Fig. 1 Metal oxidation 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. 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 from the article last month 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 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. 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 horizonal 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 dewpoint 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-1.
The top chart in Fig.1 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 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. 1, 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, dewpoint, 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 (dewpoint, etc.) had been much better (such as that represented by the other curves, C2, C3, and C4).
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 occured inside the furnace chamber during the brazing run.