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

3 This alloy starts to freeze at a temperature of about 1500⁰C. Crystals of almost pure iron form in the face-centered pattern. (In pure iron, the crystals first formed in the body-centered pattern, but changed over to the face-centered pattern at a temperature a bit above 1500⁰C.) Most of the carbon atoms keep moving around in the liquid until a lot of the iron has crystallized. As the temperature drops to a bit above 1389⁰C, all the liquid disappears. The carbon has gone into what is termed ”solid solution” in the mass of iron crystals. Apparently the carbon atoms lodge themselves in the centers of the relatively open face-centered cubes. They remain there until the temperature of the solid metal has dropped to 723⁰C (1333⁰F), the transition temperature for almost all iron-carbon alloys. (The transition temperature for pure iron was much higher: 910⁰C.) At this temperature, the iron crystals change from the face-centered form to the body-centered form. In the body-centered form, there’s not enough room for a carbon atom. What happens to the carbon? Most of the carbon atoms link up with iron atoms to form a crystalline compound called iron carbide (Fe₃C). If cooling is slow, this iron carbide forms plates between plates of pure body- centered iron. If we cut, polish, and etch a specimen of this steel after it has cooled completely, and examine it under a microscope, we find that it is made up almost entirely of grains of pearlite. Each grain of pearlite has a layered structure, with layers of iron separated by layers of iron carbide.