The big four failure modes and how the build prevents them

Visual: scratched, eroded, or polished surfaces in the metal-flow path. The flash land thins on the entry side and shows directional scoring aligned with the flow. The gate face shows washout where high-velocity metal abrades the cavity. Aluminum forge dies show smeared metal pickup on the cavity surface where workpiece material has adhered to the die and pulled away. Brass and copper dies show pickup in a different pattern but the same mechanism. The depth of removed metal is typically 0.01 to 0.1 mm by the time the die is pulled.

Mechanism: abrasive wear from oxide scale carried by the workpiece, plus adhesive wear from the workpiece sticking to the cavity and tearing off die material on separation. The Rolinski failure-mode breakdown in Forging Magazine puts abrasion at roughly 60 percent of forge die pulls in steel forging. Scale particles act as a third body between the workpiece and the die, the particles are harder than the substrate even at 50 HRC, and they cut the cavity surface on every cycle. Adhesive wear is the dominant mode on aluminum because aluminum bonds readily to steel at forging temperatures, and the workpiece-die bond exceeds the cohesive strength of the die surface layer when the part lifts off.

Build decisions that shift it. Nitriding is the primary lever. A controlled compound layer of 5 to 10 micrometers with epsilon-phase dominance in the outer zone is the standard answer for sliding abrasive wear, because epsilon is the harder and more wear-resistant phase. For impact-dominated service the trade-off goes the other way and gamma prime wins, but for flash-land and gate-washout failure the epsilon-rich compound layer is what extends the life. PVD on top of nitride (duplex) extends further: a 3 to 5 micrometer AlTiN or CrN coating cuts adhesive pickup on aluminum by 5 to 10 times in the peer-reviewed studies, and the diffusion-zone-supported coating survives the contact stress that would crack a coating on a soft substrate. Polish to the working Ra by feature: flash land Ra 0.8 to 1.6 micrometers holds enough lube to break the adhesion mechanism without scoring deep enough to trap scale; gates and runners polished to Ra 0.2 to 0.4 micrometers break the flow geometry that traps abrasive particles. Cavity geometry that does not pocket scale (no sharp inside corners where particles accumulate, no dead zones in the flow path) cuts the abrasive supply at the source.