Heat Treating Music Wire

[from July-August 2001 newsletter]

by Roy Vaillancourt -- submitted by Bill Melcher

The music wire used by sailplane modelers (and R/C modelers) to make landing gear and cabin struts is medium carbon steel heat treated to spring temper or about 45 on the Rockwell C scale of hardness (RC45). On this scale, RC20 is soft, RC45 is tough, and RC60 is hard. Tough wire can be bent and cut using the proper tools and techniques, but sometimes it's just too difficult to work with.

One way to soften steel music wire is to heat it, which makes it easy to bend and form. But after heating and forming, the subsequent cooling, often at an uncontrolled rate, can make the finished wire too hard or too soft since its hardness is determined by the rate at which it cools. For some parts, the final hardness isn't critical. But a landing gear formed from wire softened too much won't spring back to its original position; and a gear made from wire cooled to a harder than normal state will snap on its first use. To restore the wire to its original specific spring temper, it must be heat treated a second time and cooled at a controlled rate.

Three Steps:

To form wire easily, first anneal it; next, form or bend it to the desired shape; and then heat treat the part back to spring condition, that is temper it.

First the wire should be annealed at the location to be bent. To anneal it, heat the wire until it becomes a bright cherry red, about 1400 degrees Fahrenheit. Let it cool completely to the touch. Don't quench it or blow on it. Just let it cool naturally away from any drafts. The wire should now be in the RC25 soft range, and it will bend easily. After forming, once again heat the wire with a torch until it becomes bright cherry red, but this time quench it--that is, cool it rapidly by immersing it in room temperature water. Plunge the steel into the water with a twisting, swirling motion to keep water vapor from insulating the wire against the cooling action of the water.

At this point the wire should be very hard, probably above RC60. To test the hardness, try to make a mark on the worked area with a file. The file should slide off without cutting the steel at all. If it cuts the wire, try the heat and quench cycle again. If the file still cuts the wire, it isn't high carbon steel. Get another piece of wire and start over--you won't be able to add the necessary carbon to low carbon steel. When the file test signals success, the wire is ready for the final step, but not for use, because it's very hard and quite brittle, and will probably snap off.

The final step is to temper the wire back to the desired hardness. Tempering is a form of annealing

but is controlled so that the steel achieves a specific hardness. Start by sanding the wire with steel wool or emery cloth. Then beat it gradually with the torch. Watch for the following colors as a guide: straw color (350 degrees), followed by dark blue (600 degrees), and the medium blue (750 degrees). At this point, remove the wire from the heat and allow it to cool slowly. Don't quench it or blow on it?, just let it cool naturally in still air. Once the steel returns to room temperature, it should at the target RC45 hardness, which has a good spring temper. Try the file test again. You should be able to make a mark now, but only with some effort. If it passes the test, the wire is properly tempered.

Besides parts for model planes, tempered music wire can also be used to make special purpose tools. Instead of tempering to 750 degrees (medium blue), stop at the straw color stage. The wire will be at about RC60, which is still very hard, but not brittle. Wire at this temper can be used to drill wood and plastics, and most aluminum and copper.

Notes:

1. Anneal: To heat and then cool (as steel or glass) usually for softening and making less brittle.

2. Quench: To cool (as heated metal) suddenly by immersion (as in oil or water).