Pre-hard vs through-hard: which workflow, when

Hardness ceiling sets most of the tradeoff. Pre-hard H13 caps at roughly 44 HRC because the mill cannot ship harder stock that is still machinable with carbide on standard equipment. Through-hard H13 lands wherever the temper places it on the secondary-hardening curve, with 48-50 HRC the working range for hot forge dies and 50-52 HRC for hot-trim cutting edges. The 6-8 HRC delta is the difference between a cavity that holds shape under hot contact and a cavity that washes out at the corners.

Distortion budget runs the other way. Pre-hard stock distorts only as much as machining stress relief allows, which on a finished die is well under 0.001 inch per inch. Through-hard distortion on vacuum-quenched H13 runs 0.001-0.003 inch per inch on symmetric geometries and 0.003-0.008 inch per inch on asymmetric or thin-section parts. The through-hard path needs explicit stock allowance on every critical surface, plus EDM allowance of 0.005-0.010 inch per side on internal cavities that cannot be ground after heat treat.

Lead time favors pre-hard by two to four weeks. The heat-treat cycle itself runs about a week at most commercial vendors, and the second finish machining pass adds another one to two weeks of shop time. Finish-machining hours on through-hard work are also non-trivial, because hardened H13 burns through endmills faster and requires either hard-milling tool packages or EDM finish on intricate features.