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Lesson 08·Surface treatment

Surface treatment sequencing: nitride, PVD, polish, in what order

The build order that takes an H13 forge die from rough block to coated, polished, ready-to-run, and why getting any step out of sequence is a failure mode rather than a slow ramp to nominal life.

9 min readLesson 8 of 13

Step 1 of 5The 13-step build sequence

The correct order for a fully-treated H13 forge die runs as follows. Reorder any step and the lesson below covers what breaks.

  1. Receive rough block, forged and annealed, around 22 HRC.
  2. Rough machine to within 0.5-1.5 mm of final on all surfaces. Leave generous stock on critical features.
  3. Stress relieve at 650-680°C, 2-4 hours, slow cool. This is the cycle most shops skip and pay for later.
  4. Semi-finish machine to within 0.25-0.50 mm of final. Drill all holes, mill peripheral features.
  5. Heat treat: austenitize, high-pressure gas quench, double temper to target hardness, typically 46-50 HRC for hot-forge service.
  6. Inspect for distortion and hardness on a witness coupon processed in the same load.
  7. Finish grind all reference surfaces, datums, parting line, outside-of-cavity features.
  8. EDM the cavity to final shape. Rough EDM, then finish EDM with low-energy settings to drive recast under 10 µm.
  9. Polish or lap the cavity to remove EDM recast, typically 25-75 µm per side via diamond paste or vapor honing. This is polish step one. Target Ra 0.4-0.8 µm.
  10. Nitride the cavity, almost always plasma for a forge die. Target diffusion zone 0.30-0.50 mm, compound layer 5-8 µm γ' or mixed-phase.
  11. Light polish of the compound layer if needed, 1-3 µm per side. This is polish step two and is optional for steel forge work, required for PVD prep.
  12. PVD coat, only if the application calls for it. Typical pick is AlTiN at 3-5 µm for hot-work, deposited at 450-500°C.
  13. Final inspection, light degreasing, ship.

Heat treat goes before EDM, because EDM on a soft block followed by heat treat distorts the cavity. EDM goes before nitride, because EDM through a nitrided case burns the diffusion zone at the heat-affected layer. Polish off recast goes before nitride, because nitriding inherits every microcrack in the recast layer. The compound layer gets thinned before PVD, because thick compound layers crack under the PVD interface load. Each step protects the step that follows.

Quick check

A shop EDMs a cavity after heat treat, ships it directly to a plasma nitride vendor without polishing, and the compound layer comes back with visible cracks under a loupe at 10x. What step did the routing skip, and what is the mechanism?