- 學位論文
IN-738超合金高溫雷射銲接及銲補研究
Studies of High Temperature Laser Welding and Repairing on IN-738 Superalloy
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摘要
近年來電廠基於成本考量，將發電機組中之受損葉片，以銲接修補方式進行再生處理已成為必然的趨勢。本研究主要專注於電廠氣渦輪機內高溫區段之IN-738葉片，採用高功率二氧化碳雷射，配合高溫預熱對其進行雷射銲補與銲接，評估製程之可行性，並探討銲補(接)件品質及其相關機械性質。IN-738為業界廣泛應用之鑄造型鎳基超合金葉片材料之一，其銲接性差，施銲過程中易發生銲道凝固熱裂與熱影響區液化龜裂，故一般傳統銲接方式無法避免銲接裂縫的生成。雷射銲接擁有能量密度高且低熱輸入量的特性，可使熱影響區與變形量降至最低程度，並配合高溫預熱以降低銲接應力，因此可大幅提高銲補(接)成功率。 經長時間運轉後之IN-738葉片，表面常因高溫腐蝕、高溫疲勞、潛變，與異物的衝擊造成損傷。受損葉片必須以銲補方式進行再生，使用雷射同軸送粉技術，在預熱800℃或以上的情況下，可成功完成IN-738送粉銲補與銲接施工，不會有裂縫產生，但由於本製程中使用大量保護氣體，故銲件仍有少量氣孔存在。雷射銲補(接)件經適當的銲後熱處理(1180oC/2 h + 850oC/16 h 或 1120oC/2 h + 850oC/16 h)，可使內部之γ´與碳化物的析出狀況與原材料相近。使用IN-738填料銲補(接)之雷射銲件於熱處理後經機械性質測試顯示，在拉伸強度與硬度方面皆有不遜於原始材料之表現。拉伸試驗均未於銲道位置斷裂，表示銲道相較於母材強度較佳。銲補(接)區之高溫疲勞裂縫成長速率較母材略高，但沒有明顯加速的現象。潛變破斷試驗在不同應力下所得到之破斷時間也能達到銲件之要求範圍，顯示以高溫雷射銲補(接)IN-738具有相當之可行性。由於雷射銲接與銲補區晶粒較細，在高應力(310 MPa)之潛變破斷試驗時易於銲道處以沿晶方式斷裂，降低抗潛變性。雖然雷射製程有晶粒細化之缺點，但由於IN-738運轉時所承受之應力遠低於本實驗之最低應力(275 MPa)，因此並不影響本製程之應用性。
並列摘要
During recent years, power generation plants have developed various repair welding techniques to refurbish damaged blades in order to reduce their operational costs. IN-738, a cast nickel base alloy, is one of widely used blade materials for hot sections in industrial gas turbines. However, the weldability of IN-738 is poor due to the formation of hot cracks in the weld metal (WM) as well as liquidation cracks in the heat-affected zone (HAZ). As a result, it is difficult to weld IN-738 alloy by traditional welding methods without forming cracks. Laser processing with high power density and low heat input results in a narrow HAZ and produces minimum distortion on the workpiece. It is known that residual stresses can be reduced significantly with preheating at elevated temperatures and using a low heat input process. Therefore, the tendency to form cracks is lowered and the success rate for weld repairs is greatly increased. This research focuses on the feasibility of using high power CO2 laser with high-temperature preheating to perform welding or repairing of IN-738 alloy. In the repairing process, laser cladding with additive powder was employed to performed weld repairs. Additionally, tensile, fatigue crack growth rate (FCGR) and creep rupture tests were conducted on the welded/repaired specimens at 850oC to assess their performance. After long-term service, turbine blades may suffer from damages due to various causes and require refurbishment. Experimental results indicated that the use of laser welding and cladding techniques with preheating temperature of 800oC or higher could avoid cracking of IN-738 in the simulation process. After suitable post-weld heat treatments (PWHTs), e.g., 1180oC/2 h + 850oC/16 h or 1120oC/2 h + 850oC/16 h, the microstructure of the WM was comparable to that of the BM for both welded and repaired IN-738 specimens. Mechanical test results of the welded/repaired specimens were within the range of published data for IN-738 samples tested at elevated temperatures. The strength of WM was higher than that of BM, which was evident by the fracture location within the BM in the tensile tests. Even though the FCGRs at high temperatures of the WM were slightly higher than those of the BM, but they were within the same scattered band. For the creep rupture test, it clearly revealed the satisfactory performance of the welded/repaired specimens in terms of rupture times at two stress levels. Nevertheless, a reduced rupture life of the welded specimens at a higher stress level (310 MPa) was noticed due to the presence of fine grains in the WM. For similar specimens tested at a lower stress level (275 MPa), the effect of grain size became negligible. Apparently, laser processing associated with rapid cooling results in a decreased grain size of the WM to enhance creep at high stresses. Despite such an adverse effect, it did not affect the use of laser welding/cladding techniques for refurbishing IN-738 blades owing to that the highest stress of the blade generated in the operation was considerably lower than the applied stress in the experiment. The foregoing results demonstrated that the laser welding or repairing process with preheating at elevated temperatures can be used to refurbish damaged IN-738 blades. Other than that, the huge amount of shielding gas used in the process in order to prevent oxidation also caused the formation of some porosity in the welded/repaired regions. However, this can be improved by processing in a vacuum or argon chamber and is the subject for further studies.
參考文獻
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