FLAW INSPECTION APPARATUS FOR STEEL PLATE AND CAST PIECE

PURPOSE:To prevent the flux density at a detecting part from fluctuating due to the fluctuation of gap between the detecting part and the surface of a steel plate and the gap between a magnetizer and the surface of steel plate, and to prevent the saturation of a signal when a high sensitivity detector is employed. CONSTITUTION:A magnetic detector 23 is loaded to a magnetizer 11 through a vertical moving mechanism 14 and a horizontal scanning mechanism 15 and then the vertical moving mechanism 15 operated to sustain a constant gap between the magnetic detector 23 and a steel plate 22. A compensation coil 22 disposed closely to the magnetic detector 23 generates a compensation field. The magnitude of the compensation field is regulated by a signal obtained by smoothing the field detected by the magnetic detector 23 thus suppressing the increase of flux density. Subsequently, the horizontal scanning mechanism 15 is operated to scan the magnetic detector 23 horizontally and leakage flux from the steel plate is detected based on an output signal from the magnetic detector 23.