Stress corrosion cracking of simulated heat-affected zone in a CF8A weld in high temperature water

Abstract

In this study, thermal simulation was performed to conduct thermal cycles on CF8A cast austenitic stainless steel substrates with different ferrite contents to simulate the microstructures that form near a weld fusion boundary. The effects of ferrite content and morphology on the mechanical properties of CF8A were investigated. The stress corrosion cracking (SCC) susceptibility of the specimens was evaluated with slow strain rate tensile (SSRT) tests in a simulated boiling water reactor (BWR) coolant environment.The results indicated that an increase in the ferrite content of the CF8A base metal led to a minor increase in hardness. Thermal simulation caused a marked increase in ferrite content, especially in the high ferrite CF8A substrate. The yield strengths of the tested samples increased obviously with increasing ferrite content, but the ductility decreased. The ultimate tensile strengths of the samples reached a plateau when the ferrite number (FN) was greater than 18. The applied thermal simulation greatly improved the impact toughness of high ferrite CF8A in comparison with the original substrate. The results indicated that the samples with high ferrite content had a shorter rupture life but greater fracture strength than those with low ferrite content strained in high temperature water. Moreover, cleavage-like fracture was found in all of the samples that suffered from SCC. The SCC cracks were observed to be more likely to propagate along the δ/γ interfaces of a CF8A. Furthermore, crack initiation and growth inside the γ of low ferrite CF8A contributed to a great loss in strength in a high temperature water environment.