Dan Kay's blog series on eliminating "holds" in brazing cycles continues. Read parts 1 and 2 here.
One shop was heating parts using a heating rate of approximately 40°F (22°C) per minute and had several holds in their heating cycle. The holds ranged from 30 minutes to over an hour. When added together, the total holding time was over two hours on the way up to brazing temperature.
I suggested that they slow down their heating ramp rate to approximately 15°F (8°C) per minute, eliminate all the holds during the heating cycle, and see how the parts looked after the brazing cycle was completed and the parts were removed from the furnace. The parts looked good, were well brazed and no distortion was noticeable. We then tried heating at a faster rate (25°F/15°C per minute), again with no holds, and slight distortion was noticed on the test pieces run.
In the third run, the heating rate was adjusted to about 20°F (12°C) per minute, again with no holds, and the parts brazed very nicely. The shop then used this rate for their regular production, and their productivity increased because their total brazing cycle (using the slower ramp rate and eliminating the holds) was actually shorter than their original cycles that had been using the higher ramp rates and the built-in holds. It was an amazing discovery for that brazing shop!
The following conclusions were reached.
- “Holds” are not actually required for burning off (volatilizing) brazing-paste binders in a furnace brazing cycle. Such holds are often necessary because the ramp rate being used is so fast that it does not allow the binder to fully volatilize. A much slower ramp rate will allow the binder to fully volatilize without the need for a built-in hold.
- “Holds” are not actually required for stress-relieving machined components. Once again, such holds are often necessary because rapid heating ramp rates do not allow sufficient time for such stresses to be thermally relieved. With a slow ramp rate, the stresses can be nicely removed gradually and steadily so that a temperature hold isn’t needed.
- Determine the initial starting ramp rate for heating. Begin with your current brazing cycle. Calculate the total time it currently takes for your typical brazing cycle to reach brazing temperature. Divide that number representing your brazing temperature by the total number of minutes to get there (in your current program), and then try an experimental run using that calculated rate (with no holds in that test cycle).
For example, let’s assume that your current brazing cycle takes three hours to reach a brazing temperature of 2000°F/1095°C. Let’s further assume that you are using a ramp rate of 50°F/25°C per minute and have three built-in holds (shown in Fig. 1) on the way up to brazing temperature. To find an appropriate slower ramp rate with which to experiment, divide the 2000°F by 3 hours (180 minutes). The new test rate should be just over 11°F/minute. Therefore, try a 12°F/6°C per minute ramp rate in your experimental run, and see how the test braze works out. If you find that it works well, then in-crease your test ramp rate to 15°F/8°C per minute, etc.
Break the paradigm, and become an agent of change. Just because you’ve “always done it that way” does not mean that it is the correct way to heat the parts for brazing. Be willing to try something new. See if your shop’s productivity can be improved by using the slower ramp rate that allows the removal of all holds during heating.