Silver Soldering - reheating.

Hi guys,

I'm about to start work on my very first project, a Hero Engine, basically a small capped cylinder that you put water in, heat and it spins around (if you get it right).

The main component is a simple sealed tube... a short length of copper tube that is has flat ends silver soldered on... I'm thinking through the operations required and it appears to be sensible to solder one end on, chuck it and clean the join up in the lathe, before doing the 2nd end, then finally the other fittings.

This approach means that I'd be heating the compoents several times and the whole thing is small enough that I'm sure the existing silver soldered joints would be heated up a lot each time.

I think I remember reading that this is Ok as Silver solder, through some arcane process, actually needs a higher tempretaure to melt the 2nd and subsequent times... unlike soft solder which pretty much always melts at the same temp.

Am I correct here and would this approach work? Or should I try to get all the various bits arranged and heat it once for all joints?

cheers. Jerry

Hi Jerry,

When you reheat a small object to silver solder again, you could use a a silver solder with a lower melting point than you used the first time. Don't know how localized you can manage to heat with your equipment, but you may be able to heat just the new part. SInce copper conducts heat well I think you should start with a silver solder with high melting point, and use one with a lower melting point for the second. It will be wise to put some wire around to hold already silver soldered parts together when you reheat.

Regards

Thor

When you make a silver soldered joint some of the solder combines with the copper/zinc in the part assuming its brass or copper and forms a new alloy at the joint that has a higher melting than the silver solder used to form the joint, so you are correct in this but it is only a small increase, so if you need to be carfull in follow on operations not to put too much heat into it with a small part this will be very dificult hence it will be better to step solder, ie use solder of lower melting points for follow on operations.

There is a very good article comming up in the Model Engineer magazine that goes into step soldering very well.

Hope this helps

Stew

Hi Jerry

Suggest you design the end pieces like a top hat that fits inside the tube Joint gap 0.1mm radial.

Flux the joint with silver solder flux eg EF flux.

Drop a piece of 38% silver solder rod (eg 438) say 1.5mm dia into the joint. Heat from the outside moving the torch around to get the whole joint hot. The alloy will melt and flow through the joint.

All you will see is a witness of the solder at the joint line. No machining!

Repeat at the other end with a lower melting point alloy 55% silver alloy (eg 455).

However I suspect that you could use just one alloy the 455. The first joint will have to get rod hot to melt. Ensure this doesn't happen by controlling the heat.

If you require any further information go www.cupalloys.co.uk and see "Best Brazing Practice"

regards

Keith

Silver solders alloy with the base metal enough to raise the re-melt temperature by a small amount. One-shot soldering is best if you can be sure the parts won't move during the operation, but reheats won't affect the joints already made so long as they don't melt and accidentally move ! Mike.