face-harden

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive verb To harden the surface of (a metal).

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb to harden (steel) by adding carbon.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb harden steel by adding carbon