soldering stainless steel to copper

I want to soft solder (tin electrical solder) copper pipe into a stainless steel fitting for a vacuum system.

I don't know the SS grade - it came from a refrigeration supplier. I read that a phosphoric acid flux is needed but it is not obvious from looking at fleabay which if any is suitable. So, if possible a product and a source please, not too expensive either.

Harris's "Stayclean" you can get it from CuP

Get the flux from most hardware outlets i.e. JENOLITE rust killing liquid. It is mainly phosphoric acid which is what you need as a flux for SS. Also look up phosphoric on this site.
BobH

Hydrochloric Acid with as much Zinc as will dissolve into it (AKA Killed Spirits) is the ideal flux for soft soldering to all grades of Stainless Steel, my first batch was made for me in the laboratory at Firth Vickers Stainless Steels in Sheffield when I asked what to use for soft soldering to stainless bar.

Don't breathe the fumes as you're soldering though, use extraction, full face protection or do it in a large space/outdoors ,

- Nick

Edited By Nick Hulme on 31/03/2017 15:24:31

I'm not sure if this is true but might you need a special kind of flux to get any metallurgical bond on stainless steel, a bit in the same way aluminium can't be soldered without a very aggressive type of flux (and the appropriate solder ofc)?

Always smells like pine resin when you burn flux.

Michael W

Edited By Michael-w on 31/03/2017 15:37:07

What is being suggested IS a more aggresive flux which IS suitable for stainless, from the Harris site:

"Harris Stay Brite® and Stay Brite® #8. Stay Clean is an active soldering flux. It works well on copper and brass but is more frequently used on base metals that require a more aggressive flux, such as steel, nickel, and stainless steel, (it is not suitable for aluminum or magnesium)."

The other two home brews will do the same thing.

Edited By JasonB on 31/03/2017 15:43:04

That's because it's Rosin, which is made from pine resin

As for flux, bring the Fluxite Quinns back to ME!

Neil

Ah I see, you learn something new everyday, it's great to know theres a lot of fellow pyromaniacs on here who know a lot about burning (in a controlled fashion).

Michael W

Hi Roger,

I would suggest silver soldering the parts.

There are specific grades of silver solder used for stainless to copper particularly for radiant superheater elements to copper tails, though I think J-M silverflo 55 would do especially in your application.

All my vacuum pipe runs have been silver soldered on miniature locos.

Cheers,

Julian

.

Where id the photo of that boiler you 'constructed' Michael. ............ That was an educational masterpiece for anyone wishing to know anything about soldering.

Nick

Cheers, it's good to know you haven't forgotten me yet

Michael W

Hello Roger

If you require only a small quantity use a suitably flux cored wire e.g. 2207.

A flux specifically for soldering stainless steel can be sent in the post.

2207 is a silver tin alloy with excellent flow characteristics to produce strong leak-tight joint with small neat fillets

It will also give a good colour match on your stainless.

It is readily available

For more info **LINK**

Regards

Keith

Very much agree.

If the joint needs to be more structural than can be achieved with soft solder then a hard solder would be advisable, if not then the max it can be subjected to is 14.5psi so anything with greater tensile strength than that of used chewing gum would suffice

Thanks for the advice all. I have some Jenolite and also a tin of Fry's is lurking somewhere. Will give it a try.

I have soft soldered copper wire to stainless steel by "tinning" the stainless with silver solder first. Colin