Abstract: To carburize the inner surface of hollow, narrow work pieces, such as valve seats in blind bores, and avoid the use of salt baths which have residues containing Prussic acid, a mandrel is made fitting with little clearance in the order of 0.1 mm into the blind bore, and having a carbon content in the range in which carbon is yielded which is higher than the carbon content of the work piece to be carburized, the mandrel or carburizing element having a surface which fits or matches the surface of the work piece with the clearance. The work piece and the carburizing element are heated to carburizing, incandescent temperature and the carburizing element is removed from the work piece after carburizing thereof.