Distortion budget: how it propagates through the build

The figures below are typical for H13 in hot-work practice. They move with section thickness, fixturing, and quench medium, and the right answer for a specific block is to put it on a cycle trace and measure it.

Rough machining. Removing material redistributes residual stress from the rolled or forged stock. On a 500 mm block, expect 0.05 to 0.20 mm of dimensional movement during and after rough machining, larger if the supplier did not stress relieve the block. A pre-machining stress relief at 650°C for four hours, slow cooled, cuts post-machining movement by roughly half.

Heat treat. Vacuum harden plus double temper produces 0.05 to 0.15 percent linear dimensional change on H13. On a 500 mm dimension this is 0.25 to 0.75 mm of length change, plus 0.05 to 0.30 mm of out-of-flat depending on quench symmetry and fixture support. High-pressure gas quench at 6 to 10 bar nitrogen produces the lowest distortion. Oil quench can run 2 to 3 times higher. The figure is measured between annealed stock and the fully tempered final part, and it includes the volume change from martensite transformation plus the consequence of the quench gradient.

Finish grind after heat treat. Not a source of distortion. The operation that reclaims distortion the heat treat created. Grinding 0.20 to 0.30 mm off the cavity face and the datums returns the part to flat, provided the heat-treat movement was contained inside the stock allowance.

EDM. Sinker or wire EDM imposes no mechanical force, so it does not deflect the workpiece. Bulk dimensional change is well under 0.01 mm on a cavity feature. What EDM leaves behind is a recast layer 5 to 25 µm thick with tensile residual stress, hard, micro-cracked, and brittle. Lesson 7 covers it in detail.

Post-EDM stress relief. Recast tensile stress can release into bulk distortion at the next thermal step. A relief at 550 to 580°C for two hours, below the second temper temperature, dissipates it without affecting bulk hardness. Skipping this is acceptable on small inserts and malpractice on a 500 mm cavity going to nitriding.

Nitriding. Gas or plasma nitriding produces 0.02 to 0.05 percent linear growth on H13, driven by lattice expansion as nitrogen diffuses into the diffusion zone and the compound layer forms on the surface. On the 500 mm cavity this is 0.10 to 0.25 mm on the longest dimension and 5 to 15 µm per side perpendicular to the surface. The growth is reasonably uniform if the atmosphere is uniform around the part. It is not reclaimable by grinding without breaching the case.