Handbook-Structure of Steel

Hard- Surfacing, Building Fusion Welding Carbon Welding Non-Ferrous Metals Heating & Heat Treating Braze Welding Welding Cast Iron Welding Ferrous Metals Brazing & Soldering Equipment Set-Up Operation Equipment For OXY-Acet Structure of Steel Mechanical Properties of Metals Oxygen & Acetylene OXY-Acet Flame Physical Properties of Metals How Steels Are Classified Expansion & Contraction Prep For Welding OXY-Acet Welding & Cutting Safety Practices Manual Cutting Oxygen Cutting By Machine Appendices Testing & Inspecting

1 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 1530⁰C, 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) The Freezing of Pure Iron