Long term thermal aging effect on SCC initiation susceptibility of L-grade 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効果

加治 芳行  ; 青木 聡  ; 藤村 由希; 近藤 啓悦  

Kaji, Yoshiyuki; Aoki, So; Fujimura, Yuki; Kondo, Keietsu

本研究では、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について検討した。BWR模擬環境におけるすき間付曲げ試験の結果、316系低炭素ステンレス鋼の冷間加工+長時間熱時効材にSCC発生感受性の増加が認められた。透過電子顕微鏡等を用いたミクロ組織観察の結果、316系低炭素ステンレス鋼の冷間加工材は、長時間熱時効によりミクロ組織が変化し、局所的な変形が律速になることがSCC感受性が増加した原因の1つと考えられることが分かった。

In this study, the influence of the long term thermal aging (LTA) on SCC initiation susceptibility of L-grade SSs was examined. Results of CBB testing on Type 316L CW and CW + LTA under the BWR simulated water condition showed the increase of SCC susceptibility to initiation after the LTA treatment. The material examinations on those materials were conducted to identify the SCC accelerating factors. The initial microstructure in Type 316L CW and CW + LTA was characterized to dislocation tangles and dislocation cell structures, respectively, indicating that the LTA treatment induced the rearrangement of dislocations toward the energetically favorable state. Whereas, no sensitization of the grain boundaries of Type 316L CW + LTA was confirmed by means of TEM/EDX analysis. The examination of surface deformation microstructure by bending deformation was conducted by the consecutive SEM observation before and after CBB testing. The more heterogeneous deformation was enhanced by the LTA treatment, that is, the reduction in dislocation motion and increase of deformation twinning. It was considered from those obtained results that the concentration of local stress around grain boundaries caused by the more heterogeneous deformation might be one of the promoting factors for increasing the SCC susceptibility in non-sensitized Type 316L CW + LTA.