Stainless Steel To Copper Pipe Weld

[TBP]

Hi: I'm not getting much response to this question on the piping board, so I'll try it here.

I need to get from 304 or 316 SS to type 'L' or 'K' copper. This transition piece would be the final step of a shop fabricated assembly. It's important that joint left for the field installation crew be a copper pipe joint that can be silver soldered or brazed. (This copper end could be male or female.) Sizes will range from 1/2" to 3" pipe. The vast majority of these will be 1/2" & 3/4" pipe. This assembly needs to meet ASME piping codes for oxygen service.

Is there a recognized welding procedure that will allow me to join copper and SS and keep the pressure piping inspector happy?

 

[Hush]

Why weld? Why not use compression fittings for your field install?

 

[TBP]

Thanks for your response, Hush. I'm not aware that compression fittings like Parker and Swagelok come in sizes larger than 7/8" (3/4" copper pipe is 7/8" OD). The cost of this type of fitting can be also be a big factor. (Some fittings are pretty pricey.) Of course, this would have to be balanced off against the labour savings as compression fittings install quickly.

 

[Hush]

I know Swageloks are available to 2" tube OD, above that could be a problem though.

 

[GRoberts]

I don't know quite what you mean by "recognized welding procedure", but most likely for piping, you would be working to ASME Section IX for welding, which has SWPS (Standard welding procedure specification), which do not have to go through a full qualification. They would not have one for copper to stainless steel though, only common applications. So you will probably have to preform full qualification for your weld. Something else you might consider though, is if you are solder/brazing all your copper/copper joints, why not braze the copper/stainless joint also. ASME Section IX also covers brazing qualification.

 

[TBP]

I guess I should have said "established welding procedure" for joining copper and SS. I'm trying not to break much new ground, if I can avoid it.

 

[Rich2001]

Stainless steel is difficult for solders and braze filler metals to wet. The surface chromium oxides that protect it from corrosion also prevent the filler metals from wetting the surface. Special fluxes are needed to eliminate these stainless oxides.

In the past, I have had a problem of getting the stainless steel hot enough to solder. The thermal conductivity of the stainless is poor compared to the copper.

Soldering Stainless Steel to Copper

The silver solder commonly sold for home plumbing with copper pipe will work on stainless, however a different flux is needed. Fluxes containing hydrochloric acid or one that says it is for fluxing nickel alloys or stainless steel should be used.

1. "Tin" the fitting first with solder.
2. Apply flux to the stainless pipe.
3. Fit the two pieces together.
4. Heat the joint, and feed more solder into the joint once it is hot.

With this method, the stainless steel surface is protected from the air until it is hot enough to be wetted by the solder.

Brazing Stainless Steel to Copper

Silver-based brazing alloys have lower melting temperatures than copper or zinc brazing alloys, so the silver-based alloys are the more practical choice.

Keep these two isuues in mind when brazing.
1. Most brazing temperatures are right in the temperature range that causes sensitization of the austentic stainless steel.

2. Brazing must be done efficiently to reduce the time is the criticakl temperature range.

Use a slightly reduced flame and AWS-type 3A flux, which has the higher useful temperature range needed for brazing (1050-1600 °F).

Both surfaces to be joined must be cleaned and fluxed for best results.

Prebraze the copper fitting, because it has the higher thermal mass in the localized area. Preheating the fitting helps reduce the amount of time that heat is applied to the joint.

The AWS BAg-5 is generally recommended for this purpose and is readily available. The usage temperatures for BAg-5 is 1370 to 1550 F.

 

[Hush]

This idea seems obvious so there's probably a reason you can't do it but, why not thread it or weld a threaded coupling to it?

 

[beadhand]

This welding problem isn't as hard as some are making it out to be, ss of any grade welds very easy to copper. Use the tig welding process and use 304 ss wire. You will find that this works.