Lab Equipment Design Incorporating Safety for the
Demonstration of Phase Changes in Steel Wire
Author: Brian M. Harris, Student, Mechanical Engineering Technology
Affiliation: Oregon Institute of Technology Seattle Campus
Author: Casey A. York, Student, Mechanical Engineering Technology
Affiliation: Oregon Institute of Technology Seattle Campus
Advisor: John W. Bridge, PhD, PE Program Director and Assoc. Professor
Affiliation: Oregon Institute of Technology Seattle Campus
Introduction
Previous demonstrations of the phase change that occurs in spring steel—as described in an
article by Jearl Walker published in Scientific American —have been conducted in open air
environments and often used screws and washers to hold the spring steel 1085 piano wire during
testing (2). This design differs by addressing several safety concerns with running these
experiments, with the top two safety concerns being electrocution and burns. This design also
seeks to include gauges as well as an infrared thermometer in order to extract more
engineering/material science information for students, and pressure clamps to hold the wire in
place during testing. The experiment requires a variable autotransformer intended to
conveniently vary the output voltage for a steady AC input voltage. The variable
autotransformer, or variac as it is also called has a rheostat for adjusting the voltage being
applied to the wire. Due to overall circuit resistance the wire heats quickly and will continue to
increase in temperature as the rheostat is turned up.
Experiment Trial Setup and Testing
The goal of the trial setup is for proving equipment operation and wiring of selected components.
Demonstrations include the phase change from BCC to FCC in the test wire, the loss of
magnetism in the test wire, and a quench hardening of the test wire. Initial experimentation has
been conducted on a setup consisting of a variac, wood blocks, screws and washers, a repurposed
power chord for a desktop PC, a two foot wooden dowel with a magnet attached at one end,
gloves, a drip pan, a sponge, and galvanized test wire .032 in. thick. The wood blocks were
spaced 3 feet apart and attached to a wooden base. The power chord was stripped to expose the
wires, which were then stripped at the ends and soldered into a u-shape for leads from the variac
to the test wire. The screws and washers were used to secure the leads and test wire between the
posts. This setup worked well as a proving ground to test components and configurations
quickly, as well as different thicknesses of wire.
In selecting a size of test wire for final lab experiment testing, the following concerns arose. If
the thickness were too small it is possible that air quenching of the material would occur.
Conversely if the thickness were too large two things are likely to occur. One concern for thicker
test wire is the voltage applied would need to be higher and while the variac is capable of
producing the required amount, the concern is that running the variac at a higher voltage reduces
the unit’s life. The other concern with thicker test wire is in locating and acquiring the material,
as with increasing thickness so increases the wire cost and above 0.063 in. it is not commonly
stocked. Wire sizes selected for testing in this experiment are 0.063 in. and 0.045 in. which are
easily acquired and not expected to be greatly affected by air quenching, nor require excessive
voltage for heating.
Test 1
Initially we measured the resistance across the complete setup using a 0.032 galvanized wire to
be approximately 9Ω. With the variac turned up briefly to 15 volts the wire sagged and was
visibly orange. Next the variac was quickly turned up to 30 volts which produced arcing inside
the variac. It is suspected that downward pressure on the variac rheostat caused the arcing and as
such should be avoided in follow on testing. With the variac turned up a final time to 25 volts the
wire exhibited a total loss of magnetism. After tuning off the variac the wire was quenched at its
left end until it was at room temperature. This was done using a wet sponge while wearing a
leather welding glove. After the wire was removed it was bent in the area where quenched,
however the wire did not break. It is suspected that the galvanized wire was of the low carbon
variety and in conjunction with unknown alloying elements we believe this attributed to the wire
not breaking.
Test 2
This test used 0.063 in. thick 1085 piano wire in addition to hold down clamps instead of screws
and washers to secure the wire as shown in figure 1 below.
Figure 1. Trial experiment setup.
The resistance across this setup measured 7Ω. With the variac at 15 volts the wire was mildly
orange in color, and there was an audible resonance coming from the heated wire. When turned
up to 20 volts the wire was glowing brightly orange. At 25 volts the wire exhibited a complete
loss of magnetism. After turning off the variac the wire was quenched to room temperature.
After being removed, when bent the wire did break in the area where quenched. The broken
piece of hardened wire was able to scratch the surface of glass.
Many more tests were conducted using the wooden block setup with both 0.063 in. and 0.045 in.
thick 1085 wire to validate parts upon arrival for inclusion in the overall circuit. From this testing
it was determined that gradual progression in the voltage applied produced the best results
overall. The testing also showed there were definite improvements needed for the safety of the
user of this experiment.
Safety addressed in the design
The safety concerns of this experiment are the potential for electrocution due to the electrified
test wire, and burns due to temperatures in excess of 700 degrees Celsius. Further concerns are in
assuring the wire is not energized while quenching occurs.
The criteria for the final design were as follows:
1) Incorporate safety in the design to reduce or eliminate the risk of electrocution or burns
from occurring.
2) Demonstrate the material relaxing and retracting as a phase change occurs in the wire. It
is required to be able to repeat this demonstration several times without replacing the
wire.
3) Demonstrate the loss of magnetism in the material with the use of a magnet when the
Curie temperature is reached or exceeded.
4) Demonstrate the martensitic change in the material due to water quenching while the wire
is at or near the Curie temperature. The wire must not be energized during quench. After
quench it is shown that in the area quenched the wire can be broken as it has become
brittle, as well as its ability to scratch glass due to its increased hardness.
Safety concerns in previous designs are addressed to meet the current design criteria for user
safety when operating the experiment. With the primary risk being electrocution we felt it was
necessary to create an enclosed environment for the experiment to be conducted in. This is
accomplished using a clear polycarbonate front cover. This cover provides a barrier between the
user and the experiment while still allowing many students to view the experiment being
conducted. Polycarbonate exhibits high mechanical properties, namely it has a high impact
resistance and will thus provide for a long life for the cover. Polypropylene was selected for the
remainder of the box and exhibits both chemical and impact resistance. The clamps selected to
hold the wire inside the box are capable of applying 90 lbs. of clamping force, which ensures the
wire will not come loose during testing when the clamp posts are properly adjusted. The clamps
also make it relatively easy and quick to load and unload wire for each experiment.
Safety concerns are addressed in the demonstration of the Curie temperature by using a magnet
attached to a two foot wooden dowel. The magnet is inserted through a three inch by three inch
cutout in the front cover which keeps the user at a safe distance while using a non-conductive
device. The size of the cutout in the front cover allows for safe testing while the wire is
energized while also making it difficult to fit a person’s hand through the opening. From this
cutout users are also able to use an infrared pyrometer to get approximations as to the
temperature of the material with a given amount of current running through the wire as shown by
the analog voltage and ammeters installed on the back wall of the lab equipment.
A criterion of the experiment is to perform a quench on the wire. This aspect of the experiment
can be very dangerous in the case that the wire is still energized while the user performs the
material quench. In thinking about human factors design we decided the best way to show the
wire is not energized was to include in the circuit an LED indicator light in the area where the
quench will be performed on the wire. This allows the user to clearly see there is no power to the
wire when the indicator light goes out. The indicator light is wired to a safety switch that is
mechanically attached to the front cover. When the front cover is opened by approximately 15
degrees the safety switch disengages power to both the receptacle where the variac plugs inside
of the box, and the LED indicator light. This way quenching can be performed by simply
opening the front cover. While the user wears a leather glove, a wet sponge can be used to
quench the wire. Keeping human factors design in mind we also decided to include the handle to
open the clear front cover on top over the left side of the wire where the indicator light is located
and quenching is to be performed. With the enhancements as previously described, all in all we
feel this design will provide a safer experience for students and instructors using this lab
equipment.
Wiring for Safety
A final wire size of 10 gauge stranded was selected as it will be capable of conducting the
maximum output of the variac. Initially while testing prior to the final wiring of the poly box, we
wired the safety equipment so as the receptacle inside of the box is controlled by the opening and
closing of the safety switch using the following wiring path:

Neutral lead and ground from pre wired plug attached directly to receptacle. Hot lead
from prewired plug to NC position 21 in safety switch. Two wires then lead out from
position 22 in safety switch with one wire out to hot lead side of receptacle, the other
wire from position 22 in safety switch goes out to LED indicator light, and then from
indicator light out to the ground on receptacle completing the circuit.
When the safety switch is activated the power is then disrupted from both the receptacle and the
indicator light as a means of providing confirmation that power is disengaged. However, in
evaluating possible failures in the circuit a problem arose. To alleviate this possible false
indication, changes were made to the wiring path as follows:

Neutral lead and ground from pre wired plug attached directly to receptacle. Hot lead
from prewired plug to NC position 21 in safety switch. A single wire then leads out from
position 22 in safety switch out to LED indicator light. Two wires then lead out from
LED indicator light with one wire out to hot lead side of receptacle, the other wire out to
the ground on receptacle completing the circuit. Figure 2 shows the initial path before
final wiring using this configuration.
Figure 2. Preliminary wiring configuration.
The main change from before is we moved the LED from being a secondary path to being in line
with the remainder of the circuit. This change provides a safer environment for the user. This
change eliminates the possibility for a false indication that there is no power flowing to the test
wire, and provides a positive indication of a safe user environment.
Final Design Testing
The final experiment operates as designed. The variac plugs into the receptacle inside the
enclosure, and in order for the variac to operate the front cover must be fully closed which allows
current to run through the safety switch. During operation, when the front cover is opened the
safety switch disengages power to the receptacle and LED indicator thus shutting off the variac
while the cover is opened. The variac dial should then be turned to zero and the power switch
turned off before closing the front cover again to prevent surge voltage to the system and
equipment. While testing the wire in the enclosure the amperage and voltage can be read from
the analog panel meters installed on the back wall of the box. Magnetization and temperature
tests can be safely done from the cutout in the front cover of the enclosure.
Issues occurred in the final installation of components within the poly box designed for the
experiment. Modifications had to be made to the front cover and the base of the box to allow for
a full range of motion in the safety switch lever arm that attaches to the front cover. Initially it
was overlooked that due to the rotational axis of the front cover and that of the switch lever not
being coincident, interference would be caused by the safety switch lever arm when the front
cover was far less than fully open and allowed to hang down in front as designed. In order to
overcome this design issue, modifications to the cover and base were made to allow for
clearance. Also the location of the switch was moved so as the axis of rotation of the lever arm of
the safety switch and the rotational axis of the front cover hinge were coplanar. This minimized
the area required to be removed for clearance and due to the length of the guide slot in the lever
arm a full range of motion was now possible. The final box design is shown in figure 3.
Figure 3. Completed experiment.
Several experiments were run to assure all equipment was in working order and behaved as it
was designed for, and from which no problems were found with the final configuration. This
final product meets all of the initial criteria for design and experimentation, and provides a new
level of safety for the user.
Material Changes Observed During the Experiment
Phase Changes
The experiment heats a selected size of 1085 music wire to the point where a phase change
occurs in the material. Initially upon heating the wire relaxes, as the BCC (body centered cubic)
crystal structure changes to an FCC (face center cubic) crystal structure the wire briefly tightens
then relaxes again after the phase changes back to BCC. This change can also be seen during
cooling of the wire. During the experiment this demonstration will be repeated several times.
Loss of Magnetism
The 1085 wire is heated to at or above the Curie temperature which for Iron occurs at
approximately 768 degrees C. At this temperature there is a loss of magnetic properties in the
material. This is shown by touching a magnet to the wire while energized.
Formation of Martensite
The 1085 wire while heated to at or above the Curie temperature is de-energized and quickly
quenched with water. Quenching causes the formation of martensite by preventing the diffusion
of carbon in the material. This makes the material very hard and brittle which will be
demonstrated by breaking a section of the wire by hand and using a portion of the broken section
to scratch a piece of glass.
Material Properties
Piano wire— also known as music wire —is a specialized type of wire which is used in piano
strings, as well as for other numerous purposes. Piano wire is made from tempered high-carbon
steel, and is also referred to as spring steel. For example it is often the case that very hard tools
are tempered at low temperatures, whereas springs are tempered at much higher temperatures.
The 1085 piano wire consists of two crystal states, termed allotropy. As stated by Callister,
“some metals, as well as nonmetals, may have more than one crystal structure, a phenomenon
known as polymorphism. When found in elemental solids, the condition is often termed
allotropy. The prevailing crystal structure depends on both the temperature and the external
pressure. Pure iron has a BCC crystal structure at room temperature, which changes to FCC iron
at 912C (1674 F). Most often a modification of the density and other physical properties
accompanies a polymorphic transformation” (1). This particular material property is exhibited
when the test wire is heated and cooled. Figure 4 below shows a BCC crystal structure, and
figure 5 shows a FCC crystal structure.
Figure 4. BCC crystal structure (1).
Figure 5. FCC crystal structure (1).
The volume density changes are determined based on the materials change from a BCC to an
FCC crystal structure, and Callister goes on to discuss these changes as follows; “For facecentered cubics, the coordination number is 12. The coordination number for the BCC crystal
structure is 8; each center atom has as nearest neighbors its eight corner atoms. Since the
coordination number is less for BCC than FCC, so also is the atomic packing factor for BCC
lower—0.68 versus 0.74” (1). With heating, the wire lengthens, gets taut and then lengthens again
based on these properties.
The loss of magnetism in the wire can be attributed to temperature having an effect on the
magnetic characteristics of the material. When raising the temperature of a solid you get an
increase in the thermal vibrations of the atoms. The effects of temperature are further discussed
by Callister where he says, “the atomic magnetic moments are free to rotate; hence, with rising
temperature, the increased thermal motion of the atoms tends to randomize the directions of any
moments that may be aligned. For ferromagnetic, antiferromagnetic, and ferrimagnetic materials,
the atomic thermal motions counteract the coupling forces between the adjacent atomic dipole
moments, causing some dipole misalignment, regardless of whether an external field is present.
This results in a decrease in the saturation magnetization for both ferro and ferrimagnets. The
saturation magnetization is a maximum at 0 K, at which temperature the thermal vibrations are a
minimum. With increasing temperature, the saturation magnetization diminishes gradually and
then abruptly drops to zero at what is called the Curie temperature” (1). This is due to the mutual
spin coupling forces being completely destroyed. Thus at temperatures above the Curie
temperature, both ferromagnetic and ferrimagnetic materials are paramagnetic. For the 1085
piano wire it is assumed this occurs at or near the Curie temperature of pure iron which is 768
degrees C.
Initially the tempered 1085 piano wire is of an approximate spheroidite microstructure as shown
in figure 6. When steel is tempered for a sufficiently long enough time at a temperature near the
eutectoid spheroidite will form, and appears as sphere like particles of cementite mixed in an
alpha phase matrix. This transformation is due to additional carbon diffusion and yields a
relatively soft material.
Figure 6. Spheroidite (1).
Figure 7. Iron Isothermal Diagram (1). Figure 8. Martensite (1).
When the 1085 wire is quenched at or above the Curie temperature of the material, martensite is
formed as shown in figure 8. As stated by Callister, “martensite is a nonequilibrium single-phase
structure that results from a diffusionless transformation of austenite. It may be thought of as a
transformation product that is competitive with pearlite and bainite” (1). This transformation
occurs at a quench rate that is rapid enough to prevent carbon diffusion as shown on figure 7.
Due to this material change the hardened wire can now be broken instead of bent and will now
scratch the surface of a piece of glass.
Conclusion
The final experiment design functions as planned and meets all of the design requirements.
Safety features incorporated provide the user with an environment in which tests can be
performed without the risk of electrocution due to the inclusion of a safety circuit that will
disengage power when the front cover is opened. Injuries due to burns have been reduced so long
as operating procedures are followed during the material quench. With these additions, an old
experiment can be conducted in a portable, safe environment.
Much was learned during the design and build of this lab experiment. Initial challenges in the
safety circuit and box design were met and overcome, and provided insights for consideration in
future designs. All in all the final product meets every one of the initial design criteria. This
design provides a new level of safety for all users in demonstrating the phase and material
changes in steel wire, and will last for many years to come.
References
1.
Callister, Jr., William D. Materials Science and Engineering an Introduction. 7th ed. York, PA: Quebecor
Versailles, 2007. Print.
2.
Walker, Jearl. "In Which Heating a Wire Tells a Lot about Changes in the Crystal Structure." Scientific
American, The Amateur Scientist. May 1984: 148-155. Print.