輥軋與測試溫度對301不銹鋼及其雷射銲件之氫脆影響研究

本研究係針對301不銹鋼母材、雷射銲道及經輥軋 (30%厚度減縮) 後之試片，於不同測試溫度 (25、70、100℃) 下，進行大氣與氫氣中之慢速率缺口拉伸試驗，以評估輥軋與測試溫度對材料氫脆的影響。室溫與150℃輥軋之試片，在顯微組織上有很大差異，麻田散鐵的變態量及其類型 (α'- 與 ε- martensite) 會對氫脆造成影響。氫脆效應除了使材料之缺口拉伸強度下降外，亦會造成延性損失。母材 (B) 試片於氫環境中塑性變形時，氫原子隨可動差排遷移至應力集中區，並促進局部塑性變形，產生應變誘發 α' 麻田散鐵，但其變態量與氫脆敏感性則隨測試溫度升高而遞減。母材經25與150℃輥軋後，即B-CR與B-HR試片，在氫氣中之缺口拉伸曲線均無塑性變形，顯示氫原子係經由擴散移至裂縫前端而導致脆斷。由於氫在 α' (BCC) 中之擴散速率較γ (FCC) 快了很多，導致B-HR試片 (α' = 0.3%) 之抗氫脆性優於B-CR試片 (α' = 23.8%)。由此可知，大量 α' 之生成增加了301不銹鋼之氫脆敏感性。經雷射銲接之銲道 (W) 試片的氫脆機制與母材 (B) 試片相似。然而，銲道凝固組織含有δ-ferrite，使α' 變態量降低，可捕集氫原子之γ/δ 界面增加，導致W試片之氫脆敏感性低於B試片。此外，W試片之金相並無B試片之退火雙晶組織，減少了裂縫沿雙晶界面成長的路徑，此為其氫脆敏感性相較於B試片降低的另一原因。銲道與母材輥軋試片於氫環境測試下，氫原子之傳輸皆以擴散為主，但前者之部分滑移帶受到凝固次晶界的阻擋，導致W-CR與W-HR試片之 α' 變態量及氫脆敏感性亦較相對應之B-CR與B-HR試片為低。值得注意的是，W-HR試片在100℃氫環境中測試，缺口拉伸強度與空氣測試者接近，但其延性損失仍有10%。在大氣中，所有試片的破斷面皆呈現窩穴狀之延性破裂；在氫氣中，試片破斷面受氫脆影響區域則呈現脆性破裂模式。當測試溫度升高後，試片缺口附近之氫原子吸附率及 α' 變態量則隨之下降，氫脆敏感性與破斷面脆性均有減緩的趨勢。

關鍵字

301不銹鋼；雷射銲接；輥軋；缺口拉伸強度；氫脆；應變誘發麻田散鐵變態

並列摘要

The effects of rolling and testing temperature on hydrogen embrittlement (HE) of AISI 301 stainless steel and its laser welded specimens were evaluated in both the unrolled and rolled conditions. Slow displacement rate notched tensile tests were carried out in air and gaseous hydrogen at 25 to 100℃ to assess HE of various specimens. The rolled specimens were processed at either room temperature or 150℃ with 30% thickness reduction. Due to the temperature dependence of strain-induced martensitic transformation in 301-type stainless steel, the microstructure showed substantial differences between the specimens rolled at distinct temperatures. The HE was seen to be affected by the amount and type of martensites (α' and ε) transformed in the specimens. Apart from reducing the notched tensile strength (NTS), the effect of HE also caused ductility loss of the specimens. For the base metal specimen (the B specimen), hydrogen atoms could be transported to the stress concentration region through mobile dislocations during plastic deformation in hydrogen. This would promote localized plasticity and lead to the formation of strain-induced α'. It was also found that the amount of strain-induced α' and HE susceptibility decreased as the test temperature increased. For the base metal rolled at 25 and 150℃ specimens, i.e. the B-CR and B-HR specimens, their stress-displacement curves revealed no plastic deformation during notched tensile tests in hydrogen, implying that hydrogen atoms were diffused to the crack tip and caused brittle fracture. Because hydrogen had a much faster diffusivity in the α'-phase (BCC or BCT) than in the γ-phase (FCC), the HE resistance of the B-HR specimen (0.3% α') was better than that of the B-CR specimen (23.8% α'). The results clearly demonstrated that the formation of considerable amounts of α' increased the HE susceptibility of 301 stainless steel. The HE mechanism of the weld metal specimen (the W specimen) was similar to that of the B specimen during notched tensile tests in hydrogen. However, the presence of δ-ferrite after weld metal solidification the led to a reduction of α' transformation in the W specimen. Moreover, the γ/δ interfaces could also act as hydrogen traps, resulting in the HE susceptibility of the W specimen less than that of the B specimen. Unlike the B specimen, the W specimen does not contain annealing twins to serve as potential crack propagating paths along twin boundaries. As a result, the reduced HE susceptibility for the W specimen could be expected. For the notched tensile testing in hydrogen, hydrogen transport was controlled predominantly by diffusion for both the weld metal and base metal specimens after rolling. However, some slip bands in the welded specimens were impeded by solidification sub-grain boundaries so that both the content of strain-induced α' and the HE susceptibility of the W-CR and W-HR specimens were lower than those of the corresponding B-CR and B-HR specimens. It is noteworthy that the NTS loss of the W-HR specimen was close to 0% at 100°C, but its ductility loss was approximately 10% in hydrogen. For testing in air, the fracture surface exhibited ductile dimples for all specimens. However, the region affected by HE on the fracture surface changed to brittle fracture in hydrogen. When the test temperature was raised, the hydrogen adsorption and the amount of strain-induced α' adjacent to sharp notches of the specimens decreased. Accordingly, the HE susceptibility and the extent of embrittlement on the fracture surface of a given specimen reduced.

並列關鍵字

AISI 301 stainless steel ； laser welding ； rolling ； notched tensile strength ； hydrogen embrittlement ； strain-induced martensitic transformation.

參考文獻

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9. G. B. Olson and M. Cohen, J. Less-Common Metals, 28 (1972) 107.

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國際替代計量

輥軋與測試溫度對301不銹鋼及其雷射銲件之氫脆影響研究

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