銲接製程對308L沃斯田不鏽鋼銲道疲勞裂縫成長行為之影響

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研究生：	林劭品 Lin, Shao-Pin
論文名稱：	銲接製程對308L沃斯田不鏽鋼銲道疲勞裂縫成長行為之影響 Influence of welding processes on fatigue crack growth behavior of 308L stainless steel weldments
指導教授：	黃嘉宏 Huang, Jia-Hong 喻冀平 Yu, Ge-Ping
口試委員:	黃俊源蔡履文
學位類別：	碩士 Master
系所名稱：	原子科學院 - 工程與系統科學系 Department of Engineering and System Science
論文出版年：	2012
畢業學年度：	100
語文別：	英文
論文頁數：	95
中文關鍵詞：	銲接製程、308L不鏽鋼、電銲、氬銲、疲勞成長速率
外文關鍵詞：	308L weldments, shielded metal arc welding
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本研究利用施作電銲、氬銲與寬隙氬銲件來探討銲接製程對於308L沃斯田不鏽鋼銲道在室溫與288 °C之疲勞裂縫成長行為的影響。電銲與氬銲件的金相皆顯示FA固化模式的枝狀晶結構；因為電銲條有較高的鉻鎳當量比且銲接時有較低的熱輸入量，其金相呈現長條狀肥粒鐵模式，而氬銲則呈現蠕蟲狀肥粒鐵。拉伸測試中可發現銲道的降伏應力有明顯上升，其主要原因為銲接的快速固化過程使銲道轉變為肥粒鐵與沃斯田鐵的雙相結構而有強化降伏應力的效果。
疲勞測試結果則顯示銲道不論在室溫或288 °C皆有較母材304L好的疲勞抗性，其主要原因與銲道的非均質結構造成疲勞裂面與路徑的粗糙化與複雜化，增加了實際的疲勞路徑，而銲件不論在疲勞或拉伸試驗中都幾乎沒有因應變而麻田散鐵化的強化現象。
寬隙銲件由於第一道需要較長銲接時間而形成了平行銲接方向成長的粗化枝狀晶，並造成一開始較電、氬銲件快的疲勞裂縫成長速率，但此差異隨著裂縫成長與應力強度因子範圍的上升而逐漸減小。
由結果可發現由銲接造成結構上的差異主要影響疲勞裂縫成長的初期，而後期隨著塑化區增大、裂面粗糙化並呈現大量的山脊紋路與準劈裂特徵，應力狀態成為主要的影響因素。

Shielded metal arc (SMA), gas tungsten arc (GTA), and wide-gap tungsten arc weldments were produced to study the influence of welding process on fatigue crack growth behavior of 308L austenitic stainless steel weldments at room temperature (RT) and 288 °C. Both SMA and GTA weldments showed dendritic microstructure with FA solidification mode; however, the lower heat input with larger (Cr/Ni)eq in SMAW process led to lathy ferrite morphology and more residual ferrite in the SMA welds, while vermicular ferrite morphology was shown in GTA weldments. The yield strength of the welds significantly increased with the decreasing elongation, which was mainly due to the dual phase strengthening effect after rapid solidification during welding.
All the weldments showed better fatigue resistance than the 304L base metal at RT and 288 °C in air, which was attributed to the non-homogeneous structure leading to a rougher and more complex crack path. In addition, no strengthening effect by deformation-induced martensitic transformation was observed in both welds. Moreover, due to the slower welding speed in the first pass on the wide-gap GTA weldment, larger spacing dendritic structure was produced along the welding direction at the center, which led to higher fatigue crack growth rate (FCGR) than that of SMA and GTA welds at lower ∆K region; however, the FCGR of all welds converged as ∆K reached 32 and above.
The influence of microstructure on the FCGR was more distinct at lower ∆K region, which could be observed only on specimens without side-grooves, where crack growth direction was not limited, and then the crack growth behavior switched to stress-state controlled mode where the plastic zone became sufficiently large and the fracture surface displayed more ridges and quasi-cleavage fracture.

List of Tables    ........................................viii
List of Figures    ........................................viii
Chapter 1    introduction...............................1
Chapter 2    Literature Review..........................3
1    Austenitic stainless steels........................3
1.1     Characteristics....................................3
1.2     Elemental composition..............................3
1.3     Strain-induced martensitic transformation..........6
1.4     Solidification mode and solidification cracking....7
2    Arc welding process................................9
2.1     Effects of welding parameter......................10
2.2     Effects of welding process to characteristics of weldments.................................................14
2.3     Weldibility of austenitic stainless steels........15
3    Fatigue failure of austenitic steel weldments.....19
3.1     Fatigue failure mechanism.........................19
3.2     Fatigue failure of austenitic steels weldments....21
4    Methods of residual stress measurement............22
Chapter 3    Experimental Procedures...................24
1    Materials.........................................24
2    Specimen preparation..............................25
3    Metallography.....................................28
4    Microhardness test................................28
5    Ferrite number evaluation.........................29
6    Tensile test......................................30
7    Fatigue test......................................31
8    Scanning electron microscopy (SEM)................32
9    Residual stress measurement.......................33
9.1     XRD sin2ψ method..................................33
9.2     Neutron diffraction method........................35
Chapter 4    Results...................................37
1    Metallography.....................................37
1.1     Base metal microstructures........................37
1.2     Macrostructure of weldments.......................37
1.3     Microstructure of weldments.......................37
2    Microhardness and ferrite content.................42
3    Tensile properties................................45
4    Fatigue crack growth rate.........................47
5    Residual stress measurement.......................49
6    Fatigue fracture surface morphology...............52
6.1     Macroscopic fracture surface morphologies.........52
6.2     Fracture morphology of weldments tested at room temperature...............................................52
6.3     Fracture morphology of weldments tested at 288 °C.53
Chapter 5    Discussion................................63
1    Metallography of the weldments....................63
2    Microhardness and ferrite contents................63
3    Tensile properties................................64
4    Residual stress measurement.......................65
5    Fatigue crack growth rate (FCGR)..................66
5.1     Factors affecting the fatigue growth rate.........66
5.2     Fatigue crack growth at room temperature..........67
5.3     Fatigue crack growth at 288 °C....................68
Chapter 6    Conclusions...............................70
References................................................72
Appendix 1................................................77
Appendix 2................................................79

                                

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