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Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/13496

Title: 麻田散鐵變態對沃斯田鐵不□鋼疲勞裂縫成長特性影響研究
Austenitic Stainless Steel Fatigue Crack Growth Affected by Martensitic Transformation
Authors: Yi-Chu Liu
劉益助
Contributors: NTOU:Institute of Materials Engineering
國立台灣海洋大學:材料工程研究所
Keywords: 麻田散鐵變態;沃斯田鐵不□鋼;疲勞裂縫成長
martensitic transformation;austenitic stainless steel;fatigue crack growth
Date: 2002
Issue Date: 2011-06-30T07:22:15Z
Abstract: 本研究主要探討大氣與氣態氫環境下,AISI 304、316與316L不□鋼板材,與經雷射熱處理和電漿銲接試件之疲勞裂縫成長特性。 316不□鋼經雷射熱處理試片,熱處理區前方區域因殘留壓應力場作用,使疲勞裂縫成長具有減緩效果且具有較高的ΔKth,即使在氣態氫脆化環境中仍有高阻擋裂縫成長的能力,而在硫化氫環境中裂縫成長初期,則有略高的裂縫成長速率。316不□鋼板材或經雷射熱處理試件,在氣態氫中之裂縫成長行為,實驗結果與大氣環境中差異不大,此一結果應與破裂面上不完全麻田散鐵變態有關。由於氣態氫環境與大氣中之疲勞裂縫成長特性一致,導致破壞面呈現相同的破壞特徵,主要為沿雙晶邊界破裂與穿晶疲勞破壞。經雷射熱處理試件於低ΔK範圍,出現因殘留應力造成的壓合面,而在裂縫成長後期,疲勞條紋越來越明顯。 304不□鋼銲件微觀結構為凝固的沃斯田鐵基地內,含有骨骼狀肥粒鐵(skeletal δ-ferrite),然而母材基地內只有等軸之沃斯田鐵晶粒與些許雙晶。未經應力消除之電漿銲件(AW),於低ΔK範圍其阻擋裂縫成長的因素,應歸究於裂縫尖端之殘留壓應力所造成,且於氣態氫環境下依然具高阻擋裂縫成長的能力。經900℃一小時之熱處理之回火銲件(SR),初始高ΔKth現象及高Paris Law斜率消失。304不□鋼無論板材、銲件或回火銲件,實驗結果顯示:在相同的ΔK範圍內304不□鋼在氣態氫環境下有明顯加速現象。銲件巨觀疲勞破斷面出現鋸齒狀裂縫成長路徑,而母材則呈現較平整裂縫成長路徑。X-ray繞射結果說明304不□鋼疲勞破斷面,大體上均顯示出高度α’麻田散鐵變態,僅殘留少量未變態的沃斯田鐵,破裂表面生成大量麻田散鐵,變態之麻田散鐵因氫脆導致304不□鋼裂縫成長加速。 316L不□鋼未回火銲件(AW)無論在大氣或氣態氫環境下其裂縫成長速率都隨著ΔK升高而急速爬升。除此之外,AW試片之ΔKth遠大於母材(BM)與經應力消除銲件(SR)試片,而其da/dN~ΔK斜率更是遠高BM與SR試片。316L不□鋼疲勞破斷面經由X-ray繞射分析,破斷面上為不完全的α’麻田散鐵變態,且殘留大量未變態的沃斯田鐵,於氣態氫環境中α’/γ比例亦未有明顯升高的趨勢,而沃斯田鐵能夠固溶大量的氫原子,因此α’麻田散鐵未造成氫脆化,且其疲勞破斷面亦沒有觀察到板條狀α’麻田散鐵脆性破壞特徵,也可證實316L不□鋼在本實驗條件下並未發生高氫脆性,亦未發現裂縫成長加速的現象。Knoop硬度計量測(microhardness test)疲勞破斷面側邊硬度分佈,推測得知316L不□鋼在大氣與氣態氫環境中,兩者裂縫尖端幾何形狀應大致相同具有相同塑性區,亦證實裂縫尖端應未受氫脆影響,導致裂縫尖端幾何形狀發生改變。
Fatigue crack growth tests were performed to evaluate the fatigue behavior of AISI 304, 316 stainless steel plate and welds in laboratory air and gaseous hydrogen. The microstructures of austenitic stainless steel weld consisted of lacy ferrite in the columnar grain and subgrain boundaries within the austenite matrix, instead of equiaxial austenite grains with some twins inside the parent metal. The high threshold ΔK of the as-welded (AW) specimen to initiate crack growth was clearly higher than that of base metal and stress-relieved weld, regardless of testing environments. The considerable retardation of crack growth vanished for welds subjected to stress-relief treatment. For AISI 304 stainless steel (SS) the fatigue crack growth rate (FCGR) of a stress-relieved weld was lower than that of the base metal, within the experimental ΔK range. Moreover, enhanced crack growth was disclosed for all 304 SS specimens tested in gaseous hydrogen. However, all the 316 SS specimens were insensitive to hydrogen-accelerated crack growth. In addition, the fatigue crack growth rate of stress-relieved weld was similar to that of the base metal within the experimental ΔK range in 316 SS. The macroscopic appearance of fatigue-fractured welds was very rough and showed a zig-zag crack path, unlike the straight path in the base metal. For 304 SS, the thin martensite layer formed on the cracked surface suffered from hydrogen embrittlement, leading to the accelerated crack growth in gaseous hydrogen. Extensive quasi-cleavage fracture in hydrogen, in contrast to transgranular fatigue fracture in air, clearly indicated that strain-induced martensite strongly affected the fatigue crack growth behavior of the AISI 304 SS weld. However, X ray diffraction pattern of 316 SS revealed that on the cracked surface just partial transformation of austenite to martensite occurred within a narrow depth. The presence of higher extent of residual austenite in highly strained region to trap large amount of hydrogen, would help to reduce the degree of hydrogen embrittlement. As a result, AISI 316 SS showed a high resistance to hydrogen-enhanced crack growth. The presence of residual compressive stress field ahead of crack front in laser-treated 316 SS results in the reduced fatigue crack growth rate in the compact tension specimen.
URI: http://ethesys.lib.ntou.edu.tw/cdrfb3/record/#G0000000126
http://ntour.ntou.edu.tw/ir/handle/987654321/13496
Appears in Collections:[材料工程研究所] 博碩士論文

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