Heat-treat fundamentals for forge dies — austenitize, quench, temper, secondary hardening

Gas quench hardness, abbreviated GQH, is the as-quenched hardness achievable on a given section thickness at a given quench bar pressure. The number captures whether the gas quench was severe enough to clear the carbide-precipitation nose through the full section. Vacuum furnace OEMs publish GQH curves that plot section thickness against the bar pressure required to reach full martensite. For standard H13, 5 bar nitrogen handles sections up to roughly 200 mm thick. Sections above 300 mm require 10 bar minimum.

The shop floor decision is whether the heat treater's furnace can deliver the quench severity your section requires. A 6 bar furnace cannot harden a 400 mm hammer die to spec at the centerline. The center transforms to bainite during the quench and reads 38 HRC where the surface reads 52 HRC, and a certificate that shows a single 48 HRC surface reading hides the gradient. Require core hardness verification on heavy sections, not just surface.

Oil quench and salt-bath interrupted quench remain valid for specific cases. Oil delivers higher quench severity than gas at the cost of 2 to 3 times the distortion. Salt-bath interrupted quench at 200-250°C between austenitize and martensite finish gives the lowest distortion of any medium, but it adds operational complexity and is rare outside premium tooling shops.