Here at Altair Technologies most of our products are for high vacuum applications and we therefore do most of our brazing in Hydrogen Gas or in a high vacuum atmosphere. During the braze run, while assemblies are being heated up to braze temperature, the furnace “bell” sees a steady flow of Hydrogen gas. This process continues until parts are below a certain temperature at which time the bell is backfilled with Nitrogen gas. Products or assemblies that are commonly hydrogen brazed include X-Ray Tubes, Traveling Wave Tubes, Linear Accelerators for medical, research and security applications and similar ion or electron beam devices. For reasons to be explained below, Hydrogen brazing is arguably the best process for bonding metals and ceramics regardless of the application.
Hydrogen (H2) gas acts as a fluxing agent reducing native oxides and removing hydro-carbon contamination producing an ultra clean raw metal surface. Many oxides, like Fe, Cu, Ni, and Co are easily reduced by H2 whereas many others like Al, Be, Ti & Si can be very tenacious and will not braze or reduce properly in wet or dry H2. Here we have a reactive family of elements that may form undesirable compounds and are therefore typically brazed in high vacuum or with other inert gases like Helium or Argon. Chromium (Cr), which is a large constituent of Stainless Steel (SST), occurs near the middle of the oxidation/reduction equilibrium space that hydrogen furnaces can produce. As desired, we can form Chromium Oxide or reduce that oxide by the atmosphere’s dew-point for temperatures greater than 800 ºC. For Chromium rich SST, we braze in dry hydrogen atmosphere or if plated with Nickel or another suitable metal, we can alternately braze in a wet Hydrogen atmosphere.
The chart below shows the temperature and/or dew point where the native oxides for various metals can be reduced. FYI, dew points below -60 C are not achievable.
Cr2O3 stable even in Dry H2
The decision to braze in “wet” or “dry” Hydrogen can depend on a few important aspects such as the base or substrate materials being used and/or the filler alloy type, as well as the application or performance requirements. In cases where the removal of oxides is predominantly important or necessary, dry Hydrogen is used and if the user is more concerned with the removal of hydro-carbon contaminants, it is advisable to braze in wet Hydrogen.
Perhaps the most important aspect or lesson to be learned from this Post is that “Hydrogen Brazing” requires special vacuum grade OFE Copper 101. We’ve learned over the years that not all OFE is equal and more importantly not all suppliers of OFE Copper are reputable. Here at Altair Technologies, we source direct all of our Copper and virtually any material to be used in a high vacuum application. When non OFE Copper is used, oxide inclusions form water-vapor in a hydrogen furnace. This phenomena results in “blisters,” “bulges,” or a rough surfaces, where the copper part(s) expand permanently and the braze alloy/filler disappears into opened grain boundaries. In this scenario, vacuum leaks are inevitable.
It should be noted that vacuum leaks due to non-OFE Copper that has been brazed in Hydrogen gas are easily detected, but can be hard or impossible to pin-point.