supercarbonize

from The Century Dictionary.

• To impregnate locally with carbon (a mass of steel) which should not elsewhere contain so high a percentage of carbon, or of carbon combined with the iron. This method is used in the case of armor-plates or other steel elements which must resist perforation, or impact, or abrasive attack on the surface exposed, but which would be inconveniently brittle if the hardening effect of a high percentage of carbon was operative all through the mass or thickness.