Help me eliminate voids in brazed copper socket joints
May 22, 2017 6:09 PM   Subscribe

I am trying to qualify a brazing procedure to ASME standards which require less than 20% unbrazed surface. However, the tests I've been sending to the lab are all showing voids. I am at a total loss as to how I can correct this and get my procedure passed.

I am brazing copper ACR tubing, 1-1/8" OD, in copper fittings. I am using Silphos-15. The only position I am having trouble qualifying is vertical upflow. My joint preparation procedure is: grit cloth to remove oxidization, wire brush the fitting, wipe all with alcohol to remove grease and oil. Fit together, braze. I am bringing the whole assembly up to light red/orange and trying to keep the whole joint hot. I have used a half a rod per joint; I have also used the minimum required to flow around the joint. My test results from the laboratory all seem the same. Is there something simple I am missing? I am licensed and have attended school for HVAC work, I am fairly familiar with the process but I don't get much field experience - maybe once or twice a year. I have run dozens of samples of this joint and it's getting expensive.
posted by Naib to Work & Money (6 answers total) 2 users marked this as a favorite
 
I am not a brazing expert - I've done it a few times, that's all. So really all I have are some questions

1. you don't mention flux, are you using some kind of flux? Paste or other? (Maybe silphos is a brazing compund that doesn't require flux?)

2. The silphos page I read says


Designed primarily for those applications where close fit-ups cannot be maintained.
It has ability to fill gaps and form fillets without adversely affecting joint strength.
Recommended joint clearance: .003" to .005" (.076 mm to .127 mm).
Slow flow.


Maybe you have too little clearance in the joint (or too much?) It says "slow flow" so maybe you need to give it more time with the heat?

3. To what extent can you do preliminary testing yourself? Like, if you cut the joint open do you see the voids, or is it something you need a lab/equipment to determine?
posted by RustyBrooks at 7:14 PM on May 22, 2017


Are you wearing gloves so you're not contaminating the joint by touching it? I also wonder if you might be overheating the joint.
posted by aramaic at 7:19 PM on May 22, 2017


Best answer: Not an HVAC/plumbing expert by any means, I'll definitely defer to someone who is, my only experience with having my joints inspected is when induction brazing, and upjoints are a PITA, but...

I also wonder if you might be overheating the joint - "light red/orange" sounds a bit high (getting up towards 850-900°C+), where you probably want bright-cherry-red (~800°C) for silver-loaded alloys.

Cleanliness +. We used to steel wire brush, clean cloth wipe, brass wire brush, wipe, emery cloth, alcohol wipe.

One trick an old hand taught me that helped immensley with upjoints is to heat the tube first, then the fitting, then sweep the joint area to temp. Solder while sweeping from fitting to tube, and try not to heat the down-side too much. That helps the alloy wick into the joint.
posted by Pinback at 7:46 PM on May 22, 2017 [1 favorite]


Best answer: Never had my joints tested but I've pulled plenty apart and pretty well had 100% coverage of the joint area if it was a good joint.

In agreement with Pinback. Reduce your metal temp, approaching orange is too hot; direct most of your heat at the fitting (like 70-80%) with the end of the flame directed to the tubing; evenly heat around the joint, cold spots can cause flow problems. Try using a torch with a wide dispersal pattern. I mostly used an acetylene turbo torch.

And whenever I had flow problems with a copper-copper silphos joint (usually the result of oil contamination) i'd use a regular acid flux to wet the joint though I imagine you are using new copper for this.

I've pretty much only cleaned with Emory cloth; both the female and male part. Maybe you are getting steel contamination from the wire brush? I'm not sure of the purpose if the copper is shiny from the emory cloth. Try skipping that.
posted by Mitheral at 8:21 PM on May 22, 2017 [1 favorite]


My experience with brazing is limited, but I join with RustyBrooks in wondering about flux. I also wonder if it would make sense to flux and tin the tube, let it cool, clean and flux it again before assembling the joint and proceeding to braze.
posted by Bruce H. at 6:49 AM on May 23, 2017


I do not have experience with that filler rod - My advice is to keep the joint under reducing flame as much as you can.

If it is permitted, use braze flux - The good stuff is the dark-colored stuff. I use Superior no. 601B/3411 which gives you more time at temperature then the white stuff. It's a bear to clean off, so I try to get it clean while the part is still hot with some water and a wire brush. Make a thick-ish paste and then use light heat to dry it completely before kicking up the heat flux to get to braze temp. If the braze paste cracks while drying you need to start over and maybe thin your paste out a little.

By my experience, 'pre-tinning' with braze with the intention to braze the two tinned pieces together is a recipe for disaster.

Watch out when sanding copper - soft metals will pick up grit and then you have to braze to the grit as well. Consider just a stainless steel brush, or a citric acid based clean like Citranox to bite into the metal a little bit. If you NEED to sand, then use an acid etch to (hopefully) clean the grit off the part.

Pictures would help.
posted by Dmenet at 5:01 PM on May 23, 2017 [1 favorite]


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