I'm no expert but 455 is the 'new' Easiflo No 2 and the 1,5 or 1,6 size is ideal for most boiler making.

I think that CUP would recommend the 430 for the initial heats but, personally, I only use the 455 with a Propane torch and the appropriately sized nozzle.

For a large boiler, I would get a friend (or even my wife) to add background heat with another torch and the whole thing should be well surrounded by thermal bricks but NOT the ones that came out of a storage radiator as these are designed to absorb heat and that is not a good idea.

I always use Tenacity No5 flux as this will still do its job even after prolonged heating.

It is important that you can heat the item quickly to the correct heat and maintain it throughout the soldering operation.

Quantity of solder is a difficult one because it depends on the number and size of the joints and the amount of gaps in them.

Brian

Edited By Brian H on 14/08/2020 19:12:05

It depends how neat you are. I have seen boilers that have so much silver solder where it should not be, such that there is very little copper showing.

I use a chalk solution (suspension) in meths and paint this around the joint. so the solder is confined to the area that I want it to be in.

Andrew.

Keith

I agree in principle with your comments but you do not want any undercutting of the solder in the joint as this creates a nasty stress concentration which could result in a long term fatigue failure as the joints in a boiler are never in pure shear and always have some element of peel in them which will try to cleave the joint open. So a 'small' fillet is going to be preferable and I know our club boiler inspectors will insist on it. Just my opinion from experience of vacumn brazing aerospace parts.

Dave

I'll buy the need for a fillet on cosmetic grounds. Keep it small by using a narrow melting range silver solder. Just as well because these alloys are the most expensive.

A boiler inspector normally wants to see a "witness" of silver solder on a joint that implies total penetration of the silver solder through the joint and a sound strong joint. That is not the same as wanting to see a "fillet". Personally I'd question the the integrity of any joint with a manufactured fillet at both ends.

Undercutting in the joint area is normally caused by the filler metal dissolving the parent materials. This in turn is caused by a poor temperature/time profile in the furnace that leaves the molten filler in contact with the parent metals for extended periods.

Undercutting is a potential problem with all vacuum and controlled atmosphere furnaces.

The number of model engineers using a furnace for brazing is very small.

This is not a problem that the vast majority of modellers will encounter.

Keith