Table 1 shows a chart that I developed many years ago which shows how to interpret the results of five different scenarios that can occur inside the brazing furnace when conducting a high-temp furnace brazing run. The first four vertical columns (left to right) in the table show possible surface conditions that might be observed for the following three items:
a. base metals being brazed (the “part”)
b. 304 stainless-steel “on-site atmosphere QC-specimen” (the “control”)
c. brazing filler metal (BFM)
The remaining columns of the table describe the conclusions that can be reached based on what is observed in each of the different scenarios shown in the first four columns in any particular row.
Each of the horizontal rows in the chart describe a particular “observed scenario” when the brazed parts are removed from the furnace and inspected. These scenarios are as follows:
1. The base metal (parent metal of the part/assembly being brazed) is bright and clean, the BFM flow on the metal part looks good, and the 304-stainless QC-coupon also looks bright and clean with good BFM on it. The conclusions that can be readily drawn from that scenario are quite favorable: Good furnace atmosphere, good-quality BFM, and proper furnace brazing cycle.
2. If base metal comes out of the furnace dark or discolored with no BFM flow (or very poor BFM flow), but the 304-stainless QC-coupon is bright and clean with good BFM flow, then you know that the furnace atmosphere is good and that there is a problem with the base metal (perhaps it contains aluminum or titanium, etc.) which is easily oxidized, even in normally “good” vacuum atmospheres.
3. IF both the base metal and the stainless QC-coupon come out of the furnace discolored, but the BFM has flowed well on both, then you know that the base metal, BFM, and furnace atmosphere were all good up through the end of the time the parts were being held at brazing temp (because the BFM flowed out well), but that an atmosphere problem occurred somewhere during the cooling cycle. There may be a leak in the gas-cooling/quench line that is being used to rapid cool the parts, or perhaps the parts were pulled “too-hot” from the furnace.
4. If both the base metal and the 304 stainless QC-coupon come out of the furnace dark or discolored, and the BFM has not flowed at all, then you know that you had an atmosphere problem right from the start of the heating cycle (since the BFM will not melt and flow if it (or the base metal) becomes oxidized heavily during heating, and those oxides can’t be “reduced” prior to reaching brazing-temp. Thus, you may have a bad furnace leak, or the furnace is badly contaminated with material that readily outgasses during heating (a “virtual leak”) and contaminates the atmosphere and the parts being brazed.
5. If the base metal and the stainless QC-coupon come out of the furnace bright and clean, but the BFM has not melted or flowed, then either the furnace was never turned on, or the BFM may be bad. Including another 304 stainless atmosphere QC-coupon with some known good-flowing BFM as a “control specimen” can quickly verify this.
Okay, put your understanding to the test: In the photos below in Fig. 3, what was the problem in each of the two furnace runs represented by the two different stainless coupons shown? (The answers are at the end of the article)