The big four failure modes and how the build prevents them

Visual: a fine network of shallow cracks across the heated surface of the die, with crack spacing measured in tenths of a millimeter to a few millimeters, oriented in a roughly orthogonal pattern. The network appears first on the hottest contact zone, expands outward with cycles, and deepens. Under 10 to 30x optical the cracks look like dried mud, hence the trade term craze. A die in late-stage thermal fatigue shows the network linked across most of the cavity surface and beginning to spall flakes between intersecting cracks.

Mechanism: cyclic thermal stress. Each forge cycle heats the die surface to 500 to 700°C while the bulk stays near 200 to 300°C, then quenches the surface as the workpiece leaves and lubricant or coolant hits the cavity. The surface expands and contracts against a constrained bulk, accumulates plastic strain on each cycle, and cracks initiate when the local strain exceeds the substrate's low-cycle fatigue capacity at temperature. Crack density grows with cycle count, crack depth grows with peak temperature delta.

Build decisions that shift it. Steel selection sets the ceiling. Standard H13 carries the baseline. Premium H13 (ESR or VAR remelted to NADCA #207 grade) with tighter inclusion content and finer carbide bands runs roughly 2 to 4 times the heat-check life on the same geometry because non-metallic inclusions are crack-initiation sites and clean steel has fewer of them. Dievar and W360 ISOBLOC push further by trading silicon and adjusting alloy chemistry for higher hot toughness. Heat treat sets the substrate's hot strength: a properly executed double or triple temper that lands cleanly on the secondary-hardening peak (Course 3 Lesson 5) holds yield strength at die-surface temperature where a single-temper or under-tempered die crashes. Nitriding with a controlled compound layer of 5 to 8 micrometers, dominantly gamma prime phase, shifts the crack initiation site from the substrate surface into a brittle skin that the diffusion zone can still arrest. Preheat strategy (Course 4) cuts the surface-to-bulk delta on every cycle and is the single largest operational lever once the build is fixed.