Step 2 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 2 of 4Common tool steels and how they line up
H13. Tempered at 595-620°C (1100-1150°F) to 44-48 HRC for hot-work service. A 525-565°C gas cycle leaves 30-95°C of margin; plasma at 480-510°C leaves more. The default nitride substrate.
Nitralloy 135M (EN41B, 32CrMoV13). Designed from the start for nitriding. The ~1% Al drives the alloy-nitride precipitate network that gives Nitralloy its case hardness. Temper at 600-650°C to 30-36 HRC, nitride at 510-540°C. Surface microhardness routinely reaches HV 1100-1200.
P20. Pre-hardened at 28-32 HRC, tempered around 600°C, supports a 525-560°C cycle. The watch-out is the case-to-core ratio: HV 900-1000 surface over a 30 HRC (~HV 300) core is a steep gradient, and Hertzian contact stress on a high-clamp-tonnage mold can punch the case into the soft substrate. Specify enough case depth (0.20-0.30 mm minimum) to keep the load path in the case.
A2 and D2. Tempering temperature varies. Secondary-hardened (tempered ~510-540°C) is marginal for any cycle at 540°C and above. The standard low-temperature temper (175-200°C) for maximum hardness cannot be gas-nitrided, since the core would over-temper through the floor. A2 and D2 work usually means low-temperature plasma (480-510°C) on a secondary-hardened temper, with the trade-off documented.
420 stainless. Pre-hard mold tooling tempers anywhere from 250°C (high-hardness) to 600°C (corrosion-priority). The high-hardness temper is incompatible with conventional nitriding. Plasma at 480°C or below on a 600°C-tempered 420 is the standard call, and stainless mold work is where low-temperature S-phase nitriding (Lesson 5) earns its keep to preserve corrosion resistance.
Pre-hard grades in general. "Pre-hard" tells you the mill quenched and tempered the bar. It does not tell you the temper temperature, which varies by grade and supplier. A 4140 pre-hard tempered at 600°C is nitride-ready; the same grade tempered at 540°C for higher delivered hardness will soften during a 540°C cycle. Read the certificate.
Quick check
A shop is quoting nitride on three substrates: H13 at 47 HRC, D2 secondary-hardened at 58 HRC, and 420 stainless tempered at 300°C for 50 HRC. Which require process changes from a default 540°C gas cycle?