号数	ばね論文集 59 号（ 2014 年）
ページ数	pp.19-27
種類	論文
論文名	オーステナイトステンレス鋼SUS321-Bの700℃における高疲労寿命領域でのフィッシュアイを伴う破壊形態
Title	Fracture Morphology Accompanying Fish-eye of Austenitic Stainless Steel SUS321-B under Long Fatigue Life Region at 700℃
著者	高橋幸男 1)，井原　透 1)，金澤健二 1)
Author	Yukio TAKAHASHI 1), Tohru IHARA 1)and Kenji KANAZAWA 1)
抄録	オーステナイトステンレス鋼の700℃における２段のＳ-Ｎ曲線において，低寿命領域では表面破壊に，高寿命領域では破面にフィッシュアイが形成される破壊になることが明らかにされてきた．破壊形態におよぼす酸化などの効果を明らかにするために，ＳＵＳ321-Ｂ鋼のすでに得られている試験片の破面に対してＥＰＭＡなどを用いて詳細な観察・分析を行った．破壊の起点が試験片表面直下にある小さなフィッシュアイが試験片表面に達した後は，２通りの破壊過程があった．一つは表面き裂となって第2段階のき裂として進展し破壊する場合と，他の一つはフィッシュアイ面が酸化物で覆われ進展しない閉塞状態のき裂になる場合があった．後者のタイプでは，閉塞状態になってフィッシュアイによって誘起されて，試験片表面で新たなき裂が発生・進展し破壊を起こした．いすれの場合も，フィッシュアイの周囲の破面には，酸化度合いの強い半円状領域が形成されていた．破壊の起点が試験片表面から深い位置にあり大きなフィッシュアイが形成される状況下では，フィッシュアイ先端が試験片表面に到達後，表面き裂となって進展して内部破壊になった．この場合，フィッシュアイの周囲の破面には，酸化度合いの強い半円状領域は形成されなかった．どの破壊形態になるかを支配する因子としては，フィッシュアイの起点の深さとフィッシュアイが表面に達した時点におけるフィッシュアイ先端の応力拡大係数を挙げることができた．
Abstract	It has been clarified that surface fracture and fracture accompanying a fish-eye on its fracture surface occurred under short and long fatigue life regions, respectively, in the stepwise S-N curve of an austenitic stainless steel obtained at 700℃. In order to clarify the effect of oxidation on fracture morphology, fracture surfaces obtained already for SUS321-B steel were examined in detail by EPMA. After a small fish-eye, of which origin was located at the subsurface of the specimen, had reached the specimen surface, two types of fracture processes occurred. One type was that the fish-eye became a small surface crack and propagated as a Stage�U crack. The other type was that fracture surfaces of the fish-eye were covered with oxides and the fish-eye became a non-propagating occluded crack. In the latter case, initiation of a new crack at the specimen surface was induced by the occluded crack and then fracture occurred. Oxidized semi-circular area was formed near the fish-eye in both types, which was brought by behavior of oxide-induced crack closure. After a large fish-eye, of which origin was located at the interior of the specimen, had reached the specimen surface, the fish-eye became a large surface crack and propagated as a Stage�U crack. In this case, oxidized semi-circular area was not formed near the fish-eye. The depth of fish-eye origin from the specimen surface and stress intensity factor at the tip of fish-eye when it reached the specimen surface were pointed out as factors controlling which type of fracture processes occurred.
著者の所属	1) 中央大学
Belonging	1) Chuo University
Key Words	High cycle fatigue, Elevated temperature, Austenitic stainless steel, Internal fracture, Fisheye, Fatigue crack propagation, Oxide-induced crack closure