I'd like to start doing some low temperature OA whuzzit fixits. Just enough
to get some bonding metal to hold the thing together, max working pressure
400 psi. Like soldering, but just a little stronger. Is there a guide for
me to flux coated wire electrodes that might help me with my selection? I
figure that a supply of four different fluxes, and four different diameters
would cover most situations, and most probably a lot less than the 16
permutations you get from those variables.
If you had to pick four rods and flux combos to cover 75% of common
situations, what would they be?
If by "low temperature" you mean at less then the melting point of the
parent metal then you are probably talking about brass, bronze or
You can but flux coated brass rods and I believe bronze but I think
that all silver comes as bare rod, or wire.
Flux for silver depends on the parent material and you often require a
special flux for stainless while a silver rod made for copper usually
contains phosphorous (I think) and is self fluxing.
Interesting approach, but not workable.
You can "solder" using tin, lead, or silver alloys.
Low temp soldering on copper and steel is done with tin or tin/lead
There are acid paste fluxes, but the liquid zinc chloride flux works
the best. The solder is sold in 1 lb. spools of different diameters,
depending on the scale of the work you are doing.
You can also get "acid-core" solder that is a hollow tube with the flux
inside. Hobbyists use it, professionals don't.
Be careful of the fumes, they can be nasty.
These alloys of solder are intended for soldering copper water pipe
using the pure Tin alloys or steel sheet metal using the Tin/Lead
Next up in strength are the low content silver solders known as
"silver-bearing" or "soft" solders. They are still mostly tin or
tin/lead, with enough silver to increase the strength while still
allowing them to flow at temperatures below 700 degF.
These can be used for higher strength repairs than the tin or tin/lead
Brass, bronze, copper, and steel can be "brazed" using LF (low fuming)
bronze, and a flux. The rods can be purchased pre-coated with flux, but
these have a short shelf life as the flux will absorb ambient moisture
and break down. The better way to go is to purchase 1 lb. cans of flux.
There is a standard flux used for most applications and a high
temperature flux used for brazing cast iron.
Warm the end of the rod with the torch, and dip it into the powdered
flux to adhere it. Reapply flux at regular intervals as you braze.
High temp soldering is done using high content silver solders (aka
"hard solders") and is often referred to as "silver brazing". This is
used for gun parts, knife hilts, and most silver jewelry. The silver
solder can be ordered in many different temperature ranges so you can
solder multiple times on the same part by moving down in temperature
each time. The 3 main ranges are Hard, Medium, and Easy. Hard silver
solders use a higher temperature flux than the silver bearing solders.
I like the Sta-silv product line of paste fluxes for work on copper
alloys, steel and stainless steel.
For actual jewelry work Batters flux works very well.
Be careful with the hard silver solders. Silver is a much better heat
conductor than copper, so don't try holding the end of the solder with
your bare hand, use pliers or forceps.
Applying solder with a long length of wire is good for low temp work,
but for high temp solder I prefer "chip" soldering where short pieces
of solder are snipped off and applied to the joint using a small brush
dipped in flux. The entire area is then carefully heated to first dry
out the flux to a white crust, and second to melt that crust and the
solder until it flows. This is a tricky task and tiny solder chips will
often pop off the joint as you boil the water out of the paste flux.
Solder can only flow where there is flux removing the oxides.
There are also aluminum solders. The simplest are the "miracle" rods
which are mostly zinc. they can work for simple low strength repairs on
many metals, but they have no real strength and the joint is quite
brittle. Handy for patching a hole in a aluminum canoe or water tank.
Higher strength aluminum solders such as Alstate #30 comes with a
matched flux. It is used mostly for aluminum refrigeration, and air
You need to get the work quite hot to braze with brass - this can
sometimes just about be done with a propane/air torch, or better a
mapp/air or mapgas/air torch, but it can be a real problem if you don't
have a big enough torch, and/or a large selection of torches.
Silver soldering, also known as hard soldering or silver brazing  is
done at a slightly lower temperature, about 670 C instead of 950 C, and
is a good bit easier as far as torch selection goes .
Soldering with propane/air is eminently practible for the lower
temperature silver solders, and possible even for the higher temperature
ones, though MAPP/air or mapgas/air is better there.
The OP was talking about OA, which doesn't really have that problem -
and even oxy-propane is okay, for both brazing and silver soldering, if
the work can't take the heat of brazing.
Oxy-propane can be very useful when getting hold of acetylene is a pain
and you don't need it to weld with.
Silver solder is a lot more expensive than brazing brass (which isn't
exactly cheap either) as it contains a goodly bit of silver, but it is
plenty strong enough for almost anything at 180 MPa shear 400 MPa yield
500 MPa UTS in as-soldered state (that's like a good mild steel, and
with three times the shear and yield strengths of as-brazed brass), and
for small jobs it is still affordable.
It's a bit too expensive for large jobs though - not so long ago I
repaired the mounting of a washing machine drum, used about $25 worth as
I had to go all around the mounting. Wouldn't want to use silver solder
for anything much larger, unless it was already expensive stuff, eg
joining LOX pipes.
Silver soldering stainless can be a little tricky, but really you just
need the right materials. In the UK attention is focussed more on the
flux, while I believe in the US more attention is placed on the solder,
but really just get a combination which is meant for stainless and
experiment a little, it isn't hard if you use the right stuff.
 the names vary. I mean joining with a filler containing 56% to 24%
silver and a melting point of 620 to 780 C.
When they banned lead in water pipe joints a new form of solder was
introduced containing 3-5% silver, a smidgin of copper and the rest tin,
with a mp of about 220 C. Similar solders are used in electronics after
RoHS. while both are sometimes called silver solder, they are soft
solders with only 5% of the strength of the hard silver solders (25 MPa
vs 500 MPa) and they are not at all what I am talking about.
There are about four different formulae for hard silver solder in common
use, with melting points differing by about 50 degrees, so you can step
braze: solder using a high temp solder then solder again with a solder
with a slightly lower MP and not melt the first joint - step brazing can
also be done using diffusion brazing, but you'll need an accurately
electronically temperature controlled furnace to do that one properly.
There are also cadmium-bearing silver solders, which are slighly
cheaper, have slighly lower mp's, flow better and which wet some things
better (but others worse); but they have recently been banned in the EU,
and I never much liked using them anyway.
 At these temperatures most of the heat is lost through radiation,
and the radiant heat loss goes up as the fourth power of absolute
temperature, so to go from 670 C to 950 C you need to overcome nearly
three times (2.83744 x) the heat loss.
Also, the difference between the flame temperature and the work is less,
so the transfer of heat from flame to work is less (about half as
effective), so overall to reach 950 C in the work the flame has to be at
least six times the power needed to reach 670 C.
That's some of the theory - in practice you'll probably need even more
torch power, say ten times, as you need to reach temperature in a
reasonably short time.
For OA or oxy-propane the difference in power needed is less, as the
flame is hotter and therefore the difference between flame temperature
and work temperature is higher, so heat transfer between flame and work
is more efficient. Recommended for brass brazing, or the higher
temperature silver solders.
-- Peter F
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