Gas nitriding — the workhorse and its limits

Ammonia flows into a sealed retort holding the parts. The retort runs 450-590°C. At those temperatures NH₃ cracks at the steel surface, releasing atomic nitrogen that diffuses into the ferrite. There is no quench. The case forms by diffusion, not by phase transformation. Total cycle time runs 40-80 hours depending on case depth target and whether the recipe is single-stage or 2-stage. A typical H13 die comes out with a 5-15 µm compound layer over a 0.20-0.60 mm diffusion zone, surface microhardness 1000-1200 HV, core hardness essentially unchanged from the pre-nitride temper.

The equipment is the simplest of the four nitriding families. A retort, a heating element, an ammonia supply, a vent, a thermocouple. The combination of mature equipment and high commercial volume is why gas nitriding still owns the largest share of nitride throughput. It is also why "we nitride" can mean almost anything, and why two certificates with the same word on them can describe very different cases.