臺灣博碩士論文加值系統

本研究採用Nd:YAG雷射，在三種不同雷射輸出功率(1250、1500和1750W)下，搭配矩形脈衝波(PW)及連續波(CW)，對鎳基690合金進行走銲實驗(BOP)，探討上述不同雷射參數對690合金之銲縫剖面，熔融率(MR)，微結構和微硬度之影響，以期獲得對接銲臨界貫穿走速。接著，利用最佳臨界貫穿走速條件對690合金進行雷射銲接(LBW)與鎢極氣體電弧銲接(GTAW)之對接銲實驗，於銲接過程中精確地量測銲件各點所經歷的銲接熱循環曲線(即峰值溫度、加熱速率和冷卻速率)和溫度分佈(即溫度梯度)。本研究目的為探討銲接熱循環曲線和溫度分佈結果，與其殘留應力分佈、晶界特性分佈(GBCD)、敏化程度(DOS)及碳化物種類之相關性，以建立LBW與GTAW兩銲接製程之峰值溫度、加熱速率、冷卻速率及溫度分佈對690合金銲件之抗沿晶腐蝕(IGC)和抗應力腐蝕開裂(SCC)的影響。
BOP實驗結果顯示，當固定雷射平均輸出功率條件下，由於脈衝波試片具有較連續波試片為高的熔融比，使其臨界貫穿走速較高且銲道較為狹窄，所以，脈衝波試片之銲道可獲得較細的枝晶結構和較高的硬度值。而對接銲實驗結果表明，由於LBW製程功率密度高達104?105 W/mm2，可大幅降低入熱量(112 J/mm)，並提供非常陡峭的溫度分佈、非常高的加熱速率(16080 ?C/s 在銲道處)及非常高的冷卻速率，故相較於GTAW銲道，LBW銲道則具有較細緻的枝晶結構與較高的硬度值。另一方面，相較於GTAW銲件，其LBW銲件陡峭的溫度分佈可有效地降低縱向拉伸殘留應力區之範圍。藉由LBW和GTAW兩銲件之雙環動電位再活化法(DL-EPR)實驗曲線得知，LBW銲道與銲接衰化區的Ir/Ia值分別只為0.15%、0%；GTAW銲件的相應區域卻高達0.41%、0.6%，故其LBW銲件之敏化程度明顯遠較GTAW銲件為低許多。根據GBCD結果顯示，在LBW銲道處，其低能量Σ重位點陣(CSL)晶界(1≦Σ≦9)之比例明顯高於GTAW銲道。而由改良式惠式試驗結果表明，相較於GTAW銲件，其LBW銲道與銲接衰化區皆具有顯著抵抗沿晶腐蝕與枝晶間腐蝕(IDC)的能力。這是因為在LBW銲道與銲接衰化區處皆具有非常高的冷卻速率，導致通過Cr23C6碳化物析出溫度範圍620–1020?C的時間不足，只為1.2–1.6秒，大幅抑制了碳化物析出和沿晶界缺鉻區出現之所致。最後，U-Bend固定變形與慢應變速率試驗(SSRT)結果皆表明，LBW製程技術可顯著地改善690合金銲件抗應力腐蝕的能力。

This study initially investigates the influences of three mean Nd:YAG laser output powers (1250, 1500, and 1750W) combined with rectangular pulse wave (PW) and continuous wave (CW) on weld bead profiles, melting ratios (MR), microstructures, and microhardnesses of alloy 690 for bead on plate (BOP) welding specimens, in order to achieve the critical welding speed for the just fully penetrating the butt weld. Moreover, precise measurements of the welding thermal cycles (i.e. the peak temperature, the heating rate and the cooling rate) and temperature distributions (i.e. temperature gradient) continuously at various points of the alloy 690 butt weldment during laser beam welding (LBW) and gas tungsten arc welding (GTAW) processes were taken. The resulting thermal cycle and temperature distribution profiles were then correlated with the distribution of residual stress, grain boundary character distribution (GBCD), degree of sensitization (DOS), and carbide type with an aim to study and establish the combined influences of peak temperature, heating rate, cooling rate, and temperature distribution on the intergranular corrosion (IGC) and stress corrosion cracking (SCC) resistance of alloy 690 weldments.
The results showed that PW specimens have significantly higher MR values than those of CW specimens under constant mean laser output power. The PW specimen is characterized by its full penetration capability at higher critical welding speed, which results in a narrower weld bead. As a result, a denser dendritic structure and higher microhardness can be obtained in the fusion zone (FZ). For the butt welding specimens, the LBW process, with a power density as high as 104?105 W/ mm2, has significantly lower heat input (112 J/mm) on the weldment than the GTAW process, which provides a very steep temperature distribution, a very great heating rate (16080?C/s in the FZ), and a very high cooling rate. As a result, the sub-grain structure in the FZ of the LBW weldment appears to be denser than that in the FZ of the GTAW weldment. Moreover, the very steep temperature distribution in the LBW weldment effectively reduced the size of the longitudinal tensile residual stress zone relative to that in the GTAW weldment. A comparison of the double-loop electrochemical potentiokinetic reactivation (DL-EPR) curves on the FZs and weld decay zones (WDZs) of the LBW and GTAW weldments revealed that the Ir/Ia values in the FZ and WDZ of the LBW specimen were about 0.15% and 0%, respectively, while those of the corresponding regions of the GTAW specimen were about 0.41% and 0.6%, respectively. Therefore, the DOS value of the LBW specimen was much lower than those of the GTAW specimens. Furthermore, the GBCDs by orientation image maps showed that the fraction of low energy Σ coincidence site lattice boundaries (1≦Σ≦9) in the FZ of the LBW weldment is significantly higher than that in the FZ of the GTAW weldment. In particular, the modified Huey test results revealed that in the LBW weldment, compared with the GTAW weldment, interdendritic corrosion (IDC) and IGC were significantly arrested in the FZ and the WDZ, respectively. This occurred because the very rapid cooling rates in the FZ and WDZ during welding leads to an insufficient exposure time of around 1.2–1.6s through the Cr23C6 carbide precipitation temperature range of 620?1020?C, suppressing Cr-carbide precipitation and Cr-depletion along grain boundaries in the FZ and WDZ, respectively. Consequently, both the U-bend constant deformation and slow strain rate testing results showed that the LBW technique makes a significant improvement in the SCC resistance of alloy 690 weldments.

摘 要 I
Abstract III
Acknowledgements (Chinese) V
Contents VII
Table captions X
Figure captions XI
List of symbols and abbreviations XVIII
1. Introduction 1
1.1. Investigation background 1
1.2. Literature review 7
1.3. Motivation and goal of this study 10
2. Principles of temperature fields, residual stress, laser beam welding 12
2.1. Welding temperature fields 12
2.2. Mechanisms of thermal stress and welding residual stress 16
2.3. Principle of laser beam welding 20
3. Experimental procedures 25
3.1. Experimental procedures and design 25
3.1.1. Base metal (BM) 27
3.1.2. Laser beam welding (LBW) 27
3.1.3. Gas tungsten arc welding (GTAW) 28
3.1.4. Temperature distributions and thermal cycles during welding 28
3.1.5. Measurement of residual stress by the hole-drilling strain-gage method (HDSGM) 29
3.1.6. Microstructural observation and analysis 30
3.1.7. Corrosion tests 31
3.1.8. Microhardness distribution 32
3.1.9. Calculation of TTT and CCT diagrams 33
3.1.10. Stress corrosion tests 33
3.2. Experimental instruments, equipments, and software. 45
3.2.1. Nd:YAG laser system 45
3.2.2. GTAW welder 45
3.2.3. Data logger and data acquisition system 46
3.2.4. RS-200 high speed assembly for HDSGM 46
3.2.5. Strain gage 47
3.2.6. Strain indicator 47
3.2.7. Potentiostat/Galvanostat 47
3.2.8. Environmental scanning electron microscope (ESEM) 48
3.2.9. Surface roughness tester 48
3.2.10. Electron backscatter diffraction (EBSD) / Orientation imaging microscopy (OIM) system 48
3.2.11. Thermodynamic software JMatPro 49
3.2.12. Focused ion beam (FIB) 49
3.2.13. Scanning transmission electron microscope (STEM) 50
3.2.14. Material testing system (MTS) 50
4. Results and discussion 59
4.1. Study of laser output powers and waveforms 59
4.1.1. Effects of various laser output powers and waveforms on welding speed and weld profiles 59
4.1.2. Effects of various laser output powers and waveforms on melting ratio 62
4.1.3. Effects of various laser output powers and waveforms on microstructure and microhardness 64
4.2. Comparison of the LBW and GTAW butt weldments 67
4.2.1. Effect of power density on heat inputs induced 67
4.2.2. Effect of power density on thermal cycles and temperature distributions 71
4.2.3. Effects of power density and heat input on microstructure 77
4.2.4. Direction and magnitude of residual stress 81
4.2.5. Degree of the sensitization (DOS) in the FZ and WDZ 87
4.2.6. Susceptibility of intergranular corrosion (IGC) 92
4.2.7. Texture and grain boundary character distribution (GBCD) 99
4.2.8. U-bend constant deformation SCC test 107
4.2.9. Effect of peak temperature and heating/cooling rates on Cr carbide precipitation 112
4.2.10. Microstructures of FZ, CGZ and WDZ 122
4.2.11. Slow strain rate test (SSRT) SCC test 133
5. Conclusion and future work 140
5.1. Conclusion 140
5.2. Future work 142
Reference 143
Curriculum Vitae 149

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