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
10 Tempering. Steel is tempered by heating it to a temperature below the transformation range and then allowing it to cool at a controlled rate. The purpose of tempering is always to improve the toughness and ductility of steel which has been hardened by quenching, or even by normalizing. Usually tempering reduces the hardness of the steel, although some alloy steels can be tempered without loss of hardness. Tempering is seldom applied to the low- carbon steels. Surface Hardening. Several processes are used in industry to produce a hardened surface on a tough, unhardened steel core. Some of these are carburizing processes, whose aim is to increase the carbon content, and therefore the hardness, of the steel surface. Flame-hardening, which will be covered in more detail in a later chapter, has a similar aim, but does not depend upon a change in the carbon content of the surface. Surface- hardening processes are applied to finished parts, usually forged or machined, and hence are of no direct interest to a welder. Hardness vs. Hardenability In closing, we’d like to call attention to the distinction between two terms widely used in talk about steels: hardness and hardenability. To a considerable degree, the hardness of a carbon steel depends on its carbon content; any high-carbon steel is naturally hard, no matter how it is cooled. It is usually heat-treated to give it the best combination of hardness and toughness required for a particular application. Hardenability usually refers to the ability of alloy steels to form martensite through heat treatment, and thus acquire higher strength. This ability is often not directly related to carbon content; to put it another way, an alloy steel with relatively low carbon content may be more hardenable than another alloy steel containing two or three times as much carbon.