Melting/Forming/Joining

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Brazing vs. Braze Welding

Daniel Kay – Kay & Associates Brazing; Simsbury, Conn.

This article discusses the differences between brazing and braze welding of nonferrous material.

Question: I was torch brazing a large-diameter copper tube into a copper fitting and built up a nice fillet on the top of the joint at my customer’s request. Someone told me that the joint I made was not a properly brazed joint but was a braze-weld instead. What did he mean? How does braze welding with a torch compare to what I thought I was doing?

Answer: Proper torch brazing involves three important steps:

  1. Broadly heating the entire joint with one or more rosebud (multi-flame) torch tips, which are held far enough away from the assembly so that strongly localized heating of the tube or fitting will NOT occur.
  2. The brazing filler-metal (BFM) wire/rod is NEVER fed directly through the torch-flame.
  3. The torch heat is kept away from the top edge of the joint as much as possible so that the broad wraparound heat from the torch can keep as much of the entire joint hot. The molten BFM can then be drawn down into and through the joint, resulting in only a tiny fillet at the top edge of the joint.

As you found out, people often think they are “torch brazing,” but they are actually using a “braze-welding torch technique” without realizing it. This is what a typical torch “braze-welding” procedure looks like.

  1. The heat from the torch tip (which can be a single-hole tip or a rosebud 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, which is used to melt the BFM and puddle it into the top of the joint.
  3. The torch tip is slowly moved around the top of the joint, and more BFM may be fed through the flame, as needed, to be sure that a significant BFM fillet is built up on the top of the joint.
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This is illustrated in Fig. 1. Notice the fillet being built up at the top of the joint and the lack of capillary action by the BFM down into the joint. 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, a large fillet at the top of a brazed joint is incorrect brazing practice.

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Fig. 1. Torch brazer using a braze-welding torch technique to melt the BFM with the flame and build up a fillet at the top of the joint.

Remember that braze welding has little to do with the design of the joint being “brazed.” Instead, it’s about the technique of how the torch is handled and moved around the outside of the joint. Thus, your method of handling and manipulating the torch determines whether you are brazing or if you are braze welding instead.

Back in 1991, I remember that the C3-Committee of the American Welding Society (AWS) 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 Chapter 19 (pp. 359-368).

Although those chapters describe what the process is, what various types of equipment are used (torches, spot welders, etc.) and what kind of filler metals might be appropriate, they were not written with the intent of showing someone how, with the same identical torch, they might use it for braze welding. By adjusting the torch settings and hand positions, the torch can be used for effective torch brazing of a different kind of assembly. Modifying the torch settings and hand-positioning techniques once again, a proper torch-welding process can be conducted 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 have 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 have often watched many self-proclaimed torch “experts” show me their alleged torch-brazing skills to join some large tubular assemblies when they were actually demonstrating their braze-welding skills instead.

The brazer heated up a 2-inch-diameter (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). The flame was moved around the top of the joint to preheat it, 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 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 not the intent of braze welding.

Proper Brazing Heats the Whole Joint, not Just the Top

Figure 4 shows the proper way to heat up a large fitting for brazing. Note that the flame is very broad and 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 rosebud (multi-flame) torch tip is used or, better yet, a dual-tip torch, which allows for evenly heating up large fittings from all sides. In braze welding, only the top of the joint is heated (Fig. 3).

 Brazing vs. Braze Welding Fig 2

Fig. 2. Chamfered joint

 Brazing vs. Braze Welding Fig 3

Fig. 3. Braze-welding technique showing top of joint being heated

 Brazing vs. Braze Welding Fig 4

Fig. 4. The use of a dual-tip torch, with rosebud (multi-flame) tips on each arm, is ideal for brazing larger tubes and fittings.

Another option would be the use of a circular C-type torch head (Fig. 5) that can completely wrap around the fitting/piping to uniformly heat the entire joint area.

When brazing (not braze welding), notice how the heat is applied down on the fitting (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.

 Brazing vs. Braze Welding Fig 5

Fig. 5. The use of a C-type torch allows the flames to directly heat the fitting (courtesy of Uniweld).

 Brazing vs. Braze Welding Fig 6

Fig. 6. For correct torch brazing, the flame is directed down on the fitting, not on the top of the joint (courtesy of J.W. Harris).

Braze Welding Feeds BFM Through Flame

When using a braze-welding technique, as you can see in Fig. 7, the BFM wire/rod is fed through the torch flame while the flame is held at the top of the joint rather than keeping it on the bottom of the joint (Fig. 6). Even though a fuel-rich (non-oxidizing, reducing) flame can remove surface oxidation on the tubes in the region of the heating (Fig. 7), it is very important that you also hold the flame in the joint area until the base metal itself begins to change to a bright red-orange heat. This indicates that the base metal has been heated high enough in temperature so that the hot base metal can melt the BFM rather than having the flame melt the BFM.

 Brazing vs. Braze Welding Fig 7

Fig. 7. Proper braze-welding technique involves heating the top of joint and then feeding BFM wire/rod through the flame to melt the BFM.

By failing to watch the base-metal temperature (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 to form a fillet at the top of the joint (Fig. 8).

 Brazing vs. Braze Welding Fig 8

Fig. 8. Cross section of a typical braze-welded joint, showing retention of BFM at the top of the joint. Sometimes people build up a fillet (shown at left) on top of the joint thinking that it helps make a stronger joint (courtesy of J.W. Harris).

Temperature Guide

Table 1. A color chart showing the approximate base-metal temperature when that base metal glows a certain color. This chart is a nice approximation for base-metal temperatures of copper alloys and steel (courtesy of SimplyToolSteel.com).

By comparison, when doing proper torch brazing, the heat is held down on the fitting being brazed well below the top of the joint (Fig. 9). The BFM is then fed into the top of the joint, allowing the hot base metal 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 (1600-1700°F/870-925°C). This means that the base metal itself will be hot enough to melt any phosphorous-copper BFM rod touched against it, thereby allowing the BFM to be pulled into the joint by capillary action.

 Brazing vs. Braze Welding Fig 9

Fig. 9. Torch heat is held down on the fitting, away from the top of the joint. The BFM is fed from the top of the joint, and the heat draws it down into the fitting (courtesy of J.W. Harris).

 Brazing vs. Braze Welding Fig 10

Fig. 10. Small fillet

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 the key differences between proper torch brazing and a braze-welding technique that is not actually good brazing.

Here 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., joint diametrical clearances in the range of 0.001-0.006 inch/0.025-0.15 mm). All joint surfaces must be thoroughly cleaned prior to brazing to ensure no oil, lube, grease or dirt remains on any of the joint surfaces. Braze-welded joints often do not meet such criteria, and that’s a good clue 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 (Fig. 4) not just the top (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 wraparound flame is hitting the entire fitting. By contrast, in braze-welding, the BFM is fed through the flame, which is held at the top of the joint. The heat from the flame melts the BFM, which then puddles into the top of the joint where it can solidify as a nice fillet.
  5. Drawing BFM into joint: When a proper torch-brazing technique is used, the 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. 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.

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 450°C (840°F) 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.

All photos/graphics provided by the author unless otherwise noted.

For more information: Contact Daniel Kay, Kay & Associates Brazing, Simsbury, CT; tel: 860-651-5595; fax: 860-651-1919; e-mail: dan.kay@dankay.com; web: www.kaybrazing.com.