Ahmad K Elshennawy, George F Schrader
The heat treatment process changes the strength, hardness, ductility, and other properties of metals. Hardening (or strengthening) is done by heating an alloy to a high enough temperature, depending on the material, and cooling it rapidly. Major properties of steel, such as strength, hardness, durability, and toughness, influence the metal to withstand scratching and resist wear. A continuous cooling transformation curve is a modified S-curve. It shows the changes that occur when austenite is transformed over a range of temperatures rather than at one temperature. Quenching: heat may be removed from hot metal by immersion in brine, water, oil, molten salts, or lead, or by exposure to air or gases, or by contact with solid metallic masses. Annealing is a heat treatment process that is used to reduce brittleness and residual stresses, while improving ductility and toughness. Hardenability refers to the degree and depth of hardness obtained in a heat treatment. Annealing in its broadest sense means heating a metal to where a change occurs and then cooling it slowly. Induction heating is done by passing a high-frequency alternating current through a water-cooled coil or inductor around the work piece or over a surface. Cyaniding or liquid carbonitriding imparts a file-hard and wear-resistant case to steel by immersing it in a molten cyanide salt bath for a time and then quenching it. Current trends in heat-treating technology indicate a great potential for automating the processes and other aspects of heat treatment. Types of heat treat furnaces are detailed.
By Gary Conner
By George N Bullen FSME