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).