As can be seen in Fig. 4, the smaller-diameter tube, having a thinner wall thickness, and thus a much lighter cross-sectional mass than the larger-diameter tube, will absorb heat faster and transform sooner than the heavier tube which will take longer to reach its transformation temp. When the smaller-diameter tube reaches its transformation temperature and begins to shrink, the larger tube to which it is attached is still growing. It should become apparent that when the smaller tube is shrinking and the larger tube is growing, something needs to “give-way”. Who will win — the small tube or the larger tube? They BOTH will win!
The smaller-diameter tube WILL continue to shrink as it transforms from BCC to FCC. The larger-diameter tube WILL continue to expand because it hasn’t yet reached its transformation temperature. The “loser” is the projection-welded bracket! It’s trying to hold onto both tubes as one is shrinking and the other one is still expanding, and the stress will be too much for the bracket, which then has to break apart from one side or the other.
How to solve this problem?
To prevent this distortion problem in your brazing furnace, you must either slow-down your heating and cooling rates in your furnace programing as you approach the transformation temps for the particular steel with which you are working, so that the smaller-diameter tube and the larger-diameter tube will go through their transform together, at the same time, or you must make the two tubes equivalent in diameter, or you must somehow find a way to make two different diameter tubes go through their metallurgical transformation together (at the same time).
The most logical solution would be to use the same diameter tubes for both sides of this rail assembly. But in the world of automotive brazing, once a commitment to a design is made, you may be stuck with this kind of situation for a year or two before any design correction could be made. Thus, you must find another way in which to try to make these two different-diameter tubes behave the same in your brazing furnace.