face-harden

American Heritage® Dictionary of the English Language, Fourth Edition

1. v. To harden the surface of (a metal).

Century Dictionary and Cyclopedia

1. To make hard on the surface; harden a thin layer on the face of (a piece of metal); subject to a face-hardening process.

GNU Webster's 1913

1. v. to harden (steel) by adding carbon.

WordNet 3.0

1. v. harden steel by adding carbon