Dan Kay fields a reader question about the differences between brazing and braze welding.
Question: I was torch brazing a large-diameter copper tube into a copper fitting, and I 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:
- 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 of the tube or fitting will NOT occur.
- The brazing filler metal (BFM) wire/rod is NEVER fed directly through the torch flame.
- 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 are actually using a “braze-welding torch technique” without realizing it. This is what a typical torch “braze-welding” procedure looks like:
- 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.
- The BFM wire/rod is fed through the flame, which is being used to melt the BFM and puddle it into the top of the joint.
- 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.
This is illustrated in Figure 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 WRONG BRAZING PRACTICE!
Remember: 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.