Fracture Properties of Welded 304L in Hydrogen Environments

Austenitic stainless steels are used for hydrogen containment of high-pressure hydrogen gas due to their ability to retain high fracture properties despite the degradation due to hydrogen. Forging and other strain-hardening processes are desirable for austenitic stainless steels to increase the material strength, and thus accommodate higher stresses and reduce material costs. Welding is often necessary for assembling components, but it represents an area of concern in pressure containment structures due to the potential for defects, more environmentally susceptible microstructure, and reduced strength. Electron beam (EB) welding represent an advanced joining process which has advantages over traditional arc welding techniques, through reduced input heat and reduced heat-affected zone (HAZ) microstructure, and thus present a means to maintain high strength and improve weld performance in hydrogen gas containment. In this study, fracture coupons were extracted from EB welds in forged 304L and subjected to thermal gaseous hydrogen precharging at select pressures to introduce different levels of internal hydrogen content. Fracture tests were then performed on hydrogen precharged coupons at temperatures of both 293 K and 223 K. It was observed that fracture resistance (JH) was dependent on internal hydrogen concentration; higher hydrogen concentrations resulted in lower fracture resistance in both the forged 304L base material and the 304L EB welds. This trend was also apparent at both temperatures: 293 K and 223 K. EB weld samples, however, maintain high fracture resistance, comparable to the forged 304L base material. The role of weld microstructure solidification on fracture is discussed.