Technical Letter Report on the Cracking of Irradiated Stainless Steels in Low-Corrosion-Potential Environments

Abstract

Irradiation-assisted stress corrosion cracking (IASCC) is a major degradation mechanism for reactor internal components of austenitic stainless steels (SSs) in light water reactors. Crack growth rate (CGR) and fracture toughness J-R curve tests were performed on four irradiated SS specimens in high-purity water with low dissolved oxygen and in a simulated pressurized water reactor (PWR) environment. The samples had been previously irradiated from ~5 to 8 dpa in the BOR-60 reactor. The materials were cold-worked (CW) 316 and solution-annealed (SA) 304L SSs, which are commonly used in PWR core internals. Cyclic and constant-load CGRs were obtained from these samples to assess their cracking behaviors in low-corrosion-potential environments. A Ti-stabilized specimen, CW 316-Ti SS, was found to be highly vulnerable to IASCC, and its cracking susceptibility increased with neutron dose between ~5 and 8 dpa. The CW 316 and SA 304L SSs showed moderate cracking susceptibilities in the low-corrosionpotential environments up to ~7 dpa. Periodic partial unloading (PPU) had a significant impact on the crack growth behavior of all irradiated specimens, and the measured CGRs were more than one order of magnitude higher in the test periods with than without PPU. A "stair-step" crack growth behavior was also observed for all irradiated specimens during the test periods with PPU, suggesting a strong effect of dynamic loading on IASCC. The SA 304L SS showed the best fracture resistance, and the CW 316 SS was the most brittle among the tested specimens. The fracture toughness J values of the two CW 316-Ti SS specimens were nearly identical at ~5 and ~8 dpa.

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“…Cast austenitic stainless steels (CASSs) are used in light water reactors (LWRs) for components with complex shapes, e.g., pump casings, valve bodies, elbows, control rod guide tube spaces, etc. CASSs in LWR core internals can experience hardening and embrittlement due to extended exposures to elevated temperatures and/or neutron irradiations [1][2][3][4][5]. Previous studies have shown that thermal aging increased the yield and tensile strength, and decreased the ductility, impact energy and fracture toughness of CASS materials [1][2][3][4].…”

“…However, the ferrite is susceptible to embrittlement during thermal aging. The mechanisms of thermal aging embrittlement of CASSs have been studied extensively [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. It is suggested that thermal aging embrittlement at 300-500°C are associated with several microstructural processes in the ferrite phase: (1) the formation of Cr-enriched αʹ phase through spinodal decomposition, (2) precipitation of G-phase (M 6 Ni 16 Si 7 , M = Mn, Cr), and (3) the precipitation of carbides at the austenite-ferrite phase boundaries.…”

Phase stability in thermally-aged CASS CF8 under heavy ion irradiation

“…CASSs in LWR core internals can experience hardening and embrittlement due to extended exposures to elevated temperatures and/ or neutron irradiations [1][2][3][4][5]. Previous studies have shown that thermal aging increased the yield and tensile strength, and decreased the ductility, impact energy and fracture toughness of CASS materials [1][2][3][4].…”

“…Previous studies have shown that thermal aging increased the yield and tensile strength, and decreased the ductility, impact energy and fracture toughness of CASS materials [1][2][3][4]. A recent study suggests that a combination of thermal aging and irradiation may further reduce the fracture resistance of CASS alloys [5]. A potential interaction between the two degradation processes of irradiation damage and thermal aging needs to be evaluated for the long-term service of CASS components.…”

“…However, the ferrite is susceptible to embrittlement during thermal aging. The mechanisms of thermal aging embrittlement of CASSs have been studied extensively [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. It is suggested that thermal aging embrittlement at 300-500°C are associated with several microstructural processes in the ferrite phase: (1) the formation of Cr-enriched a 0 phase through spinodal decomposition,…”