Effect of long term thermal aging on SCC susceptibility in austenitic stainless steels

オーステナイト系ステンレス鋼のSCC感受性に及ぼす長時間熱時効の効果

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SCC感受性に及ぼす長時間熱時効の影響についてCBB試験により検討してきた。用いた材料は304Lと316Lステンレス鋼であり、溶体化処理あるいは20%冷間加工後、288C大気中で14000時間時効処理を行った。SCC感受性評価は、BWR模擬高温水中288Cで1000時間のCBB試験により実施した。CBB試験の結果、304L溶体化材、304L冷間加工材、316L溶体化材ではほとんど感受性を示さなかったが、316L冷間加工材では高いSCC感受性を示した。TEM/EDXによる粒界分析ではCrの偏析等が見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。機械的性質、圧延組織、化学組成等の点から検討を進め、長時間熱時効材においてSCC感受性が高くなる原因について議論する。

The influence of long term thermal aging on SCC 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 288C 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 288C for 1000 hours. It was revealed by CBB tests that long term aged 304L-SA, 304L-CW, and 316L-SA showed low susceptibility of cracking, whereas long term aged 316L-CW showed high SCC susceptibility. And so far, it is considered that the low temperature sensitization was not the main cause of SCC in long term aged 316L-CW, because the previous TEM/EDX analysis on the long term aged 316L specimen showed no chromium depletion in the vicinity of grain boundaries. Further investigations are in progress from the viewpoint of mechanical properties, cold-rolling texture, microstructure and michrochemistry after the long term thermal aging in order to discuss the factors increasing SCC susceptibility.