White layer

The surface of nitrided steel which has been converted to a complex iron–nitrogen compound.

It is called white layer because it does not etch (i.e. remains white) when a nitrided microstructure is prepared.

During the nitriding cycle (the length of which is dictated by the case depth required), a surface coating is produced on the component, known as the ‘white layer’, Fe4N. This tends to be brittle and is often better removed after nitriding by polishing, an allowance of 0.002˝ per surface is usually sufficient for this.