Polishing and surface finish for forge service

Polishing removes material. After nitriding, every micron the polisher takes off comes out of the compound layer or the upper diffusion zone. Typical removal from a post-nitride finishing pass is 2-5 µm per side, which is most or all of a controlled compound layer.

The white layer budget set in Lesson 8 has to include the polishing allowance. If the spec calls for a 5 µm γ' compound layer at delivery to the polisher and the polisher removes 3 µm, the die ships with 2 µm of compound layer. That may be the right answer for some service, but it has to be designed in rather than discovered. The nitride vendor and the polisher both need to know the as-delivered case structure and the post-polish target.

For PVD-bound dies the calculation is sharper. PVD adhesion wants a thinned or removed compound layer (see Lesson 8), and the polish before PVD is doing that removal job. A 5 µm compound layer minus a 4 µm polish leaves 1 µm of compound layer plus the diffusion zone underneath, which is the supporting layer the PVD coating sits on. Skipping the polish puts the PVD on top of an intact compound layer, with predictable adhesion failure within hundreds of hits.

The traveler should specify compound layer thickness at delivery from the nitride vendor, removal target at polish, and inspection method for the post-polish surface, typically Ra measurement plus a cross-section on a witness coupon.