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

A forge die builder sends an H13 cavity block to two heat treaters with identical print callouts. Both certificates come back at 48 HRC. The die from vendor A runs 220,000 cycles before the cavity wears out of tolerance. The die from vendor B cracks through a corner radius at 35,000 cycles. The hardness numbers were identical, the steel was from the same mill heat, and the cavity geometry was the same. What separated them is what happened inside the cycle: how clean the austenitize was, how fast the part cleared the carbide-precipitation field on the quench, and whether the temper schedule actually transformed the retained austenite or just hid it.