I haven’t contributed in a while due to being very busy with
graduate school, but I am having an issue that my tutor just wants
to solve with flux but here’s the deal–
I am scoring and folding stainless steel sheet into jewelry-sized
work and am having an issue with getting the silver solder to flow.
The first time it didn’t work because the flux failed due to the
heat. The second time I tried Firescoff. It was still working when I
gave up. The solder just balled and flattened a little bit.
A German flux has been recommended to me (by my tutor) but I don’t
recall the name right off. I don’t know if it would work better than
the Firescoff.
I am using british medium silver solder but I don’t know the alloy
of stainless… My torch is natural gas and compressed air.
Is it a fluxing issue? An alloy problem? Do I need to direct the heat
differently? Is it contamination of some sort? From the torch
perhaps? Do I need to prepare the surface differently? I chemically
cleaned and sanded the joins. Is that enough?
I have silver-soldered stainless before, 18-10 tableware
specifically, and I was using an American high-heat paste flux (can’t
recall the name of it either. Its been years…) and an acetylene
torch. That’s why I ask about the torch.
Hey Raewhat you are talking about is probably the Handy flux type B1.
it is a Black paste flux instead of the white paste flux, and is
specifically made for silver or gold solder on stainless steel. the
torch type is not as important as the flux in this case. Rio Grande
has it in America, any disent tool supply house or welding supply
house should have it, you can also plug it in the Net to see where
you can find it in your area.
I would assume that the stainless alloy will be a 300 series simply
because they are amongst the most common. You will need a stainless
steel formulated flux to successfully solder the piece, something
like Johnson Matthey Tenacity which is formulated to cut the
stainless oxides and allow the solder to wet the stainless surfaces.
A fabrication technique that can assist is to “tin”, that is wet the
individual joint surfaces as separate steps with the solder and then
position the sections to be joined and reheat until the solder on
both sides of the joint melts and coalesces.
For precious metals I use a flux called Auflux, but I found it
doesn’t work for stainless steel. For stainless I found that standard
Easyflo (the white powder mixed to a paste with water) works fine. If
the solder just balls without flowing into the joint then its because
the joint is not hot enough; you are heating the solder more than the
joint. It is essential to get the joint hot enough to melt the
solder. Maybe your flame isn’t supplying sufficient heat to get the
joint to the correct temperature before the flux looses its
effectiveness. If you keep the flux in its clear glass-like state for
longer than about 30 seconds or so then it will no longer work, in
which case you should try a flux called Tenacity, which is specially
formulated for prolonged high temperature. But be aware that it’s
very difficult to remove afterwards. If your current project is
physically bigger than your previous successful project then it
certainly could be that your flame is simply not supplying sufficient
heat (though this seems unlikely with gas and compressed air); what
heat it can supply is possibly leaking away. Since stainless is such
a poor heat conductor, pay attention to conserving what heat you
have: put fire bricks around the joint so as to direct the heat onto
the joint area.
Is it a fluxing issue? An alloy problem? Do I need to direct the
heat differently? Is it contamination of some sort? From the torch
perhaps? Do I need to prepare the surface differently? I
chemically cleaned and sanded the joins. Is that enough?
Things to think about when soldering stainless
1 work fast, the longer you have the work at soldering temps the
worse the oxide layer will be and the less likely your flux can do
the job. So you may need a larger tip on your torch for more heat.
2 Use a boron modified paste flux also known as black flux. The
boron aids in high temperature oxide removal on stainless. It is
almost required to get a good joint on stainless.
3 stainless is a crappy thermal conductor so you need to directly
heat the area you are trying to solder rather than the whole item.
This also makes the flowing of longer joints a problem.
The trick to successfully silver soldering stainless steel is,
indeed, the flux. Here in the U.S. We get black flux, which is a
high heat flux that controls the oxides, at welding suppliers and Rio
Grande. So, if you cannot find it at a local welding supplier, you
could order it from Rio. I learned this from Munya Upin, who learned
it from Arline Fisch. (In conversations, Jim Binnion has confirmed
and agreed with this to me.)
