Stress-corrosion Cracking Of Austenitic Stainless Steels Research Articles

Role of residual ferrites on crevice SCC of austenitic stainless steels in PWR water with high-dissolved oxygen

The crevice stress corrosion cracking (SCC) susceptibility of austenitic stainless steels was evaluated in simulated pressurized water reactor (PWR) environments. To simulate the abnormal condition in temporary clamping devices on leaking small bore pipes, crevice bent beam (CBB) tests were performed in the oxygenated as well as hydrogenated conditions. No SCC cracks were found for SS316 in both conditions. SS304 also showed good resistance in the hydrogenated condition. However, all SS304 specimens showed SCC cracks in the oxygenated condition, indicating poor crevice SCC resistance. It was found that residual ferrites were selectively dissolved because of the galvanic corrosion coupled with the neighbouring austenite phase, resulting in SCC initiation in SS304. Crack morphologies were mostly transgranular assisted by the damaged δ-ferrite and deformation-induced slip bands.

Fatigue Life of the Strain Hardened Austenitic Stainless Steel in Simulated Pressurized Water Reactor Primary Water

Over the last 20 years or so, many studies have revealed the deleterious effect of the environment on fatigue life of austenitic stainless steels in primary water reactor (PWR) primary water. The fatigue life correlation factor, so-called Fen, which corresponds to the ratio of fatigue life in air at room temperature to that in water under reactor operating conditions, has been standardized to consider the effect on fatigue life evaluation, and the formulations are function of strain rate and temperature due to their noticeable negative effect compared with other factors (Chopra and Shack, 2007, “Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials,” Final Report, Report No. NUREG/CR-6909, ANL-06/08; Codes for Nuclear Power Generation Facilities, 2009, "Environmental Fatigue Evaluation Method for Nuclear Power Plants," JSME S NF1-2009, The Japan Society of Mechanical Engineers, Tokyo, Japan). However, mechanism causing fatigue life reduction remains to be cleared. As one of the possible approaches to examine the underlying mechanism of environmental effect, the authors focused on the effect of plastic strain, because it could lead microstructural evolution on the material. In addition, in the case of stress corrosion cracking (SCC), it is well known that the strain-hardening prior to exposure to the primary water can lead to remarkable increase of the susceptibility to cracking (Vaillant et al., 2009, “Stress Corrosion Cracking Propagation of Cold-Worked Austenitic Stainless Steels in PWR Environment,” 14th International Conference on Environmental Degradation of Materials in Nuclear Power Systems; Couvant et al., 2009, "Development of Understanding of the Interaction Between Localized Deformation and SCC of Austenitic Stainless Steels Exposed to Primary PWR Environment," 14th International Conference on Environmental Degradation of Materials in Nuclear Power Systems). However, its effect on fatigue life has not necessarily been cleared yet. The main effort in this study addressed the effect of the prior strain-hardening on low cycle fatigue life in the primary water. A plate of 304LSS was strain hardened by cold rolling or tension prior to fatigue testing. The tests were performed under axial strain control at 300 °C in primary water including B/Li and hydrogen, and in air. The effect on environmental fatigue life was investigated through a comparison of Fen in experiments and in regulations, and also the effect on the fatigue limit defined at 106 cycles was discussed.

Effects of Potential and Stress on Stress Corrosion Cracking of Austenitic Stainless Steels in Concentrated Chloride Solutions

The chemical-mechanical correlation model for stress corrosion cracking (SCC) was used to analyze the effect of electrode potential (E) on SCC of austenitic stainless steels (SS) in concentrated chloride solutions. The analysis produced a value of the mechanical constant (Qc/m = 20.6 kJ/mol) which was the same as that obtained at open-circuit potentials (OCP). Analysis also revealed that the effect of E on SCC mainly affected the environmental factor (β) of the model. A theory developed about the physical meaning of β was able to predict the dependence of β on E in concentrated chloride solutions, based upon repassivation properties.

海水における二, 三のオーステナイトステンレス鋼の応力腐食割れ -その試験法について-

Some instruments exposed to hot sea water have showed fracture due to SCC (stress corrosion cracking). On the other hand, fracture of austenitic stainless steels due to SCC has not been observed in a marine environment. The SCC of austenitic stainless steels has been known to occur in some environment such as concentrated sulfuric acid containing sodium chloride at room temperature. This may suggest crack propagation due to SCC even in low temperature sea water. So far, Scheil solution and hot distilled water have been widely accepted as testing enviroments for SCC. But quantitative testing method of SCC in mild environment has not yet been reported. Therefore, the present study concerning a testing method of SCC in mild environment was carried out and the results were compared with those obtained by using Scheil solution.