Fig. 1. Metal-oxide Equilibrium Curves.
Let me make two important statements right at the start: 1. Surface-oxidation of metals will prevent effective brazing. 2. Brazing filler metals (BFMs) do not like to bond to, or flow over, oils, dirt, greases, or oxides on metal surfaces.
Thus, if any of the surface contaminants just mentioned are present on the metal surfaces to be brazed, effective brazing will not occur. Effective brazing requires the BFM to be able to alloy with (i.e., diffuse into) the base-metal being joined in order to form a strong, leak-tight metallurgical bond. The amount of alloying required is not large, e.g., copper BFM on steel actually alloys/diffuses much less than 5% and yet forms very strong, leak-tight brazed joints on steel.
Surface-oxidation is a common source of problems in commercial brazing, especially in those shops where production personnel say: “Don’t worry about that oxidation; the furnace will take care of that!” Wishful thinking, and highly impractical, since furnace atmospheres may be able to “clean up” the outside surface of the assembly, but will NOT be able to effectively clean deep down inside a braze-joint if any of those inside surfaces (faying surfaces) were oxidized or contaminated prior to assembly. Parts to be brazed must be cleaned BEFORE assembling the parts for brazing, and then must be kept clean during the brazing process.
One very effective tool that brazing engineers and shop personnel must understand and learn to use is the famous “Metal / Metal-Oxide Equilibrium Curves” published in 1970 in the AWS Welding Journal (N. Bretz and C. Tennenhouse, AWS Welding Journal, Research Supplement, pp. 189s-193s, May, 1970.) as shown on the left in Figure 1. Its correct use can help insure freedom from oxide-contamination during the brazing process.
Although this chart was initially developed using a hydrogen atmosphere, subsequent testing revealed that the curves on the chart apply quite well for other atmospheres also, such as argon and nitrogen. It was also found that, perhaps due to the high partial pressure of water vapor in a vacuum furnace, these oxide dissociation curves also applied to a continuously pumped vacuum furnace as well, and is so indicated along the right-hand vertical axis of the chart.
All metals have a driving force to react with oxygen to form oxides as the metal gets hotter and hotter during brazing operations. The degree of that oxidation varies considerably from metal to metal, with some metals showing little reaction with oxygen, whereas other metals may show extensive reaction with oxygen. In the brazing world, it is very important to know how each metal reacts with oxygen as that metal is heated in a furnace atmosphere.
The plot of each curve on the chart shows the dewpoint at which the oxide and the metal are in equilibrium. At dewpoint and temperature combinations above and to the left of any given metal-oxide curve that particular metal will oxidize and remain oxidized. At dewpoint and temperature combinations to the right of any given metal-oxide curve that metal-oxide will be reduced/dissociated.
Recall that a dewpoint is a temperature to which any gas must be cooled (at a given pressure) in order for the first drops of moisture to condense. Obviously the word “dewpoint” originates from the fact that cool night air causes some of its daytime moisture content to condense out onto the ground as dew, since cooler gas cannot hold the same amount of water vapor as a warmer gas. Dewpoint therefore represents the presence of water, and water represents the presence of oxygen. And obviously, the presence of oxygen represents the potential for oxidation of metal surfaces during brazing operations.
Therefore, the job of all brazing shops is to be sure that they operate their brazing cycles to the right of any given metal-oxide equilibrium curve. It is strongly recommended that furnace operation be at least “one-diagonal” to the right of any given oxide curve, the “one-diagonal” being a diagonal line drawn from the upper-left corner to the lower-right corner of one of the little boxes shown on the chart between the intersecting vertical and horizonal lines forming the grid lines of the chart. Example: operating a furnace at 2050F (1100C) and a dewpoint of -60F/-50C can be seen on the chart to be about “one-diagonal” to the right of the Cr2O3 (chromium-oxide) line, and should therefore yield favorable brazing results.
Which particular oxide’s equilibrium-curve should be used when brazing a complex base-metal? Answer: choose the oxide-curve of the constituent in that alloy that represents the most difficult to reduce oxide in that alloy’s composition. For example, in a 304-stainless steel, the chrome-oxide curve should be the one to use, since the oxide curves for the other constituents in 304 stainless, i.e., iron and nickel, are far to the left of the chromium-oxide curve, and thus much easier to reduce/dissociate. Always be sure a brazing furnace has the capability to deal with the “worst” of the oxide-curves in any base-metal’s composition.
Important Note: Only look at the oxide-curves for those elements in each base-metal composition that represents about 0.5% or more in the base-metal’s composition. When an element’s composition is less than this amount, it usually does not have a negative effect on the brazeability of the part.
As another example, suppose you are trying to braze Inconel 738. Looking at the base-metal chemistry of that alloy, it will be noted that it contains small additions of titanium and aluminum in addition to the nickel, chromium, etc.. Since the titanium and aluminum metal-oxide curves are the furthest to the right on the chart of all the significant constituents in that Inconel’s composition, then those are the curves to deal with in evaluating your furnace’s capability to braze that base metal. But alas, it appears that to get to the right side of those curves would require furnace operating temperatures well above the operating range of most commercial brazing furnaces, and thus, not easily achievable. In such a situation, it is wise to pre-plate the faying surfaces of the Inconel 738 with a layer of electrolytic nickel prior to brazing, in order to prevent any oxygen in the vacuum furnace from reacting with the metal to form tenacious Ti-oxides or Al-oxides that can prevent any effective brazing.