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