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 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 an 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 dew point at which the oxide and the metal are in equilibrium. At dew point and temperature combinations above and to the left of any given metal-oxide curve that particular metal will oxidize and remain oxidized. At dew point and temperature combinations to the right of any given metal-oxide curve that metal-oxide will be reduced/dissociated.

Recall that a dew point 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 term “dew point” 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. Dew point, 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 horizontal lines forming the grid lines of the chart. Example: operating a furnace at 2050F (1100C) and a dew point 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.

Dew point vs. ppm water content.

Fig. 2. Dew point vs. ppm water content.

This illustrates why it is critically important that the oxygen content of any gaseous atmosphere be measured right at the brazing furnace. This is easily done by use of a dew point meter since the measurement of the dew point of a gas is a good indicator of how much oxygen is in that atmosphere. The chart in Figure 2, taken from the same AWS article (N. Bretz and C. Tennenhouse, AWS Welding Journal, Research Supplement, pp. 189s-193s, May, 1970.), shows this clearly.

Please note, too, that vacuum is an “atmosphere”, in that there can be plenty of oxygen present in the partial pressure remaining in the furnace chamber during brazing. A perfect vacuum is only available in deep outer space. In most vacuum furnaces, operated at about 10-4 Torr, for example, there are still lots of oxygen atoms moving around in the furnace chamber. It’s just that the “mean free path” of those oxygen atoms is such that not enough “hits” occur on the metal surface to cause damaging oxidation during a normal brazing run. But, another very important part of vacuum-furnace brazing is the “leak-up rate” of the furnace, because that item (the subject of another upcoming article) is key to keeping the oxygen level low in a vacuum chamber. Vacuum furnace “leak-up rates” are the equivalent, in many respects, in importance to that of “dew point” in regular gaseous atmospheres used in brazing.

Next, we will look further into the interpretation and use of the metal/metal-oxide equilibrium-curves shown in Fig. 3, and describe a bit more about the oxidation/reduction reactions that may be occurring inside the brazing furnace throughout the brazing cycle. Again, let me make two important statements as stated at the beginning of this article: 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. Surface-oxidation is a common source of problems in commercial brazing. 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. Please recall that all metals have a driving force to react with oxygen to form oxides as the metal gets hotter and hotter during brazing operations. 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 dew point at which the oxide and the metal are in equilibrium. At dew point and temperature combinations above and to the left of any given metal-oxide curve that particular metal will oxidize and remain oxidized. At dew point and temperature combinations to the right of any given metal-oxide curve that metal-oxide will be reduced/dissociated. That chart also showed similar relationships for levels of vacuum in a vacuum furnace.

Remember, 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 last month’s chart between the intersecting vertical and horizontal lines forming the grid lines of that chart. The particular metal-oxide curve to use on the chart was the curve for the most sensitive element in the base metal composition.

As an example, when brazing a metal such as 304-stainless, the element chromium is the most sensitive-to-oxidation component in its composition. Chromium will oxidize readily as it is heated, as compared to iron and nickel, the other two primary ingredients in its composition. Thus, when 304 is being brazed in a hydrogen atmosphere with a dew point of -60F/-50C and a furnace temperature of 1950F/1050C, the chromium-oxide will be effectively reduced, i.e., dissociated, and the stainless surface will be free enough from oxides to be brazed.

Let’s look at this phenomenon in a bit more detail. Look at the curves in Figure-3.

The top chart in Fig.3 is a representation of the M-MO curve shown in last month’s article and is limited to only showing the chromium-oxide curve from that chart (which I just discussed once again in the paragraph above). This chromium-oxide curve is the equilibrium curve at which the equation MO↔M+O exists for chromium. Thus, along any such curve, there might be an equal probability for metal-oxides to break up into the pure metal plus liberated oxygen, or the metal to react with oxygen to form that particular metal-oxide. It could theoretically go either way. But as we move further to the right of that curve, the reaction becomes more strongly one of oxide-reduction instead of oxide formation. And, when we are at least one-diagonal (the diagonal of one of the blocks) to the right of that oxide curve, as represented by curve B, there is a stronger and stronger probability for the reaction to go only one way, namely MO→M+O. The further to the right one goes, the stronger should be that dissociation/reduction reaction.

Now, let’s look at the bottom chart in Fig. 3, showing several variations of a “Curve C”. These curves represent the progressive oxidation that occurs to metals as they are being heated in an atmosphere, be it a gaseous atmosphere or a vacuum atmosphere (there is still lots of air molecules present in any vacuum-brazing cycle). If the atmosphere quality is poor for any reason, then the amount of oxidation that occurs in the heat-up portion of the brazing cycle might be quite significant, such as that suggested by the upper curve (C1) in that group. Note that as the heating approaches the theoretical equilibrium line for that particular oxide (chromium-oxide is used in this figure), the rate of oxidation slows down, and then stops. As you progress to a higher temperature (for that particular given atmosphere condition, dew point, etc.) in that furnace run, you will note that you will be moving to the right of the equilibrium curve and there will be a driving force to dissociate those oxides that have formed. Please note that by the time curve C1 re-crosses the horizontal line (representing the surface condition of the part at the start of the brazing cycle) a much higher temperature is required than if the atmosphere quality (dew point, etc.) had been much better (such as that represented by the other curves, C2, C3, and C4).

Metal oxidation curves.

Fig. 3 Metal oxidation curves.

From this lower chart, it can be seen that the better the quality of the atmosphere, the less will be the amount of oxidation that occurs in the cycle, and the easier it will be (temperature-wise) to reduce/dissociate those oxides.

An interesting visual evidence of all this was noted in a brazing cycle in which the furnace quality was somewhat poor, and the brazing temperature had to be quite high to get “one-diagonal to the right” of the given curve, and when the stainless component was removed from the furnace, it had an etched surface look to it, instead of the bring shiny look it had when it was loaded into the furnace. An identical piece brazed in a high-quality atmosphere came out of the furnace still bright and shiny as when it went in. Thus, an added quality-control item might be to look at the “degree of etching” you note on the surface of the stainless (or other metal) when it comes out of the furnace. That can be a direct indicator to you of the what occurred inside the furnace chamber during the brazing run.

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