Hydrogen Embrittlement of Cold-worked 304L Stainless Steel and its Welds
|Keywords:||304L不?袗?;銲接;冷加工;氫脆;缺口拉伸強度;麻田散鐵相變態;304L stainless steel;welding;cold rolling;hydrogen embrittlement;notched tensile strength;martensitic transformation||Issue Date:||2011||Abstract:||
本研究針對介穩定之304L沃斯田鐵型不?袗?母材、銲道及經冷加工 (20與40%厚度縮減) 後之試片，在不同溫度 (25、80與135℃) 與環境 (大氣與氫氣) 下進行慢速率缺口拉伸實驗，以評估其氫脆敏感性。在25℃空氣中測試時，所有試片於塑性變形開始即有 α'' 麻田散鐵相變態; 但於80℃測試時，則須超過缺口拉伸強度後，α'' 才開始增加且變態量較25℃測試者少。測試溫度上升至135℃時，則幾乎無應變誘發 α'' 生成。試片若於氫氣中測試，氫原子可隨差排傳輸及藉由擴散移動至應力集中區，為導致氫脆之主要原因。在25℃測試時，母材 (B試片) 於低應力下即塑性變形，氫隨大量可移動差排傳輸，使局部塑性變形更為容易，導致應變誘發 α'' 變態並沿此破裂，使其缺口拉伸強度損失率 (NTS loss) 最高。經20%冷加工 (B20R試片) 後，因可移動差排減少，捕集氫原子之缺陷密度增加，使氫脆敏感性降低。經40%冷加工者 (B40R試片)，由於氫原子在其中係以擴散方式移動，試片中70% 之 γ 與 ε 大幅降低了氫原子擴散速率，若以NTS loss為評估基準，B40R較B20R試片有較低之氫脆敏感性。此外，氫原子經由擴散移動之速率較隨差排傳輸者低了很多，亦為B40R試片NTS loss降低之另一因素。
銲接試片方面，由於 γ / δ 界面亦具捕集氫原子的作用，且δ-ferrite降低應變誘發 α'' 變態量，導致銲道試片 (W試片) 及經20%冷加工者 (W20R試片) 之氫脆敏感性均較相對應之板材試片為低。銲道經40%冷加工者 (W40R試片)，由於氫原子易沿著 α'' 與 δ 所構成之連續路徑擴散，並弱化材料鍵結，導致氫脆敏感性較W20R試片略增，此為與板材試片相異之處。不論板材或銲接試片，在大氣環境測試者，破斷面皆為延性之韌窩狀破斷形貌;氫環境測試者，受氫影響區域主要皆呈現具二次裂縫之準劈裂破斷形貌。隨測試溫度上升，所有試片之氫脆現象皆逐漸減緩，此係因氫原子於材料表面之吸附速率與缺口前端應變誘發 α'' 含量隨測試溫度上升而下降所致。介穩定型沃斯田鐵不?袗?在缺口拉伸試驗中，氫原子隨差排移動至應力集中區，並促進局部塑性變形，使 α'' 變態集中於斷口處，為導致本實驗材料氫脆之成因，此現象與HELP理論相符合。在沒有 α'' 變態之310S沃斯田鐵不?袗?，25至135℃之氫脆現象並不明顯，充分印證 α'' 變態為介穩定型不?袗?之氫脆敏感性大幅提升之重要原因。
The susceptibility to hydrogen embrittlement (HE) of AISI 304L austenitic stainless steel and its welds was evaluated by slow displacement rate notched tensile tests in gaseous hydrogen. The notched tensile tests were carried out in different combinations of temperature (25 to 135℃) and environment (air or hydrogen) to assess the HE of various specimens. The base metal and weld metal specimens were conducted in both the unrolled and cold rolled (20% and 40% reduction in thickness) conditions. During testing in air at 25℃, all specimens underwent strain-induced α''-martensite transformation at the beginning of plastic deformation. For the specimens tested at 80℃, such a transformation started to increase only after reaching the notch tensile stress (NTS) and was considerably less than at 25℃. When the test temperature was further increased to 135℃, the α''-martensite transformation was generally not observed. In hydrogen-containing environments, hydrogen atoms can be transported through dislocation or by diffusion to the stress concentration region, resulting in HE of the specimens. In the case of the base metal specimen (the B specimen), plastic deformation occured at low stress levels. Hydrogen atoms were transported through mobile dislocations and facilitated localized plastic deformation. This caused the formation of strain-induced α'' in front of the notch tip and resulted in cracks along a narrow α'' region, leading to a significant NTS loss in the B specimen at 25℃. For the B specimen after 20% thickness reduction, i.e. the B20R specimen, the density of hydrogen traps increased and the number of mobile dislocations decreased, hence the HE susceptibility was lowered. In the 40% cold-rolled base metal specimen (the B40R specimen), hydrogen was transported mainly by diffusion, and the presence of 70% γ and ε in the specimen greatly decreased the diffusion of hydrogen. Therefore, the B40R specimen had lower HE susceptibility than the B20R specimen if the NTS loss was adapted to index the relative HE susceptibility. The much slower movement of hydrogen by diffusion compared with dislocation transport also explains why the B40R specimen has better resistance to HE.
For the welded specimens, the γ / δ interfaces are trapping sites for hydrogen atoms and δ-ferrite inhibits strain-induced α'' transformation in the deformation process. Consequently, the weld metal specimen (the W specimen) and its 20% cold-rolled specimen (the W20R specimen) exhibited lower HE susceptibility than the corresponding base metal specimens (the B and B20R specimens). As for the 40% cold-rolled specimen (the W40R specimen), the hydrogen atoms could diffuse along a tunnel or continuous path which consisted of α'' and δ, resulting in a slightly higher susceptibility to HE than the W20R specimen. This tendency is different from the cold-rolled base metal specimens, where the B40R specimen had slightly lower susceptibility to HE than the B20R specimen. Regardless of the experimental group, the fracture appearance displayed ductile dimples for the specimens tested in air, but changed to quasi-cleavage with secondary cracks in gaseous hydrogen. Additionally, the effect of HE decreases as the test temperature increases for all specimens. This is due to the fact that both the hydrogen adsorption on metal surface and the α'' content in front of the notch tip are reduced. During the notched tensile test of metastable austenitic stainless steels, hydrogen atoms enhanced localized plasticity and caused further strain-induced α'' transformation. This is the main cause of HE of the experimental material and generally agrees with the hydrogen enhanced localized plasticity (HELP) theory. In 310S austenitic stainless steel with no strain-induced α'' transformation, HE was not observed between 25 and 135℃, which clearly indicates that the formation of α'' is an important factor to cause HE in metastable austenitic stainless steels.
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