The big four failure modes and how the build prevents them

Visual: a small number of discrete macro-cracks, often just one, running from a high-stress geometric feature into the bulk of the die. The crack initiates at an internal corner, a flash-land inside edge, the base of a rib, or any feature with a small radius and high local stress. There is no surface network around it. The fracture face shows beach marks consistent with cyclic propagation and a final overload region.

Mechanism: cyclic mechanical stress concentrated at a geometric stress riser. Hot-forge contact stresses run 1500 to 2000 MPa peak. A sharp internal corner with R 1 mm puts Kt at 4 or 5, which drives local stress above the yield strength of H13 at temperature on the first cycle. The first cycle plastically deforms the corner root, the next 10,000 cycles propagate a crack through the deformed zone, and somewhere between 5,000 and 50,000 cycles the crack reaches critical length and the die fails through-section.

Build decisions that shift it. Radii rules (Lesson 3) are the dominant lever. Opening an internal radius from R 1.5 mm to R 3 mm cuts Kt from roughly 4 to roughly 2.5 and typically doubles or triples the fatigue life at that feature. Draft angles interact with radii because a steeper drafted wall spreads stress over a longer transition into the bottom radius. Gate placement that does not dump metal velocity into a corner reduces the local impact stress contribution. Stress-relief sequencing matters across the whole build. An un-relieved part going into final heat treat carries residual machining stress that adds to service stress and shortens fatigue life. Stress relieve at 650 to 700°C after rough machining and again after EDM, and the substrate enters service with a clean residual-stress state.