Not if you use Tenacity5 or HT5 flux as it contains something that removes the chrome oxide that forms on the stainless as it is heated.

Not really my thing but superheaters are often brazed not silver soldered as the melting point is higher.

I used high temp silver solder on one of mine, CuP alloys to the rescue.

You've got to use plenty of heat and get it up to temp quickly or you exhaust the flux

I am curious Peter. Do you mean in the firebox, ie radiant superheaters, or within the flue at the firebox end? If within the flue, I think copper would be better. In that case I would bronze braze.
I want to replace the copper elements in my Simplex with radiant stainless but I don’t want to bother to have them TIG welded. I want to bronze braze them and perhaps protect the block or spear points with a stainless cup over the joint area simply slipped over the last inch as the elements are pushed home to the back of the firebox. Does anyone have experience of bronze brazed joints within the firebox, ie, do they last?

Yes my use of "braze" is as Dave describes. Not only does it need a higher temperature to form the joint it therefore resists the heat of the firebox better and in the case of spear point type joints is able to form a decent fillet which the "silver solders" don't do so well.

Dave's pink stuff is the flux normally used for brazing of copper alloys, available from most welding suppliers. At a guess the melting point for sif bronze is about 850- 900c. noticably higher than silver solder and will give a good fillet, it does need a good heat to get it to flow.. Noel.

And brazing is soldering!

Much engineering terminology has ancient Trade roots. Craft definitions rather than scientifically accurate, and not to be taken too seriously.

Soft soldering, silver soldering and brazing all joint metal by melting a different filler metal into a gap. Soldering isn't as strong as welding, where the filler is the same metal, but soldering is often more convenient because it requires much less heat.

I think of it thus:

• Soft Soldering – the filler is a low melting point alloy usually containing Lead and Tin. Excellent for semi-permanent joints such as electronics, plumbing and making tin cans etc. Mechanically weak and melts at relatively low temperature – downright iffy for boilers!
• Brazing – the filler is a medium melting point alloy usually Brass. Considerably stronger than soft solder, but much more heat needed. Poor for semi-permanent joints like electronics, good for strong-ish high temperature requirements like model steam boilers.
• Silver Solder is confusing. Usually means a brazing alloy containing Silver as a way of reducing the melting point whilst maintaining strength. Considerably easier to Braze with than plain brass but expensive! However, the term also refers to a type of soft solder! Silver is added to Lead/Tin soft solder to increase melting point and strength slightly, which is good for jewellery and similar repairs. And to eliminate Lead: most modern electronics solder is 95% Tin and 5% Silver. Soft silver solder joints are much weaker than Brazing type Silver Solder.

Although Brazing used properly is an excellent technique, it can be misused and once had a poor reputation. In the 'good old days' bodgers often applied it as a cheap, nasty substitute for welding. In consequence Brazing is illegal for many car repairs and other safety critical applications. When welding is appropriate, insist on it!

Dave

Highest melting silver solder Cup do is 418 at 776-815degC To get into the temps Noel mentions you need CZ6 Brass rod that is 890-910 deg C and no silver content.

Dave, on the choice of welding or brazing for car repairs, the MOT rules apparently allow MIG brazing but not gas brazing (Appendix A, sections 10 and 11). For highly stressed components, even "welding isn't normally acceptable, other than where the component is made up of sections that are welded together" (App. A, section 6).

Edited By Bill Phinn on 24/12/2021 13:41:16

If I can resurrect this thread, I have just had a disaster trying to silver solder a stainless steel boiler band on my large scale steam lorry.

Step 1 was to try a wee test piece with two offcuts of stainless. Silver solder worked OK (Cup alloys 455, All state flux – wish I could tell you more but the label is not readable contents are still powdery though). Sifbronze sort of worked but didn't flow very well, so decided not to go down that route.

So tried doing the first end of a boiler band with silver solder, that worked OK using SS and flux as above.

Tried the second end and just no chance of getting flow, tried a bit of hot rodding extra flux but still no joy. I have just dismantled the joint and sure enough some areas had got oxidised.

So my question is in several parts:

Any tips on cleaning in the first place? I used a combination of belt sander plus emery cloth with elbow grease.

I have some Thessco F flux, would that be a better choice?

Cleaning up after silver soldering was a bit of a nightmare on the first end. Is there a better way? Pickle seemed to make no difference at all! The dremel with a drum sander did a not very good job, and how I clean up the second unsuccesful joint I really don't know.

All advice gratefully received. If not I shall have to revert to plan C and make up some ally angles and rivet them on.

Martin

Tenacity No5 or CuP's HT5 contains something that removes the chrome oxide that forms as you heat the stainless

Brick cleaner works for me softening these fluxes in about 1/2hr

This was a stainless steel boiler that I made for a 5" Firefly loco. 10 gauge backhhead and tube plate thickness, all formed over 'jab rock' formers using a club hammer. Most of the joints were silver soldered with the old EasyFlo silver solder and Tenacity 5 flux. PLENTY of heat from a 4 pint paraffin blow lamp and a BIG propane Bullfinch torch on propane.

Use plenty of heat to bring the joint up to temperature quickly, but not too much (Oxy Acetylene is great, but can burn the flux out before the silver solder flows)

Unfortunately (In ignorance) the boiler could not be certificated as at the time (1966) UK inspectors were not prepared to issue certificates for stainless boilers.