Step 1 of 5
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Cavity design and stress concentration: radii, draft, flash land, gates
Where forge dies actually crack and the geometric rules that move the failure point: minimum radii at stressed corners, draft angles by cavity depth, flash land and gutter dimensions, and gate placement that does not dump metal into a corner.
Step 1 of 5Internal radii and Kt
The working rule for hot-forge cavities is internal radius R greater than or equal to 3 mm wherever the geometry permits, R 1.5 mm absolute minimum on any feature seeing direct material flow or impact, and never a sharp corner where a nitride or PVD case has to transition. The reason is stress concentration. At a sharp internal corner the elastic stress concentration factor Kt goes to infinity in theory and runs 3 to 5 times the nominal stress in real geometry. Hot-forge contact stress runs 1500 to 2000 MPa peak, and H13 at die-surface temperature has a yield strength of 800 to 1100 MPa. Multiply nominal by Kt 4 at a sharp corner and the local stress is well above yield on the first cycle. That cycle initiates a sub-surface crack at the radius root, and subsequent cycles propagate it.
Course 2 covered why the compound layer wants to be thin and ductile under impact. Here the geometry decides whether even a thin compound layer survives. The compound layer is brittle and has effectively zero fatigue tolerance over a stress riser, so a Kt 4 corner cracks the compound layer in the first hundred cycles regardless of nitride process quality.
Opening the radius pays off fast. A move from R 1 mm to R 3 mm cuts Kt from roughly 4 down to roughly 2 to 2.5 on a typical forge feature. The next move from R 3 mm to R 5 mm cuts Kt to about 1.8. Returns diminish after R 5 mm. The first 3 mm of radius is where most of the die life comes from.
Two specific failure modes. A radius that reads R 2 mm in plan and R 2 mm in section can intersect at a sharp three-dimensional corner where the two radii meet, and the crack starts at the point. Spec a spherical radius at any 3D intersection. The other mode is the print radius that is not the as-built radius. EDM electrode wear at a deep blind corner tightens the radius at the bottom of the pocket, and finish grind cleans up the face while leaving the corner sharper. Verify the as-built radius before nitride, with an optical CMM on accessible features or a silicone replica on blind ones.
Quick check
A cavity has two perpendicular R 2 mm radii that meet at a 3D corner. The CAD model shows both radii on the print but the two cylindrical surfaces intersect at a sharp point at the corner. What is the predictable failure location on the first die out of production, and what is the geometric fix on rev B?