本研究之主要目的為探討鎳基Alloy 690合金及AISI 304不銹鋼銲接熱循環曲線和溫度分佈結果，其與敏化(Sensitization)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Alloy 690合金及AISI 304不銹鋼銲件之抗沿晶腐蝕(IGC)的影響。因此，採用GTAW及雷射兩種不同銲接製程，對鎳基Alloy 690合金及AISI 304不銹鋼進行實際水平對接銲(Butt Welding)實驗，經由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和溫度分佈，探討銲接溫度場之熱循環對銲件熱影響區敏化現象所造成的影響。接著，配合JMatPro冶金資料庫所得之熱物理性質，使用ANSYS軟體平台，進行銲接移動熱源之銲接溫度場數值計算模擬，並藉由其結果與真實銲接實驗比對，以確認其一致性，達成預測銲接敏化之位置及範圍之目標。
實驗結果顯示：銲接時，由於LBW製程功率密度高達104~105 W/mm2，因此加熱速率非長高，連帶產生非常快之冷卻速率與，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。在Alloy 690抗腐蝕測試方面，由改良式惠式試驗結果表明，相較於GTAW 銲件，LBW試件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。在AISI 304方面，藉由ASTM 262 A抗腐蝕試驗顯示：GTAW銲件經由第二道次銲接熱履歷後，銲件的HAZ有IGC現象產生，且其敏化鋒值溫度範圍為500~960 oC，銲件HAZ在最低IGC溫度範圍加熱及冷卻時間為67.2秒，相對而言，GTAW第一道次銲件及LBW之銲件HAZ在敏化溫度範圍加熱及冷卻時間較短，分別為34.2秒及1.6秒，兩者僅有輕微孔蝕(pitting)現象，並未有IGC現象產生。此外，銲接模擬時，以非線性模擬方式所得結果較以常溫之熱物理性質模擬結果一致性較佳，而銲接走速高的LBW其因冷卻速度快，採用線性與非線性熱物理性質模擬差異較小。使用模擬銲接暫態溫度場模式，可以提供一種可靠的方式來預測及估計銲接後，HAZ敏化的位置及範圍，以避免銲接敏化或提供銲接敏化修補方法(雷射表面處理, LSM or LSP)參考。

This study initially investigated the effects of the temperature field within Alloy 690 and AISI 304 butt welds fabricated using the two-pass gas tungsten arc welding (GTAW) and the laser beam welding (LBW) methods, respectively. Precise measurements were taken 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 weldments during GTAW and LBW processes. In addition, the welding thermal cycles of the two welding methods were simulated using ANSYS software based upon a moving heat source model, and the high-temperature thermal physical property data were derived in the JMatPro database. The validity of the numerical model was confirmed by comparing the simulation results with the corresponding experimental findings. The resulting thermal cycle and temperature distribution profiles were then correlated with sensitization, 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) resistance of weldments.
The results showed that for the butt welding specimens, the LBW process, with a power density as high as 104~105 W/mm2, has significantly lower heat input (131J/mm) on the weldment than the GTAW process (936J/mm for Alloy 690, 1330J/mm for AISI 304), which provides a very steep temperature distribution and a very high heating rate and cooling rates. As a result, the modified Huey test results revealed that in the LBW weldment, compared with the GTAW weldment, IGC was significantly arrested in the WDZ. This occurred because the very rapid cooling rates in the FZ and WDZ during welding led 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. Results showed that the LBW technique significantly improved the ICC resistance of Alloy 690 weldments. Oxalic acid etch test results have shown that the region of the HAZ in the 2nd pass of the AISI 304 GATW weldment is susceptible to IGC. It was also found that the range of the peak temperature of the sensitization is between 500 and 960oC. In contrast, there was no evidence of IGC in the 1st pass of GTAW and LBW weldments. The total durations of heating and cooling of the HAZ in the 1st pass of GTAW and LBW weldments wrer 34.2 and 1.6 seconds, after which there was no evident IGC ditch structure. In the 2nd pass weldment, however the total heating and cooling duration was 67.2 seconds, and a ditch structure (indicating IGC) was quite evident. In conclusion, the simulated transient temperature fields of the FEM models provide a reliable and convenient means of estimating the range and size of the sensitization zone following welding, and therefore should facilitate the subsequent alteration or repair of the IGC-affected weldments using LSM or LSP methods.

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