ULTRASONIC FLAW DETECTION FOR ROUND STEEL BAR

PURPOSE:To perform flaw detection right under the surface of a material with good sensitivity by making ultrasonic waves incident to a round steel bar at a 45+ or -8 deg. angle of refraction thereto, and detecting the reflection signal from the area near the 1st reflection position where ultrasonic waves converge. CONSTITUTION:While a round steel bar 2 which is held sandwiched from above and below at the front and rear of a water tank 1 is revoleved around its axial center by means of skew rolls 3 and pinch rolls 4, the bar 2 is conveyed at a constant speed in a longitudinal direction, and ultrasonic waves are transmitted from a probe 9, whereby the flaw detection is accomplished. In this case, the ultrasonic waves are made incident to the bar 2 from the outside surface thereof in such a way that the angle alpha of refraction attains 45+ or -8 deg., and these are converged at the 1st reflection position 1B where the ultrasonic waves entering the inside of the bar 2 reflect on the material surface. A flaw detector 12 gates the range G before and behind the point 1B, detects the reflection signal generated from the range G and detects the non-metallic inclusions existing within the range of about 10% below the surface in the bar 2. Thereby, defects are detected with high sensitivity and the influence of surface roughness is eliminated.