IN-738超合金FIC處理與硬銲修補之研究

鎳基超合金IN-738具有優異的高溫機械性質及抗蝕性，為火力發電廠氣渦輪機熱段組件之主要材料。葉片經長期運轉後，表面常有高溫腐蝕、高溫疲勞、潛變、異物的衝擊等損傷。由於此材料價格昂貴，為降低成本故有必要進行葉片之再生處理。IN-738合金為高鋁鈦的鎳基超合金，屬於銲接性差的材料，以傳統銲補製程易生成龜裂，故本研究採取真空硬銲製程進行修補。為了得到良好的硬銲效果，試片在硬銲前需以氟化物離子清洗(Fluoride ion cleaning，FIC)製程進行去除表面氧化物。硬銲製程使用IN-738/DF4B的混合粉末填料，以50/50、40/60、30/70、20/80重量比，進行真空硬銲製程及效果評估，並以50/50和20/80的填料，進行顯微組織觀察、EPMA成分1180℃之熱處理及高溫(850℃)拉伸試驗。硬銲層的四種顯微組織由γ + γ' (Ni3(Al, Ti))基地、鉻硼化合物、細碎散佈碳化物及最後凝固的網狀共晶組織(BCr + BNi3 + (Ni))所組成。當試片經過FIC處理與硬銲後，硬銲層與母材間會出現明顯的界面，此界面是來自於FIC處理後殘留的氧化物和氮化物。在高溫(1180℃)進行熱處理時，硬銲層中的鉻硼化合物並不隨熱處理時間增長而分解，但共晶組織會隨熱處理時間增長而逐漸消失，其中硼元素擴散至近界面之母材，並增加鉻硼化合物在此區域之含量。在850℃高溫拉伸試驗中，IN-738母材抗拉強度為710 MPa；對硬銲層而言，50/50填料硬銲層之抗拉強度約為500 MPa，20/80填料硬銲層約為440 MPa，分別為母材強度的70 %與60 %。拉伸實驗結果顯示，提高低溶點填料(DF4B)於混合粉末中所佔比例，將生成更多硬脆的鉻硼化合物，使拉伸強度降低。

關鍵字

鎳基超合金IN-738 ；真空硬銲；氟化物離子清洗；熱處理；高溫強度

並列摘要

IN-738 is a nickel-base superalloy, which has excellent mechanical properties and good corrosion resistance at elevated temperatures. It is one of the most widely used materials in hot sections of gas turbines in power generation plants. The sophisticated blade design, in order to increase efficiency, inevitably leads to a high blade cost. After long-time service, turbine blades may suffer from various damages and require refurbishment to reduce the operation cost. IN-738 alloy with high Al and Ti contents has poor weldability in traditional repair welding processes. In this study, a vacuum brazing process was used to simulate repair brazing of the alloy. In order to have better brazing results, the surface of IN-738 specimens was cleaned using a FIC (Fluoride ion cleaning) process to remove oxides prior to brazing. Mixed IN-738/DF4B powders (50/50, 40/60, 30/70 and 20/80 in weight percentages) were used as brazing filler metals in the process. Microstructural observations, EPMA analyses and 850℃ tensile tests were conducted on some brazed joints (50/50 and 20/80 powder mixtures). In addition, a high-temperature heat treatment at 1180℃ was carried out on the brazed specimens to study the changes in microstructures with time. The microstructures in the brazed region consisted of γ' (Ni3(Al, Ti)), eutectic (BCr + BNi3 + (Ni)), chromium borides and dispersed carbides in the matrix of γ. After FIC and brazing, some residual oxides and nitrides in the interface between the brazed region and the base metal still could be observed. Microstructural observations indicated that chromium borides were intact; however, the eutectic areas were decomposed gradually in the brazed specimens subjected to a long-time heat treatment at 1180℃. Furthermore, the amount of chromium borides in the base metal near the interface was increased due to boron diffusion from the brazed region to the base metal. For tensile tests at 850℃, the ultimate tensile strengths (UTSs) of butt joints with 50/50 and 20/80 powder mixtures were about 500 and 440 MPa, i.e. 70 % and 60 % of the base metal (710 MPa), respectively. The lower UTS of the joint with 20/80 powder mixture could be attributed to a higher volume fraction of brittle chromium borides in the brazed region.

並列關鍵字

IN-738 superalloy ； Vacuum brazing ； Fluoride ion cleaning ； Heat treatment ； High-temperature strength

參考文獻

51. 汪鼎凱，IN738及IN939鎳基超合金之真空硬銲研究，民國94年，台灣大學碩士論文。

11. 莊旻洲，高熵合金Fe-Co-Ni-Cr-Al-Mn(-0.1Mo)之高溫氧化宇潛變分析研究，民國93年，台灣大學碩士論文。

33. William F. Smith, “Structure and Properties of Engineering Alloys”, McGraw-Hill, 1993, pp.498-507.

45. David R. Gaskell, “Introduction to the thermodynamics of materials”, 4th ed., Taylor & Francis, 2003, pp.359.

2. William D. Callister, JR., “Materials Science and Engineering An Introduction”, 6th ed., pp.591-594.

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IN-738超合金FIC處理與硬銲修補之研究

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