Abstract: Cracks and surface flaws in billets are detected by heating the surface of the work-piece by means of high frequency current, the heating taking place while the work-piece is brought to pass in its longitudinal direction through a high frequency induction coil. That part of the work-piece, which has just left the coil is immediately, while the work-piece is still passing through the coil, scanned by means of infrared recording equipment which records the temperature distribution across the work-piece. The emission coefficient for the different work-pieces is increased and differences equalized by moistening the surface of the work-piece with an appropriate liquid, for instance water, to which have been added components for reduction of the surface tension.