Abstract
Avoiding low austenite fractions and nitride formation are major challenges in laser welding of duplex stainless steels (DSS). The present research aims at investigating efficient means of promoting austenite formation during autogenous laser welding of DSS without sacrificing productivity. In this study, effects of shielding gas and laser reheating were investigated in welding of 1.5-mm-thick FDX 27 (UNS S82031) DSS. Four conditions were investigated: Ar-shielded welding, N2-shielded welding, Ar-shielded welding followed by Ar-shielded laser reheating, and N2-shielded welding followed by N2-shielded laser reheating. Optical microscopy, thermodynamic calculations, and Gleeble heat treatment were performed to study the evolution of microstructure and chemical composition. The austenite fraction was 22% for Ar-shielded and 39% for N2-shielded as-welded conditions. Interestingly, laser reheating did not significantly affect the austenite fraction for Ar shielding, while the austenite fraction increased to 57% for N2-shielding. The amount of nitrides was lower in N2-shielded samples compared to in Ar-shielded samples. The same trends were also observed in the heat-affected zone. The nitrogen content of weld metals, evaluated from calculated equilibrium phase diagrams and austenite fractions after Gleeble equilibrating heat treatments at 1100 °C, was 0.16% for N2-shielded and 0.11% for Ar-shielded welds, confirming the importance of nitrogen for promoting the austenite formation during welding and especially reheating. Finally, it is recommended that combining welding with pure nitrogen as shielding gas and a laser reheating pass can significantly improve austenite formation and reduce nitride formation in DSS laser welds.
Highlights
Duplex stainless steels (DSS), with a nearly equal amount of ferrite and austenite, have received much attention in recent years, thanks to offering both high corrosion resistance and superior mechanical properties [1]
The effects of using pure argon or nitrogen as shielding and backing gas and that of laser reheating were investigated in laser welding of 1.5-mm-thick FDX 27 DSS
4- An almost fully ferritic microstructure was seen at the surface of Ar-shielded samples, while the N2-shielded samples had an acceptable distribution of austenite at the surface
Summary
Duplex stainless steels (DSS), with a nearly equal amount of ferrite and austenite, have received much attention in recent years, thanks to offering both high corrosion resistance and superior mechanical properties [1]. In DSS, a balanced phase fraction is of vital importance as the best combination of mechanical properties and corrosion resistance comes by approximately equal fractions of ferrite and austenite [2]. Laser welding has the potential to accelerate the fabrication of components [3, 4]; some metallurgical challenges. The effect of shielding gas on welding of DSS, as one of the most important factors of controlling weld properties, has been studied [15,16,17].
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