena, ENCOMAT Research Group, School of Industrial Engineering - University of Vigo, 36310-Vigo, Spain. E-mail: gpena@uvigo.es † Paper published as part of the ECASIA 2013 special issue. a ENCOMAT Research Group, School of Industrial Engineering, University of Vigo, 36310 Vigo, Spain b AIMEN Technology Centre, Joining Technology Plant, Relva 27A, 36410-O Porriño, Spain Surf. Interface Anal. 2014, 46, 892–896 Copyright © 2014 John Wiley & Sons, Ltd. ECASIA special issue paper Received: 30 August 2013 Revised: 13 December 2013 Accepted: 7 February 2014 Published online in Wiley Online Library: 1 April 2014 (wileyonlinelibrary.com) DOI 10.1002/sia.5442 892