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Effect of long term thermal aging on SCC initiation susceptibility in L-grade austenitic stainless steels

低炭素ステンレス鋼のSCC発生感受性に及ぼす長時間熱時効の影響

加治 芳行  ; 青木 聡  ; 近藤 啓悦  ; 山本 正弘 

Kaji, Yoshiyuki; Aoki, So; Kondo, Keietsu; Yamamoto, Masahiro

低炭素ステンレス鋼の応力腐食割れ(SCC)発生感受性に及ぼす長時間熱時効の影響をすき間付曲げ(CBB)試験を用いて検討した。用いた材料は、溶体化及び冷間加工した304Lと316L対炭素ステンレス鋼を288度で4000時間熱時効したものを用いた。SCC発生感受性は、288$$^{circ}$$C、BWR模擬高温水中における1000時間のCBB試験により評価した。CBB試験結果により、冷間加工し長時間熱時効した316Lステンレス鋼のみで高いSCC発生感受性が見られた。その原因を材料の機械的特性、冷間加工材料特性、ミクロ組織及び塑性変化等の観点から検討した。その結果、粒界近傍での偏析が認められなかったことから、低温鋭敏化が原因ではないことが分かった。本発表では、その他に考えられる要因の検討結果について述べる。

The influence of long term thermal aging (LTA) on SCC initiation susceptibility in L-grade austenitic stainless steels (SSs) has been investigated using Creviced Bent Beam (CBB) tests. Test materials were type 304L and 316L SSs, and aging heat treatments were conducted at 288$$^{circ}$$C in air for up to 14000 hours followed by solution-annealing (SA) or 20% cold-working (CW). The evaluation of the SCC initiation susceptibility was conducted by CBB tests in BWR simulated high-temperature water at 288$$^{circ}$$C for 1000 hours. It was revealed by CBB tests that 304L-SA, 304L-CW, 304L-SA+LTA, 304L-CW+LTA, 316L-SA 316L-CW and 316L-SA+LTA showed no SCC initiation susceptibility, whereas 316L-CW+LTA showed high SCC susceptibility. To clarify the cause of SCC initiation in 316L-CW+LTA, several investigations are in progress from the viewpoint of mechanical properties, cold-rolling texture, microstructure and michrochemistry after LTA. It is considered that the low temperature sensitization was not the main cause of SCC in 316L-CW+LTA, because the TEM/EDX analysis on the 316L+LTA and 316L-CW+LTA specimens showed no chromium depletion in the vicinity of grain boundaries. In this presentation, we will explain the further investigation results for effect of LTA on SCC initiation susceptibility in L-grade austenitic SSs.

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