Cold Work and Temperature Dependence of Stress Corrosion Crack Growth of Austenitic Stainless Steels in Hydrogenated and Oxygenated High-Temperature Water

Abstract

The rate of stress corrosion cracking (SCC) was measured for nonsensitized, cold-worked Type 316 (UNS S31600) and Type 304 (UNS S30400) in both hydrogenated pressurized water reactor (PWR) primary water and oxygenated water, each containing standard boron and lithium additions. First, the stress dependence of 5% to 20% cold-worked Type 316 (CW316) was determined in the PWR primary environment at 320°C. The rate of crack growth increased both with increasing cold work and stress intensity. Intergranular morphology was observed for 5%, 10%, 15%, and 20% CW316. Second, the dependence of crack growth rates on temperature in the range from 250°C to 320°C were measured in both hydrogenated PWR primary and oxygenated water. More rapid crack growth rates were observed in oxygenated water than that in hydrogenated PWR water. The crack growth rates could be correlated with a 1/T dependence on temperature for both hydrogenated PWR primary and oxygenated water; apparent activation energies of crack growth were similar in both environments. To assess what appeared to be common dependencies on temperature and cold work in both hydrogenated and oxygenated water, grain boundary creep (GB creep) was studied in air using CW316; intergranular creep cracking (IG creep cracking) was observed after low-temperature creep tests in air. The apparent activation energy of IG creep cracking was similar to that of intergranular stress corrosion cracking (IGSCC) in high-temperature water. The similar dependencies on temperature, cold work, and rolling direction of SCC and creep suggest that creep by grain boundary diffusion plays a critical role in the growth of SCC. The extent to which grain boundary diffusion interacts with other electrochemical processes is probably important but is not defi ned.

Multi-scale characterization of stress corrosion cracking of cold-worked stainless steels and the influence of Cr content

“…The orientation of the deformation bands with respect to the propagation direction seems to play an important role. (7) No evidence for selective grain boundary oxidation ahead of the crack tip was found. However, enhanced oxidation was observed at the highly stressed regions around the crack tip.…”

“…Cold work has also been shown to change the grain boundary diffusion rates of the alloy constituents, due to the increase in the proportion of high-angle sub-boundaries, which are faster diffusion paths [38]. Arioka also found similar dependencies of SCC and creep on temperature, cold work and rolling direction, and suggested that creep by grain boundary diffusion plays a critical role in the growth of SCC [7].…”

“…Increase of crack growth rate by cold work has been reported for several alloys under BWR [6] and PWR [7,40,42,43] conditions, with crack growth rates tending to be faster in oxygenated water.…”

“…The yield strength is higher in samples with higher cold-work levels, due to the increase in dislocation density and the formation of more twin deformation bands. The threshold stress intensity factor for crack initiation was found to be around 15 MPa m 1/2 for all of the austenitic steels studied [7,53]. For this reason, no sample with less than 5% CW was found to crack (so far) during autoclave simulated experiments in PWR primary water conditions.…”

“…Terachi [7,37,52,54] reported the observation of double-layered spinels in the form of a coarse-grained Fe 3 O 4 outer layer and a FeCr 2 O 4 inner layer. Diffusion of Cr from the bulk to the Cr-rich inner oxide layer was not observed and the surface of the original base material remained at the outer/inner oxide interface.…”