Too many companies still use built-in “holds” when heating up parts in their brazing furnaces – such as that shown in figure 1 for a typical furnace brazing cycle.
I recently asked the production manager at an aerospace company, “Why do your brazing personnel have those temperature holds in their furnace brazing cycles?”
He responded that such holds were needed to help equalize the temperatures of all the parts being brazed in that particular furnace load. He said the heavier parts (or heavier portions of a component) would heat up more slowly than thinner parts or small parts, and the holds allowed those heavier parts to come up to the same temperature as the smaller (or thinner) parts before continuing with the heating cycle.
Such reasoning appears good on the surface, but when I asked him why the temperatures were so different between the thinner/smaller parts and the heavier parts, he indicated that the rapid heating rate used resulted in the larger/heavier parts falling behind the thinner/smaller parts as far as part temperature was concerned.
I then asked him why his company chose to use a heating rate that necessitated the use of holds in order to prevent the temperature differentials from getting too large. His response was typical of what I usually hear when I ask that question. He said, “We’ve always done it that way.” The only other answer I hear from people is “I don’t know.” Nobody has ever given me a reason that truly justifies the rapid heating rates used. Ever!
As a metallurgical engineer who has worked in the brazing field for 50 years, I know from experience that metal components do not handle large heat differentials within their structure very well. All too of-ten it leads to distortion of the parts being brazed in the furnace. This can happen on the way up to brazing temperature, and not just during the cooling cycle.