高性能板式熱交換器硬銲製程之研究

Abstract

使用改良型的非晶質鎳鐵基VZ2106箔片硬銲304不鏽鋼基材，以提升傳統不鏽鋼銅硬銲道的抗蝕性，並且改善傳統BNi鎳基填料的硼化物脆性破壞的問題。實驗結果顯示新型的VZ2106填料不僅可成功硬銲304不鏽鋼，亦可避免銲道生成大量硼化物，其拉伸強度可提升至433MPa。由於目前不鏽鋼板式熱交換器的性能，已逐漸無法符合工業界高溫高壓的需求，研究中亦選用Inconel 625 (IN-625)合金開發次世代高性能板式熱交器。實驗中的填料分別使用非晶質鎳基MBF51箔片及鎳鐵基VZ2106箔片與銅箔片進行硬銲研究。MBF51填料之銲道主要析出物為硼化物及Nb6Ni16Si7。兩者在高溫及長時間持溫之下，硼化物可逐漸粗化並在晶界中形成非連續排列，而Nb6Ni16Si7析出相則由於IN-625基材對Si固溶度提升造成大量消失，但過高的硬銲溫度將造成基材晶粒粗化，反而使拉伸強度下降。此外，以VZ2106箔片做為填料時，結果顯示銲道主要析出物同樣為硼化物及Nb6Ni16Si7相，兩者在高溫及長時間作用下的變化狀況與在MBF51銲道中的變化相似，但剪力測試中則顯示Nb6Ni16Si7析出相為主要破裂起始源，適當的硬銲溫度控制可使Nb6Ni16Si7相大量消失，使剪力強度提升至超越IN-625基材的降伏強度。使用純銅箔片硬銲IN-625基材時，銲道的中央為富Cu/Ni相，而銲道介面上的析出物為Cr/Mo/Nb/Ni四元化合物，其剪力測試結果顯示破斷位置主要為富Cu/Ni相，銲點的剪力強度取決不同的硬銲溫度對填料流動性的影響。硬銲溫度過低時，Cu填料流動性不足，因此容易在銲道中產生缺陷；但是當硬銲溫度過高時，則Cu填料發生恆溫凝固效應，而使銲道介面出現大面積未接合處，導致剪力強度劇烈下降至200~250 MPa。研究中發現最佳的硬銲溫度為1160℃、持溫30min，其剪力試片強度可達到最大值約470 MPa。

The modified Ni/(Fe)-based amorphous brazing foil, VZ2106, is utilized for improving corrosion resistence and mechanical properties of brazed stainless steel plate heat exchanger. The result shows that VZ2106 foil can reduce boride precipitates of brazed joint, and tensile strength of the joint is increased to 433 MPa. Because stainless plate heat exchanger can not meet the requirement of heat exchanger industry, IN-625 alloy is chosen for fabricating high performance plate heat exchanger. In this research, Ni-based amorphous foils, MBF51, V2106, and copper foil are applied in brazing IN-625 substrate. IN-625/MBF51/IN-625 brazed joints include two types of precipitates which are boride and Nb6Ni16Si7. As increasing the brazing temperature and dwell time, boride is coarsened and grows along the IN-625 grain boundary, but the amount of Nb6Ni16Si7 compound is decreased. Nevertheless, increasing the brazing temperature causes IN-625 grain coarsening which damages the tensile strength of MBF51 brazed joint. For IN-625/VZ2106/IN-625 brazed joint, both boride and Nb6Ni16Si7 phases are observed, and their microstructural evolution is similar to IN-625/MBF51/IN-625 joint. However, shear test shows that the crack is originated from Nb6Ni16Si7 phase in the IN-625/VZ2106/IN-625 brazed joint. As Nb6Ni16Si7 precipitates are disappeared from high temperature brazing, the bonding strength of VZ2106 brazed joints can exceed the yield strength of IN-625 base. For IN-625/Cu/IN-625 brazed joint is also investigated in this research. Cu/Ni rich pahse located in the middle of joint is crack initial and Cr/Mo/Nb/Ni quaternary compound is observed in the joint. Shear test indicates that different brazing temperatures affect the shear strength of brazed specimen apparently. Lower brazing temperature makes Cu filler without enough fluidity, and over elevated brazing temperature results in isothermal solidification of copper braze which induce defects of the joint. IN-625/Cu/IN-625 joint brazed at 1160℃ for 30min have the best shear strength.