From: email@example.com (J. Kimberlin)
Subject: Re: MIG vs. brazing for bike frames
Date: 4 Oct 1995 17:20:06 -0700
In article <firstname.lastname@example.org>, Mike Rehmus <email@example.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???