AL-6X及AL6-XN超級不銹鋼之氫脆研究

Hydrogen Embrittlement of AL-6X and AL-6XN Superaustenistic Stainless Steel

本研究比較二種超級不銹鋼之氫脆敏感性，探討不銹鋼中含氮量的影響。實驗材料為AL-6X (20Cr-24Ni-6Mo)及AL-6XN(AL-6X+0.3%N)二種不銹鋼，使用標準拉伸試片進行拉伸試驗，在不同應變速率及應力狀態下，量測試件在充氫下的性能，並配合光學顯微鏡及掃瞄式電子顯微鏡，進行顯微組織與破斷型態觀察。結果顯示：在充氫拉伸試驗中，AL-6XN較AL-6X有較大的延性損失；定負荷充氫拉伸的結果也顯示AL-6XN隨定負荷時間增加而有顯著的延性損失。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，尚可在脆性層觀察到階梯狀斷面，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，而氮則助長差排的平面滑移，可將氫直接送至裂紋成核處，形成脆性破壞，而此塑性變形與脆性裂紋交互發展直至材料破斷。

The objective of this work has been to compare the hydrogen embrittlement (H.E.) susceptibility of two superaustenitic stainless steels (AL-6X and AL-6XN), and to study the effect of nitrogen content in the steels. Standard tensile specimens were prepared from these steels, and tensile testings under different stress conditions and strain rates were carried out in air and in hydrogen charging environment. Moreover, the microstructure and fractography of the specimens were studied using optical microscopes and scanning electron microscopes. Results from the slow strain rate tensile testing show that the sensitivity to H.E. of AL-6XN was higher than that of AL-6X. In constant load rupture testing, AL-6XN has a higher ductility loss. There were many parallel brittle cracks on the specimen surface and step-like markings were found on the brittle region of the fracture surface. The mechanism of hydrogen embrittlement of this N-containing superaustenitic stainless steel can be suggested as follow: Hydrogen absorbed in the steel induced local plasticity, and nitrogen on the other hand enhanced the planar slip of dislocations, thus allowing hydrogen to be transported via dislocations to the crack tip efficiently, giving rise to brittle fracture.