Avaliação da soldagem multipasse de chapas espessas do aço inodável lean duplex UNS S32304 soldadas pelos processos SMAW, GMAW E FCAW

Abstract

The duplex stainless steels (DSS) have been presented as an excellent alternative for applications where high corrosion resistance and high mechanical strength are required. In parallel, it has been seen a rush of the DSSs manufacturers to obtain ever cheaper alloys. In this context, the lean duplex stainless steels (LDSS) come up with lower levels of high cost alloying elements, such as nickel and molybdenum. However, the DSS, including the LDSS, has worse weldability in comparison with the austenitic stainless steels, due to the difficulty in maintaining sufficient proportions of austenite in the heat affected zone (HAZ) in parallelwith no precipitation of deleterious intermetallic phases in the microstructure. Thus, this study aims to evaluate the multipass welding of 22mm thick plates of the lean duplex stainless steel alloy UNS S32304. Initially, it was carried out the characterization of the base metal andwelding consumables. Then, in order to determine the heat input to be applied in later stage, specimens were welded by bead on plate technique using autogenous GTAW process with four different heat input (HI): 1.0; 1.5; 2.0; 2.5 kJ.mm-1. Lastly, six specimens were weldedthrough the SMAW, GMAW and FCAW processes applying consumables of two different chemical composition, 2209 (22% Cr, 9% Ni and 3% Mo) and 2307 (23% Cr and 7 % Ni). It was determined the productivity by acquiring the open arc time, while the soundness was evaluated through the x-ray tests. After that, it was extracted specimens for tensile, bending.Charpy, macrography, microhardness, optical microscopy and SEM with EDS tests. Both the base metal and welding consumables have presented chemical composition and mechanical properties within the ranges specified by the standards for each material. The GTAW weldmetal (WM) presented coarse ferrite grains and low level of austenite, while the HAZ showed high ferritization, but at a lower intensity than the WM. The HI of 2.0 kJ.mm-1 has been chosen by its relatively higher proportion of austenite. Acceptable discontinuities according ASME B31.1 were found in some of the experiments, but it did not influence the mechanical results, which have presented above the base metal and standards. The metallographic analysis showed the austenite proportion above 48% in the WM microstructure, while it was obtained in some cases 17% of austenite in the heat affected zone. The welding process that produces slag (SMAW and FCAW) have presented higher levels of non-metallic inclusions, identified by EDS as oxides of Mn, Si, Ti and Al. So, it was possible to obtain sound welds that met the mechanical requirements in all experiments, however, all experiments had lowproportions of austenite in the HAZ, which suggests increasing the heat input in future work.