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研究生:劉澤英
論文名稱:氮對AL-6X及AL-6XN氫脆性質影響之研究
指導教授:陳立業陳立業引用關係
學位類別:碩士
校院名稱:國立臺灣大學
系所名稱:材料科學與工程學研究所
學門:工程學門
學類:材料工程學類
論文種類:學術論文
論文出版年:1995
畢業學年度:83
語文別:中文
論文頁數:100
中文關鍵詞:氫脆性質
外文關鍵詞:AL-6X
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  本研究比較二種超級不銹鋼(superaustenitic stainless steel)之氫脆敏感性,並對含氮不銹鋼中氫與差排之交互作用作一探討。選用材料分別為AL-6X(20Cr-24Ni-6Mo)及AL-6XN(20Cr-24Ni-6Mo-0.3N),加工成二類型試片(未開槽標準拉伸試片與開槽拉伸試片),利用各種拉伸試驗機,給予試片不同之應變速率及應力狀態。這些變數包括:空氣中之拉伸(應變速率=10-3及6.5*10-6l/s)、慢速率動態充氫拉伸(應變速率=10-6l/s)、不同應力(90%YS、115%YS、130%YS)下定負荷充氫後再拉伸及不同時間(1week,2weeks,4weeks)定負荷充氫後再拉伸。對於上述部份試片,以高溫真空熱萃取定氫設備測定試片中之氫含量,最後,分析前述拉伸與定氫數據,並配合光學顯微鏡及掃瞄式電子顯微鏡對其顯微組織與破斷型態的觀察。
  結果顯示:兩者之抗氫脆能力皆比304與316不銹鋼更好。以未開槽標準拉伸試片而言,動態充氫拉伸後,AL-6XN較AL-6X表現出較大的延性損失;定負荷充氫拉伸的結果也顯示AL-6XN隨定負荷充氫的應力或時間增加而有顯著的延性損失,可能是因為AL-6XN中的氮使SFE下降,提高了加工硬化與Planar slip (Clustering-N也有助於後者形成)之頃向,兩者都提高了氫在材料中之破壞性。
  然而,開槽的拉伸試片則呈現了相反之結果,AL-6X反而展現了較大的動態充氣延性損失,這可能是因Plastic constraint的現象阻礙了差排攜氫效應。
  定氫的結果顯示:有承受應力之試片與未承受應力之試片的氫含量差異頗大。各組試片氫含量隨定負荷充氫時間增長並非每組都有明顯持續增加之趨勢,但皆明顯隨充氫應力上升而上升,尤其以AL-6XN130%定負荷充氫試片最明顯。以上結果可推斷此實驗中之差排攜氫效應大於晶格擴散而位居主導地位。
  上述的結果提供一證據說明氫與差排有強烈之交互作用,而且此交互作用深受氮降低SFE及提高Planar slip傾向的影響。又透過SEM的觀察,在AL-6XN高應力充氫拉伸試片破斷面的脆性層中,除可觀察到層狀裂紋外,尚可在脆性層高倍率照片觀察到step-like marking,因而推斷此含氮不銹鋼的氫脆機構如下:氫導致局部塑性變形容易發生,差排透過Planar slip將氫輸送至裂紋成核處,形成脆性破壞,而此塑性變型與脆性裂紋交互發展直至材料破斷。此時氮所扮演之角色與在Metasable ausenitic stainless steel中之角色(氮為γ-Phase穩定元素)有所差異。
  The objective of this work is to compare the hydrogen embrittlement susceptibility (H.E.) of two superaustenitic stainless steels, and to study the interaction between hydrogen and dislocations in these materials. Two high Ni-Cr-Mo alloys with and without nitrogen addition designated as AL-6XN and AL-6X respectively were used. Smooth standard tensile and notched specimens were prepared from these steels, and tensile testings with different stress conditions and strain rates were carried out in air and in hydrogen charging environment. The hydrogen content of these specimen were determined by vacuum hot extraction method. Moreover, the microstructure and fractography of the specimens were studied using optical microscopes and scanning electron microscopes.
  Results on the tensile tests of smooth standard specimen show that the sensitivity to H.E. of AL-6XN was higher than that of AL-6X. In constant load rupture test, AL-6XN has a higher ductility loss when subjected to a loading corresponding to 130% yield stress. The hydrogen content increased with stress, and this was very significant when the AL-6XN specimens was stressed beyond its yield stress. There were many parallel brittle cracks on the specimen surface and step-like markings were found on the brittle region of the fracture surface. However, the AL-6XN notched specimens showed a higher resistance to H.E. as compared to its AL-6X counterpart. It was probably resulted from the plastic constraint phenomenon caused by the notch effect.
  Above results provide an evidence to illustrate that there was an interaction between hydrogen and dislocations in these superaustenitic stainless steel. It is possible that the nitrogen decreased the stacking fault energy of the steel and encouraged planar slip hence assisting H.E. The role of nitrogen in AL-6XN and metastable stainless steel was different, where in the latter case the austenite phase was considered as metastable, and might transform to martensite under stress. In AL-6XN, hydrogeninduced local plastic deformation took place easily. Thus, planar slip carried the hydrogen to some defects and caused the crack to nucleate. Consequently, the ductile behavior and crack propagation proceeded alternately until the specimen failed.
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