Since it’s been a while, let’s review once again and compare in more detail the differences between correctly performed manual torch brazing and a typical braze-welding process. You might even need to go back and review earlier posts (part 1 and part 2).
1. 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 it is 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 and, better yet, a dual-tip torch (Fig. 4), which allows for evenly heating up large fittings from all sides.
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 (Fig. 5). Additionally, 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.
2. Braze welding feeds BFM through flame. As you can see in Figure 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 Figure 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. 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.
By failing to watch the base-metal temperature 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 Figure 8.