Thermally Induced BCC to FCC Transformation in 1080 Steel Wire

Location:

Concepts Shown:

steel characteristics. To demonstrate thermal expansion and thermally induced crystal structure changes in eutectoid (1080) steel.

Equipment:

steel "piano" (1080) wire (d = 0.031-0.049 microns), fire resistant wood, black fire retardant paint, 2 0.25" x 4" steel bolts, 10 0.5" bolts 2 0.5" wing nuts, 8 washers, extension cord, transformer (see attached). Cost without transformer: $40.

Procedure:

Construction: The basic setup is a wire stretched between two terminals. Use two 1' x 4' sheets of 3/4" plywood (fire retardant), and put them in an L shape, using a 2" x 4" to support the back of one. Drill two 1/2" holes 1" square from each top corner. Cut the female end of the extension cord off, strip about one inch off. Put a washer on the bolt and wrap the wire below the washer. Place the bolts through the holes and lock each down with 6 bolts. Runt he steel over top of bolt and lock down between two washers with the wing nuts. (White strips in the middle of the board as a gauge will help to show the wire sag. A flag hung on the wire in front of the strips also helps).

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Presentation:

Plug the extension cord into transformer and turn the transformer up to about level one and let the wire sag. Shut the transformer off after about 5 seconds. This will show thermal expansion. Turn the transformer back on and turn it up to level two and let the wire sag. Let the wire go through the structure change. This is apparent when the wire, as it sags, suddenly pulls back up an inch, and then continues to sag further. Let the wire sag for a few more seconds, and then shut the transformer off. The wire will go through the reverse processes. As it cools, though, the jump that occurs during the structure change is much more apparent.

Science:

As the current from the transformer runs through the steel wire the resistance inherent in the wire causes energy in the form of heat to be released. As the wire heats more the molecules become more excited and farther apart, thus thermal expansion occurs. When the temperature exceeds 800 degrees C the wire proceeds through its crystal structure change from BCC alpha ferrite to FCC austenite. When the crystal structure goes from BCC to FCC the packing factor (percentage of space filled by atoms in a unit cell) changes from 0.68 in BCC to 0.74 in the FCC. This means that instead of 32% of the space in the unit cell being called "free space", it is only 26% free space. Thus this causes the density to increase in the FCC structure and causes the wire to tighten, which causes the flutter seen in the wire.

This transformation can be seen more clearly though the use of a density vs. temperature diagram. As the density decreases the length of the wire increases.

As seen in the diagram, the density in the alpha-ferrite region decreases until it the temperature reaches 727 degrees C. The wire then jumps back up as the structure goes to FCC. If you look even further on, there is a region called delta-ferrite. It can not be obtained in lab experiment, yet it has a BCC structure, so at 1450 degrees C the density jumps down. This delta-ferrite is only formed in hypoeutectoid (below 0.8% carbon) steels.

A phase diagram used in conjunction with the experiment will help show students the importance of studying the processing of steel (see attached). As well it will help to make them aware of heat in design considerations.

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Other: Before the first demonstration run the experiment through so that the wire is annealed and quenched once. This changes the microstructure slightly, and allows the experiment to be more dramatic.

Level one - a voltage that makes the wire barely glow red. Make sue that the wire doesn't go through a phase transformation.

Level two - a voltage that causes the wire to sag and go into a phase transformation. This is apparent when the wire suddenly pulls back up an inch or two from it�s sagging.

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