301不銹鋼及其雷射銲件經輥軋後之內部氫脆研究

Abstract

中文摘要 AISI 301不銹鋼係屬介穩定型沃斯田鐵系不銹鋼，其受到應變發生麻田散鐵變態之Md30溫度約67°C，故於室溫下輥軋或拉伸，會產生應變誘發之 α' 與 ε 麻田散鐵 (α'- and ε- martensites)。本研究針對301不銹鋼母材與雷射銲接試片，經30%厚度縮減 (Thickness reduction) 之25°C冷軋與150°C溫軋後，進行300°C熱充氫與一般大氣 (不充氫) 之熱處理試驗，量測大氣中之缺口拉伸性質，並評估不同條件處理試片之內部氫脆 (Internal hydrogen embrittlement) 敏感性。經300°C一般熱處理者，各試片之缺口拉伸性質與未經熱處理之對應試片，並無明顯差異。當熱處理溫度提高至450°C，母材及銲道冷軋試片之拉伸強度大幅下降，此係因冷軋造成試片內部大量缺陷，降低碳化物 (Cr23C6) 的析出溫度，弱化了晶界所致。 301不銹鋼母材試片於300°C熱充氫後，氫原子被捕集於晶界或雙晶晶界，進行缺口拉伸試驗時，這些位置優先應變誘發 α' 麻田散鐵，於拉伸過程中易發生破裂。母材試片經不同條件輥軋，其氫脆敏感性與顯微組織有密切關係。冷軋之母材試片有26%之 α'，且 α' 的抗氫脆性較 γ 與 ε 為差，而降低了抗氫脆性；溫軋之母材試片僅有 ε 相生成，基地大部分仍為 γ 相，故氫脆敏感性相較於冷軋母材試片為低。銲道試片方面，由於銲接凝固過程會有少量δ-ferrite殘留，增加了氫捕集的γ/δ 界面，使銲道試片及其經輥軋試片之氫脆敏感性，均較相對應之母材試片為低。 未經輥軋之母材或銲道試片，經一般熱處理後，均無碳化物析出者，缺口拉伸之破斷面皆呈現韌窩狀破斷形貌。若晶界有碳化物析出 (如冷軋試片經450°C一般大氣熱處理)，破斷面則呈現沿晶破斷形貌。熱充氫測試者，在受氫影響區域之破斷面呈現脆性破斷形貌，並有許多二次裂縫，且 α' 變態集中於斷面兩旁的狹窄區域，可印証氫促進局部塑性變形的效應，此與HELP理論相符。若提高熱充氫溫度，各試片之氫含量增加，α' 變態量減少，而導致氫促進局部塑性變形現象更加明顯。

Abstract AISI 301 stainless steel is a metastable austenitic stainless steel and has the Md30 temperature of approximately 67°C, i.e., the temperature at which 50% of the austenite phase transforms into martensite during tensile testing at a true strain of 0.3. As a result, rolling and tension testing of 301 stainless steel would strain-induce transformation of γ to α'- and ε- martensites at room temperature. In the present study, the base metal (the B specimen) and laser welded specimen (the W specimen) were subjected to either cold rolling at 25°C (the B-CR and W-CR specimens) or warm rolling at 150°C (the B-HR and W-HR specimens) to reach a 30% reduction in thickness. The specimens were then thermally hydrogen charged at 300°C, or treated with an air furnace heat at 300°C (without hydrogen charging). Under various processing conditions, the internal hydrogen embrittlement (IHE) of the specimens was evaluated by the notch tensile test in air. The notch tensile properties of the specimens after heat treating at 300°C in air furnace showed insignificant differences to the specimens without heat treatment. Increasing the heat treatment temperature to 450°C, the tensile strength of the B-CR and W-CR specimens significantly reduced because the carbides (Cr23C6) precipitated at the grain boundaries. After cold rolling , the specimens generated many defects in the matrix, which resulted in a decrease in temperature for carbide precipitation, leading to weakened grain boundary regions. For the B specimen, hydrogen atoms were trapped at the grain and twain boundaries after thermal hydrogen charging at 300°C. These locations fractured easily during the notch tensile test due to the formation of brittle α'-martensite. The susceptibility to IHE of the B specimens under various rolling conditions was closely related to their microstructure. The B-CR specimen transformed into about 26% of α', which has worse resistance to IHE than the γ and ε phases. The B-HR only formed ε-martensite, the martrix was mainly γ phase so that exhibited lower susceptibility to IHE than the B-CR specimen. The strain-induced α'-martensite led to higher IHE of the B-CR specimen. As for the welded specimens, a small amount of residual δ-ferrite still existed during the weld solidification process. The γ/δ interfaces increased the trapping sites for hydrogen, as a result, the W specimen and its rolled specimens (the W-CR and W-HR specimens) would exhibit lower IHE than corresponding base metal specimens (the B, B-CR and B-HR specimens). The notch fracture surface of the B and W specimens without carbides precipitation showed dimple fracture after the air furnace heat treatment. The fracture surface displayed intergranular fracture (as seen in B-CR specimen after air furnace heat treatment at 450°C) was resulted from the carbides precipitated at grain boundaries. As for the thermal hydrogen charging specimens, the fracture surface of regions that were influenced by hydrogen exhibited brittle fracture with many secondary cracks. The strain-induced α' transformation localized in a narrow region in front of the notch tips indicates that hydrogen promotes localized plastic deformation, which agrees with the Hydrogen Enhanced Localized Plasticity (HELP) theory. If the thermal hydrogen charging temperature was increased, the hydrogen content in all specimens increased and the amount of α' transformation decreased, resulting in the localized plastic deformation more obviously.