Analysis of Helium-Induced Cracking in Laser Beam Weldments Performed on Irradiated 304L Austenitic Stainless Steel

Abstract

This document presents the results obtained under an extension (Tasks 6–10) of the research contract with the Electric Power Research Institute (EPRI), under US Department of Energy proposal no. NFE-18-07249. The document finalizes the research activity performed earlier under two Rapid Turnaround Experiment proposals and the original research contract with EPRI (research tasks 1–5, reported previously). Section 1 briefly discusses the importance and necessity of performing welding on the preirradiated austenitic steels—materials of nuclear power plants’ internal components. Section 2 describes the specimens selected for the current project and briefly documents key experimental methods and tools. Section 3 discusses the results obtained during the work; each subsection presents the results related to a particular task and provides a limited analysis. Of special interest are in situ mechanical tests, revealing the complexity of crack development and fracture processes in the irradiated and welded specimens containing helium. Preexisting cracks tended to blunt and usually did not propagate at small strains. Fractography analysis revealed the complex morphology of the fracture surface. The locations inside the heat-affected zone (HAZ) demonstrated intergranular cracking or transgranular cleavage fracture. Degraded grain boundaries (ones with a high density of helium bubbles) showed specific fracture surfaces—so-called helium puddles—with shallow dimples. Instead of a relatively small fraction of observed degraded grain boundaries, most of the HAZ exhibited a brittle (either intergranular or transgranular cleavage) fracture type, suggesting a very low fracture resistance of the HAZ in welded specimens with helium. The results presented in the document help elucidate the degradation processes and microstructure evolution during the welding of austenitic stainless steels containing helium.