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

H13 and the 5% Cr hot-work family hold hardness at die-face temperature because their alloy carbides precipitate during tempering, not because the as-quenched martensite is hard. The cycle exists to dissolve the alloy carbides at austenitize temperature, lock the alloy into supersaturated martensite by quenching fast through the carbide-precipitation nose, and then precipitate the carbides back out in controlled form during tempering. The second temper is not a finishing touch. It transforms the retained austenite that the first temper conditioned, and without it the die grows in service and softens at the wrong rate.

The cycle is four steps in sequence: preheat to equalize the part through-section, austenitize at 1010-1030°C to dissolve the carbides into the matrix, quench fast enough through 1010-540°C to suppress bainite and pearlite, and temper twice (three times for premium grades) on the secondary hardening shoulder to precipitate the fine alloy carbides and transform the retained austenite. Skip any step or run it outside the window and the structure that leaves the furnace is not the structure the spec called for, even if the hardness number is.