Fig. 1. Expansion curves for some common metals
We are using a heating rate of approximately 25-30°F per minute in our atmosphere retort-type brazing furnace for low-carbon steel assemblies we are brazing. Each assembly consists of a fairly large tubular part that needs to be brazed along its entire length to another tubular part (same alloy) next to it with a somewhat smaller diameter. We projection weld a small clip onto the two tubes to keep them aligned with each other and include several load-thermocouples (load-TCs) in the load. We find that we have to include at least two built-in “holds” during the heating phase (about 20-minutes “hold” each time) in order to allow the load to equalize in temperature before we can continue heating once again. Even so, we still get a high percentage of the tubular brazements “pulling apart” somewhere during the cycle, i.e., the smaller-diameter tubing pulls away from the larger-diameter tubing, even snapping the welded clips off one of the tubes, so that they are not brazed together along their length. What’s happening, and how can we fix this problem?
This is really a two-part question. First, let’s look at the heating rates you are using. Why are you using 25-30°F/minute heating rate for these parts? Unfortunately, it is fairly common for companies to use a “standard” heating rate for assemblies in their brazing furnaces, and then build in the number of “holds” necessary along the way up to temp in order to keep all the load TCs within a certain allowed temperature spread. I challenge this thinking and ask you to do the same.
When a “standard heating rate” is used, it induces thermal stresses not only throughout the entire furnace load but also within each assembly, as thinner sections heat up faster, and thicker parts come up to temp much slower. These thermal stresses are powerful and can result in distortion and cracking (which we’ll look at next week). Using “standard heating rates” is often just a habit that gets carried over from one generation to another, so to speak.
Try to break the heating-rate paradigm by trying the following: use a much slower heating rate and also eliminate the built-in holds you’ve scheduled in along the way. Find out the heating rate that will allow all load-TCs to stay with the allowable tolerance band without the need for any built-in holds. A few test cycles may be needed to determine this, using dummy loads of the same mass you are currently using.
You may be very surprised to see that not only will the parts appreciate this greatly (from a stress point of view), but by eliminating the holds, your slower heating rate may actually result in a shortened brazing cycle overall.