The effect of thermal aging and boiling water reactor environment on Type 316L stainless steel welds

Abstract

The thermal aging and consequent embrittlement of materials are ongoing issues in cast stainless steels and duplex stainless steels. Spinodal decomposition is largely responsible for the well known "475°C" embrittlement that results in drastic reductions in ductility and toughness in these materials, and this process is operative also in welds of either cast or wrought stainless steels where 6-ferrite is present. While the embrittlement can occur after several hundred hours of aging at 475°C, the process is also operative at lower temperatures, at the 288°C operating temperature of a boiling water reactor (BWR) for example, where ductility reductions have been observed after several tens of thousands of hours. An experimental study has been completed in order to understand how the spinodal decomposition may affect material properties changes in BWR pipe weld metals as well as the effects of the BWR environment on Type 316L stainless steel welds. This thesis also represents the first systematic and rigorous investigation of environmental fracture. In addition, weld metal centerline SCC crack growth rate has been quantified. Material characterization includes SCC crack growth, in-situ fracture toughness, fracture toughness in air, as well as Charpy-V and tensile property evaluation as a function of aging time and temperature. SCC crack growth rate results in BWR normal water chemistry indicate an approximately 2X increase in crack growth rate over that of the unaged material. In-situ fracture toughness measurements indicate that environmental exposure can result in a reduction of toughness by up to 40% over the corresponding at-temperature air values. This has been termed "environmental fracture" Detailed analyses of the results strongly suggest that spinodal decomposition is responsible for the degradation in properties measured ex-environment. SCC crack growth rate and fracture toughness have been linked to the microstructural features of the Type 316L weld metal. Analysis of the results also strongly suggests that the in-situ properties degradation is the result of hydrogen absorbed by the material during exposure to the high temperature aqueous environment.