Can low pressure steam boilers be soft soldered ?

I thought that all steam boilers had to be hard soldered ; I think silver soldering and brazing are also terms for the same thing.

But according to the website, this boiler by Forest Classics can be soft soldered :

**LINK**

Is this because it is low PSI ? How high does the pressure have to be before hard solderering becomes mandatory ?

My View ... Pressure plays a part ... but the main factor is the method of firing the boiler.

This one is externally fired by tablets under the boiler. (bit like a saucepan) The flame will not touch (or come close to) any soldered joints.

If it was internally fired I think that it must be Silver-Soldered.

Dave

soft or silver solder it

Mamod boilers themselves are made of a good grade of quality brass, and are soft soldered together with a high melting point soft solder (Pewter grade), not silver solder.

Hi Brian, the guys are right obviously soft solder melts and therefore remelts at a lower temperature than silver solder. It is also correct that a soft soldered joint won't withstand the pressures a silver soldered joint would, so therefore you have to make a judgement based on these facts. However, you do not need to be lead free as a model engineer, tin lead alloys are not banned and are freely available eg **LINK**

Ordinary No-lead 'soft' solder has a rather higher melting point than old-fashioned soft solder (183C is the lowest for tin/lead, 227C for the cheapest tin+ 0.7% copper solder, more complex alloys can be lower (or higher)), with strength not much different. Some soft solders do contain a small % (less that 4%) of silver, but it is confusing to call them 'silver solders'. Soft solders rely on resin-based fluxes, or 'killed spirit' which is zinc chloride solution with some free hydrochloric acid. Some early bicycles were soft-soldered together (eg Dursley Pederson).

Silver solder (also called 'hard solder' is quite a different set of alloys, requiring a different range of fluxes, usually borax based, and applied as a paste in water or alcohol, or dry powder. As well as a much higher melting point range, they are stronger, and tougher, so more suited to the higher pressures as well as temperatures. Many silver solders - which contain 50% silver or more - are silver-white in colour, some a bit nickel-pale-yellow.

Brazing involves a further range of brass, bronze, and similar alloys, with an even higher melting temperature. But the distinction between soft and hard solder is much clearer that between silver solder and braze. The answer to your question is 'yes, but don't expect worthwhile performance'.

Cheers, Tim

Edited By Tim Stevens on 28/04/2014 18:00:43

Soft solder is accepted for any model boiler (unless the regs have changed recently and I missed it) regardless of pressure. The key point being that it is only used to make the boiler water tight. The mechanical properties of holding the thing together must therefore be rivets. Would I soft solder a 15opsi boiler...no, but that doesnt make it wrong.

I think I will hard solder this boiler when I buy it. I need to learn how to do it some time and this will be as good as any place to start.

hi brian,

it will take but a few minutes heat up and work to silver solder your boiler then you can rest assured the boiler wont fail under most conditions. i am not too sure about all those superfluous rivets though.

soft solder is only a caulking medium and has no inherent strength. as Ady1 has pointed out the Mamod boiler used something quite different to soft solder though not silver solder.

cheers,

julian

Soft solder is ok... until the boiler runs dry of water and gets a bit too hot.

.

Brian,

I'm out of my depth here, but I am sure there are others with the detail knowledge.

I recall reading that contamination by residues from previous soft-soldering can spoil a silver-solder joint. ... Therefore; if you do hard solder this existing boiler, you need to first mechanically remove every trace of lead from the joint areas.

... If this is wrong, them someone will soon correct me.

MichaelG.

Hi Brian,
I agree with Michael's comments about contamination with soft solder. Also I do not think brass is a good material for a high pressure boiler as it can loose part of it's zinc content over time and becomes brittle.

Les.

Perhaps the Midwest Steam vertical boiler might be a better option. It uses a copper boiler.

Interesting article OK it's related to plumbing but it still applies

**LINK**

Up to a working pressure of around 2 bar-g, soft solder is OK. Look at toy steam engines. This method of soldering is still used by Wilesco and there are plenty of toy steam boilers made from very thin brass and soft soldered which are over 100 years old and still in working condition today.

If you are making your own boilers you have to decide on the odds of your boiler running out of water. Commercial steam toys are designed so the burner runs out of fuel before the boiler runs out of water. So long as there is sufficient water in the boiler no harm will come to it regardless of the method used to heat it. Just look at the Wilesco D455.

This is because of the laws of physics. At 2 bar-g water cannot exceed a temperature of 135C. Most soft solders melt at 183C so there is a significant margin of error.

In fact by the time water has reached a temperature of 183C the pressure would be around 10 bar-g and mechanical failure of the soft soldered joint more likely than failure due to melting before you get to that point!

Edited By Henry Artist on 04/05/2017 08:49:36

My concern with that boiler would be, not the soft solder, but that I can't see any safety pressure relief valve in the pictures. If there isn't one I would fit a Mamod type valve.

Russell.

You would hope that the relief valve has done its job looooonnggg before the pressure reaches that critical point where failure is imminent!

Car radiators are an example of pressure sealed vessels running at pressure.....they are soft soldered, ok the temps/ pressures are not as high as 2barg......

Edited By John Rudd on 04/05/2017 09:52:43

I read about this recently in one of my old Model Engineer magazines, I think from the late 1950's.

Originally most model boilers were soft soldered, the main disadvantage being that soft solder can melt if the boiler runs dry.

Brazing with brass was said to be melt proof and stronger but much harder to do than soft soldering.

Brazing with phosphorous/brass is melt proof, strong and easier to do BUT the joint is seriously attacked by firebox sulphur. Examples of boiler failures were given. Avoid!

Silver solder (cadmium/silver/brass) was recommended because it's strong, melt proof, resists corrosion and - apparently - is not that much harder to use than soft solder, just requiring more heat. I say "apparently" because I've never made a boiler and am only reporting what I've read.

Welding was mentioned as producing the best possible joints. However, it was said to be impractical for most amateurs as well as being unnecessary in practice.

That's what the old guys thought. Has anything changed?

Dave

You should try building one some time. Not that difficult and loads of fun!