Effect of heat input and post-weld heat treatment on surface, texture, microstructure, and mechanical properties of dissimilar laser beam welded AISI 2507 super duplex to AISI 904L super austenitic stainless steels

Abstract

In this study, AISI 2507 super duplex and AISI 904L super austenitic stainless steel sheets were successfully joined using a laser welding method with two different laser powers and laser speeds. The effects of different welding heat inputs and post-weld heat treatment on the surface, texture, microstructure and mechanical properties of laser welded joints were investigated. The results obtained in this study showed that the weld metal microstructure predominantly composed of austenite phase. In addition, the occurrence of γ-austenite (dendritic), interdendritic, ferrite, and planar, coplanar, and stacking fault was observed and also Cr23C6 precipitate was formed in the microstructure with the effect of heat treatment. In the sample joined with low heat input, an increase was observed in tensile strength, yield strength and hardness compared to the samples joined using high heat input, and a decrease in impact toughness occurred. With the effect of heat treatment, a decrease was observed in tensile, yield strength and hardness and an increase was observed in ductility and toughness. In addition, there was no significant change in the bending forces of all laser-welded samples, and the bending occurred on the super austenitic stainless steel side. While the austenite grains exhibited random orientation in the non-heat-treated and heat-treated AISI 2507 base metal and weld metal, the non-heat-treated and heat-treated AISI 904L base metal showed an orientation in the [101] and [111] directions in the transverse direction, respectively. The change in the ratio of the Schmid factor and coincidence site lattice (CSL) boundaries has a significant effect on the mechanical properties. In addition, since the kernel average misorientation (KAM) values in the interdendritic, ferrite grains and grain boundaries were higher than the austenite grains, the residual stresses were found to be higher. When the γ-austenite (dendritic), interdendritic and δ-ferrite textures were examined in the 3D-orientation distribution function maps of the non-heat-treated and heat-treated base metal and weld metal, γ-fibre and α-fibre orientations were observed at high density, and recrystallisation component (112) 〈111〉 at high density and component (113)〈332〉 with high toughness at high density appeared. The γ-austenite grains in the base metal and weld metal of the non-heat-treated and heat-treated samples showed a closer Kurdjumov–Sachs (KS) orientation relationship (OR) compared to the interdendritic and δ-ferrite grains. In the γ/δ and γ/interdendritic ORs, KS OR was generally observed, while Bain OR was observed in the heat-treated super duplex base metal. ORs known in the literature were not observed in the non-heat-treated weld metal. Densities observed in the non-heat-treated and heat-treated weld metal showed that as the sputtering time increased, the O1s spectrum decreased, and the Fe 2p, Cr 2p, Ni 2p and Mo 3d spectra increased.