cast elbow to carbon pipe

[Jayare123]

I've got 2 unions in a pipe system that customer wants welded. It's threaded but they can't get 2 of the joints leak free so he asked if I'd weld it. They are installed about 12ft up off the floor. I've asked several friends what the preferred method would be. All say preheat and wrap fiber blanket post. But differs on who you ask as to 309 tig or nickel rod stick. Pipe has to hold at max 15psi. Don't have a mig welder so what would be better? 309 tig or nickel rod stick? Or other options? I know the best option is cut off unions and install proper weld unions. But customer wants to try welding these first.

[aametalmaster]

Myself save a lot of headaches and replace the threaded unions...Bob

[ryanjones2150]

Aluminum bronze tig braze.

[Jayare123]

What about silicon bronze?

[MAC702]

I don't think it matters. Use the process you want to use and then whatever you have for it. It'd probably last forever with whatever steel filler you already have handy. That said, yes, a stainless or nickel filler will be better, especially if under vibration. But this is a job with a low pucker factor.

Which joint are you welding, the pipe threads? Are they contaminated?

[ryanjones2150]

Silicon bronze will work too. I just prefer aluminum bronze. I think it's stronger and flows better. The AC welding current cleaning action will help with either of those filler metals. <br />
<br />
Mac says you should be fine with any steel filler metal. That would be easiest, but I don't like to weld any cast iron, I prefer the braze process and have had success with it in the past. Sounds like Mac has done this before, so that experience is valuable.

[MAC702]

I've successfully welded mild to cast with standard filler, but it's not fun, and takes a long time, and you don't want to screw much with it afterward. The more I think about the OP, this may be more difficult than desired. If this union is already in place with goop in the threads, though, that changes things significantly. If it can be removed to get the threads clean from Teflon and grease, then I'm not sure why it isn't being replaced with a new union that doesn't leak.

Since it's available, it seems, the nickel stick is the process I would use.

Caveat: I've no experience with the silicon bronze or aluminum bronze, so these may be ideal, depending on prep conditions and availability.

[Oldgrandad]

15 psi Mac's right low pucker factor, I've done a bunch of repairs like this and have used both silicon bronze and ss rod (not on the same repair) 304 or 316 cause that's what was on my shelf. Always used TIG, be sure to burn into the root of the threads to avoid any leaks and clean the &amp;*%$ out of the threads BEFORE getting it hot. If there is a lot of dope on the threads expect it to bubble out when things get hot and do the bottom first so the goo bubbles up into your weld, not flows down into it. New unions are your best option but welding will stop the drip. Hope repair goes smooth for you.

[ryanjones2150]

I suggested the braze process because you're not melting the base metal, which is the part that makes you cuss with cast iron repairs. <br />
<br />
How big is this pipe? I wouldn't think preheat would be necessary unless it's a large pipe, but preheat will help burn out the schmoo before you attempt to stab some filler metal in there.

[Jayare123]

It's about 4 inch schedule 40. Threads have been very well cleaned from the fitting back. I have some silicon bronze and I also have some 309. Undecided on which to use as of now. Figuring I can't really go back and try the other on top after I choose one and try it. Could stop at lws and get nickel rods tho. Supposed to be going down there tomorrow am to see what I can do. So got till then to decide.

[Jayare123]

Update. 309 successfully joined them. We'll see how it holds up after it heat cycles a few times

[ryanjones2150]

Look for cracks at the toe of the weld on the cast iron side. That will be the prime suspect if it's going to fail.

[Jayare123]

Yeah. First round had one tiny leak on cast side. Went back over it and it held. Honestly I think it won't hold through continued heat cycling. But he is fully aware that I welded something that metallurgicly isn't really supposed to work. So if it fails it's getting cut out and weld fittings installed.

[aametalmaster]

But he is fully aware that I welded something that metallurgicly isn't really supposed to work. So if it fails it's getting cut out and weld fittings installed.

Perfect...Bob

[Oldgrandad]

Awesome! Glad the repair went well.