Garden Railway Forum

Hi, I'm restoring an Archangel Princess and want to change and remake parts that will be silver soldered again.
I'm having trouble de-soldering parts that need to come off and I think it's because the parts say the meths tank inlet and outlet tubes are part of quite big assemblies and I'm just not getting enough heat local to the part.
Generally silver soldering is not my strong point and I'm putting it down to the butane/propane flame is A too big and B not hot enough quick enough if you know what I mean.
I'm thinking of getting a smaller Oxy Acetylene kit so that I've got a hotter smaller flame that will bring the local area to solder up to cherry red to get the solder to flow. Before you say yes I know all about clean joints/Borax flux etc.
Any thoughts on this? Or am I barking up the wrong tree?

Regards

John

I think you will find that oxy/acetylene will provide too much heat in a very localised area and that you run a high risk of melting the parent metal, presumably brass in this instance. In the hands of a skilled gas welder this would not be a problem but by your own admission, this is not one of your strong points. Packing around the job with heat reflecting material (fire brick etc.) to contain the heat will make all the difference. What sort of gas flame are you using? It is surprising how much heat is required for successful silver soldering.

As long as you are not soldering anything too large a decent propane torch should work okay.

I'm ok with soft soldering, but unfamiliar with Silver soldering. Is it the same, but with higher temperature and different solder?

Ian

cncmodeller wrote: Wed Jul 18, 2018 11:08 pm Any thoughts on this? Or am I barking up the wrong tree?

What size of torch are you using at present?

As LBSC always said about brazing 'you need plenty of therms...' An oxy-acetylene flame has a higher flame temperature than air-propane, but that isn't necessarily the same thing as heat output. You need oxy-acetylene gear for ride on size boilermaking, but as noted already it can easily get out of hand with small parts if you aren't trained in it's use. The equipment is not cheap either, plus you have to rent the gas cylinders which is an ongoing cost.

I do a fair bit of silver brazing when building locos and use two sizes of butane torch for fabricating parts, plus a large LPG (propane) torch with a couple of burner sizes for gas tanks and boilers.

The smallest butane torch is a refillable pencil torch that I use for fabricating small detail parts, the larger one is the type that fits on the butane cans I use for locos. This mostly uses up the gas left in loco fuel cans and can manage things like fabricating steam tees, valve bodies, etc.

The LPG torch I have is a Sievert one, sold here in Oz under the 'Tradeflame' name, which is supplied with gas from a 8kg tank. Here in Oz propane is sold as LPG and cylinders are available from the local petrol station on a changeover basis. I won't quote part nos. of the torch as they are different here.

The smaller burner I use for silver brazing gas tanks is a 22mm burner with a heat output rated at 7kW, which uses about 0.5kg/hr of gas. This has easily handled brazing the gas tanks I've built, including the large one in the Garratt.

The larger burner I use for boilers is a 35mm burner with a 43kW output, using 3.4kg/hr of gas. This puts out serious amounts of heat and I use it at about half power when brazing a 2" dia. boiler. Gas usage sounds high, but brazing a boiler doesn't take long and I think I've used less than half a kg of gas per boiler so far.


The other question is what sort of firebrick are you using in your brazing hearth? The type you want is called insulating firebrick and is fairly light in weight, usually off-white in colour. This rapidly heats up on the surface and reflects the heat back into the workpiece, unlike the heavy dense type of firebrick used in heaters and fireplaces which absorbs heat.

When I was setting up the workshop for silver brazing, I found the book 'Soldering and Brazing' by Tubal Cain very useful. It is No. 19 in the Workshop Practice Series, published by Special Interest Model Books.

In a lot of cases, it is probably easier to make new parts using the old one as a pattern, rather than trying to rework old dodgy joints, as you don't know what alloy was used originally, or what bodged repairs have been done to it since.


Regards,
Graeme

Thanks Guys for all your very useful advice, I've got quite a few tips now to work with.
Kind regards
John