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
6 While low-carbon steels exhibit relatively high ductility as measured by the methods described above, they are also subject to brittle failure under some conditions. A structural member may break suddenly when subject to stress which is below the expected yield point of the metal – that is, before any measurable permanent deformation has taken place. Such failure (fracture) always starts at a slight crack or notch in the metal. The ability of a steel to resist this type of fracture is termed notch ductility. Notch ductility is somewhat dependent on the composition of the steel. It is always related to temperature, (all steels lose notch ductility rapidly as temperatures drop below the 0-200C range) and to the grain structures within the steel, especially the structures which are formed as the result of welding. Stress-relieving – the reheating of the weld zone to a temperature of not more than 6000C – is widely used to reduce the possibility of brittle fracture in welded structures. Hardness Where metals are involved, hardness is usually defined as the ability of the metal to resist indentation or penetration by another material. In itself, the exact hardness of a steel is not of great importance in most applications. However, hardness can be measured much more readily than can tensile strength, there is a very close relationship between hardness and tensile strength, and between hardness and ductility. Usually, the harder the steel, the higher its tensile strength, and the lower its ductility. Three methods of hardness testing are widely used: The Brinell method, in which a steel ball is forced against the surface of the specimen by a heavy load, and hardness determined by measuring the diameter of the impression left in the surface; the Rockwell method, in which a diamond cone is pressed into the surface, and hardness determined by a gauge, built into the testing unit, which registers the depth of the impression; and the Scleroscope method, in which a diamond-pointed cylinder of steel is dropped onto the surface of the material from a fixed height, and hardness determined by measuring the height of the rebound. Continued on next page... Fig. 8-7. The springs or elastic bands which secure this punching bag to floor and ceiling are being subjected to repeated, but not alternating, stress.