From: kimberln@crl.com (J. Kimberlin)
Newsgroups: rec.crafts.metalworking
Subject: Re: MIG vs. brazing for bike frames
Date: 4 Oct 1995 17:20:06 -0700

In article <44upqi$8ci@news1.svc>, Mike Rehmus  <mrehmus@grayfax.com> wrote:
>While designing a boiler, I had occasion to question the
>strength of a silver-soldered joint.  Looking at the
>container of Silvaloy brand of silver-solder, I discovered 
>no statement about joint strength in the familiar 'psi' 
>rating we see on most welding supplies.
>
>I talked with one of the engineers for Englehardt, the supplier
>of Silvaloy and a major supplier of brazing and silver-soldering 
>supplies about joint strength.
>
>He said that a properly prepared joint is 3X stronger than
>the base metal.  Period, with no definition of the base
>metal type, etc.  This was for silver-solder.  Brazing
>should even be higher, shouldn't it?

You will have to define brazing as opposed to silver-soldering.  This 
always comes up on this newsgroup, of course.  Generally, brazing using 
brazing rods melts both the base metal and the rod and fuses them 
together.  This is not very strong in the case of the base metal being 
copper.  Handy and Harmon, in *The Brazing Book* state that brazing is 
defined as using a filler metal to join two workpieces without melting 
those workpieces.  Brazing filler metal runs in by capillary action.

Silver braze and silver solder mean the same thing to me.  Both use a 
filler rod containing anywhere from about 3% to about 55% silver, with 
the other metal being copper, cadmium, zinc,tin, and sometimes manganese 
or nickel.  Some contain phosphorous and copper only and melt around 1550 F.

If you are making a model boiler, you always want to use a silver/copper 
solder on the joints that see flames.  No zinc, no phosphorous, and this 
is because coal fired boilers normally see a lot of sulfur which can 
displace the phosphorous and eat up the zinc to eventually cause leaks.  
On the outside of the boiler jacket, phos-copper is frequently used 
because of low cost.  The mud ring can be done with phos-copper too but 
it has less fluidity and won't be sucked in by capillary action as easily 
as a silver containing solder.  If you are making a propane fired boiler 
the same is true about the sulfur.  Propane contains methyl mercaptan as 
an oderant at about 15 ppm (watch it draw flies) and this sulfur will 
decompose in the flame  to form SO2/SO3 in a 95/5% mixture.  This is acid 
gas and attacks zinc, copper, and the sulfur will replace phosphorous.  I 
say this knowing that the joint area seeing the acid gas is very small 
and the etch rate is very slow, relatively.  A copper boiler may well 
fail for other reasons such as abrasion 30 years before the joints fail.  
But we build these things to outlive us...don't we???


JerryK