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Lesson 08·Pre-treatment

Pre-nitride heat treatment — you can't nitride a soft die

Why the temper temperature has to sit above the nitride cycle by 30-50°C, why EDM without stress relief is the single most common cause of premature nitrided-tool failure, and what the print has to say before the part ships out for treatment.

8 min readLesson 8 of 12

Tying it together

What this means on the shop floor

For forge dies in H13 or Nitralloy 135M, confirm temper at 600-625°C from the cert, stress-relieve at 570°C after rough machining, finish-machine, ship. If EDM detailed the impression, polish the EDM areas before the stress relief. For stamping dies in D2, A2, or P20, pull the temper temperature; if it is below 540°C the part is not a gas-nitride candidate and the quote moves to plasma at 480-510°C. EDM-detailed punch and die openings always get stress relief. For plastic mold cavities, 600°C-tempered P20 is nitride-ready at 0.20-0.30 mm case depth; 420 stainless goes to plasma at 480°C with the corrosion-resistance requirement on the print explicitly. For gears in Nitralloy N or 31CrMoV9, the pre-nitride risk is residual machining stress driving distortion on the tooth flanks, not core softening; stress-relieve after hobbing or grinding. For aluminum extrusion dies in H13 with long EDM-detailed bearing surfaces, stress relief is not optional.

Pushback questions to send with the part

  1. What is the temper temperature and core hardness on the heat-treat certificate, and does it sit at least 30-50°C above your planned nitride cycle?
  2. Has the part been stress-relieved after machining and EDM, and at what temperature and time?
  3. What prior coating or surface treatment, if any, has the part seen, and how was it removed?
  4. What surfaces require masking, and which masking method is acceptable for the process being quoted?

A vendor who reads those four answers off a traveler knows what they are working with. A vendor running a generic cycle on whatever shows up that morning does not.

Common confusions

Nitriding after EDM without stress relief is the single most common cause of premature nitrided-tool failure on the shop floor. The certificate looks clean and the part fails anyway, because the failure mechanism is in the substrate, not the case. Bench-stoning the EDM recast and running a sub-temper stress relief is one extra step in the routing that prevents the network cracking ending tool service life on the first or second run.

A short nitride cycle does not compensate for a soft substrate. Reducing cycle time leaves the case thinner but does not raise the core hardness back to where it was before the cycle.

Up next: application playbook.

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