Analysis of the GTAW process influence on UNS S32760 superduplex steel microstructure and corrosion resistance

Abstract

The use of Superduplex Stainless Steel, on Oil & Gas productive process equipment and piping, meet the requirements of high corrosion resistance, due to their high PREN (Pitting Resistance Equivalent Number), and excellent mechanical properties. However, these steels, because of the high alloy addition, can suffer loss of corrosion resistance and embrittlement when subjected to localized heating. This occurs due to the unbalance of the ferrite (δ) and austenite (γ) phases; formation of deleterious phases - "igma (σ), Chi (χ), secondary austenite (y2) and precipitates of Chromium Nitrides (Cr2N). In this context, the objective of this paper was to analyze the welding process on a UNS S32760 Superduplex steel specimen, subjected to the arc heat of GTAW process. The specimen was produced with the heat input of 1.116J/mm. The samples were analyzed by optical microscopy and scanning electron microscopy (SEM), before and after the corrosion test - according the method A of ASTM G48 Standard. Also, a micro hardiness test was performed to determine the changes on localized mechanical behavior, due to microstructural changes. The study determined: (a) the percentage of each phase present in the sample - before process; (b) the Vickers hardness profile - after welded; (c) the possible precipitation of deleterious phases, it is associated with de chemical composition of the alloy; (d) the zones and phases preferably attacked on corrosion test; and (e) Pitting morphology.