Effects of Nickel Addition on the Microstructure of Laser-Welded UNS S32750 Duplex Stainless Steel
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lex stainless steel (DSS) has a biphasic microstructure with equal proportions of austenite and ferrite, which gives it excellent mechanical strength and corrosion resistance compared with austenitic stainless steel.[1,2] In general, after being subjected to welding processes, DSS presents an unbalanced microstructure, predominantly ferritic, which compromises its properties.[3] The Nd:YAG pulsed laser welding process offers some advantages over conventional processes, namely precision in the process, short cycle time, and lower heat input. Nevertheless, the low heat input and high cooling rates, which are typical of the process, favor the formation of ferrite, resulting in an unbalanced microstructure.[4,5] To obtain a balanced microstructure,
ELI J. DA CRUZ JUNIOR is with the Instituto Federal de Educac¸a˜o, Cieˆncia e Tecnologia de Sa˜o Paulo, Campus Votuporanga, Avenida Jeroˆnimo Figueira da Costa 3014, Votuporanga, SP, 15503110, Brazil. Contact e-mail: [email protected] OTACILIO D. FRANZINI is with the UNIFEV Centro Universita´rio de Votuporanga, Avenida Nasser Mara˜o 3069, Votuporanga, SP, 15503-005, Brazil. IRENE CALLIARI is with the Department of Industrial Engineering, University of Padova, Via Gradenigo, 6/a, 35131, Padova, Italy. VICENTE A. VENTRELLA is with the Mechanical Engineering Department, Campus of Ilha Solteira, UNESP, Sa˜o Paulo State University, Av. Brasil 56, Ilha Solteira, SP, 15385-000, Brazil. Manuscript submitted November 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
we must use a post-weld heat treatment, or add austenite-forming elements, such as nickel or nitrogen. In the literature, there are several studies concerning the effects of nickel and nitrogen addition on phase balance for certain welding processes, e.g., gas tungsten arc welding, power beam welding, and plasma welding.[6,7] However, no results have yet been obtained for effects related to Nd:YAG pulsed laser welding. Due to the importance of phase balance to the properties of DSS and in its applications, this work studied the effect of using electrolytic nickel foil as an addition metal on the microstructure of UNS S32750 DSS welded by the Nd:YAG pulsed laser welding. The base material used was a UNS S32750 super duplex stainless steel (SDSS), in sheets of 1.5 mm thickness with a nominal chemical composition of C max: 0.03 wt pct; Si: 0.3 wt pct; Mn: 0.6 wt pct; Cr: 25.6 wt pct; Ni: 7.0 wt pct; Mo: 4.0 wt pct; and N: 0.3 wt pct. Electrolytic nickel foil with a 30 lm thickness was used as the addition metal placed on the contact surface of the weld joint (Figure 1(a)). An Nd:YAG pulsed laser machine with a peak power of 7 kW, a wavelength of 1064 nm, and a beam spot diameter of 0.2 mm was used. Two conditions were evaluated: autogenous welding (AW), and welding with the addition of nickel (NiW), both with the same welding parameters. Single-pass welding was applied. The welding parameters were selected to give a regular surface, without porosity, and a deep weld pool higher than 50 pct of the sheet thickness, and we
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