Iron nitride compound zone (epsilon and gamma prime phases) on top of a nitrided case, named because it etches white in nital. It resists wear and corrosion, but when thick or mixed-phase it is brittle and can spall under contact loads, so gear and die prints often cap it or require it ground off. In FNC parts the white layer is the main point of the treatment.
Print says 0.0003 max white layer on the flanks, so plan on a light grind after nitride.
Machinists use white layer for the rehardened skin left by abusive grinding or EDM, a different thing that also etches white.
Also heard as
- compound layer
- compound zone
Related terms
Nitriding
Diffusing nitrogen into steel below its critical temperature to form a hard case with no quench.
FNC
Ferritic nitrocarburizing: adding nitrogen and carbon below the critical temperature for a thin, hard layer.
Ion nitriding
Nitriding with a glow-discharge plasma in a vacuum vessel, which allows close control of the white layer.
Grinding burn
Heat damage to a hardened surface from abusive grinding, which softens it or creates brittle rehardened spots.