Brazing Inconel 625 Using the Copper Foil
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THE chemical composition of Inconel 625 (IN-625) alloy is 61Ni-21.5Cr-2.5Fe-9Mo-3.6Nb in wt pct, and it is featured with excellent corrosion resistance, especially for the presence of chloride ions (e.g., sea water).[1–4] The IN-625 alloyed with Mo and Nb belongs to a solidsolution strengthened alloy, and it possesses excellent formability and weldability compared with other Nibased alloys such as Inconel 738.[5–9] Therefore, IN-625 has the potential to be applied in production of corrosion-resistant plate heat exchangers. The plate heat exchanger is a crucial component in the heat exchanger industry.[10,11] The brazing-type plate heat exchanger has advantages of compact volume, high reliability and high heat exchange rate. Furnace brazing is one of the most common approaches to manufacture a plate heat exchanger. Traditional copper brazing used to produce a 316 stainless steel (316SS) plate heat exchanger is well developed in the past two decades.[12,13] However, the copper-brazed 316SS plate heat exchanger has failed to satisfy all requirements in modern industry because of its poor corrosion resistance.[14,15] Because the Cu is almost insoluble with Cr and Fe, the chemical composition of the Cu-brazed 316SS joint is almost pure Cu.[16] The combination of Cu and 316SS easily results in
galvanic corrosion of the Cu-brazed joint. Accordingly, the application of Cu-brazed 316SS plate heat exchanger is significantly confined by the type of coolant and environment. Only selected noncorrosive refrigerants are suitable for the Cu-brazed 316SS plate heat exchanger. The Ni-based IN-625 alloy can be brazed using the copper filler foil because nickel and copper are completely miscible.[16] It is well known that the corrosion resistance of pure copper is significantly improved by alloying with Ni.[17,18] It is expected that the Cu-brazed IN-625 joint mainly consists of Cu-Ni alloy. When Ni is added to Cu, a series of solid-solution alloys forms of approximately 10 pct, 20 pct, and 30 pct Ni called cupronickels.[2] The Ni additions increase the strength, oxidation, and corrosion resistance of Cu. The cupronickels are used for marine condensers and tubing for conducting sea water because of their moderate high strength and resistance to the corrosive and erosive effects of high-velocity sea water.[2] Therefore, pure copper foil has the potential to be used as the filler metal in brazing IN-625 substrate for plate heat exchanger application. Microstructural evolution and the shear strengths of IN-625/Cu/IN-625 joints with different brazing conditions are performed in the study.
II.
WEN-SHIANG CHEN and CHENG-YEN WANG, Graduate Students, and REN-KAE SHIUE, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted March 30, 2013. Article published online September 4, 2013 5724—VOLUME 44A, DECEMBER 2013
MATERIAL AND EXPERIMENTAL PROCEDURES
IN-625 alloy is used as the base metal with the dimensions of 15 mm i
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