The Collaborative International Dictionary of English v.0.48:

Carbonize \Car"bon*ize\, v. t. [imp. & p. p. Carbonized; p.
   pr. & vb. n. Carbonizing.] [Cf. F. carboniser.]
   1. To convert (an animal or vegetable substance) into a
      residue of carbon by the action of fire or some corrosive
      agent; to char.
      [1913 Webster]

   2. To impregnate or combine with carbon, as in making steel
      by cementation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

carbonize
    v 1: unite with carbon; "carburize metal" [syn: carbonize,
         carbonise, carburize, carburise]
    2: turn into carbon, as by burning; "carbonize coal" [syn:
       carbonize, carbonise]