Character of Low Cycle Fatigue and High Strain Rate Deformation for Fe Based Stainless Steel Welds(III)

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Project title

Code/計畫編號

NSC93-2216-E019-003

Translated Name/計畫中文名

特用型不銹鋼之機械與抗蝕性應用研究---子計畫四:鐵基不銹鋼銲件之低週期疲勞及高應變速率變形特性健會(III)

Project Coordinator/計畫主持人

Shing-Hoa Wang

Funding Organization/主管機關

National Science and Technology Council

Department/Unit

Department of Mechanical and Mechatronic Engineering

Website

https://www.grb.gov.tw/search/planDetail?id=1028251

Year

2004

Start date/計畫起

01-08-2004

Expected Completion/計畫迄

01-07-2005

Bugetid/研究經費

784千元

ResearchField/研究領域

材料科技

SAF 2205 及SAF 2507 雙相不銹鋼，其素材與銲件的銲道熔融區硬度顯示 2205 較硬。在不同拉伸應變速率下，增加應變速率會使 2205 的素材及銲件極限抗拉強度有略升的趨勢。降伏強度方面，二者素材在ε=1×10-4 s -1 到ε =1×10-3 s -1有大幅提升，但ε =1×10-3 s -1到ε =1×10-2 s -1則維持持平狀態，至於銲件則呈現相反情況。應變速率增加對素材及銲件伸長量均會下降，且顯示 2205 有較好的延伸率。疲勞應變比R值的改變對 2205 和 2507 都呈現循環軟化現象，疲勞壽命在應變振幅 1%以上無明顯變化，但在應變振幅 1% 以下R= -0.2 及R= -0.5 呈現比R= -1 有較高的疲勞壽命。疲勞後之微觀組織顯示許多雙晶生成且差排多為PLB差排組織。高鎳鉬之超級沃斯田鐵不銹鋼 254 SMO素材與銲件，在不同應變速率，其拉伸應變速率越快延展性越好；相反破裂疲勞應變速率越快壽命越短，其斷口面疲勞條紋越為明顯。雙相鋼和 254 SMO在室溫下以 5×103 s -1及 8.5×102 s - 1二種不同高應變速率下的動態塑性變形行為。高速率應變會產生絶熱剪切帶僅在硬度 2507 表面出現一條約 45 度shear band。沃斯田鐵不銹鋼 254 SMO金相中發現到受到高應變速率撞擊後，在基地上出現了折曲細線狀碳化物析出。在熔融區由應變所引發之γ相變態，其局部硬度的變化視應變速率而定；超級沃斯田鐵不銹鋼因有穩定的沃斯田鐵相，所以不受撞擊影響。撞擊過的銲件在高應變速率 5×103 s -1 下，沿著固化曲線的圓柱狀晶界在表面顯現出羽毛狀的褶痕，起因於粗糙的結晶材料在猛烈的塑性變形下所產生散佈的呂德帶(Lüders bands)。Two duplex stainless steels SAF 2205 and SAF 2507 with similar microstructure are used for weld metal in 2507 steel consists of more γ phase than that in 2205 steel. Therefore the microhardness values of weld metal are higher than that of base metal for 2205 steel. The ultimate tensile strength increases with increasing strain rate for 2205 steel and their welds, but 2507 steel exhibits opposite trend. 2205 steel and their welds appear better elongation than 2507 steels. Fatigue endurance limit varies with different strain ratio. The materials undergo cyclic strain exhibit the hardening at initial cycles but cyclic softening later. The areas of hysteresis loop increases with increasing strain amplitude because of large plastic deformation. In general. The tensile properties of super austenitic stainless steel 254 SMO and weldments, show an abnormal behavior. It exhibits a better strength and ductility as the strain rate increases. Conversely the fatigue life reduces as increasing strain rate. The dynamic deformation behavior of the special stainless steel impacted by means of a Split-Hopkinson bar was studied based on one dimensional elastic wave theory. The experimental materials are the mixed α and β phases of duplex stainless steels（SAF 2205 and SAF 2507）and super-austenitic stainless steels （254 SMO）with a higher nickel and molybdenum content. The tests were performed at room temperature under strain rates 8.5×102 s-1 and 5×103 s -1. A shear band was found at surface of SAF 2507 only with both strain rates. Single phase of austenitic stainless steel 254 SMO shows the grain after impact deformation due to instantaneous heat generation. The flow stress, work hardening increases with increasing strain rate to a maximum value, then decreases for both duplex stainless steels. Inversely austenitic stainless steel 254 SMO shows work hardening completely. A considerable volume fraction of γ phase increases in the fusion zone (weld metal) for two duplex stainless steels after a high strain rate impact. The strain induced γ phase formation in the fusion zone results in the trend of local hardness variation depending on strain rate. In contrast, super austenite stainless steel is unaffected by impact because of the fully stable austenization. The impacted welds at high strain rate of 5×103 s -1 reveal the feather-like surface creases along the solidified curved columnar grain boundaries. The apparent surface creases form due to the diffused Lüders bands, which are caused by heavily plastic deformation in coarse grain materials.

Keyword(s)

雙相不銹鋼
銲接
拉伸特性
疲勞
應變比
高應變速率
duplex stainless steel
weld
tensile property
fatigue
strain ratio
high strain rate

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Character of Low Cycle Fatigue and High Strain Rate Deformation for Fe Based Stainless Steel Welds(III)

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