The Study of Fatigue Crack Characteristics of 316L and A508 Steels
|Keywords:||316L不?袗?;A508低合金鋼;應變誘發麻田散鐵相變態;疲勞裂縫成長速率;熱處理;冷加工;氫脆.;316L stainless steel;A508 steel;strain-induced martensitic transformation;fatigue crack growth rate;heat treatment;cold work;hydrogen embrittlement.||Issue Date:||2012||Abstract:||本研究係針對核電廠常用的 316L不?袗?與A508低合金鋼，進行以大氣為主之疲勞裂縫成長速率試驗，並探討不同熱處理或冷加工之差異性。實驗用316L不?袗?試片包含：經過1050℃/ 20 min固溶處理之母材（316L-BM）試片、冷軋母材（316L-CR）、冷加工後敏化（316L-CRS）與1100℃/ 1 h晶粒粗大化（316L-CG）試片等。A508低合金鋼方面，主要有母材（A508-BM）、母材經900℃/ 1h固溶後水淬（A508-SQ）與淬火後621℃/ 24 h回火（A508-QT）等試片。上述316L與A508試件皆於空氣與氫氣中進行25℃與300℃之疲勞裂縫成長試驗，以探討氫脆效應對裂縫成長速率的影響。實驗結果顯示，25℃大氣下測試之316L試片中，冷加工與敏化熱處理對常溫之裂縫成長速率無明顯影響（316L-BM-25A、316L-CR-25A和316L-CRS-25A）；而粗晶試片（316L-CG-25A）因差排移動受晶界阻礙減少，在高ΔK（應力強度因子範圍）區之裂縫成長速率有加速的情形。於空氣中300℃測試者，經敏化之試片（316L-CRS-300A）FCGR和冷輥試片（316L-CR-300A）相比無加速之現象；輥壓試片（316L-CR-300A）受應變誘發麻田散鐵強化，裂縫成長速率較未變態之母材試片（316L-BM-300A）低；及粗晶試片（316L-CG-300A）因粗糙度誘發裂縫閉合效應，速率和母材試片（316L-BM-300A）相當外，其他則與25℃大氣中試驗結果大致相同。另一方面，經不同熱處理的A508試片，在大氣中之疲勞裂縫成長行為並無明顯差異，300℃測試結果亦大致相同。上述試片於25℃氫氣中，則與大氣中有相當的改變，316L-BM-25H試片為316L四組試片中氫脆敏感性最低者，而A508淬火試片（A508-SQ-25H）則因麻田散鐵變態量高，其氫脆敏感性提升，裂縫成長速率較母材試片（A508-BM-25H）為快。整體而言，大氣中測試之316L和A508試片，其300℃之速率皆較25℃測試者為高，除材料於高溫塑性較佳，使裂縫成長較快外，316L於高溫測試過程中沒有無相變態誘發之裂縫閉合效應亦為原因之一。另一方面，雖然316L之敏化熱處理之效應不明顯，但於低溫氫環境下使用時，仍有裂縫成長加速現象，此係因氫原子隨差排移動至應力集中區，促進局部塑性變形，使應變誘發之麻田散鐵變態集中於裂縫尖端，而加速疲勞裂縫成長，此現象與氫促進局部塑性理論相符合。
This study focused on two commonly used materials, 316L stainless steel and A508 low-alloy steel, in the nuclear power industry. In the fatigue tests, several differently treated 316L specimens were used: 20% cold rolled (316L-CR), cold rolled and sensitized (316L-CRS), grain-coarsened (316L-CG), and solution-treated base metal (316L-BM). The A508 steel was tested in three conditions including as-received (A508-BM), solution-treated and water-quenched (A508-SQ), and quenched and tempered (A508-QT). A series of fatigue crack growth rate (FCGR) tests was conducted at 25°C and in air 300°C, and the results were compared. The FCGR tests for various 316L and A508 specimens were also conducted in gaseous hydrogen at 25°C to investigate the influence of hydrogen embrittlement (HE). To identify the specimens tested at various conditions, the numbers (temperature) together with A (air) or H (hydrogen) were attached to the specimen''s designation for simplicity. For instance, the 316L-CR-25H represented the 316L-CR specimen which was tested in hydrogen at 25°C. Experimental results revealed that the 316L-BM-25A, 316L-CR-25A and 316L-CRS-25A specimens had similar FCGRs, implying that cold-working and sensitization treatment had little influence on FCGRs in air at 25°C. Under the same condition, the 316L-CG-25A specimen exhibited higher FCGRs at high ΔK (stress intensity factor range), possibly due to fewer grain boundaries to retard the motion of dislocations. For specimens tested in air at 300°C, both the 316L-CR-300A and 316L-CRS-300A specimens possessed similar FCGRs. Due to the lack of strain-induced martensitic transformation at 300°C, the 316L-CR-300A specimens with pre-existing (alpha)'' had higher strength / hardness and lower FCGRs than the 316L-BM-300A specimen. In addition, the 316L-CG-300A specimen had FCGRs close to that of the 316L-BM-300A specimen, possibly owing to the effect of roughness-induced crack closure. The FCGRsof the A508 specimens did not differ from each other significantly under various heat treatments and test temperatures (25°C and 300°C) in air. Nevertheless, the results of the 316L and A508 specimens tested in H2 at 25°C were remarkably different from those tested in air at 25°C. The 316L-BM-25H specimen had the lowest FCGR among the four groups of 316L specimens. Furthermore, the A508-SQ-25H specimen, with a greater amount of untempered martensite, possessed higher FCGRs than the A508-BM-25H specimen due to the effect of HE. In general, the average FCGR of a given 316L or A508 specimen tested at 300°C was higher than that at 25°C, owing to improved ductility at elevated temperatures. Aditionally, the absence of strain-induced martensitic transformation, i.e., no phase transformation-induced crack closure, could also contribute to the higher FCGRs of 316L specimens at 300°C. Although the sensitization is not apparent for 316L steel, the FCGRs of specimens tested at low temperatures in hydrogen were accelerated. This phenomenon is associated with strain-induced martensite formation at the crack’s front and agrees with the Hydrogen Enhanced Localized Plasticity (HELP) theory.
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