Silver soldering rod for boiler

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Nikhil Bhale

Well-Known Member
Nov 15, 2019
Reaction score
Vidharba, India

I am planning to built a model boiler for my PM research steam engines. I would not like to exceed working pressure of boiler more than 3 bars. I have gone through some plans on for the same.
After seeing the plans I am confident that I can get all the material locally for the built.
Only question is brazing/ soldering supplies.
I have some rod of Harris safety silv 45 blue flux coated, some bronze brazing rod, and Harris Phos copper rods.
1. Harris Safety-Silv 45 1/16" x 18" Flux Coated Mini Pak - 3 Stick Tube 45F318MPOP
2. FC-BRONZE 261 2.0 X500MM 1,0KG - Wilhelmsen
3. Dynaflow®.

Which rods will be suitable for brazing the boiler? I have only a butane gas torch which I think will be insufficient for the job.

Or I have to take help of a professional welder/ fabricator for brazing the boiler? He will have all the gas and other accessories.
The other option may be TIG welding the boiler locally.

What will be your suggestions?

Depending on the gauge of copper you are going to use a regular home propane torch should work fine with the Harris 45 silver solder. I would get everything assembled and ready for soldering with one heat cycle.
What will be your suggestions?
Blue tip will be ok but..............what is the boiler size..???
We did a few here a few years back and used firebricks to retain any heat the best we could AND a 75kw burner.
Sounds like this is a first attempt by you and as such I'd be getting some local advice from qualified /experienced boiler builders, possibly even their build and you "watch".
Likewise, any precoated silver solder would be out, use purpose made liquid flux on the joints before applying heat.
Two different silver solder colour tips were used at differing temperatures due to construction requirements, same heat cycle just different applications within the one.
Think you'll find strict engineering checks/requirements/ inspections depending on your country but for sure any TIG welding would be a no-no.
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Thanks for all the suggestions.
I have a 2.5"/ 65mm pipe gauge 16 or 1.6mm thick pipe.
So that's the size of my boiler.
It will be an all copper construction.

I think I will do some practice soldering with the blue rod and my butane torch.
If I am not successful I will approach a professional welder.

Hi Nikhil. I do not know what boiler regulations apply in India.
I am based in the UK, and as an Engineer I would not contemplate making any boiler without making the components from materials that meet regulations.
I did a few times, but now I know better!
There is a lot of work in making a boiler...
But if small enough it can power small displacement models and be within the scope of your Butane torch, but I have never made anything that a regular shop-bought butane torch can heat adequately. These are usually rated just below 2kW heat output, and you will probably need 2 to 3 times that for silver soldering the end plates to the outer cylinder.
A proper 6kW blow torch with hose run from a larger propane tank is the minimum I should use to attempt a 3inch diameter boiler.
I bury one end in sand to contain the heat, and surround the top inch or so with firebrick, and use 2 x 2kw (old, paraffin) blow-lamps to pre-heat and a 6kW Propane blow-torch to do the localised soldering.
I am an amateur and have never managed a single soldering session on one end of a boiler. I always need to repair a leak - pin-hole - as when working quickly when the boiler gets hot enough, the flux can start to degrade rapidly and the last bit seems to have some inclusion that leaks under test.
So a professional coppersmith (NOT your local steel welder!) is worth considering.
On material thickness, I am currently planning to repair 2 or 3 boilers by the addition of more stays, as they were used "without certification", and have experienced internal pressures that have domed or buckled end plates.
Using end plates on a 3 inch diameter boiler, with flue tubes, it is folly to consider anything less than 1/8in thick end plates unless many stays are fitted. And all that needs calculations to determine maximum spacing, depending on the end plate thickness and the stays selected.
I have repaired/re-built 3 boilers, (One had never been successful due to poor design!) and made 5 of my own, and my early boilers used materials that were simply too thin... hence the planned repairs!
So please learn from my errors, and I suggest you find out what regulations apply in your country.
Failing that, the best "simple" guide I have seen and used is a magazine article by Kozo Hiraoka: Live steam and outdoor railroading: Nov/Dec. 2008. - Worth every penny of purchasing a back-copy from the publishers (on-line).
ASK when you have had a think about what you really want to do? Just have 1 boiler?, or - like me - get involved in this as a part of your hobby.
I can help with advice on how to design your boiler, so it is SAFE and will last your lifetime.
The pipe you have will be OK for an outer tube. But are you planning on "simple ends"? = Horizontal or vertical? (I recommend "horizontal" to increase the surface area and steaming capacity). With or without flue tubes, or other means of increasing performance?
I have looked at a couple of PM engines - 1" bore and 1 1/2" stroke, and larger.... What is the largest you want to power? I believe you will struggle to power these engines satisfactorily unless you have a 3" diameter boiler with flue tubes etc. and adequate heater. With a 2 1/" boiler you will struggle to get enough steam and the pressure will drop straight away.... in my experience! But there are some "performance" designs that may be adequate? Please advise the boiler design you think will suit you, and I can advise "my humble opinion"...
But are you planning on "simple ends"? = Horizontal or vertical?
I am looking at many boiler designs. Mainly from It has many boiler projects, independent or as a part of some other model. I am leaning towards Stuart like horizontal boiler with water tubes beneath the boiler for extra heat transfer.
I was thinking of buying copper end caps and use it as boiler ends.
I was planning for 2 1/2" boiler as I have some tube on hand. For 3" boiler I will have to buy a larger tube.
I do not know what boiler regulations apply in India
One of a guy from my college days used to work as an industrial boiler inspector. I will have to find him and ask for information.

Failing that, the best "simple" guide I have seen and used is a magazine article by Kozo Hiraoka
I have book by K N Harris. I will also buy the back issues as you suggested. I also have books from my college days for machine design, strength of materials, thermodynamics etc. I will brush up my engineering knowledge.

It sounds like you are approaching the project with caution and research, so, very good.
My copper brazing is limited to copper pipe and craft projects, but I did receive expert advice from the welding and brazing experts at work.

The butane temperature will melt the brazing alloy, but only if the output from the torch supplies enough BTU's for the heat that the copper will suck up. Because of the limited heat, an alloy with good gap filling properties and a wide melting range is important. The Safety Silv 45 meets those requirements.

However, I prefer one of the BCup-x classifications like BCup-5. (BCuP-5 is 15% silver) The reason is that it meets the previous requirements but it also has phosphorus in the alloy, which acts as the flux, and eliminates the need of any additional flux coating or having to mess with and clean additional flux.
EDIT by Lloyd-- It has come to my attention that copper brazing filler metals that contain phosphorous (Like the BCuP-x series) are not suitable for use in a sulphur environment, such as a boiler. They will degrade over time from the sulfur and its compounds. Coal and crude oil, and other petroleum fuels do contain sulfur. The BCuP-x series still works fine for other copper to copper and brass applications.--

Here is a copper project I made years ago using copper water tube and BCuP-5 brazing alloy. If you look at the detail picture you will see that the alloy produces a very nice fillet, and also tolerates stopping and starting the brazing process as the project was turned over multiple times to reach the tricky areas of the tubing. Cleanup was almost unnecessary, but I did apply a patina for appearance. The fillet shape and flow of the detail joint are as-brazed, with no sanding or grinding.

One thing I would strongly suggest is a hydraulic pressure test of the finished project, and also of a sample at the beginning of the project. A simple hydraulic Porta-Power or similar will work well. Because applicable boiler specs for your project will probably be hard to find, one approach would be to estimate the worst-case accidental over-pressure condition and then triple that for a test pressure. But of course improperly done pressure tests can weaken the assembly; calculations beforehand should alleviate that. I have used that method to test small diameter air storage tubes to destruction. I trust that you will be careful.


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You better watch Blondihacks build her boiler, you will learn a gartof posted..
I built a few boilers and used something called phoscopper rod. Used 8 inch copper pipe and plate for the ends concaved in. It static pressure tested to 300 pounds and the sides bulged a little but not at the joints so it held. It took a lot of heat using bricks and a gas burner to create enough heat for the welding torch to be able to work.
I built a few boilers and used something called phoscopper rod.
I read somewhere that rods with phosphorous should not be used for boiler construction. Then I read somewhere that these same rods were used in earlier boiler construction before advent of silver brazing.
Then I read somewhere that brass should not be used due to de zincification. But I think silver brazing rod have some amount of zinc in it.
Its very confusing as what to use. The rule of thumb keeps changing every decade. So I post it here in the forum as people have more experience than me and they can guide me.

Also considering the cost of silver brazing rod and availability in India (at my hometown), pure silver wire will be cheaper option for me. 🤣

Hi Nikhil,
To be clear:
  1. Bought end caps for "water piping" Are NOT thick enough to withstand the NWP of 3 bar with the recommended factors of safety.... YOU NEED 1/8" Copper ends. (Unless you build-in a lot of stays to manage the pressure)
  2. Phosphor loaded silver soldering rods for brazing are NOT permitted by every country with Modern Regulations. SO Best avoid that!
  3. Talk to your mate: "One of a guy from my college days used to work as an industrial boiler inspector. I will have to find him and ask for information". He KNOWS the right answers - it is his job. We are all just well intentioned amateurs by comparison. Even though much of our advice is good, we do not have the training and expertise to avoid telling you some things that are wrong.
  4. A NORMAL boiler will have a factor of safety of 8: But Copper at STEAM Boiler pressure and TEMPERATURE is only 2/3rds to 1/2 the strength of "room temperature" copper. So realistically the strength of copper for calculations is limited - AT 3 Bar (Steam) NWP to 4000psi tensile stress, and because of its peculiar strength characteristics is limited to 1160psi. for tubes in COMPRESSION (e.g. flue tubes, firebox inners, etc.).
  5. In addition, you MUST include a water-level gauge and Safety relief valve in the boiler. To make the tapped holes for these use PHOSPHOR-BRONZE bushes - not copper - and the bushes should reinforce the copper shell to at least twice the diameter of the drilled hole in the shell. In addition, where there are penetrations in the shell (e.g. for these bushes!) the shell calculations must include a stress concentration factor of 3.3 (American Regs.).
And some cautionary comments.
K.N. Harris is a very worthwhile read, and gives good useable guidance on how to design boilers. BUT he is "out-of-date" to Modern Regulations. - so please use the notes above to modify "his" calculations and limits.
Frankly, I have heard of boiler failures, nothing tragic or explosive, but was given one boiler that had NEVER managed to steam without leaking because the copper was too thin, and the design was overstressing the end joints. The intent of the maker seemed as good as many comments here, but it failed completely, and I ended up stripping out the useable metal and making a completely different design of boiler. With thickened end plates and joints that are good design. Even so, to Regulation limits it is only good for 15psi NWP.
Other boilers for which I have checked the strength have had to be de-rated from 45psi (as used) - to 7psi!, 100psi (Planned) to 45psi (too low a pressure to operate the engine as planned), 90psi to 80 psi (made for a slower ride on the track), etc.
So please calculate and use THICKER copper, proper joints, correct materials (appropriate silver solder, flux, etc.), and the bit I am sure you will achieve - good workmanship.
I suggest you think of boiler making as the same as making a small hand bomb. It probably won't kill you, but has enough stored energy to destroy a hand.... Make it well, use it appropriately.
Ask me when you want more advice or calculations.
Boilers are engines: They convert chemical generated heat into "Steam power". The steam "engine" then converts the "steam power" into rotative power. a 5in locomotive can have a coal fire developing 30kW of chemical energy generated heat.... and we enjoy that by riding trains with the 30kw between our knees... So boilers must be made to be safe.
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You better watch Blondihacks build her boiler, you will learn a gartof posted..
Quinn at Blondihacks has built two boilers, an upright and a horizontal for a Pennsylvania A3 switcher locomotive. The playlist is at:

The A3 Switcher follows the plans from the classic Kozo Hiraoka book:
Quinn mentions repeatedly through the A3 Switcher series that Kozo's book is invaluable. Particularly in the suggested order of operations for assembling the boiler parts. Kozo is a master builder with an enormous amount of experience and the book is geared toward helping a novice boiler-builder achieve a safe and working boiler.

Hi Nikhil,
To be clear:
  1. .....snip
  2. Phosphor loaded silver soldering rods for brazing are NOT permitted by every country with Modern Regulations. SO Best avoid that!
  3. .........snip
Can you cite a reference to the no phosphorous in copper boiler brazing alloys recommendation, please ?
I can only find anecdotal information, but nothing authoritative about no-phosphorous. Just very curious.
Thanks, Lloyd
Hi Lloyd, I think it is in material regs which I found once but would have to find again. Something to do with corrosion....? Or embrittlement with temperature ctcling...? Any metallurgist have any idea? (Especially if I am wrong!).
Craig, I think we should put this book on our Xmas lists!
But don't believe me as I may have it completely back to front.
I am going through other sites on internet for information.
What I could gleen from them is " A coal fired boiler produces a hot Sulphur bearing atmosphere that destroys joints made with copper phosphorus alloys.".

So I think phoscopper rods are not used on steam locomotive boilers nowadays.
The A3 Switcher follows the plans from the classic Kozo Hiraoka book:
I looked up the link and it cannot be delivered to India. I will look for other source for this book.

Screenshot 2023-09-08 083041.jpg
Hi Nikhil.
A quick calculation of the 2 1/2 in tube - with penetrations - and the boiler - except for end plates should be just OK for NWP = 3 bar (45psi).
BUT the end plates do not work at 1/16in thick.
They must be at least 1/8in thick.
Sorry, but that is what the numbers say.