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Is Brazing Welding ?– Part III – Braze Leaks

Posted January 08, 2013 12:00 AM by larhere

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.

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Guru

Join Date: Jan 2008
Posts: 1753
Good Answers: 59
#1

Re: Is Brazing Welding ?– Part III – Braze Leaks

01/08/2013 10:44 PM

Brazing is brazing, and welding is welding. And the two shall never meet.

In welding you melt the basic metals, maybe add to it to ioin them in the right form.

In brazing you use different metal capable producing strong intermetallic compounds below the melting point of the base metals.

Can brazing be strong? Not nearly as strong as welding. As you have little contol over the intermetallic compound's physical properties. At the same time, I would hazard the guess, that 80-90% of all weldings would be perfectly adequate with brazing.

Quality control? Mostly the same. I learned from a HVAC piping artist. Never, ever a leak in the time I was able to observe. It takes a craftsman with pride in his work.

Well, it is mostly out of fashion, while its lower temp. characteristics are good.

Go figure.

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Guru
Hobbies - DIY Welding - New Member

Join Date: Sep 2007
Posts: 1601
Good Answers: 57
#2
In reply to #1

Re: Is Brazing Welding ?– Part III – Braze Leaks

01/09/2013 9:36 AM

The twain has met. There is a third joining method called braze welding - look it up. BTW there are corrosion related defects that can occurr in braze joints. They are often the result of poor choices of base material and/or braze metallurgy.

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