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 Heat of Fusion. The amount of heat required to completely melt a unit mass of a metal once it has attained its melting point. Here again, more heat is required for a light metal, such as aluminum, than for a heavier metal such as iron. Thermal Conductivity. As everyone knows, the handle of a sterling silver spoon left in a hot cup of coffee gets hot in a hurry, whereas a stainless steel spoon handle heats up only a little in the same period of time. Silver is an excellent conductor of heat, while stainless steel is a poor conductor. In fact, silver is twice as good a conductor as aluminum, and nearly 10 times as good as a conductor as low-carbon steel. Copper and gold are the only metals that come close to silver in thermal conductivity. In fact, the high conductivity of copper is quite a complication when it comes to welding. Thermal Expansion. The increase in dimensions of a solid body due to an increase in temperature is termed thermal expansion. This property is of much significance in welding operations, since the metal close to the weld zone is heated to a higher temperature, and therefore expands more than the metal at a greater distance from the weld zone. Furthermore, the molten metal deposited during welding must shrink – or least try to shrink – as it cools down in the solid state. Mathematically, the term used to express the tendency of a metal to expand when heated is ”coefficient of thermal expansion”. By comparison with zinc, lead, and magnesium, this coefficient is relatively small for steel; an iron bar one meter long increases in length a little more than one millimeter when heated 1000C. The expansion and contraction of steel when heated and cooled are matters of great importance in welding, and will be covered in more detail in Chapter 12. Electrical Conductivity. As stated earlier, a metal must be a conductor of electricity. Some are much better than others; generally, the metals which are the best conductors of heat, such as copper, silver and aluminum, are also the best conductors of electricity.