Can H13 tool steel be nitrided?

H13 is one of the best steels to nitride. Its 5% chromium, 1.1-1.75% molybdenum and 1% vanadium are strong nitride formers, so nitrogen diffusing in at 500-540 °C (930-1000 °F) produces a very hard diffusion zone, about 1000-1100 HV at the surface, beyond the useful top of the Rockwell C scale and far above the 550-650 HV of nitrided 4140. Just as important, H13 is hardened from about 1010-1030 °C (1850-1890 °F) and double or triple tempered at 540-620 °C (1000-1150 °F) to its working hardness, typically 44-50 HRC, so its tempering temperature already sits above the nitriding temperature and the core does not soften. The high chromium also slows nitrogen diffusion, so cases are shallower than on low-alloy steel for the same time: about 0.1-0.2 mm (0.004-0.008") in a typical gas or ion cycle, up to about 0.3 mm (0.012") with long cycles.

How deep, and what kind of surface, depends on the job. Die-casting dies and core pins get a shallow case, about 0.1-0.15 mm (0.004-0.006"), with no compound layer: the nitrided surface resists erosion and aluminum soldering, but a deep or brittle case accelerates heat checking under thermal cycling, and a white layer spalls. Extrusion dies are nitrided on their bearing surfaces and routinely re-nitrided between campaigns after cleaning. Forging dies, plastic-mold components, ejector pins and wear parts gain the same hard surface plus a large improvement in fatigue and galling resistance. Ion (plasma) nitriding (AMS 2759/8) is often preferred on H13 because it controls the compound layer closely and masks simply; gas nitriding with nitriding-potential control (AMS 2759/10) and ferritic nitrocarburizing are also used. Growth is small, from a few microns up to about 0.02 mm (0.0008") per surface, so finished and polished parts are nitrided last.

Surface preparation decides the result. Remove the EDM recast layer before nitriding, because a nitrided recast film spalls in service. Stress relieve after heavy finish machining or grinding so the part does not move during the cycle. The surface must be free of oil, scale and decarburization; a decarburized skin from open-furnace hardening nitrides poorly. Nitriding slightly dulls a mirror-polished mold surface, so polish after as well as before where the finish matters. Threads, surfaces to be welded later and areas to be machined are masked. And a nitrided H13 surface cannot be weld repaired directly: grind the nitrided layer off the repair area first, weld to a hot-work tool steel repair procedure, temper, and re-nitride.