氫氣與凹槽效應對沃斯田鐵系不銹鋼機械性質之影響

Effects of Hydrogen and Notch on the Mechanical Properties of Austenitic Stainless Steels

謝忠霖(Chung-Lin Hsieh)

指導教授 ： 林志光

國立中央大學/工學院/材料科學與工程研究所/碩士(2007年)

摘要

本研究主旨在探討用來製作鎂基儲氫合金儲氫罐的AISI 300系列不銹鋼之適用性。本實驗選擇了三款沃斯田系不銹鋼(AISI 304、AISI 316、AISI 316L)，在室溫 (RT) 至300oC測試氫氣對其機械性質之影響。除此之外，凹槽效應及拉伸速率效應也一併討論。
結果顯示，在拉伸速率 0.015 mm/min下，氫氣一般不會影響室溫下的抗拉強度(UTS)，可是在200oC和300oC會降低UTS。然而，AISI 304在200oC的氫氣測試條件下，UTS比空氣中的值大，推測可能是間隙強化所致。在300oC的一個較慢拉伸速率0.0024 mm/min下，氫氣對於UTS沒影響，甚至還使UTS增加，顯示氫的強化效應。
對AISI 304和AISI 316來說，在0.015 mm/min的拉伸速率下，氫氣降低其延伸量，但是隨著溫度升高，氫降低延伸量的效應減少了。AISI 316L的延伸量變化和其它兩款材料有著相反的趨勢，乃是因為有較低的碳含量。在300oC的一個較慢拉伸速率0.0024 mm/min下，氫沒有降低延伸量，甚至還增加了，這是因為氫助局部塑性(hydrogen-enhanced localized plasticity, HELP)機制作用的關係。
對凹槽試片而言，在拉伸速率0.004 mm/min下，氫氣大致上弱化了凹槽抗拉強度(NTS)，而且當Kt從2增加到3.6時，氫效應增加了。在300oC的一個較慢拉伸速率0.0006 mm/min下，對AISI 304而言，在Kt = 2的狀況下氫造成較大的NTS減低量，而Kt = 3.6的狀況下氫造成較少的NTS減低量，這可能是間隙強化和弱化機制(遮蔽效應或在動態應變時效(DSA)中的氫陷阱作用)的交互作用所造成的。
破斷面觀察結果顯示，在室溫中，氫氣會導致平滑試片在表面形成裂紋，而在凹槽試片中產生劈裂破壞形態。當溫度上升到300oC時，氫會在AISI 304和AISI 316L中，形成低密度、大而平的拉長型酒窩狀破斷形態。然而，對AISI 316而言，和在空氣中測試的試片比較起來，氫氣幾乎沒有改變凹槽試片的破斷面形態。

關鍵字

不銹鋼 ； 凹槽 ； 氫氣

並列摘要

The purpose of this study is to investigate the applicability of AISI 300 series stainless steels to fabricate the storage tank for Mg-base hydrides. Three commercial austenitic stainless steels (AISI 304, AISI 316, and AISI316L) were tested in the current study to investigate the hydrogen effect on the mechanical properties of such alloys at room temperature (RT) to 300oC. Furthermore, notch effect and stroke rate effect were also studied.
Results showed that hydrogen generally did not affect the ultimate tensile strength (UTS) at RT and reduced the UTS in most conditions at 200oC and 300oC under a stroke rate of 0.015 mm/min. However, for the case of AISI 304 tested at 200oC, the UTS was found to be increased in hydrogen, presumably, due to an interstitial strengthening mechanism by hydrogen. Under a slower stroke rate of 0.0024 mm/min at 300oC, no detrimental hydrogen effect and even an increase in UTS by hydrogen were found, indicating a strengthening effect by hydrogen.
Hydrogen reduced the elongation, but the hydrogen effect was decreased as the temperature increased for AISI 304 and AISI 316. AISI 316L exhibited an opposite trend of hydrogen effect on elongation to that of the others as a result of a lower carbon content. Under a slower stroke rate of 0.0024 mm/min at 300oC, no hydrogen effect and even an increase in elongation by hydrogen were observed, presumably, due to a hydrogen-enhanced localized plasticity (HELP) mechanism.
For notch specimens, hydrogen generally degraded the notch tensile strength (NTS) and its effect increased when the Kt was increased from 2 to 3.6 under a stroke rate of 0.004 mm/min. Under a slower stroke rate of 0.0006 mm/min at 300oC, for AISI 304, the results showed a greater NTS reduction for Kt = 2 and a less NTS reduction for Kt = 3.6. This might be due to an interaction between the interstitial strengthening mechanism and the softening mechanisms, such as a shielding effect and/or trapping of hydrogen during dynamic strain aging (DSA).
Fractographic observations showed that hydrogen could induce surface cracks for smooth specimens and a cleavage fracture pattern for notch specimens at RT. When the temperature was increased to 300oC, hydrogen resulted in flat, elongated, lower-density, and bigger-size dimples for AISI 304 and AISI 316L. However, for AISI 316, hydrogen barely changed the fracture surface morphology of notch specimen in comparison with that in air.

並列關鍵字

austenitic stainless steels ； notch effect ； AISI 304 ； hydrogen embrittlement

參考文獻

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