This post concludes our discussion on this topic.
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. 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 ofn Table 1 that you should be able to see the base metal start to glow with a color ranging from light-red to orange for proper torch brazing (1600-1700°F/870-925°C). 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.
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 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 inch (0.025 mm) to about 0.006 inch (0.15 mm) max). All joint surfaces must be thoroughly cleaned prior to brazing to ensure 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, not just the top of the joint.
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. 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 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. 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!
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. Reader: Please learn the significant difference between torch brazing and torch-braze-welding, and don’t confuse the two!