Heat treatment and heat input effects on the dissimilar laser beam welded AISI 904L super austenitic stainless steel to AISI 317L austenitic stainless steel: Surface, texture, microstructure and mechanical properties

Abstract

In this study, AISI 904L super austenitic - AISI 317L austenitic stainless steel sheets were successfully welded by laser welding method using six different heat inputs. Welded samples went through 1 h of post-weld heat treatment (PWHT) at 1100 °C and then left to cool at room temperature. The effect of PWHT and welding heat input on surface, texture, microstructure and mechanical properties has been examined. According to the results obtained in the study, it was observed that the weld metal microstructure of the samples welded at the lowest and highest heat input values consisted entirely of austenite (dendritic + interdendritic) grains. With PWHT, Cr23C6 precipitation was observed in both grain boundaries and austenite grains. In addition, austenite as well as dislocation and stacking fault formations were observed in the microstructure. While the highest austenite content was observed in the microstructure of AISI 317L, the content of austenite increased in all samples with PWHT. It was observed that most of the grains that were orientated in the [111] direction were orientated in the direction of [101] and [001] after PWHT. However, the KAM values of the dislocation in grain boundaries decreased with the effect of PWHT. In addition, with the reduction of the ratio of dark grains and the effect of re-crystallisation, (112) <111> components representing austenite grains appeared in FCC/BCC texture. It was identified that due to these textural changes, the tensile strength and resistance to bending of the samples were negatively affected but their toughness was positively affected. AB3 sample with the highest tensile strength, hardness, bending strength and lowest toughness had values of 615 MPa, 214 HV, 1100 N and 57 J, respectively, while these values changed to 550 MPa, 181 HV, 720 N and 60 J with the effect of PWHT. Due to the presence of the chromium-oxide layer, it was identified that the density of O− ion and H− ion was at high levels on the surface of the weld metal. It was identified that the density of these ions decreases with increasing depth, while Fe−, Cr− and Ni− ions increase. Excellent K–S and N–W orientation relationship (OR) was observed between FCC/BCC-structured grains present in the microstructure of without PWHT base metals and weld metal, while Bain OR was also observed in weld metal with or without PWHT. In the microstructure of the base metals with PWHT, there was no OR relationship between grains with FCC/BCC structures.