SUMMARY OF THE SESSIONS ON AUSTENITIC ALLOYS: STAINLESS STEELS – Chaired by Ulla Ehrnstén, Anders Jenssen and Katsuhiko Kumagai
The first part of the stainless steel session on Thursday afternoon comprised four papers.
The session started with a presentation by Changheui Jang (KAIST, Korea) entitled “Improvement of EAF life of austenitic stainless steels in simulated PWR primary environment – Effect of Zn and strain holding”. The presentation summarized results from fatigue testing in environment under PWR conditions using smooth specimens with and without Zn addition (30 ppb) and post-test analysis of the 316 stainless steel specimens. Based on three specimens (two without and one with Zn addition), a three times longer fatigue life-time was observed using Zn. The post-test analysis verified the presence of Zn on the fracture surface and at the crack-tip and impedance measurements showed a higher surface reaction resistance in the specimen with Zn addition compared to those without. More tests will be performed to verify the results.
The second paper was presented by Wen Chen (PSI, Switzerland) and dealt with “Environmentally-assisted fatigue of austenitic stainless steel in high-temperature water/air with/without mean stress”. This paper summarized results performed in the PhD work by the presenter on the role of mean stress on fatigue life. He has used hollow specimens made from 316L stainless steel and performed low- and high-cycle fatigue tests in simulated BWR/HWC conditions. The results show a positive effect of positive and negative mean stress in low-cycle fatigue in air and environment, where the positive effect is larger than the negative effect of the environment. The post-test characterization showed development of Corduroy structure, which can explain the secondary hardening typically observed. He proposed that the SWT (Smith-Watson-Topper) model can be used to analyze the results.
The third talk was given by Catherine Guerre (CEA, France) on the “Effect of oxygen transients on oxidation and SCC of pre-strained stainless steels in PWR primary water”. The investigations focused on the effect of oxygen transients in PWR environment on the oxide film and EAC susceptibility of 316L. The test method was very slow strain rate testing. The results showed a change in the oxide structure after switching from PWR conditions to a period with oxygen addition. The structure did not recover after switching back to PWR conditions. The oxide structure in transient conditions is more porous than in PWR conditions with a Cr-rich layer on the metal, and some internal oxidation, which was not observed in fully aerated conditions. The crack density was highest in transient conditions. Further work will be done to verify these results using, e.g., constant load testing.
The forth presentation was held by Litao Chang (University of Manchester, UK) and had the title “Effect of strain-induced martensite on SCC initiation in austenitic stainless steels in high-temperature water”. In his PhD work he has investigated the role of deformation-induced martensite on EAC susceptibility of 304 and 316 stainless steel under PWR conditions. 20% deformation was performed at RT and 200°C, and the test method was SSRT. The results show an increase of strain needed for IG cracking in the 20% RT deformed 304 stainless steel with martensite. The detailed post-test characterization showed preferential oxidation of the martensite, which is evaluated to be the reason for the higher strain needed to initiate IG cracking.
The stainless steel session continued on Friday morning with three papers.
Qi Huang (MINES ParisTech, France) started off by giving a presentation entitled “Stress corrosion cracking initiation of cold-worked austenitic stainless steels: Influence of the combination of microstructure and mechanical fields at the polycrystal scale”. The study aimed at associating stresses and strains, microstructure and oxidation at stress corrosion cracks to determine the requirements for cracking. Type 316L stainless steel was pre-strained in air at RT before being slow strain rate tested in simulated PWR primary coolant at 340°C. With the use of an Au-Pd microgrid and digital image correlation (DIC), the strains in a selected region of the specimen was measured after pre-straining, as well as after SCC testing. EBSD analysis was performed on the same area and based on this information a FE model was developed that was used to determine the stresses in the area. It was found that most cracked grain boundaries were high-angle boundaries oriented perpendicular to the tensile axis during testing. In addition, the stress at the cracked boundaries were in most cased above 410 MPa. However, the oxidation depth is also important as not all high-angle boundaries normal to the tensile axis with a stress above 410 MPa cracked, and some boundaries at lower stress sometimes cracked. Continued work will study the influence of grain boundary oxidation on the cracking.
Takuyo Yamada (INSS, Japan) presented results from a study investigating the effect of dynamic loading on cracking in blunt notched specimens. Specimens of 20% cold-worked type 316 with a 0.5 mm notch radius were exposed in simulated PWR primary water at 320 or 360°C. Following 600 h at constant K, dynamic loading was introduced by increasing K from 27.5 to 44 MPa√m over a period of 200 h, then holding K at this level for another 200 h before reducing to 27.5 MPa√m. The reload/hold time/unload cycle was repeated until the specimen failed or the test was stopped. IG SCC was observed in specimens subjected to dynamic loading, while specimens tested at a constant K of 44 MPa√m did not develop any cracking. The cracks were oriented perpendicular to the loading direction and did not form a network like SCC observed in Alloy 600 tested at constant load in a previous study. The extent of cracking was greater at 360°C compared to the lower temperature, but additional testing would be required to quantify the temperature dependency. It was concluded that the test method is suitable for studying crack initiation in cold-worked type 316 stainless steel.
Joshiyuki Kaji (JAEA, Japan) ended the session by presenting a paper entitled “Long term thermal aging effect on SCC initiation susceptibility of L-grade stainless steels”. With the purpose of understanding the cracking observed in a Japanese type 316L BWR core shroud after about 12 years of operation, a study with the objective of assessing the combined effect of hardening (from grinding) and exposure to the reactor water temperature on initiation of SCC has been conducted. Crevice bent beam (CBB) testing in 288°C high-purity water with 8 ppm oxygen showed that aging of 20% cold-worked type 316L at 288°C for 14’000 h increased the susceptibility to initiation of IG SCC. Cold-worked type 304L with and without the thermal aging treatment was much less susceptible to cracking. Examination by SEM of a non-cracked area before and after CBB testing indicated that the thermal aging treatment of cold-worked type 316L changed the deformation mode from homogeneous to heterogeneous. Heterogeneous deformation results in a localized stress/strain state at grain boundaries that can lead to higher susceptibility to SCC. A further study of the grain boundary stress/strain state and its effect on corrosion is underway.
Three more presentations after the coffee break concluded the stainless steels session, as well as the technical part of the 2019 meeting.
The first talk was presented by Tai-Cheng Chen (INER, Taiwan) entitled “Study on the stress corrosion cracking susceptibility of cold-rolled 304L stainless steel in simulated PWR water environments”. This presentation summarizes SSRT tests on 304L stainless steel in hydrogenated (2.4 ppm DH) and oxygenated (7~8 ppm DO) environments at 320 °C. Cold-rolled materials up to 10, 20 and 30%, as well as solution annealed steel were subjected to the test at a strain rate of 3∙10-7 s-1. The microstructure characterization by EBSD showed good correlations between cold-work ratio and slip band density, as well as strain-induced martensite and residual strain with the level of cold-rolling. The reduction of area decreased with increasing cold-work ratio in both SSRT tests at temperature and at room temperature. In the opinion of the session chair, the correlation between SCC susceptibility and cold-work ratio remains unclear, partly due to the nature of SSRT tests. Significant difference in the extent of TG cracking was observed, however, between hydrogenated and oxygenated environments. In hydrogenated environment, the final fracture surface showed a TG morphology and numerous micro cracks were observed on the side surface of the specimen. The author suggested hydrogen-induced cracking as possible mechanism. Multiple models have been proposed, e.g., hydrogen-enhanced localized plasticity (HELP) or hydrogen-enhanced strain-induced vacancy (HESIV).
The second presentation was given by Ho Jung Lee (KHNP, Korea) on the “Effect of microstructure and residual stress on stress corrosion cracking susceptibility of feed water heater tubes”. Multiple TGSCC incidents were experienced in 304L stainless steel feedwater heater tubes (16 mm outer diameter, 1.27 mm thickness) in a Korean PWR unit less than 2 years after the re-tubing. The feedwater heater tubes were supplied by three different manufacturers A, B and C, and failures occurred only in samples from B and C. Detailed characterization by optical microscope, EBSD and TEM revealed that samples B and C have significantly higher degree of grain size inhomogeneity, BCC phase distribution and dislocation density. It was also confirmed that the residual stress in samples B and C was approx. 350 MPa, and was significantly higher than the measured value of 15 MPa in sample A. Those observations can be attributed to the difference in the straightening procedure during fabrication. While a stretching process is used for sample A, a rolling process is adopted in samples B and C, during which additional stress and severe work hardening are directly applied on the outer surface of the tubes. It was confirmed that an additional heat treatment after straightening is effective in lowering the residual stress and will be added to the manufacturing specifications.
The very last technical presentation of the 2019 meeting was held by Mariia Zimina (CVR, Czech Republic), entitled “Effect of laboratory surface treatment on EAC initiation of type 316L steel in high-temperature water/vapor environments”. The presentation began with a brief introduction of the test facilities at CVR, including a testing system for EAC in hydrogen steam environment (HSV). Utilizing those facilities, CERT (constant extension rate tensile) tests on 316L stainless steel were conducted in both, PWR and HSV environments at pull-rod displacement rates of 2∙10-6 and 2∙10-8 m/s. Primary focus was the effect of surface finish on EAC initiation. Flat tapered specimens with two different conditions of surface finish, 1 μm diamond polish and 500-grit SiC-paper ground, were tested at 350 °C. The effectiveness and the validity of the test acceleration by applying elevated test temperature, dynamic straining and hydrogenated steam environment were also discussed. Typical cleavage-like cracks were observed on both specimen surfaces, ground and polished, after CERT testing in PWR water. A lower threshold stress for SCC initiation was observed on the ground surface compared to the polished one at the same displacement rate. The exposure of the 316L to the H2-steam environment, however, did not lead to EAC initiation.
Before concluding the techical part of the meeting, Mariia received the Group’s “Last Presenter Award” and some fine Swiss chocolate compensating a little for the late scheduling of her talk.
Please note that one additional stainless steel presentation was submitted for the meeting minutes and two more stainless steel papers were presented in one of the summary/poster presentation sessions and :
Clinton Fong (ITRI, Taiwan): “Effect of cold-work on stainless steel PWSCC”
Kuniki Hata (JAEA, Japan): “Oxidation in the vicinity of crack tips of load-applied CW316L stainless steel immersed in high-temperature water at 290°C”
Jiamei Wang (SJTU, China): “Effect of δ-ferrite on the stress corrosion cracking behavior of 321 stainless steel”