Step 5 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 5 of 5Predicting cracks from the 3D model
Before cutting steel, run the cavity through a Kt audit. Tag every internal radius below R 3 mm as a candidate first-crack location. If the radius cannot be opened, the substrate has to compensate with toughness (premium H13, W360 at lower core hardness) or the surface treatment has to compensate with a thinner case. Look at every 3D intersection where two radii meet and spec a spherical radius rather than two intersecting cylindrical radii. Look at every undercut that holds scale, because trapped scale creates a local hot spot that abrades the surface on every cycle, and open the undercut into a smooth scallop where possible.
A useful exercise on rev A of any new cavity: take the 3D model into a structural FEA, apply the forging contact pressure as a uniform load on the cavity face, and overlay the peak von Mises stress contours. The contours will not predict the exact crack location, but they will rank the candidate corners by stress severity. Anywhere a contour crosses the H13 yield strength at die-face temperature (800-1100 MPa) is a corner that needs either a larger radius or a tougher substrate before the die ships.
Quick check
A FEA stress overlay on a rev A cavity shows peak von Mises stress of 1450 MPa at one internal corner with R 2 mm, and 980 MPa at a different internal corner with R 4 mm. The substrate is standard H13 at 47 HRC tempered to 580°C. Which corner is the first-crack candidate, and what does the model tell you about the rev B geometry priority?