Soft soldering stainless steel

[ChristofferB]

So, i'm in need of soldering some very small stainless steel parts together, specificly, 3mm tubing, 1.5mm tubing and hypodermic needle-size tubing.

The entire assembly needs to be insulated in a PTFE ferrule, as it's to be used as an electrode for a micro-flame ionization detector, so joining with fittings is out of the question. Epoxy and such is too.

Is there any flux that would allow soft (Sn97Ag3) solder to do this job, or do I need to silver solder/braze. If the latter, do you think it can be done with a micro propane burner? They get fairly hot and the thermal mass is tiny.

Any input appreciated! Thanks in advance

--Chris

[DTJ]

No practical experience but it seems it can be done

https://sciencing.com/silver-solder-stainless-steel-6144307.html


I've used a micro-burner for silver soldering small parts (non stainless) before.

[Conrad Hoffman]

Maybe it can be done but I wouldn't trust it as far as I could throw it. Can you find somebody to laser or beam weld it?

[T3sl4co1l]

Yes, use acid paste flux.

Tim

[ChristofferB]

Acid paste flux, gotcha. I'll try that. If it doesn't work I might try my luck with those ebay silver solder+flux kits.

I don't suppose dilute hydrochloric acid would work as flux?  That'd be delightfully simple.

Thanks all!

[T3sl4co1l]

Nah, too cooling, and it doesn't stick around.  Once it's gone, oxides are back.  You can make your own zinc chloride by dissolving the metal in it though.

Tim

[Domagoj T]

I don't suppose dilute hydrochloric acid would work as flux?  That'd be delightfully simple.

Don't dilute it. Dissolve zinc in it, as much as will dissolve (make sure you add little by little zinc and do it outside).
Careful with the final product as it will rust anything it touches and most things in proximity. Again, try soldering outside if possible. Neutralize with sodium carbonate (washing soda).

[David Hess]

Is there any flux that would allow soft (Sn97Ag3) solder to do this job, or do I need to silver solder/braze. If the latter, do you think it can be done with a micro propane burner?

A propane or butane torch is absolutely hot enough for silver brazing using roughly 50% silver alloys.