Definitions:

1. The harden the surface of, as by chilling or casehardening. VERB
   • The face-hardened surface of the tool provided excellent cutting performance and extended tool life.
2. To harden steel by adding carbon. VERB
   • The surface of the metal was face-hardened to increase the resistance to wear and tear.

Synonyms:

• fortify
• solidify
• stiffen
• strengthen
• toughen