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

9 Types of Heat Treatment The four terms most frequently used in connection with the heat treatment of steel are annealing, normalizing, quenching, and tempering. We’ll take them up one at a time. Annealing. Steel is annealed by heating to a temperature above the critical (733⁰C), so that all the iron will return to the face-centered form called austenite, and then allowing it to cool slowly through the transformation range (from just below to about 50⁰C below the critical temperature). The heating part of the process is termed austenizing; the precise temperature to which the steel is heated, and the time it is held at that temperature, are both carefully controlled. Annealing is almost always performed at the steel mill. Sheet and strip steel are usually annealed to reduce the hardness acquired in rolling, and to make cold forming easier. Annealing is also done to improve the machinability of steel. By precise control of the heating and cooling cycles, several different types of final structures can be achieved, ranging from the lamellar (where the carbon is in grains of pearlite) to the spheroidical (where virtually all the carbon winds up in small particles of cementite between grains of ferrite). Process annealing is a different process, performed for a different purpose. It is applied to parts which have been undesirably hardened by cold forming, to remove stresses locked up in the metal by the cold forming and to restore the ductility of the steel. Normalizing. Normalizing is a simpler process than annealing. The steel is heated to a temperature above the transformation range (that is, above the critical) and then allowed to cool in still air. (In annealing, cooling usually takes place in an annealing furnace, under precise control.) Normalizing may be performed to make steel softer (after casting, forging, or cold-working), or to make it harder (after annealing), or merely to improve the grain structure (in castings, for example.) Quenching. This is defined as the rapid cooling of steel from an elevated temperature, usually by immersion in oil or water. The purpose of quenching is always to harden the steel; usually that implies the transformation of the steel to the martensitic structure previously described. Quenching is usually followed by tempering.