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The time and temperature of a heat treatment vary depending on the metal or alloy being treated and the qualities desired in the finished product. Different treatments are needed for making flexible sheet metal, copper wire, and hard ball bearings. In furnaces using metal belts or wire baskets to hold the heated pieces, some processes use temperatures as low as 250 degrees Fahrenheit or as high as 2,450 degrees Fahrenheit. Some qualities can only be achieved by subjecting the metal to temperatures hundreds of degrees below zero.
Heating, or annealing, metal softens it, refines its grain, and removes internal stress points. Tempering it (reheating it after it has cooled) makes it tough and flexible. In carburizing (or case-hardening), heat treaters use gas, carbon, or chemical baths to harden the surface of steel objects to meet certain specifications.
Heat treaters know exactly how long to heat an object and how to cool it. Cooling is called quenching and may be done by immersing the object in liquids, cooling it in the furnace, or cooling it in the air. The rate of cooling greatly affects the properties of the finished product.
Heat treaters use their knowledge of metals, heating and cooling processes, and the desired qualities of the end product to determine how to heat and cool an object and for how long. Some heat treaters are responsible for both making these determinations and operating the furnace and quenching equipment needed to carry out the processes. They control temperatures and times to harden, temper, anneal, and carburize the metal. They load objects into and remove them from heat-treating equipment and decide whether to quench the objects in water, oil, or air. Workers designated as heat treaters II treat objects following the instructions devised by more skilled or experienced heat treaters. Workers may begin as heat treater apprentices and usually work under the direction of a supervisor.
Many workers specialize in one phase of heat treating. Heat treater inspectors check parts before and after treatment to make sure they meet standards and specifications. Annealers control the heating and cooling of the metal in furnaces to prevent oxidation. Flame-annealing-machine setters operate machines that anneal the side-walls of metal cartridges and shell cases before further processing.
Case-hardeners put steel objects in wire baskets and immerse them in baths of heated chemicals (such as sodium cyanide) to harden them. After objects are treated, a hardness inspector tests the objects to ensure that they have reached the desired degree of hardness. Temperers reheat annealed, quenched, and hardened metal and quench it again in water, oil, brine, or molten lead to remove stains and brittleness.
Special kinds of heat treaters include heat-treating bluers, who heat gun parts with bone chips and whale oil to rust-proof them and give them a decorative blue finish. Rivet heaters heat-treat rivets, and hardeners treat watch parts. Other heat treaters work on jewelry and ammunition components. Production hardeners case-harden mass-produced items such as buckles and screws.