Now, back to our discussion of ITS. Notice the difference between vertical lines A and B in fig. 3. On vertical line B the liquidus and solidus temperatures are the same. Such a junction is called a “eutectic point”, and represents the lowest melting point for a given BFM alloy system, i.e., it is the composition at which the lowest melting temperature BFM-liquid can exist.
Notice now the horizontal dotted-line in Fig. 3 that is labeled “Brazing temp”. The brazing temperature used in any brazing operation should be at least 100F (or 50C) higher than the “melting point” (liquidus temp.) of the BFM being used. We’ll assume that’s the case in our present example.
Now, remember that at brazing temperature the boron begins to diffuse away from the joint into the base metals of the part being brazed (as the boron atoms move, i.e., diffuse away, to achieve an equilibrium balance of boron throughout the entire structure). As you can see, the liquidus line of the BFM in Fig. 3 begins to rise as you decrease the amount of boron in the joint from just above 3.5% down to 2% or less, i.e., as the boron continues to diffuse away from the braze-joint area. The boron concentration in the brazed joint won’t go to zero, since the boron (B) atoms are merely trying to achieve an equilibrium balance throughout the structure.
Notice in Fig. 3 that the horizontal dotted line representing our hypothetical brazing temp crosses the liquidus line at about 2.5% boron (vertical line C). When the brazing cycle is held at brazing temp long enough until the boron in the BNi-2 BFM has diffused away to below 2.5%, there is then not enough boron left in the joint to keep the BFM liquid at that specific brazing temp. Notice that when the boron is less than 2.5% the brazing-temp line is no longer in the “liquid” section of the chart, but is now situated in the “Slush” zone between the liquidus and solidus lines! When this happens, the BFM will begin to solidify, even though the brazing temperature is being held constant. Isothermal solidification (ITS) has begun!
Please note that ITS will not occur by merely holding the BFM at brazing temp for a few minutes. Instead, it requires much longer times at temp, typically 30-minutes minimum, and sometimes as long as one or two hours, or even longer, depending on the size of the furnace load. Experience will indicate the time required, depending on the mass of the parts, how much BFM is present, and thus, what clearance is being used in the joint.
Note: Wide gaps are not effective for trying to implement ITS. For good ITS results, the gaps should be tight at brazing temp, typically 0.000-0.003” (0.000-0.075 mm), and the quantity of BFM applied should be just enough to fill the volume between the faying surfaces of the joint. Not only can thick braze-joints be a deterrent to effective ITS, but excessively large amounts of applied BFM can result in large, heavy braze-fillets which, because of their large size and volume, can prevent ITS from occurring in those external fillets, even though their re-melt temperatures may have actually increased a little because of some boron diffusion.
In summary, if the gaps are large, or if too much BFM is applied, there will typically be too much BFM present in the joint area to be able to effectively diffuse away enough of the boron temperature-depressant, and ITS will not occur, even for lengthy holds at brazing temp.
Please note, too, that because of the much larger size of silicon and phosphorus atoms used as the temperature-depressant in many other nickel-based BFMs, ITS will be very difficult, if not impossible, with those BFMs using silicon or phos as the temperature-depressant additives instead of boron.