Quench gas choices are nitrogen up to 12-bar overpressure or, in some instances, blended gas mixtures of nitrogen and helium (or nitrogen and hydrogen). The ratio of blended gases can be determined by the type of steel being austenitized and quenched. Ratios can be 60% nitrogen and 40% helium as well as variations on that quench-gas composition. The helium is recoverable, but the recovery method is costly. Hydrogen is the most thermally efficient gas there is, both for heat transfer to or from the steel being quenched.
H13, for example, can be carburized (for surface carbide formation with the carburizing gas and the 5% chrome) followed by a 10- to 12-bar overpressure quench using nitrogen, thereby providing a highly abrasion-resistant surface.
The combination of low-pressure carburizing in conjunction with high-pressure gas quenching creates a very clean finished surface after heat treatment, one that does not require any post washing.