In my previous blog I gave some process examples of welding and brazing on HVAC pressure applications. Now I will talk about some common causes of braze leaks.

Common Causes of Braze Joint Leaks

From my experience in the torch brazing of HVAC coils and heat exchangers there are THREE common causes of braze joint leaks. These causes occur during brazer performance qualification, product prototyping, production brazing and leak repair brazing. Here is a brief review of these causes:

1) Contaminated parts prevent the filler metal from flowing into the joint and bonding to the base metal. The contaminants are dirt, grease, oils and oxides. This is a housekeeping issue. Grease can

come from the tooling, for example in tube belling equipment excess grease from inside the expanding mandrel gets to the outside contaminating the belled parts. Manufacturing oils can come from fin presses, tube benders, saws, etc. Operators with dirty/oily hands or gloves contaminate the parts. Cleaning should always consist of chemical first and then mechanical (if needed to remove oxides). Mechanical cleaning by abrasive pad or stainless steel brush is most common. Also, Nitrogen purging during brazing is necessary to prevent oxide formation(if the service application requires it).

2) Poor joint fitup prevents the faying(mating) surfaces of the parts to meet the proper diametral gap range. Diametral gap in tubing is what you are trying to control because radial gap cannot be measured or controlled during brazing - unless precision tooling is used. Poor joint fit-up occurs when part dimensions(i.d., o.d., wall thickness) are not made to specifications, joint surfaces are not parallel and thermal expansion/contraction of the parts(at brazing temperature) has not been considered. Using a simple digital or dial caliper will measure most part dimensions.

3) Improper heating prevents the filler metal from flowing completely through the joint.

Manual torch brazing with face feeding rod is the biggest problem for inexperienced brazers. The most common fault with brazers is to melt the rod with the torch instead of heating the joint up to brazing temperature and letting the base metal melt the rod. As parts get bigger and more massive this problem is magnified. This results in a beautiful, large fillet around the outside of the joint but very little filler metal in the joint. A critical step in training is to make brazers cross-section and evaluate their joints until they learn how to apply the heat and filler metal correctly. Obviously, the manufacturer must also provide the proper heating equipment and safety instruction for the brazer.

Editors Note: CR4 would like to thank Bill Spietz of GEA Consulting, for contributing this blog entry.