Gas quenching with helium

Safety and environmental considerations make gas quenching an attractive alternative to conventional liquid quenchants. Gas quenched parts are clean and bright and do not require an additional cleaning step. There also are no disposal problems associated with gas quenching. The disadvantage of slower cooling rates can be overcome by using helium or helium-containing gas mixtures as an alternative to nitrogen or argon. The advantages of using helium when gas quenching in vacuum furnaces are basically the result of its high thermal conductivity (among gases, second only to that of flammable hydrogen, second only to that of flammable hydrogen) and its low density (less than that of nitrogen or argon). Depending on furnace pressure, cooling rate can be increased simply by switching gases. And because fans require less power to circulate helium, both furnace pressure and gas velocity (fan speed) can be optimized. Increasing pressure and/or fan speed also can increase cooling rate. The type of gas flow around the parts determines the best helium mixture to use. However, since the pattern may not be obvious, tests in production furnaces are recommended. A computer model of gas flow and heat transfer as a function of pressure and velocity is being refinedmore » at Air Liquide. It will be useful for predicting the effects of different gases and gas mixtures.« less