第 3 章 準安定オーステナイト系ステンレス鋼の疲労き裂進展特性に及ぼす
3.4 結言
本章では，2種類の準安定オーステナイト系ステンレス鋼 SUS304およびSUS316Lの疲労き 裂進展試験を0.7および90 MPaの水素ガス中（外部水素），ならびに90 mass ppm程度の水素を 一様にチャージした状態（内部水素）で行い，水素環境下での疲労き裂進展加速に対するα’マル テンサイト変態の役割について検討した．また，内部水素と外部水素それぞれによるマクロなき 裂進展加速率に関して，γ相の安定性に及ぼす水素の影響の観点から考察した．得られた結論を 以下に示す．
1. オーステナイト安定度の低いSUS304においては，すべての水素環境下で顕著なき裂進展の 加速が認められ，特に外部水素環境の場合には水素ガス圧力の増大とともにき裂進展加速 率も増大した．一方，SUS316L では外部水素，内部水素の場合ともに有意なき裂進展加速 は確認されなかった．
2. いずれの材料においても，水素環境下では α’相のラス構造に起因すると思われる擬へき開 破面が観察され，その形成頻度はき裂進展加速率の大きいSUS304において特に顕著であっ た．内部水素と外部水素の場合における破面形態に大きな相違は見られず，このことは両環 境下においてき裂進展の加速をもたらす破壊機構が等しいことを示唆している．
3. 外部水素環境の場合，き裂周辺の α’変態域は大気中のき裂周辺と比較してき裂の近傍へと 局所化した．このような α’相の局在化は，従来提唱されているように局所的な塑性変形が 助長されたことによるものではなく，α’に沿う早期的なき裂伝播によってき裂先端で繰り返 される塑性ひずみのサイクル（累積塑性ひずみ）が減少したことによって生じたものであ る．
4. 内部水素試験におけるき裂進展加速率は，約90 mass ppmという高い固溶水素量に反して，
外部水素の場合よりも低い値を示した．この特異な結果は，γ相中の水素がプラナーな塑性 変形を助長し，α’相の核生成サイトとなる変形帯交差部の数を減少させることに起因する．
5. 準安定オーステナイト系ステンレス鋼中の水素誘起疲労き裂進展加速に対しては，き裂先 端でのα’変態の生じ易さ，すなわちオーステナイト安定度が最重要因子となる．また，α’相 に関連したき裂進展の加速は局所的な水素拡散係数の増加によるものではなく，α’相自身の イントリンシックな耐水素性の乏しさに起因するものであると考えられる．
6. 予め水素をチャージした試験片を水素ガス中で試験した場合，き裂進展加速率は水素ガス
（外部水素）のみの場合よりも低下する．すなわち，内部水素によるγ相安定化の効果は，
外部水素が重畳した場合であっても有効に作用する．
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