Silver soldering stainless steel

I would use two grades of silver solder, one per phase, to avoid problems.

AWEM

Do the inside bit first, then the ends.

Unless you have an extremely humongous burner there shouldn't be any problem, and no need for staged brazing or anything - if it's 4" or more away in a 25 mm tube, the inside bit will be well solid by the time you get to do the nearest end.

-- Peter Fairbrother

I wouldn't bother myself, don't see any need to with this geometry.

Step/stage brazing, using filler metals with different melting points in order, highest first, especially with a torch, is a bit dubious unless you are well practiced - you can easily melt the first step too, by mistake.

That doesn't really happen much with two-stage brazing with the melting points far apart, but it does with more steps (braze fillers then have to have closer melting points) my advice, if you need to to do step brazing, especially by hand, use a brass rod for the first step, a 19% filler for the second, and a 56% silver filler for the third step. And a Nicrobraze filler for a zeroth step, if needed. That will give a hundred degress or more between fillers.

The "ordinary" step silver soldering method, using silver solders with about 50 degrees between melting points, while possible, is only really suitable for experts - you will often find that the whole assembly, on which you have done a lot of work, comes apart because you melted one of the higher-melting silver solders by mistake.. unless you are very skilled, of course, when it is possible to do something like 7-step brazing in 100 degrees - I'm good, but that's for cookoo-clock makers.

As you may have gathered, I've made a lot of mistakes while brazing!

Diffusion brazing is nice though - it's usually done with a Nickel/Copper/Boron filler, first the usual brazing is done then the made joint is held at a little under the melting point of the filler for about 20 minutes in an oven.

The boron in the filler then diffuses into (and perhaps eventually through, where it gets oxidised) the base material, leaving the filler boron-poor, and thus at a higher melting point than when it was originally applied.

This means that diffusion brazing works like glue - do one bit, and then do another, and then another, and so on - no need for steps/stages, after the diffusion process you can heat an already-made joint up well past the melting point of the filler metal, and it won't melt. :)

-- Peter Fairbrother

Stainless is such a poor heat conductor that staged soldering is not required on something of this size. One point though, if the end caps seal the tube completely then you might find that air expansion/contraction causes problems with the second cap. A single, tiny drilled hole will avoid any problems.

Hi

Firstly many thanks to all for their input.

My concern is more with what happens to the structure the metal, I am sure I read some wherwe that there are problems with this, does this jog any memories?

Peter

I haven't had problems silver soldering stainless (or welding it with an Inconel filler) on the few occasions that I've done it.

Only thing to note is to use a flux designed for the job, such as Tenacity No5 since the normal fluxes aren't sufficiently aggressive.

Mark Rand RTFM

Tenacity is certainly more aggressive and better able to tolerate over-heating, but it's also far more difficult to remove afterwards. I've had no problems with Easy-flo flux, but I do make sure the joint is clean.

Depends of course on the stainless, usually what we usually get is annealed 304 (aka A2) or 316 (aka A4) austentic (non-magnetic) stainless, sometimes 303 which is a freecutting 304.

For welding you should (in theory, if you need strength and especially vibration resistance, though many people don't bother for non-critical applications) use a low carbon stainless, usually 303L/304L or 316L, to prevent the formation of chromium carbides.

Sometimes stainlesses which are just marked eg 316 will be low carbon anyway, the mills often make a batch of low carbon stainless and sell it as both eg 316 and 316L.

You can also use 310 or 321, but they are expensive!

For brazing, it's not so important to prevent carbide formation, as the temperature isn't high enough. Carbide precipitation only begins at around 800 C, when it's still happening slowly and shouldn't be a problem when brazing unless you overheat, or heat for too long. That's for brazing - for silver soldering the temperature and time is less, and you should have no problems.

The other possible effect of heating stainless is to pre-anneal it - eg some types of stainless tube are supplied cold-worked, and heating can affect this. It's rare though.

-- Peter Fairbrother