Synergistic effect of phase structure and thermal aging on the micro-scale cracking mode of AISI 316LN and Fe20Cr9Ni stainless steels in simulated PWR primary water

The effect of thermal aging at 400 °C for 5000 h on the stress corrosion cracking (SCC) behavior of AISI 316LN austenitic stainless steel (SS) and Fe20Cr9Ni cast duplex SS in pressurized water reactor (PWR) primary water was investigated using slow strain rate tensile (SSRT) testing. The fracture surface of AISI 316LN SS is mainly characterized by transgranular cracking with or without thermal aging. The density of secondary cracks of 316LN specimens in simulated PWR primary water was 31 ± 5 /mm 2 , whereas that of 316LN-A specimens was 69 ± 8 /mm 2 . The fracture surface of Fe20Cr9Ni cast duplex SS changed from a mixed fracture mode consisting of transgranular cracking and ductile fracture in the unaged specimen to a mixed fracture mode dominated by austenite/ferrite phase boundary (PB) cracking in the aged specimen. The spinodal decomposition occurred in the ferrite of the aged Fe20Cr9Ni steel, leading to the formation of Fe-rich α and Cr-rich α' phases, accompanied by G-phase precipitation. Semi-continuous Cr 23 C 6 carbides were observed along the PBs, and Cr depletion was detected on the ferrite side at the crack tip. A double-layer structure oxide film composed of an outer Fe-rich layer and a Cr-rich inner layer formed on the austenite side. The density of secondary cracks of Fe20Cr9Ni specimens in simulated PWR primary water was 27 ± 5 /mm 2 , whereas that of Fe20Cr9Ni-A specimens was 50 ± 4 /mm 2 .