Microstructural Heterogeneity and Mechanical Properties of a Welded Joint of an Austenitic Stainless Steel

This research presents the microstructural and mechanical evolution throughout the welded seam of an austenitic stainless steel (ASS) tube. It was found that the main hardness decrement occurred in the fusion zone (FZ), followed by the heat-affected zone (HAZ) and the base material (BM). Optical microscopy indicated a dendritic structure in FZ and heterogeneous austenitic grain size from the HAZ towards the BM, ranging from 100 mu m to 10 mu m. The welding process generated an intense texture around the FZ and the HAZ, while the BM still showed an extrusion-like texture. In terms of mechanical behavior, the largest austenite grain size in the FZ led to the lowest strength and ductility of all zones due to the earliest strain localization manifested by heterogeneous strain distribution. However, the strain localization in all zones appeared after 0.4 true strain, indicating an overall good ductility of the seam. These high values were related to two microstructure characteristics: (1) the 10% delta-ferrite after solidification in the FZ favored by the C-req/Ni-eq=1.67 relationship that delayed the crack propagation along the austenite grains and (2) the heterogeneous microstructure made up of soft austenite and hard martensite in the HAZ and BM producing multiple strain concentrations. Kernel Average Misorientation (KAM) maps obtained by Electron Back-Scattering Diffraction (EBSD) allowed observing higher internal misorientations in the FZ than in the HAZ due to interconnected walls between the delta-ferrite grains. However, the largest KAM values were observed in the BM between gamma-austenite and the deformation-induced alpha'-martensite phases. X-ray diffraction revealed that the residual stresses in the cross-section of the welded seam were compression-type and then switched to tension-type in the outer surface.

Automated summary

This research investigates the microstructural and mechanical changes in the welded seam of an austenitic stainless steel tube. The fusion zone experiences the greatest decrease in hardness, followed by the heat-affected zone and the base material. The welding process results in a dendritic structure in the fusion zone and varying austenitic grain sizes from the heat-affected zone to the base material. The mechanical behavior of the seam is influenced by the size of austenite grains, with the larger grains in the fusion zone leading to lower strength and ductility.