Should music wire be heated before bending?
From : Don Stackhouse
Regarding the recent discussions on RCSE of the properties of music wire, I
can add some hard data.
Music wire is AISI 1060 steel. This means that it is an alloy of iron and
carbon, with no additional significant alloying elements. The carbon
content is 0.6%, which puts it in the heat-treatable category. Music wire
is NOT heat treated in the usual sense; instead, it is "work hardened", a
natural side effect of the process of drawing it into wire.
When most metals are "cold worked" (i.e.: formed into a new shape while at
a low temperature), changes occur in the metal's chrystal structure that
mimic the effects of heat treating. The wire is made by pulling it through
diamond dies of progressively smaller diameter, with each step in the
process squeezing the diameter down just a little smaller. The diameters in
the American Wire Guage (AWG) diameter scale correspond to the diameters
that the diamond dies get opened up to during re-sharpening, to make the
next larger size in the scale.
Eventually the accumulated cold-working has hardened the steel so much that
it can't safely be drawn any smaller (all the ductility has been used up),
so the wire is annealed (heated above about 1500 deg. F and slowly cooled)
and the process is continued until it needs to be annealed again.
Music wire and piano wire are the same thing, except that piano wire is
annealed even fewer times during the process than music wire, so it tends
to be slightly harder. Music wire is cold-worked to an extreme extent, so
that the resulting hardness is higher than even some heat-treated tool
steels, and also significantly harder than what you would normally get from
conventionally heat treating 1060 steel. This is why music wire can put
such nice round dents in the cutting edges of your diagonal cutters. At
diameters above approx. 1/32" the tensile strength is about 320,000 psi,
and at diameters less than 1/32" the tensile strength is around 360,000 psi.
The downside of this is that there tends to be a lot of internal stress
built up in the material, so the metal tends to be somewhat brittle if
severely hardened this way, and it also tends to lose much of this
hardening effect at a relatively low temperature, perhaps as low as 300-400
degrees Farenheit. This is why you need to be careful about soldering music
wire, chances are that it will lose most of its hardness within the "heat
affected zone" of your soldering operation. Once softened, you will not be
able to get all the hardness back by heat treatment, even if you have the
right equipment for that, because it will tend to crack. The reason it can
be safely hardened that much by cold-working without cracking problems is
because it is under severe compressive stress during the forming process,
which keeps the cracks from starting.
The bottom line is bend it cold, don't try to straighten it back out after
bending (there is usually just enough ductility left after forming it to
allow it to be significantly bent ONCE), and don't solder or braze it near
any sections that will see high stresses in service.
Don Stackhouse @ DJ Aerotech