The cooling performance required by a vacuum furnace is dictated by the type of alloy to be hardened. Most air hardening tool steels such as the A and D series can be hardened by gas quenching at pressures of 2 bar or less. Furnaces with gas quench pressure ratings of 6 bar and higher can often be used for hardening some alloys that historically required oil quenching (ie. SAE 4340). VAC AERO has conducted comprehensive research into high pressure gas quenching. We learned that, all things being equal, a significant improvement in cooling performance is achieved when the gas quench pressure is increased from 2 bar to 6 bar. Increasing gas quench pressure above 6 bar (to 10 bar, 12 bar or higher) provides further improvements in cooling performance but to a much lesser extent. The higher the pressure rating, the more expensive the furnace is to build and operate. So, a cost/value decision should also be considered.