"What do arn &
steel wire look like ?" you may ask ...
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The pictures below were all made with transmission electron microscopes at the U.S. Steel Fundamental Research Laboratory (now defunct, sadly) in Monroeville, Pennsylvania. The magnifications are about 10,000 times at the original image size of three by four inches. They're about 2-1/2 times that on an SVGA screen. The specimens were made transparent to electrons (1/10000 millimeter thick) by electropolishing. I owe a great deal to Dick Glenn for teaching me how to make thin foils like these, and I thank Chuck Spangler and Bob Sober for making the thin foils that are pictured here.
This is piano wire, wire drawn to a true strain of 3.2. That means the wire has been elongated to nearly twenty-five times its original length, its diameter reduced by a factor of nearly five, and its strength increased by a factor of square root five (2.24 times). Actually, this particular example is a special one in which I caused the pearlite lamellae (layers of soft ferrite and hard iron carbide) to be oriented nearly parallel to the long axis of the wire. I did this by transforming the wire from austenite (the high temperature phase used in the hardening of steel) to pearlite progressively along the length of the wire by pulling the wire through a steep temperature gradient. The picture shows a transverse section, looking down the axis of the wire. Everything you see here is an end-on view of a tiny fiber, either ferrite or iron carbide. The strange, swirly structure is a consequence of the crystal structure of the ferrite, whose grains want to be come ribbon-shaped as they are stretched. They don't fit together very well that way, so the ribbons wrap around each other. The iron carbide layers are trapped within the ferrite grains, so they get swirly too.
Click on the picture to get a high resolution view. This will take a long time to load, but you can then use an image-editing program to crop sections of the image to see or print them more clearly.
This is arn wire, drawn to a true strain of 4.0 (fifty-four times its original length). It doesn't get strong as fast as piano wire, but it has the property of work hardening essentially forever, at least to a true strain of ten (twenty thousand times its original length). The rate of hardening is linear in true strain, about 20,000 pounds per square inch for each unit of strain. This is another consequence of the crystal structure of ferrite and its tendency to want to become ribbon shaped as it elongates. The curly grain structure isn't so evident here as it is in piano wire, because another wonderful process called dynamic recovery causes the fibrous grains to consume each other, so at the end there are fewer of the cell-like islands seen here (all of the material inside each island is of the same crystaline orientation) than there were grains in the original microstructure.
Again, click on the picture to get the high-resolution version.
Here the strange stuff is oriented pearlite again, but this time elongated in plane strain, which is similar to rolling, except that it uses a rectangular die instead of rolls. The material is shaped like a ribbon and is elongated by making the ribbon thinner and longer, but always keeping it the same width. Again, the stuff confounds us by having an internal shape change that only barely resembles what is going on outside. There are regions of the metal which get thinner & longer OK, but the various regions tend to slide past each other discontinuously. The boundaries of discontinuity are called shear bands, and one of them in the above image has collapsed into a shear crack, which ruins the strength and ductility of the product. Let this be a lesson to the blacksmiths pounding on cold metal: if you keep the stuff round while you stretch it, it will stay together a whole lot better than if you flatten it as you stretch it. Done hot, the metal doesn't have these limitations because the crystals keep reforming themselves by a process called recystallization, which preserves the soft state of the metal. The image above is an edge section, so you're looking across the width of the strip.
As before, click on the image to get the high-resolution version.
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