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

As-quenched H13 is supersaturated martensite with the alloy elements trapped in solution. At low tempering temperatures the martensite relaxes and softens, which is what plain carbon steels do across the whole temper range. H13 does the same up to roughly 400°C. Above 480°C the alloy elements come out of solution as fine coherent carbide precipitates. The dominant phases are M6C, MC with vanadium, and M2C with molybdenum. These precipitates pin dislocations and harden the steel above its as-quenched baseline. The peak occurs near 510-540°C, where H13 hardness can exceed the as-quenched value by 1 to 2 HRC. Above 595°C the carbides coarsen, lose coherence, and the hardness drops fast.

Secondary hardening is the reason H13 holds hardness at die-face temperature where cold-work tool steels like D2 or A2 would over-temper within hours of starting work. The die surface in steel forging runs at 500 to 600°C in service, which means the material is re-tempering itself in real time during production. A temper that did not catch the secondary hardening peak leaves a structure without the fine carbide precipitates the steel needs to hold position on that re-temper curve.