Análise da influência de elemento estabilizador da austenita em aço inoxidável hiperduplex UNS S32707 soldado por processo TIG

Abstract

Stainless steels having a duplex structure are widely applicable in chemical, cellulose, oil and gas industries, and others. The interest for these materials comes from their good mechanical properties and corrosion resistance. These characteristics are attributed to its microstructure composed by equal proportions of ferrite and austenite phases. However, when submitted to welding the balance of these phases can be modified, as well as, deleterious phases can appear, damaging duplex stainless steels performance. This study aimed to investigate the efficiency of nitrogenous addition into the shielding gas of autogenous TIG welding in the maintenance of the equilibrium of ferrite and austenite phases, as well as its effects on hardness proprieties and corrosion resistance of the welding joints. Possible undesirable effects were also analyzed, such as formation of deleterious phases and nitrites. Weld beads using pulsed arc TIG welding were done on samples of stainless steel hyperduplex UNS S32707. Shielding gas used on was argon and nitrogenous, which nitrogenous was added in different quantities for each specimen, 1.5%, 2.5%, 3.5%, 4.5% and 5.5%. The techniques employed were optical microscopy, scanning electron microscopy, atomic force microscopy, semi-quantity chemical analyze by EDS (X-Ray Energy Dispersive Spectroscopy) and quantification of volumetric fraction of existing phases by Digital Image Processing. For evaluation of mechanical properties, Vickers microhardness measurements were realized. Microhardness values did not present significant variation on testing conditions, including base metal, with 95% reliability. Fusion zones obtained from shielding gas with higher N2 concentration, resulted in increasing of austenite content, 39% to 57%, using 1.5% and 5.5% nitrogen, respectively. In consequence, elevated N2 concentration contributed also to better corrosion resistance. Samples with addition of 3.5%, 4.5% and 5.5% N2, had corrosion rate down to 0.0001 g/cm2, which indicates no corrosion process by pitting. Corrosion testing has indicated values above limits established by Standard for samples obtained from 1.5% and 2.5% N2 concentration. Spite of this result, these samples were submitted to macroscopic analyze, which it was not observed presence of corrosion points. Deleterious phases or nitrites were not identified for all testing conditions.