Vendor handoff: what goes on the print to the heat treater, nitride house, and polisher

A complete nitride callout for a forge die has ten elements.

Substrate: AISI H13 per ASTM A681, vacuum hardened and double-tempered to 48-50 HRC, final temper temperature 595°C minimum. Parts supplied with surface ground to Ra ≤ 0.4 µm and EDM recast layer removed.

Process: Gas nitriding per AMS 2759/10B Class 2, with closed-loop nitriding potential (Kn) control.

Pre-nitride: Parts shall be stress-relieved at 565°C for 2 hours after final machining and before nitriding. EDM stress relief required where EDM was used. Parts delivered clean and oil-free; no additional cleaning by the vendor.

Nitriding potential: Kn target to land the compound layer in the γ' field per the Lehrer diagram on H13 at cycle temperature. Kn trace included on the certificate.

Case depth: NHD 0.30-0.40 mm per DIN 50190-3 (core hardness + 50 HV), measured by microhardness traverse on the witness coupon per ASTM E384, HV 0.5 load, minimum ten data points from surface through transition to core.

Compound layer: 4-8 µm total thickness, γ'-dominant per microsection examination, porous outer zone ≤ 30 percent of compound layer thickness. Verified on the witness coupon at 500x with Nital etch.

Surface hardness: HV 0.5 ≥ 1000 measured at 5-10 µm depth per ASTM E384. HRC equivalent acceptable as a sanity check only, not as the controlling measurement.

Distortion: Total dimensional change ≤ 0.05 percent on cavity datums; flatness deviation ≤ 0.02 mm across datum A post-nitride.

Masking: Mask threaded holes, dowel bores, and the parting surface outside the flash land per drawing zone A4. Masked areas exempt from case depth and compound layer requirements. Method at vendor discretion provided masking is verified post-process.

Witness coupon: Single coupon of AISI H13 in the same heat-treated condition as the production parts, processed in the same load. Coupon sectioned, mounted, etched, and shipped with parts. Certificate to include microhardness traverse, compound layer photomicrograph, Kn log, and coupon results.

The substrate line carries the final temper temperature, because the nitride cycle must run at least 20-30°C below it or the core over-tempers. Citing AMS 2759/10B Class 2 with closed-loop Kn control distinguishes the spec from an open-loop ammonia bleed where the vendor grows whatever compound layer the recipe happens to produce. The DIN 50190-3 NHD definition is non-negotiable, because the same case depth measured by SAE effective case depth to a fixed hardness can differ by a factor of two on the same traverse. The 4-8 µm γ'-dominant compound layer is the standard call for a high-impact hot forge cavity. A sliding-wear extrusion die wants ε-dominant and a different Kn target. The witness coupon line is what turns the certificate into evidence rather than assertion.