VACUUM HEAT TREATING consists of thermally treating metals in heated enclosures that are evacuated to partial pressures compatible with the specific metals and processes. Vacuum is substituted for the more commonly used protective gas atmospheres during part or all of the heat treatment. Furnace equipment used in vacuum heat treatment differs widely in size, shape, construction, and method of loading.
Although originally developed for the processing of electron tube materials and refractory metals for aerospace applications, vacuum furnaces are now employed in brazing, sintering, heat treating, and the diffusion bonding of metals. Vacuum furnaces also are used for annealing, nitriding, carburizing, ion carburizing, heating and quenching, tempering, and stress relieving. Furnaces for vacuum heat treating are equipped for workloads ranging from several pounds to 90 Mg (100 tons), and heated working chambers range in size from 0.03 m3 (1 ft3) to hundreds of cubic feet. Although most vacuum furnaces are batch-type installations, continuous vacuum furnaces with multiple zones for purging, preheating, high-temperature processing, and cooling by gas or liquid quenching also are used. Vacuum heat-treating furnaces also: Prevent surface reactions, such as oxidation or decarburization, on workpieces, thus retaining a clean surface intact