PSI - Issue 16

Cristian Alejandro de León Gómez et al. / Procedia Structural Integrity 16 (2019) 265–272 Cristian Alejandro de León Gómez et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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power generation. However, exposure of DSS´s to the arc fusion welding thermal cycle deteriorates these properties as a result of profound changes in microstructure and balance of phases; i.e. incre asing and coarsening δ -phase, as shown by Varol et al. (1992), Gunn (1997) and Kordatos et al (2001). The weld metal (WM) solidifies into coarse columnar structures virtually in a completely ferritic matrix and γ -phase grows at the ferrite grain boundaries either allotriomorphic (AA) and Widmanstätten (WA) shaped or acicular intragranularly (AcA) , as shown by Varol et al. (1992) and Muthupandi et al. (2003). In the HAZ, the amount of γ -phase regenerated depends on the thermal cycle and it can be up to 60 % less than the initial content of the base metal. Varol et al. (1992) and Badjii et al. (2008) reported that this effect impacts on the mechanical properties with a large reduction in ductility due to the reduction of γ -phase and the precipitation of detrimental phases in this region. Thus, it is desirable to reduce the size of the HTHAZ and increase the regeneration of γ - phase avoiding precipitation of detrimental phases whilst maintaining the δ/γ phase ratio close to 50/50 in the microstructure of a DSS welded joint. Badjii et al. (2008) and Jiang et al. (2003) have shown that promoting a grain refinemen t of δ -phase during solidification yields improved balance of phases and mechanical properties. Varol et al. (1992), Badjii et al. (2008) and Sathiya et al. (2009) showed that while there are attempts to refine the weld metal grain structure of austenitic and ferritic stainless steels there is not research in terms of grain refinement of DDS´s δ matrix during welding to promote an increase of sites for nucleation and growth of γ -phase. Instead, research efforts have focused on post-weld heat treating or adding el emental γ -phase stabilizers. This study seeks to evaluate the use of EMS during welding in terms of δ phase grain refinement and its effect on the δ/γ ratio and on the mechanical properties. The behavior of the cracks in the components and structures of use in engineering constructed with AID is strongly influenced by the mode and type of load, and the microstructural characteristics including chemical composition of the alloy, volumetric fraction of the phases, distribution, grain size and heat treatment, Miller (1987). Although some researchers have proposed to minimize this problem with post-weld thermal treatments. García et al. (2014) have proposed the low intensity electromagnetic interaction (IEMBI) during welding with inert gas by GMAW electric arc to minimize the precipitation of unwanted phases, as well as such as the redistribution and the change of the phase relationship δ / γ, which greatly improved the resistance to localized corrosion since it suppressed the precipitation of the σ and χ phases, as well as the formation of chromium carbides and nitrides. The present research evaluates the effect of the application of an external magnetic field (CME) of 3 during the process of welding GMAW of the stainless steel duplex 2205 in the microstructural evolution associated to the thermal cycle of the welding and its impact in the increased resistance to localized corrosion and the resistance to fatigue cracking. Plates of 2205 DSS (6.35 × 70 × 150 mm) with a single V groove preparation, Fig. 1a, were welded using the GMAW process with an ER-2209 electrode of 1.2 mm in diameter (160 mm/s) and as shielding gas a mixture of 98% Ar + 2% O 2 (17 L/min). Fig. 1b shows the experimental setup for welding with the application of an external magnetic field. The axial magnetic fields (AMF) applied during welding were 0, and 3 mT. The welding torch was displaced at 3.6 mm/s with a stick out of 10 mm. Current (236 to 248 A) and voltage (27.5 V) were adjusted in order to maintain an approximate heat input of 1.4 kJ/mm considering an efficiency of 75% for the GMAW process. For the preparation of the specimens, transversal cuts were made to the direction of the weld bead and machined in a prism with a length of 5 mm, a height of same measure and a length of 10 cm, containing the weld bead in the centre, as shown in Fig. 2. Subsequently, they were polished with SiC paper of different grit up to 1200 and with diamond paste until a completely smooth surface was obtained. To reveal phases and grain boundaries, an electrochemical attack was performed with the 30% HNO 3 solution. 2. Experimental Procedure