Each of the curved lines on Figure 4 represents the threshold for oxide reduction for a given metallic oxide, based on the brazing temperature (shown along the horizontal axis of the chart), and either the dew point of the furnace atmosphere (the left-side vertical axis) or the level of vacuum used in a furnace (the right-side vertical axis).
For instance, look at the line for chromium-oxide (Cr2O3) in the center of the chart. 304-stainless contains about 18% chromium in its chemistry. So, as that HCR alloy is heated, according to the chart, the chromium in that alloy will begin to oxidize and will do so more and more until we reach that curved Cr2O3 line.
For a given temperature and dew point (or vacuum level), that line represents the “average” place where Cr2O3 will begin to dissociate (reduce). At all temperatures and dew points/vacuum levels to the right of that curved line, chromium oxides should effectively be removed, thereby allowing molten BFM to wet and braze the faying surfaces of that alloy.
For example, look at the chart and determine if you should be able to effectively braze 304-stainless if you are operating a brazing furnace at -30°F dew point and 1800°F (950°C) brazing temperature. The point where the horizontal line of dew point crossed the vertical line of the chosen temperature will still be to the left of the Cr2O3 line, and you would expect to have real difficulties in effectively brazing 304-stainless with those furnace parameters.
Instead, you can readily see that you must use a much drier furnace atmosphere and a much higher brazing temperature in order to ensure a quality braze in which the chromium-oxide has been removed. Experience indicates that you should be a distance equivalent to at least the diagonal of one full “block” to the right of the Cr2O3 line on the chart. Brazing at 1950°F (1000°C) and a dew point of -60°F (-55°C) or drier will achieve that in this example.
Likewise, if you are brazing Inconel 625 or Hastelloy X, similar precautions would be needed for furnace brazing, and a reasonable combination of temperature and furnace dew point (or vacuum level) can be determined from the chart to ensure high-quality brazements.
Problems come up, however, when you are attempting to use the chart to help you specify a temperature and vacuum-level combination that will allow you to braze HCR alloys such as Inconel 738. Notice that this alloy contains some titanium and aluminum, which was added to achieve certain metallurgical benefits in end-use service (hardenability, corrosion resistance, etc.). In order to be brazeable, we have to remove (reduce) any titanium and aluminum oxides that will form while the Inconel 738 (Inc738) is heated in the vacuum furnace. (Yes, there will still be enough oxygen in a vacuum furnace at brazing temp to cause significant oxidation of Inconel 738.)
Shouldn’t that be simply a matter of brazing it at a temperature/vacuum level combination that is about one full block to the right of the Ti-oxide (TiO) line? Well, that’s a problem because that would require a brazing temperature well above the melting point of the Inconel 738 itself. This becomes the reason why you would electrolytically nickel plate the Inc738 faying surfaces prior to assembly for brazing. The plating will shield the surface of the Inc738, preventing any oxygen from being able to reach any of the titanium or aluminum atoms in the Inc738 to form oxides.
The nickel plating, as was mentioned earlier, will not oxidize at the brazing temperatures involved. Therefore, it can be a highly effective way to allow normal brazing of Inc738 components for aerospace applications since the molten BFM can readily wet and spread over the nickel-plated surface throughout the braze joint.
Always remember, however, that the electrolytic nickel plating will slowly diffuse into the base metal and the BFM, so you must complete your brazing process before the nickel plating has fully diffused away, once again exposing Ti and Al in the HCR alloy to any oxygen in the furnace atmosphere.
In part 5 (and beyond), we continue our discussion looking at topics such as carbide precipitation, honeycomb brazing and new developments in joining HCR alloys to ceramics.