Polishing and surface finish for forge service

Three mechanisms make a polished forge cavity worse than a moderate-Ra cavity in service.

Galling against aluminum and brass. Aluminum forging at 450-500°C and brass forging at 600-800°C both rely on a lubricant film, typically a graphite-water emulsion sprayed onto the cavity between hits. The lubricant needs surface roughness to anchor. On a polished surface (Ra under 0.2 µm) the film beads up, sheets off, or evaporates without depositing a continuous layer. The aluminum extrudes into intimate contact with bare H13 and welds, pickup builds on the cavity walls, transfers to the next forging, and propagates until the die is scrap.

Scale retention in steel forging. Hot-forged steel arrives at the die at 1100-1250°C with a layer of iron oxide scale that flakes off during forming. A polished cavity holds scale fragments in surface depressions where they would otherwise be swept out by the next hit. The trapped scale grinds at the cavity surface and accelerates abrasive wear. A moderate-Ra cavity sheds scale more readily.

Adhesive pickup amplification. Any time two clean metal surfaces contact at high temperature and pressure, cold welding is possible. The cleaner and smoother both surfaces are, the higher the probability of the weld. Polish drives both surfaces in the wrong direction for hot forging service.

The exception is the extrusion bearing and the gate-runner system, where flow dynamics dominate and contact-galling is secondary.