We switch directions this month to take a look at a unique problem that can be created by torch-brazers without their even being aware of it. It’s a problem I’ve observed in a number of brazing shops (including vacuum brazing shops) over the years, another example of which just came up again this past month. Torch brazing, a process I’ve taught to many hundreds of people over the last 50-years, and a process I know and understand very well, can unfortunately be easily misunderstood and misused, especially by those who have a strong welding-background. Torch-welding and torch-brazing are very, VERY different! Proper torch-brazing involves three important steps: (1) broadly heating the entire joint with one or more rose-bud (multi-flame) torch tips, held far enough away from the assembly so that strongly localized heating will NOT occur; (2) the brazing filler metal (BFM) wire/rod is NEVER fed directly through the torch-flame; and (3) the torch heat is kept away from the top edge of the joint as much as possible, so that broad wrap-around heat from the torch can keep the entire length of the joint hot, and thus can draw the molten BFM down into, and through, the joint, resulting in only a tiny fillet at the top edge of the joint.
People who think they are “torch-brazing” but are actually using a “braze-welding torch technique” without realizing it can often be seen doing the following when heating a joint: (1) the heat from the torch tip (which can be a single-hole tip or a rose-bud tip) is focused at the top of the joint, and held so close that the inner-cone portion of the flame often directly impinges on the metal surfaces; (2) The BFM wire/rod is fed through the flame, the flame is used to melt the BFM and puddle it into the top of the joint, and; (3) the torch tip is slowly moved around the top of the joint to be sure that a significant BFM fillet is built-up on the top of the joint.
Yes, there is a significant difference between brazing and braze-welding when using a torch and a BFM-wire/rod to join metal parts together. Fig. 1 is a typical photo of someone braze-welding with a hand-held torch and a brazing filler metal (BFM) rod, steadily feeding the BFM-rod into the joint area as she is heating the joint with the torch-flame. It can be readily seen that she is wearing appropriate safety protection gear (dark ANSI safety glasses, gloves, long sleeves, fume-extractor next to work-piece, etc.).
But please note, as shown by her technique in Fig. 1, and as more closely illustrated in Fig. 2, the actual process of braze-welding with a torch involves melting the BFM rod with the hot torch flame just enough so that the molten filler metal will puddle into the top of the joint into which it is being fed. As you can clearly see in Fig. 2, the torch-tip and the BFM rod are in close proximity to each other. Thus, in a manner similar to torch-welding, the torch flame itself is used to melt the filler metal, allowing it to be puddled into the top of the joint area, while at the same time the flame is also used to heat-up the base-metal joint-area enough to allow this molten BFM to react with, and metallurgically bond to, the base metal so that a “cold-joint” is not created.
DEFINITION. Braze welding (BW) is defined by the American Welding Society (AWS) in their AWS A3.0 Standard Welding Terms and Definitions as:
“A joining process that uses a filler metal with a liquidus above 450C (840F) and below the solidus of the base metal. The base metal is not melted. Unlike brazing, in braze welding the filler metal is not distributed in the joint by capillary action.”
Unfortunately, that definition leaves out a lot of information, and can be very confusing, since it starts out so similarly to the definition of brazing. So, the AWS C3 Committee on Brazing and Soldering further defined the term, adding in the proviso that the filler metal added to a braze-welded joint “..is distributed by deposition rather than by capillary action”. Huh? Hmmm… this is still NOT very clear.
What that word “deposition” is actually saying is that the BFM is supposed to be melted, and deposited as a fillet right where it was melted, and should not flow too far away from that location, just as if the filler metal had been a welding-rod instead of a brazing filler rod. Thus, the resulting braze-welded fillet should be similar to a weld, except that you don’t melt any of the base metals in the joint area. Well, okay, that might, perhaps, be a little bit clearer, I guess.
BUT – don’t be misled, and think that ALL braze-welding with a torch must have joints that are broadly chamfered like a welding-joint, such as shown in Fig. 2. Braze-welding is not determined by joint design, but instead, is determined by your torch handling technique that you use in making the joint! Look at Fig. 3, where a torch-brazer is using a braze-welding technique to join a tube into a fitting.
Notice that, as is required for a proper braze-welding technique, the heat is held close to the top of the joint to be brazed, the BFM wire/rod is fed through the flame to puddle the BFM into the top of the joint, where it stays (since BFM likes to go where it is hottest), and a large fillet is built up on top of the joint. I don’t care what your customer wants you to do, but a large fillet at the top of a brazed joint is WRONG BRAZING PRACTICE!
REMEMBER — braze-welding is NOT about joint design; it’s about the TECHNIQUE of how the torch is handled and moved around the joint! Thus, your method of HANDLING and MANIPULATING the torch determines whether you are brazing or braze-welding.
Back in 1991, I remember that the AWS C3 Committee rightly decided to incorporate an entirely new chapter on “Braze Welding” into the 4th edition of their AWS Brazing Handbook so that people might come to understand better what this process was really all about. That same chapter was then carried over (and slightly updated) into the 5th edition of the AWS Brazing Handbook in 2007 in its Chapter 19 (pp. 359-368). Although those chapters describe what the process is, what various types of equipment are used (torches, spot-welders, etc.), what kind of filler metals might be appropriate, etc., they weren’t written with the intent of showing someone how, with the same identical torch, they might use it for braze-welding, then by adjusting the torch settings and hand positions, enable the torch to be used for effective torch-brazing of a different kind of assembly, and by then modifying the torch settings and hand positioning techniques once again, be enabled to conduct a proper torch-welding process, all with the same torch! The reader of those chapters on braze-welding is left to their own imagination and experimentation to figure it all out. Yes, the same torch can be used for each process (brazing, braze-welding, and welding), but the torch-handling techniques involved — and the training — are VERY different!
PROBLEM. Nowhere in any of the literature I’ve studied on the subject of braze-welding (and I’ve read a lot) is there ANY thorough description of precisely how to physically hold and manipulate a torch relative to the filler metal rod when trying to braze-weld a large tube-in-fitting joint. Nor is there any explanation of why braze-welding might be preferred over torch-brazing for that same joint depending on the end-use situation into which the joined assembly is to operate. The differences between the two processes are HUGE, and MUST be understood by any brazer who thinks that they are able to do both brazing and braze-welding, or when trying to decide whether one process would be preferred to the other.
TORCH HANDLING EXAMPLE. Over the last several decades I’ve often watched many self-proclaimed torch “experts” show me their alleged torch-brazing skills to join some large tubular assemblies, when, in reality, they were actually demonstrating their “braze-welding” skills to me instead, without even realizing it. The “brazer” heated up a 2-inch (50-mm) diameter copper (or bronze, or brass) tubular fitting using a single-holed torch-tip with an intense flame (sometimes not properly adjusted to a non-oxidizing flame), moving the flame around the top of the joint to preheat it, i.e., while keeping the flame right against the top of the joint, so that the inner-cone of the torch flame was often touching the base metals (he told me that this helped him to heat up the joint faster). Then, while heating the joint area in that manner, he fed the BFM-rod through the flame, allowing the flame to melt the BFM, which was then deposited at the top of the joint where the large-diameter tube went into the fitting. He then slowly worked his way around the top of the joint, feeding more and more BFM around the top of the joint, as needed, until a nice fillet was neatly deposited around the entire top of the joint. He told me that his customer wanted him to build up a fairly good-sized fillet around the joint. I complimented him on his good braze-welding technique. Yes, he did indeed know how to braze-weld, but he obviously did NOT know how to properly braze.
IMPORTANT SIDE-NOTE — Does the “splash heat” of the torch flame during the braze-welding process actually cause some of the molten BFM to flow down a little into the fitting of the joint, so that, unlike pure welding, the BFM is not strictly a fillet sitting only on the top of the joint? Yes, some of the molten BFM may actually be pulled into the joint by capillary action, but such action is minor, and is not the intent of braze-welding.
Okay, so let’s review once again, and compare in more detail, the differences between correctly-performed manual torch-brazing and a typical braze-welding process.
1. Proper brazing heats the whole joint, not just the top. Fig. 4 shows the proper way to heat up a large fitting for brazing. Note that the flame is very broad, held a distance away from the part being heated so that the flame completely wraps around the entire joint area, including the capillary space below the top of the joint. Ideally, a rose-bud (multi-flame) torch tip is used, and better yet, a dual-tip torch, as shown, which allows for evenly heating up large fittings from all sides. In braze-welding, only the top of the joint is heated, as was shown in Fig. 3.
Another option would be the use of a circular “C-type” torch head that can completely wrap around the fitting/piping to uniformly heat the entire joint area (shown in Fig. 5).
Additionally, when brazing (NOT braze-welding) notice how the heat is applied down on the fitting, as shown in Fig. 6, away from the joint itself, rather than just heating the top of the joint as would be done when braze-welding. This is a VERY important and easily observed difference between proper torch brazing and braze-welding!
2. Braze-welding feeds BFM through flame. As you can see in Fig. 7, when using a braze-welding technique, the BFM wire/rod is fed through the torch flame while the torch flame is held at the top of the joint, rather than keeping the torch-flame on the bottom of the joint as was shown earlier in Figs. 6. Even though a fuel-rich (non-oxidizing, “reducing”) flame can remove surface oxidation on the tubes in the region of the heating (as you can see the flame doing in Fig. 7), it is VERY important that you also hold that flame in the joint area until the base metal itself begins to change to a bright red-orange heat, indicating that the base metal has been heated high enough in temp so that the hot base-metal can melt the BFM rather than having the flame melt the BFM.
By failing to watch the base-metal temp (as shown in Table 1), and merely feeding the BFM through the flame, you may not get the joint itself hot enough to pull the molten BFM through. Thus, if the flame is held at the top of the joint, it will melt the BFM and deposit it in the top of the joint to form a fillet at the top of the joint, as shown, for example, in the cross-section photos in Fig. 8.
By comparison, when doing proper torch-brazing, the torch heat is held down on the fitting being brazed (well below the top of the joint), as shown in Fig. 9, and the BFM is then fed into the top of the joint, allowing the hot base-metal of the fitting to melt the BFM and draw it down into the fitting. The BFM should NEVER be fed through the flame itself.
Notice on the left side in Table 1 that for proper torch brazing, you should be able to see the base-metal start to glow with a color ranging from light-red to orange, i.e., 1600-1700F (870-925C). This means that the base-metal itself will then be hot enough to melt any phos-copper BFM rod touched against it, thereby allowing the BFM to be pulled into the joint by capillary action.
CONCLUSIONS: Too many people watch brazers using a braze-welding technique to create a tube–in-fitting joint and erroneously think that they are seeing “normal torch brazing” going on. They are missing some of key differences between proper torch brazing and a braze-welding technique that is not actually good “brazing”.
Here once again are the key differences to observe:
1. Joint fit-up and cleanliness. For proper brazing to be effective in a tubular joint fitting, the joint clearances must meet the typical requirements for good fit-up, i.e., have joint diametrical clearances in the range of 0.001” (0.025mm) to about 0.006” (0.15mm) max., and all joint surfaces must be thoroughly cleaned prior to brazing to insure no oils, lube, grease, dirt, etc., remains on any of the joint surfaces. Braze-welded joints often do not meet such criteria, and that’s a good clue right from the start that this is NOT going to be normal brazing.
2. Heating. Proper brazing involves heating the entire fitting, not just the top of the joint, whereas braze-welding typically focuses the heat just at the top of the joint.
3. Flame distance. In proper torch brazing, the flame is held away from the fitting far enough so that the flame wraps around the entire circumference and length of the fitting to uniformly heat the entire joint, as shown in Fig. 4, not just the top of the joint as shown in Fig. 7.
4. BFM feeding. In proper brazing, the BFM is NEVER fed through the torch-flame. Instead, while the flame is heating the fitting, the BFM wire/rod is touched to the top of the joint (away from the flame) and the HEAT IN THE BASE METAL should melt the BFM and cause it to be pulled into the joint down to where the “wrap-around” flame is hitting the entire fitting, as shown in Fig. 9. By contrast, in braze-welding, the BFM is fed through the flame which is held at the top of the joint, as shown in Fig. 7, so that it is the heat from the flame that melts the BFM, which then puddles into the top of the joint where it can solidify as a nice fillet.
5. Drawing BFM down into joint. When a proper torch brazing technique is used, the torch flame is moved down along the fitting after the BFM has melted so that the molten BFM will be pulled down into and through the entire joint by capillary action. For proper torch-brazing you should be able to see the BFM penetrate all the way to the opposite end of the joint from which it was initially fed.
6. Fillets. Proper brazing will always cause the molten BFM to be pulled down into the joint, leaving behind only a very small external fillet (or a small recessed fillet) where the BFM was first applied, as shown in Fig. 10. Large external fillets are NEVER required for proper torch brazing, and any brazer, or end-user client, who insists on a large built-up fillet on the joint does NOT understand proper brazing. Teach your customers what proper brazing is!
So I conclude with what I’m suggesting as a new definition for “Torch Braze-Welding”: Torch-braze-welding is a joining process that uses a filler metal with a liquidus above 450C (840F) but lower than the solidus of the base metal being joined. The torch flame, which is held at the top of the joint, melts the filler metal, and this molten filler metal is then deposited as a fillet at the top of the joint in a manner that does not allow it to flow through the joint by capillary action.
Reader – please learn the significant difference between torch-brazing and torch-braze-welding, and don’t confuse the two!
DAN KAY BRAZING SEMINARS: 3-Day seminars provide intensive training in “The Fundamentals of Brazing”, covering the brazing of materials ranging from aluminum to titanium to ceramics! FIND OUT MORE
Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 50-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year to help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at dan.kay@kaybrazing.com, and his website can be visited at http://www.kaybrazing.com/
To view a listing of all Dan’s articles please click here
Read Dan Kay’s Biography
© Copyright Dan Kay 2020