本研究在自動氬銲機台上附加電磁攪拌裝置，來探討在氬銲過程中施加電磁攪拌對於309L及347沃斯田不鏽鋼銲道微結構、機械性質及腐蝕性質的影響。結果顯示，在氬銲的銲接製程中施加電磁攪拌可以細化309L銲道的晶粒及枝晶組織，但對於347銲道的晶粒尺寸影響不大，其原因可能是因為347銲道有添加Nb。在銲接製程中施加電磁攪拌可能會使309L及347銲道的晶粒方向變得更隨機分布。309L及347銲道的硬度幾乎沒有受到電磁攪拌的影響；但從結果發現309L銲道的硬度從底部至上方有逐漸減小的趨勢。309L銲道的拉伸性質因施加電磁攪拌後的晶粒細化而有提升，然因FCC結構在Hall-Petch方程式中有較小的k值，而使309L銲道的強度提升的程度較小；而施加電磁攪拌對於347銲道的拉伸性質則有不好的影響。施加電磁攪拌使得309L及347不鏽鋼銲道的抗腐蝕性降低，推論是因為晶粒細化後，肥粒鐵與沃斯田鐵間的界面面積增加進而增強加凡尼腐蝕。

Mutipass weldments of 309L and 347 were produced by gas tungsten arc welding (GTAW) equipped with an electromagnetic stirring (EMS) apparatus and a wire feeder. The objective of this study was to investigate the effect of EMS on the microstructure, mechanical properties and corrosion properties of 309L and 347 welds. The results revealed that the den-drites and grain size of the 309L welds were refined when EMS was applied, while the effect of EMS on grain size of the 347 welds was less significant, which may be due to the addition of Nb. In addition, the grain orientation of 309 and 347 welds may change from some specific orientations to random orientation when EMS was applied. The hardness of 309L and 347 weldments did not substantially affect by EMS. However, the trend of hardness of 309L welds was found to decrease from bottom to top. EMS slightly increased the strength of 309L weldments by grain refining, which is possibly caused by the relatively small locking param-eter in the Hall-Petch equation for the FCC structure; in contrast, EMS showed an adverse effect on the mechanical properties of 347 weldments. EMS lowered the corrosion resistance of 309L and 347 welds, which may be derived from the increase of interfacial area between ferrite and austenite due to grain refinement by EMS, thus enhancing the galvanic corrosion.

摘要 ii
Abstract iii
Contents vi
List of Tables viii
List of Figures ix
Chapter 1 Introduction 1
Chapter 2 Literature Review 3
2.1 Electromagnetic stirring (EMS) 3
2.2 Gas Tungsten Arc Welding (GTAW) 6
2.3 Austenitic stainless steels 8
2.3.1 Characteristics 8
2.3.2 Elemental compositions 8
2.3.3 Solidification mode 14
Chapter 3 Experimental Procedures 18
3.1 Materials 18
3.2 Specimen preparation 19
3.3 Metallography 22
3.4 Microhardness test 22
3.5 Tensile test 23
3.6 Potentiodynamic polarization scan 25
Chapter 4 Results 27
4.1 Evaluation of electromagnetic stirring frequency 27
4.1.1 High speed camera observation 27
4.1.2 Appearance of welds 29
4.1.3 Microstructure of welds 29
4.2 Metallography 32
4.2.1 Macrostructure of welds 32
4.2.2 Microstructure of welds 32
4.2.2 Electron backscatter diffraction (EBSD) 37
4.3 Microhardness 42
4.4 Tensile test 43
4.5 Tensile fracture surface morphology 45
4.6 Potentiodynamic polarization scan 48
Chapter 5 Discussion 50
5.1 Microstructure characterization 50
5.2 Effect of electromagnetic stirring on mechanical properties of weldments 53
5.3 Effect of electromagnetic stirring on corrosion properties of weldments 55
Chapter 6 Conclusions 56
References 57

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