Atmospheres for the hardening of steel

Since time immemorial, atmospheres in one form or another have been used to protect steel surfaces against degradation or to impart properties that would otherwise be unobtainable. For a heat-treatment operation that includes both clean-hardening and carburizing, the most viable atmosphere option is a combination of both cryogenic nitrogen for safety purges, and on-site generation of nitrogen--membrane or PSA--and methanol. Given that a steel part is being heated to austenitizing temperature, what is the purpose of the furnace atmosphere: to protect the steel surface from oxidation, oxide scale impedes quenching speed and contaminates the quenching media and tank; to protect the steel surface from unintentional changes in the surface chemistry caused by carburization/decarburization; to intentionally change the surface chemistry of the part by adding or removing carbon and/or nitrogen; to provide a vehicle for heat transfer or loss, if a circulating fan on a batch furnace breaks down, the importance of heat transfer becomes painfully obvious. Note that for quenching high carbon steels, no atmosphere is often better than a poor atmosphere. A minimal layer of ductile ferrite caused by total decarburization under a layer of thin scale, is less prone to quench-cracking than a less ductile and often deepermore » layer of partial decarburization with no scale.« less