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The Stress Corrosion Cracking and Corrosion Fatigue of Fe-based Stainless Steel Weldments(III)
|Contributors: ||NTOU:Institute of Materials Engineering|
|Keywords: ||2205 雙相不銹鋼;雷射銲接;機械性質;腐蝕疲勞裂縫成長;麻田散鐵變態|
2205 duplex stainless steel;laser welding;mechanical properties;fatigue crack growth;corrosion fatigue crack growth;martensite transformation
|Issue Date: ||2011-06-28T07:09:24Z
|Abstract: ||2205 雙相鋼母材及 254SMO 超級不銹鋼在不同環境中，進行拉伸與疲勞裂縫成長試驗，試驗環境包括空氣、氣態氫環境、飽和硫化氫水溶液與 80℃飽和氯化鎂水溶液。2205 母材在大氣環境中拉伸實驗結果顯示，滾壓組織對疲勞裂縫成長特性影響輕微，但不同腐蝕環境中，縱向試片比橫向試片劣化程度較低，而滾壓方向對 254SMO 的拉伸特性並無影響。然而 2205 母材在不同腐蝕環境中，造成不同程度的疲勞裂縫成長加速特性，腐蝕環境低ΔK 範圍中，當裂縫成長方向垂直滾壓方向時，其疲勞裂縫成長速度略高於平行滾壓方向試片。TEM 觀察及 X-ray繞射結果顯示，在疲勞裂縫破斷面表層，部分沃斯田鐵因應變引發麻田散鐵變態。大氣中疲勞破斷面主要為穿晶破斷與少量的劈裂破斷，而腐蝕環境中由於大範圍準劈裂破壞，導致疲勞裂縫成長加速。而腐蝕環境卻對 254SMO 超級不銹鋼的疲勞裂縫成長特性影響輕微，在氣態氫環境及 80℃飽合氯化鎂水溶液環境，皆呈現與大氣環境相當的疲勞裂縫成長特性，其原因應與穩定之沃斯田鐵相有關。
2205 銲道疲勞裂縫成長實驗結果顯示，疲勞裂縫成長特性受到銲接殘留應力影響，需要極大的裂縫起始應力強度因子值才能產生裂縫。重複低溫熱處理(600℃/2hrs)可消除部分殘留應力，且不改變原始銲道微觀結構。低溫熱處理銲道試片疲勞裂縫成長速度較母材低，其主要原因為較長疲勞裂縫路徑，且粗糙度引發閉合效應影響，而減緩裂縫成長速度。氣態氫環境中疲勞裂縫成長速度受氫脆影響而加速，且與銲道內γ相含量有明顯關係，隨銲道中γ相提升，可以降低氫脆與銲道疲勞裂縫成長速度。2205 雙相鋼銲件得以 1050℃短時間熱處理方式提昇γ相含量，母材試片經過熱處理可提升γ相含量，經熱處理銲件其疲勞裂縫成長速度較母材慢，其原因在於粗糙度引發疲勞閉合與路徑效應導致裂縫成長減速。
The tensile and fatigue crack growth tests of 2205 duplex stainless steel were performed in various environments, including laboratory air, gaseous hydrogen, saturated HB 2BS solution and saturated MgClB 2B solution at 80 P o PC. The longitudinal specimen showed a lesser degradation of tensile properties than the transverse ones in all conditions. The orientation of specimens with respect to rolling direction had little influence on the fatigue crack growth rate (FCGR) of the alloy in air. On the other hand, enhanced crack growth occurred to different degrees for 2205 duplex stainless steel tested in corrosive environments. Moreover, as the crack grew transverse to the rolling direction, a slightly higher crack growth rate than that along the rolling direction was found in corrosive environments within the low ∆K range.
Transmission electron micrograph in addition to X ray diffraction revealed that strain-induced austenite to martensite transformation occurred near the crack surface within a rather narrow depth. Fatigue fractography of the specimens tested in air showed mainly transgranular fatigue fracture with a small amount of flat facet fracture. Furthermore, extensive quasi-cleavage fracture induced in corrosive environments was correlated with the environment-assisted crack growth.
The effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of 2205 duplex stainless steel weld were investigated by slow displacement rate tensile tests. The hydrogen embrittlement susceptibility of the specimens was correlated with microstructures of the fusion zone. The results indicated that all the specimens were susceptible to gaseous hydrogen embrittlement but to different degrees. The susceptibility decreased with increasing austenite content in the weld metal. The orientation with respect to the rolling direction had a great influence on the impact toughness of the base plate. Preheating before welding or changing the plasma-assisted gas from He to NB 2B could raise the γ content of the fusion zone, and improve the impact toughness. In case of the post-weld heat-treated weld (PW), the presence of randomly oriented acicular and blocky γ in the fusion zone led to the highest impact energy and NTS among the specimens being tested. SEM fractographs revealed that all specimens underwent a significant change in fracture mode from ductile in air to quasi-cleavage fracture in HB 2B.
The fatigue crack growth test was also performed to evaluate fatigue behavior of AISI 304 and 316 stainless steels (SSs.) with or without sensitization treatment in air or gaseous hydrogen. For the air-tested specimens, the influence of sensitization treatment on the fatigue crack growth rates (FCGR) was found to be minor regardless of testing alloys. Hydrogen-enhanced crack growth occurred for all the specimens but to different degrees. With the specimens subjected to sensitization treatment, the remarkable acceleration in crack growth rates had been found, especially for the aged 304 SS. In contrast, 254 SMO stainless steel behaves the highest resistance to hydrogen-enhanced crack growth among the alloys regardless of testing conditions.
For all the specimens tested in air, fatigue fracture appearance of 304 and 316 specimens exhibited mainly transgranular fatigue fracture with few intergranular and twin boundary separation. Meanwhile, the extent of intergranular fracture increased obviously for the aged specimens tested in gaseous hydrogen. Extensive quasi-cleavage fracture related to the formation of stress-induced martensite accounted for the increased FCGR for the un-sensitized austenitic SSs in hydrogen. Accelerated crack growth for the sensitized austenitic SSs in hydrogen was associated with the intergranular fracture in the lowΔK range.
|Appears in Collections:||[材料工程研究所] 研究計畫|
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