Application playbook — forge dies, stamping, plastic molds, gears, extrusion

Typical substrate: Nitralloy 135M, EN41B (32CrMoV13), 31CrMoV9, premium grades for aerospace and high-performance automotive. Service environment: tooth-flank contact fatigue at the pitch line, root-bending fatigue at the fillet, sliding wear on the flanks.

Target case depth: 0.30-0.60 mm. Deep enough that the maximum subsurface shear stress (which sits 0.1-0.3 mm below the contact surface) is carried by hardened material, not unhardened core. Target compound layer: thin and controlled, or partially removed in post-grinding. Most industrial gears are ground after nitriding to hold AGMA tooth-quality numbers, and the grinding removes 0.001-0.003 inch from the flank, which takes most or all of the compound layer with it. The spec has to call out which surface keeps the compound layer (typically the root fillet) and which does not (the ground flank).

Preferred process: plasma for precision and aerospace gears, controlled gas (Floe or Nitreg) for industrial volume. Plasma's lower temperature and tighter distortion budget is the technical fit for precision work. Dominant failure modes: pitting at the pitch line when the case is too shallow, and root cracking when the compound layer is too thick or porous at the fillet.