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To create strong tubular joints when brazing, it is very important that the joint have enough surface area inside the joint, i.e., between the faying surfaces, so that the molten brazing filler metal (BFM) can flow into and through the joint by capillary action.

As described In previous articles I’ve written about the thermal expansion of metals, it’s important to note the obvious fact that all metals will expand (grow) when heated up, and then contract (shrink) when they are cooled. It was also noted that, unfortunately, each different type of metal expands/contracts at a different rate from all of the other types of metal, which is something that can lead to significant brazing problems when someone is trying to braze two different types of metal together (such as tungsten-carbide to stainless steel), because at the high temperatures of brazing (which may often exceed 2000°F/1100°C) the different expansion rates between those metals may cause the joint to literally close up completely, or perhaps to become too large to braze effectively. It is very important for designers of brazed components to understand, and take into account in their designs, the different expansion rates between any two metals being joined (also known as “differential metal expansion” rates, or “DME” rates) so that gap-clearances at brazing temperature will be correct, and thus allow good, effective brazing to be accomplished.

Today’s brazing technology is based on a strong foundation of the brazing experiences of many people around the world over a period of many decades (even centuries). I’ve now been very active in the brazing world for almost 50-years, and, like my predecessors in the world of brazing, I’ve learned a lot about this fascinating joining process (and I’m still learning). In this article, I’d like to share with you one of my brazing experiences from many years back, one that involved high-temperature differential-expansion between an 18” (45 cm) diameter tool steel die and a thin carbide plate (round disc) that needed to be brazed to the die’s front surface for wear-protection.

When trying to braze together materials that have widely different Coefficients Of Thermal Expansions (COE’s), the material with the higher expansion rate (COE) will grow faster than the other when heated and shrink (contract) faster when cooled down from the brazing temperature. Once the two different materials have been brazed together and cooling begins, the shrinkage-rate differences between those two materials can produce significant shear stresses at the brazed interface between them. These stresses can, in some cases, be so strong that the thin brazed joint may be torn apart at either interface, or it might cause fracturing of either base metal, or perhaps open up a large crack through the BFM layer itself. It was this extreme — and very real — differential expansion problem that we had to address in today’s “history lesson”.

There is an important guideline about brazing filler metals (BFMs) that needs to be followed when performing any kind of brazing — namely, it is very important to feed brazing filler metal (BFM) into only one side of the joint to be brazed, allow that molten BFM to be pulled completely through the joint by capillary action until the BFM is visible at the other side of the joint, and can be seen to produce a complete braze-fillet (meniscus) around (or along) that opposite end of the joint.

As shown in Fig. 1, a BFM wire is being hand-fed into one end of a joint being brazed (gap is shown much thicker than desired only for illustration purposes), and the BFM flows all the way to the other side of that joint where it can be inspected for complete pull-through. This is an example of proper brazing in which “you feed from one side, and inspect on the other”.

It is never wise to feed BFM from both sides of the joint, since by so doing you can trap air, moisture bubbles, outgassing materials, etc., inside the joint between the two incoming walls of molten BFM, thus greatly increasing the void content in that joint. Fig. 2 shows an example of a cross-section photo of a round electrical contact that was brazed onto a flat surface, where the BFM was applied completely around the disc-shaped part that was to be brazed to the substrate below it. When the BFM melted and tried to flow into the joint, it could not do so since the air and other contaminants inside the joint could escape to the outside since the “wall of liquid BFM” was blocking it. Thus, only a fillet formed around the outside of the joint but did not flow into the joint, as shown in that photo in Fig. 2.

Braze-spatter is occasionally seen in furnace brazing and usually occurs when someone is using brazing paste. I have not personally seen it happen when solid forms of brazing filler metal (BFM) such as wire, sheet or solid rings are used. When braze-spattering does happen, it might look something like the weld-spatter shown in Fig. 1, or perhaps like the soldering spatter shown in Fig. 2.

People are often surprised when they see braze-spatter on parts coming out of their brazing furnace and wonder if the spatter was due to careless application of the paste, or perhaps due to sloppy parts-handling by personnel in the brazing shop, etc., and fail to grasp that the vast majority of braze-spattering has to do with furnace heating rates in combination with the size of the brazing-paste bead that is applied to the joint area.

Shell & tube heat exchangers (S&THE) have been used for many years in a wide range of industries, including aerospace, oil refineries, chemical processing, beverage industries, and pharmaceutical industries, to name a few.

Brazing has become more and more the desired joining-method to “seal” these heat exchangers from any internal or external leaks. In this article I will look at some of the internal characteristics of S&THE’s, options for sealing them to make them leak-tight, and then at the end of the article, I will discuss some important requirements for proper joint design so that brazing can be used to cost-effectively make many tube-to-header leak-tight joints in S&THE’s at the same time! A typical shell & tube heat exchanger is pictured in Fig. 1, and a cross-sectional view of one is shown in Fig. 2, where it can be seen that an outer shell surrounds a bundle of tubes within that shell.

One of the most widely used charts in the field of brazing is the strength vs. clearance chart created from work done in the Handy & Harman laboratories in Fairfield, Connecticut back in the 1930’s. This chart is shown below, in Fig. 1:

Notice that as the joint clearance gets tighter and tighter (moving from right to left along the bottom axis), the tensile strength (as shown on the vertical axis on the left-side of the chart) gets higher and higher. Although there is a lot of experience with this over the years, and general acceptance of this information is widespread, it must be pointed out that this chart is very specific only to the actual testing performed in making this particular chart, and may not be identical to tests performed by others using similar materials or conditions. But the general principal of increased joint strength with tighter gaps can be accepted.