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This article discusses the new generation, single-chamber vacuum furnace with a high-pressure gas-quenching system (HPGQ) able to quench with an efficiency better than that achieved in furnaces with a separate gas-quenching chamber (cold chamber) and comparable to the efficiency of oil systems. The performance of the furnace cooling system as it relates to the physical properties of quenching gas at ambient and process temperatures is presented and discussed. Finally, the efficiency and technological effect of quenching in gas is compared with results obtained in typical oil-quench systems.
Gas has been expanded as a quenchant with the application of vacuum heat-treatment technology. In the present decade, development of gasquenching systems (HPGQ) progressed due to the commercialization of low-pressure carburizing (LPC), which has come into common use.
Low-pressure carburizing may gradually replace traditional atmosphere-carburizing technology and oil-quench hardening in two-chamber furnaces (sealed or integral quench). In order to achieve the same or better results, vacuum-furnace quenching-system designs must be improved to achieve the same cooling efficiency as oil using gas as a modern and more environmentally friendly medium. Gas-quenching systems outperform oil in almost every aspect. Nonetheless, current technology performance is not as strong as oil quenching given the limitations of carburized-case applications in some steel grades and/or the part dimensions.
For the purpose of measurement and comparison, many methods and coeffi- cients help to determine the efficiency of a given system and quenching medium. These include: Grossmann's Number (H), cooling rate at given temperature (typically at 705°C), λ coefficient and heattransfer coefficient (α) as the most objective. Quenching parameters of typical oil systems were determined with α coefficient within the range from 1,000-2,500 W/m2K according to traditional division for slow- (1,000-1,500 W/m2K), medium- (1,500-2,000 W/m2K) and fast-speed oil (2,000-2,500 W/m2K).[1]
HPGQ vacuum furnaces may be classified as two types depending on design. Single-chamber furnaces (heat treatment and quenching occurs in one chamber without dislocation of charge) have slower cooling due to construction and material limitations. The more efficient two- and multi-chamber furnaces utilize a separated, dedicated cold quenching chamber.
At present with current technology, an average α coefficient of 600-800 W/ m2K can be obtained in HPGQ separated chambers for nitrogen under 20 bar and slightly above 1,000 W/m2K for helium. Single-chamber furnaces with 15-bar...