Maintenance options: polish, repolish, weld repair, re-nitride, re-coat, scrap

Re-nitride restores the case. It does not restore the substrate. The job is right when the diffusion zone has thinned but is still partially intact, the compound layer is worn off or cracked, the substrate dimensions are within finish-grind allowance, and there are no propagating cracks into the bulk.

The sequence is not "send it to the nitrider." It is strip plus polish or light grind plus re-nitride plus finish to size. The strip step is non-negotiable. Any remaining compound layer, any PVD overcoat, and any oxide film has to come off before the new nitride cycle, or the new case grows into a contaminated surface and the bond between the new diffusion zone and the substrate is compromised. Chemical strip for PVD coatings, light grind or polish back to clean substrate for compound layer, then the standard nitride cycle from Course 2.

Life-extension data: Behrens et al. (Leibniz Universität Hannover, 2022, Materials) measured nitriding's durability uplift against untreated H13 in numerical wear prediction validated against shop data. The number was roughly 80% improvement over untreated. The same logic applies in reverse to re-nitride. If the substrate is sound and the case is rebuilt on a properly stripped and prepped surface, re-nitride recovers something close to the original case's contribution to die life. Call it 70 to 90% of original life from the re-nitrided state, with the variance driven by how well the substrate held up under the first cycle of wear and how clean the strip was.

Re-nitride is the wrong call in two conditions. First, when the substrate has propagating cracks. A new case on a cracked substrate hides the cracks for a few thousand hits and then the underlying defect propagates through the new case and the die fails. Second, when the die has had two prior re-nitrides and is on its third. Substrate metallurgy at the surface is now a patchwork of original heat-treat, prior diffusion zones, and re-tempered material from each strip-and-reprocess cycle. Cost per remaining hit drops below the cost per hit of a new build, and the right call is scrap.