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The Structure of Steel

In opening Chapter 7, we said that one of the properties of a metal was a crystalline structure. This means simply

that the atoms in the solid are arranged in regular, repeating patterns. The smallest group of atoms which defines

the atomic arrangement in a crystal is termed a crystal lattice. Many different lattice structures are recognized. In

pure iron, however, there are only two forms, one called the body-centered cube, the other called the face-centered

cube. They can be represented like this:

In a liquid, there is no lattice pattern. All the atoms of the liquid are in constant, irregular motion. When the liquid

has been cooled to its freezing temperature, crystals start to form. In the case of pure iron, at a temperature of

1530 0 C, something causes nine atoms to get together in the shape of a body-centered cube (one atom at each

corner and one in the center) and other atoms then start to repeat the pattern around the cube. Hundreds of

crystals start growing at about the same time. However, because each atom must give up its energy of motion as it

joins a crystal, freezing cannot take place instantaneously. The mass of liquid must pass that surplus energy on to

its surroundings, and that takes time. While that energy is being lost, competitive growth among the many crystals

is taking place. Thus, when freezing is complete, and every atom of liquid has become part of a crystal, the crystals

themselves are not arranged in any kind of an over-all pattern. Let pure iron freeze and cool, then cut it, polish and

etch a cut surface, and examine that cut surface with a microscope. We can then make out an irregular collection

of what are termed grains. Each grain is essentially a single crystal.

Continued on next page...

Face Centered

(14 atoms)

Body Centered

(9 atoms)