Effect of δ-ferrite in welded ER308 and ER316 microstructure on hydrogen embrittlement

Abstract

In this study, the hydrogen embrittlement (HE) behavior confined to focusing the weld microstructure of austenitic stainless steels (ER308 and ER316) was investigated. To investigate the microstructural influence on HE, microstructural changes were induced by different heat-treatment times at 1050 °C. These changes were prepared from microstructures containing δ-ferrite and microstructures where δ-ferrite was removed at welded parts. The results of the slow strain rate test (SSRT) indicated that the HE index, calculated from relative reduction width (RRW), was greatly reduced due to microstructure changes. To demonstrate the correlation between the deformation mechanism and HE index, deformation behavior was analyzed and verified through chemical composition, microstructure, fracture morphology, and modified Curssard–Jaoul (C–J) analysis. In all heat-treatment conditions, more mechanical twinning and a more drastic change in the HE index were observed for ER 308 compared to that of ER 316. The longest heat-treatment time conditions in a greatest decrease in HE index for ER308 and ER316. Hydrogen decohesion effects due to accumulation with dislocation were verified observing a parallel twin plane or microvoid along the twin plane in a transgranular fracture. In additional, as the heat-treatment time increased, the removal of δ-ferrite decreased the HE index for both stainless steels. Because, δ-ferrite was assisted in hindering mechanical twinning by misostrain concentrated at δ-ferrite sites in the early state of SSRT, it was enhanced to the resistance of HE.