The big four failure modes and how the build prevents them

Visual: the cavity profile is measurably different from the print. Edges that should be sharp are rounded. Internal corners are washed open. The flash land is thicker or thinner than as-built. A profile gauge or a silicone replica against a coordinate measurement shows 0.05 to 0.3 mm of dimensional change on the working surfaces. The surface itself may look intact under optical inspection. The failure is dimensional, not visible.

Mechanism: peak service stress exceeds the hot yield strength of the substrate. Each cycle leaves a small permanent strain, the strains accumulate, and the cavity geometry walks. The mechanism is concentrated where contact stress and temperature are both highest, typically the bottom of deep pockets, the top of internal bosses, and any feature where metal flow turns abruptly. Once the cavity profile drifts past tolerance, the forged part is out of spec and the die is pulled regardless of remaining surface life.

Build decisions that shift it. Steel grade picks the hot hardness ceiling. H13 at 48 to 50 HRC holds yield strength near 1000 MPa to about 550°C; above that it over-tempers in service and yields more easily. Premium H13 with controlled carbide structure holds slightly better. Dievar and W360 ISOBLOC at the same hardness hold meaningfully higher hot strength because of alloy and processing differences. For aluminum and brass forge dies running cooler, the constraint is different and the standard grades cover. Heat treat sets the actual delivered hardness and the temper structure. A die tempered cleanly through the secondary-hardening peak holds dimensional stability across the service temperature range; a die under-tempered at 400°C softens at die-surface temperature and washes within thousands of cycles. Case depth matters because the case must carry the contact load. A nitride diffusion zone of 0.3 to 0.4 mm supports the load that would otherwise plastically deform the substrate directly. Insufficient case depth produces eggshell failure where the hard case cracks over a yielding substrate and the cavity washes underneath.