Step 1 of 4
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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.
Step 1 of 4The core rule
The substrate has to be in its final hardened-and-tempered condition before nitriding, and the tempering temperature has to sit at least 30-50°C above the planned nitride cycle temperature. Nitriding runs 450-590°C, and a typical cycle is 40-80 hours at temperature. If the temper is below the cycle, the core continues to temper during the soak and bulk hardness drops measurably. The certificate will still report case hardness correctly because the case is freshly formed. The core hardness is no longer what the print called for.
The 30-50°C margin absorbs normal furnace variation and the thermal gradient across the part. A setpoint at 525°C does not mean every surface saw exactly 525°C for the full cycle.
Quick check
A vendor proposes a 565°C single-stage gas cycle on a 4140 pre-hard part with a heat-treat cert showing temper at 580°C. Should the cycle run as quoted?