In article , Dan Caster says...

I never have had any luck soldering stainless with regular lead / tin
solder either. Maybe the tin / 5% silver solders will do some
stainless steels, but I am confident that there are some stainless
steels that either take an exotic solder or can't be done with low
temp solder.

While soft soldering stainless is a bit tougher, because of the
extremely low thermal conductivity, it can easily be done with
the correct active flux. I use "Duntons" liquid stainless
flux, which works well except the mist and fumes that come
off are a bit corrosive.

I've found that the key to doing a good stainless joint
is to carefully pre-tin the part. This really takes less
heat than is imagined - the stainless has such low
thermal conductivity that even large thick parts can
be surface tinned with a small iron or gun. The part will
heat locally and the solder will wet the surface nicely.

One common mistake is using too much heat, and overheating
the flux, and burning it off before the solder has a chance
to wet down. Also using rosin cored solder is not the best,
as the rosin for this application is not really a flux, but
a contaminant that the acid flux has to overcome before it
works properly.

Once the stainless part is tinned properly, once, it can
be sweated without the use of stainless flux - ZnCl or
rosin flux can be used to attach a tinned stainless part
to copper brass or steel.

Because the Silver-Tin solder (eutectic 151 is my favorite)
is so much stronger, and does not contain any rosin, and has
a low vapor pressure (no lead) it is my favorite for doing
the odd vacuum system job where stainless parts have to
be soft soldered.

Jim

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