Hydrogen furnace brazing is also arguably the best method for brazing metals to metalized ceramics. Although Hydrogen brazing alloys and fillers do not use a flux, H2 acts as a fluxing agent to reduce oxides and remove hydrocarbons. Many oxides, such as oxides of iron (steel), copper, nickel, etc., are easily reduced by Hydrogen, whereas many others, such as those of aluminum, titanium and beryllium, are very difficult to reduce.
Dry Hydrogen Brazing
Dry Hydrogen is one of the most active agents for reducing the oxides of many metals during brazing. If an oxidized metal is heated to a sufficiently high temperature in dry hydrogen, the oxide of that metal will be reduced to native metal and the oxygen emissions will combine with hydrogen to form water. In order to create ideal conditions for brazing, in which the brazing alloy creates a joint with the native metal, a dry hydrogen environment can ensure a successful braze. The resultant parts are metallurgically bonded across the entire mating surface, offering optimized thermal conductivity as well as strong hermetic seals to 10-10 vacuum scale. They come out of the furnace in an ultra clean condition, free of surface hydrocarbon contaminants, oxide layers and chemical cleaning residues offering an aesthetically pleasing, product with a well-defined, uniform braze fillet.
Dry Hydrogen atmospheres are used instead of wet Hydrogen when materials form oxides at relatively low dew points. Moly, Tungsten and Stainless Steel are all brazed in dry Hydrogen. Stainless steel is chromium rich and readily forms chromium oxide on its surface when heated and is therefore commonly brazed in a dry Hydrogen atmosphere. Stainless steel can also be Nickel plated and safely brazed in a wet Hydrogen atmosphere.
Wet Hydrogen Brazing
Unless a very dry atmosphere is needed, wet Hydrogen, being better at removing residual hydrocarbons than dry Hydrogen, is the preferred atmosphere.
In addition to Hydrogen gas, Argon, Helium and forming gas are also used in specialty applications.