Unfortunately, as mentioned earlier, all of these temperature-lowering, eutectic-forming elements in nickel-based BFMs are also hardeners; that is, the phase structures resulting from solidification of these elements have virtually zero ductility. Thus, the last phases to solidify (in the center of the joint) will be hard and non-ductile. If the joint is thicker than only about 0.004 inch (0.10 mm), these hard centerline eutectics can actually form a continuous line down the center of the joint (as shown in Fig. 1) and cause the joint to become very prone to cracking under any kind of thermal or mechanical stress or strain in service.
Fig. 1. Variations in centerline structures in nickel-brazed joint (Metal Progress magazine, December 1967)
Look again at the thin, vertical portion of the joint shown on the far left side of the photo in Fig. 1. Because that joint is very thin (less than 0.003 inch/0.075 mm), the amount of eutectic “hardener” that remains in that portion of the joint is minimal. Most of it will readily diffuse into the base metal on either side of that thin joint. The diffusion of the very small amount of BFM in that thin joint is such that there is physically not enough of that temperature-lowering additive remaining in that thin section to enable the formation of any kind of continuous centerline eutectic. Therefore, this thin joint can behave in a very ductile fashion in service.
It is very important for all users of nickel-based BFMs to realize that joints should be very thin in order to prevent the formation of continuous centerline eutectic structures. These structures will be very hard and prone to cracking in service due to high concentrations of boron, silicon or phosphorus.
More next time.
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