Helium’s ability to transfer heat away from a part is 5.5 times greater than nitrogen, and once heat energy is in the gas helium can hold five times as much. In either case, however, if the velocity of gas passing over the part is not uniform or does not exceed the thermal conductivity of the steel, the rate of heat removal will not be equal over the gear and the martensitic transformation will occur at different times, resulting in distortion and likely NMTP.
Nitrogen’s density is 7.8 times that of helium. Translated: fan horsepower (HP) for nitrogen must be 7.8 times higher, and then add 20-bar pressure (290 psia, 275.5 psig), and the horsepower difference is huge. Fan motor HP increases proportionally with pressure (gas density), and increases by the cube of the difference in RPM; double the RPM and the HP goes up eight times. Since gas velocity is also critical, fan output (RPM) must be high to transport heated gas to the water-cooled heat exchanger and back to the load. Even for helium, to move the quantity of gas through the load sufficient to quench alloyed steel gears, almost 400 horsepower is required. Employing the same HP for heavier nitrogen necessitates lower recirculation volume (smaller fan ,or lower RPM) to stay within 400 HP. And since nitrogen already has significantly lower heat transfer properties, combined with lower gas recirculation volume one can see its limitation. Consequently, only very high-alloyed steels—and thus more expensive—or certainly smaller cross-section gears qualify for nitrogen quenching.
10) Using the same 5130H Jominy table the surface of the 2” gear would barely make the core hardness using helium gas. This will force you to upgrade to a much more expensive alloy.
Having gone through the above, it will become obvious to those skilled in the art that in many instances the core hardness of the gear cross-section may not be the critical parameter, but the core of a tooth. Here two locations are important: tooth center on the pitch diameter (PD) and midpoint on a line drawn from root-to-root. Only empirical testing will confirm HPGQ suitability for any material. And when a suitable material is selected, a trial and error test plan including part location within the loading scheme will be required.
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