Treating Music Wire
by Roy Vaillancourt
About the author:
Roy Vaillancourt is the president of
a part time venture, catering to giant scale enthusiasts. Roy and his
wife, Nancy, have operated Vailly from their home since 1986. Roy, whose
specialty is giant scale warbirds, is an active scale contestant who has
competed regularly at the Top Gun and Scale Masters Championships for
the past ten years. He is a frequent contributor -- on a variety of
topics -- to the major scale modeling magazines and to our own
Articles and Tips department. A
mechanical engineer with two advanced degrees and over thirty years
experience in design and manufacturing, Roy works full time as a senior
design specialist for
The music wire used by sailplane 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.1
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.
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 annealed2
at the location to be bent. To anneal it, heat the wire with a torch until
it becomes a bright cherry red -- about 1400 degrees Farenheit. Let it cool
completely to the touch. Don't quench3
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 into 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 heat it
gradually with the torch. Watch for the following colors as a guide: straw
color (350 degrees), followed by dark blue (600 degrees), and then 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 be 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 this 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.
hardness testing, named after Stanley Rockwell who made his first
testing machine in 1921, is a general method for measuring the bulk hardness
of metallic and polymer materials. Although hardness testing does not
measure performance properties, hardness correlates with strength, wear
resistance, and other properties.
hardness testing is an indentation testing method. An indenter is impressed
into the test sample at a prescribed load to measure the material's
resistance to deformation. A Rockwell hardness number is calculated from the
depth of permanent deformation of the sample after application and removal
of the test load. Various indenter shapes and sizes combined with a range of
test loads form a matrix of Rockwell hardness scales that are applicable to
a wide variety of materials. The Rockwell B and C scales are used for
metallic substances. (Back to
2. Anneal: To heat and then cool (as steel
or glass) usually for softening and making less brittle.(Back
3. Quench: To cool (as heated metal)
suddenly by immersion (as in oil or water).
(Back to text.)
information about working with metals in general and about hardness testing
in particular consult these books by noted metallurgist, Harry Chandler.
Hardness Testing, 2nd Edition
Harry Chandler, 1999.
Metallurgy for the Non-Metallurgist
Harry Chandler, 1998.