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The use of industrial gases for quenching value added parts offers significant environmental and performance advantages over oil quenching. For example, handling problems associated with oil such as spills and the need for special ventilation systems to take care of vapor exhausts can be eliminated when gas quenching is adopted. Gas quenched parts are clean thus eliminating the need for post-cleaning operations.
In terms of performance, the cooling rate during oil quenching is a function of temperature and can therefore vary dramatically due to the simultaneous presence of three types of cooling: vapor, boiling and convection. This produces large temperature differentials in the parts being quenched. In gas quenching however, only the convection stage is present, so the cooling rate is much less dependent on temperature. Reduced temperature differential and a more uniform cooling rate result in less distortion of parts during quenching. This can further be improved by regulation of parameters affecting the heat transfer coefficient, such as gas velocity, gas pressure and gas mixture.
High pressure, high velocity gas systems have been developed as alternatives to oil quenching for hardening low alloy steels. These cold quench chambers can use nitrogen to harden thin section parts, while the higher cooling rate properties of helium are needed for thicker sections. Quenching using hydrogen is also under development due to its very high heat transfer coefficient, but the stringent safety precautions needed are hindering its commercialization.
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