Effects of long-term thermal aging on bulk and local mechanical behavior of ferritic-austenitic duplex stainless steels

Ferritic-austenitic cast duplex stainless steels (CDSS) undergo significant thermal aging embrittlement in service in light water nuclear reactors. Cast CF-3 and CF-8 duplex stainless steels have been laboratory aged for up to 17,200 h at four different temperatures (280 °C, 320 °C, 360 °C, 400 °C) in order to characterize the evolution of microstructure and mechanical properties. Standard Charpy V-Notch (CVN) impact testing and tensile testing have been conducted to measure the progression of embrittlement of the steels. Concurrently, nanoindentation measurements were performed to measure the hardening of the individual ferrite and austenite phases. Atom probe tomography (APT) analysis of the constituent phases is used to investigate the relationship between spinodal decomposition of the ferrite and the mechanical property evolution during aging. The effects of the local microstructure and property changes in the ferrite phase on the overall bulk mechanical property degradation of these steels are discussed.