silverflo 40 is not one of the 2 grades of silver solder recommended by johnson matthey for miniature boiler work – probably due to lack of ductility.

cheers.

julian

So what about full size practise of steel boilers with copper fireboxes?
I believe they also copper stayed them. I have read about brass firetubes.

I can't talk about JM products but they do not talk about them on forums either!

Why not use 440 from CuP or even 438. Yes there is a potential danger from the differential rates of expansion between the brass and steel but that is there no matter which silver older you use.

Having made a joint, and upon cooling, the brass contracts more than the steel. That puts the joint under a tensile stress. Minimize the effect by having a slightly larger joint gap (say 0.2mm) and set out to create a thicker joint than usual (more alloy to absorb the stress).

Do not overheat the joint. Watch the flux. Get the whole of the joint hot. Ideally use a propane torch with a burner suitable to achieve the heating you require.

Of course there would be no problem if the tubes were steel and the plate copper!

Allow the joint to cool slowly. Cover the joints perhaps with an insulation blanket. At about 350 degrees C the silver solder becomes less ductile but on further cooling ductility returns. This phenomenon is called "hot shortness".

Brass fittings into a steel shell was/is commonplace. 40 years ago, It was a standard production technique in the manufacture of fire extinguishers.

Keith