Evaluation of SCC Morphology on L-grade Stainless Steels in BWRs

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  • BWRにおける低炭素ステンレス鋼のSCC形態の評価
  • BWR ニ オケル テイタンソ ステンレス コウ ノ SCC ケイタイ ノ ヒョウカ

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Recently, a large number of stress corrosion cracking (SCC) have been observed in core shrouds and re-circulation (PLR) pipings made of L-grade stainless steel in Japanese BWRs (Boiling Water Reactors) . L-grade stainless steels, which contain less carbon to avoid the sensitization due to welding, have been developed as SCC-resistant materials after many SCC experiences on sensitized type304. As L-grade materials, especially type316L and 316NG stainless steels have shown little susceptibility to SCC initiation in laboratory tests, it has to be considered that there exist some unknown factors excluding sensitization that cause SCC in these materials. <br> From the detailed investigations of plant components so far, SCC of L-grade stainless steels show different characteristics from those observed on conventional sensitized type 304 especially in their crack morphology. In this paper, classification of observed cracks according to their morphology is discussed, followed by evaluation on root causes of SCC. The present conclusions are (1) crack orientations mainly depend on the maximum principal stress direction of welding residual stresses, (2) SCC susceptibility of L-grade stainless steels is enhanced by material hardening by machining or grinding, and (3) in L-grade materials SCC can propagate into the weld metal but in many case, cracks seem to be arrested near the fusion line.

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