Caracterização metalúrgica de juntas de aço inoxidável superduplex soldadas por processo TIG autógeno

Abstract

The duplex stainless steels are of great importance in industry, mainly in oil and natural gas due to their high mechanical strength and excellent corrosion resistance. They are characterized by having biphasic structure, consisting of nearly equal proportions of ferrite and austenite. The present work characterized welded joints by autogenous TIG process of duplex stainless UNS S32760. Four groups of samples were prepared, from the variation of welding current and consequently of heat input (20A and 40A - heat input: 0.12KJ/mm and 0.19KJ/mm) and the composition of shielding gas (pure argon or argon containing 2.5% nitrogen). Techniques of characterization by colored metallography, analysis and digital image processing and Vickers microhardness tests techniques were used. To evaluate the corrosion resistance, tests were performed at open circuit potential with a solution of ferric chloride (FeCl3) and reference electrode of saturated calomel. The quantitative analysis of ferrite and austenite phases present in the welded joints showed that the addition of nitrogen in shielding gas favored the formation of austenite phase, ranging from 11% (without nitrogen) to 26% (with nitrogen) the amount of this phase In a qualitative analysis the variation of heat input (0.12KJ/mm and 0.19KJ/mm) resulted in an increase of the ferrite phase s grain size.