Brazing of Nickel and Nickel alloys
Nickel electrochemical alloys like Constantan and Manila contain in its composition of iron and copper. They on their surface are chemically unstable oxide film, is easily recovered in gaseous media, which, when high temperature solder is removed by fluxing in vacuum, decomposing into metal and oxygen, so the brazing of these alloys is very complex.
For soldering you use solder flux, gas environment, and the solders that are recommended for copper and steel. For soldering Nickel alloys requires special fluxes, because the surface of Nickel alloys, for example, nichrome (an alloy of Nickel and chromium) coated with a very resistant film on the basis of chromium oxides. In the process of alloying of nichrome, titanium and aluminum chem. the resistance of the oxide film increases, which leads to some difficulties when soldering.
When soldering Nickel alloys and Nickel need to ensure that the gaseous medium used had sulfur compounds, since the interaction of sulfur and Nickel, formed low melting eutectics, which penetrates along the grain boundaries and causes embrittlement of the metal. Soldering heat-resistant Nickel-based alloys in the gas reducing environments requires a thorough cleaning from the remainder oxygen with dunite or platinum catalyst and additional drying to the dew point (-70°C). When brazing such alloys in a vacuum or in a neutral gaseous media — they should be carefully dried by means of barium peroxide, phosphoric anhydride or zeolite. Before soldering nichrome cover with a layer of copper or Nickel with a thickness of 15 µm, to ensure wetting of soldering surfaces in neutral media and vacuum without the use of flux.
For low temperature brazing of Nickel applied lead-tin solders, which contain from 40 to 60% Sn and recommended for brazing of steels fluxes. For structures made of Nickel alloys that operate at temperatures of 350−500°C, using silver solders. Copper solders designed for brazing high-temperature alloys of Nickel, is rarely used due to their low corrosion resistance and heat resistance. In addition, the process of soldering copper Nickel dissolves in the solder. Therefore required strict control of soldering temperature and precise dosage of the solder. When brazing alloys of Nickel alloys, doped with beryllium and boron, brazed metal also dissolves in the solder, therefore, it is necessary to solder without overheating and quickly adhering to the soldering.
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