Fig. 1. Expansion curves for some common metals

Question (Part 2 from last week):
Even with built-in "holds" when we heat our low-carbon steel parts up to brazing temp, 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?

Answer:
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 temperature much slower. These thermal stresses are powerful, and can result in distortion and cracking of assemblies during a brazing cycle. Look at the graph in Fig. 1, which shows the expansion of the metals as they are being heated over time. Notice that as low-carbon steel (1018 steel on the graph) is heated, it reaches a temperature where it actually starts to shrink and then begins to expand again at a higher temperature. Under laboratory conditions, this takes place at one temperature, but under the rapid heating of production loads, this shrinkage can occur over a temperature range.

Fig. 2. Growth of two different mass tubes of 1018 steel during furnace heating

This so-called "hiccup" in the curve for 1018 steel occurs because this steel is an example of a polymorphic material, i.e. it changes its atomic orientation at certain temperatures to minimize internal stresses, and this "re-alignment" of atoms actually results in an atomic spacing that is smaller than the spacing of atoms at room temperature. Thus, the metal actually gets smaller upon heating through that temperature range. Once all the atoms have been rearranged, the metal can expand once again. The reverse happens upon cooling.

Fig. 2 shows the effects of this expansion/contraction on the assembly as it is being heated up.

Note that the expansion of the smaller-mass tubing (with its thinner wall, and slightly smaller diameter) occurs more rapidly than the larger diameter, heavier-mass tubing. During heating, the smaller tubing will begin to shrink (contract, as it transforms from a body-centered alignment of atoms to a face-centered alignment) while the heavier-walled tubing is still slowly expanding with heat. Soon the heavy-walled tubing has reached the temperature where it begins to shrink as well. The damage will already have been done during the region on the chart (Fig. 2) where the thinner tubing is shrinking while the heavier-walled tubing is still growing. The combination of a shrinking smaller tube along with an expanding heavier-walled tube can be enough to literally snap the weld of the clip that was trying to keep the two tubes in close alignment.

Conclusions:
Slow down the heating rates of the furnace when the transformation temperature zone is being reached so that both the thin-walled and heavier-walled tubing will transform together at the same time. This will prevent the tearing stresses from occurring. Repeat this on the way down from temperature as well. By doing this, a lot of scrap and rework can be eliminated.