Answer:This is really a two-part question. I'll deal with the first part today and then post a response to the second part next week.
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.
Next week, we'll look at the effect of the mass of each part on heating rates and distortion/fracture of parts.