Silver soldering stainless steel

Slowly moving on toward my super heater tubes. Would it be practical to use use stainless steel tube silver soldered into a return block at the fire box end.. Is there a problem silver soldering Stainless steel ?

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.

Silver soldering is brazing

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

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It is perhaps unfortunate, that ‘they’ chose to go with the ‘expose to fire’ theme, rather than the ‘use of brass’

… But hey, ho, there we go.

MichaelG.

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Ref. __ http://www.weldinghistory.org/whfolder/common/etymology-braze.html

Edited By Michael Gilligan on 23/12/2021 23:21:31

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?

My simplex has a radiant superheater made from stainless, the two tubes are threaded with a fine pitch into a small stainless block cross drilled to form a return bend with a plug threaded in. Then I brazed over all the joints as a sealing measure. It's been in the engine for nearly thirty years and has been run a lot , in fact it's very worn, but no trouble with the superheater. At the smokebox end it is silver soldered into copper, using Tenacity flux. I made the block a bit oversize to make it easier to hold during machining, the threaded holes for the superheater tubes are very close together, and then linished down until it would fit through the superheater tube.

By brazing I mean using SIF flux and a Sifbronze rod, I've always known it as bronze welding,which is what it was called where I served my time, but I've also heard it called brazing, bronze brazing, and a host of others. I didn't use any special flux for  brazing the stainless, just the normal pink stuff from SIF. But did heat it very quickly. Doubtless some will think it all wrong and doomed to failure, but it's been fine all these years!

Dave

Edited By Dave Wootton on 24/12/2021 05:53:44

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.

That is the type of result I was hoping for, thanks Dave. I use borax flux, presumably the “pink stuff” as it is pink, for bronze brazing. I abrade the stainless with wet&dry and immediately flux. If silver soldering I abrade and use HT5 equivalent flux.

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.

I have always been of the opinion that Soldering is with temperatures below 450c & above 450c is Brazing.

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

The silver solder I used was up in this temperature range, as I said look on CuP website

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.

I always went with soft solder, hard solder and brazing but Keith Hale informs us that silver soldering is a brazing process and he does know what he is talking about so I will go with his definition.

Mike

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

I used Cupalloys silver solder and HT5 flux to brazenlysolder my stainless superheaters on my William and it worked fine with no sign of failure over 100 miles later (Whats that in scale miles?).

The radiant/hot end was tig welded however.

Edited By Jon Lawes on 24/12/2021 14:22:19

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 5 flux is what you need to use and if job is quenched while fairly hot it usually dislodges most of the flux but the rest will have to be mechanically removed.

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