Effect of Strain Hardened Inner Surface Layers on Stress Corrosion Cracking of Type 316 Stainless Steel in Simulated PWR
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INTRODUCTION
AUSTENITIC stainless steels have, with few exceptions to date, shown excellent stress corrosion cracking (SCC) resistance in industrial service in pressurized water reactor (PWR) primary coolant environments. Nevertheless, intergranular SCC (IGSCC) has been occasionally observed[1], most notably and unambiguously in the hot leg (Primary water inlet) safe end of a replacement steam generator (SG) inlet nozzle at the Mihama unit 2, in 2007.[2] IGSCC was reported near the weld heat-affected zone (HAZ) of non-sensitized Type 316 stainless steel safe ends welded to the low alloy steel
TOSHIO YONEZAWA and MASASHI WATANABE are with the FRI, NICHe, Tohoku University, 6-6-10, Aoba, Aramaki, Aobaku, Sendai 980-8579, Japan. ATSUSHI HASHIMOTO is with the Kobe Material Testing Laboratory Co. Ltd. 47-13, Niijima, Harimacho, Kako-gun, 675-0155, Japan. M.D. OLSON and A.T. DEWALD are with Hill Engineering, LLC, 3083 Gold Canal Drive, Rancho Cordova, CA. M.R. HILL is with the Department of Mechanical and Aerospace Engineering, University of California, One Shields Avenue, Davis, CA. Contact e-mail: [email protected] Manuscript submitted April 29, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
steam generator inlet nozzle with a nickel base Alloy 82 weld metal. Additionally, IGSCC was reported near the HAZ of the non-sensitized, strain hardened Type 316 stainless steel safe ends welded to the adjacent Type 316L stainless steel elbow of the primary circuit piping.[2] The IGSCC observed at Mihama 2 was detected in a strain hardened area with hardness greater than 250 Hv (0.01), although the crack depth was determined to be less than or equal than 1.4 mm.[3] Consequently, it is necessary to determine whether similar IGSCC of Type 316 stainless steel is a generic or unique issue and to prove whether the detected IGSCC had already arrested, as claimed in Reference 3 or was continuing to grow in the SG safe end of Mihama Unit 2. It is also essential to determine whether IGSCC initiated from heavily cold-worked zones could continue to propagate through the thickness of normal ductile base materials made of Type 316 stainless steel [with typical hardness < 180 HV (0.01)]. In Japanese commercial BWRs with a classical oxygenated Normal Water Chemistry (NWC), it has been reported that transgranular SCC (TGSCC) initiated in surface hardened layers of non-sensitized type 316 L stainless steel used for core shrouds.[4] The
hardening due to high plastic strain was caused by machining and/or grinding. Cracking also tended to propagate intergranularly into regions with high hardness and subsequently into the soft base metal. Following these operating experiences in commercial light water reactors, many research papers have reported on the effect of material strength on SCC growth rates in austenitic stainless steels exposed to high-temperature water environments.[4–13] A correlation has been observed between the SCCGR and yield strength of the material under simulated (oxygenated) BWR water environments[3–6] and also between SCCGR
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