According to the constancy of sound velocity propagation in the same medium and the proportional relationship between propagation distance and propagation time, the position of waveform reflected on the oscillograph screen can be located. The size of casting defects (usually used to measure the length of weld defects) generally adopts the “6dB method”, that is to reduce a certain DB value to determine the length of casting defects. After moving the probe to find the maximum reflected wave of casting defect, adjust the attenuator to increase the sensitivity of the instrument by 6dB, and move the probe along the direction of casting defect. When the wave height of casting defect drops to the reference wave height, the distance between the center lines of the probe is the indicated length.
If the echo amplitude of casting defect is equal to or greater than the distance amplitude curve, the range of casting defect shall be drawn on the flaw detection surface by 6dB method, so as to calculate the area of casting defect. When longitudinal wave detection is used, the boundary and depth of the casting defect near the flaw detection surface are determined by the amplitude of the front edge of the casting defect echo pulse rising to a position 6dB lower than the distance amplitude curve. When the transverse wave angle probe is used for detection, the boundary of casting defect is determined by the rise of the front amplitude of casting defect echo pulse to a position 6dB lower than the maximum casting defect echo amplitude displayed at the same time.
The size of casting defects in the thickness direction of cast steel joints is obtained from the upper and lower boundaries of casting defects determined by detection in different directions. Due to the limitation of steel casting geometry, the casting defects that cannot be detected from different directions must be described in the flaw detection report.
For the position where the bottom echo decreases by 12dB or more, check whether the bottom is inclined or uneven and whether the coupling contact is good. If the bottom echo is reduced by 12dB or more due to defects, the range of casting defects shall be drawn on the flaw detection surface on the condition that the bottom echo is reduced by 12dB. The depth of casting defect is determined by the position of the leftmost wave crest in a continuous defect echo. The size of casting defect in the thickness direction of cast steel joint is determined by the width between the leftmost and rightmost wave peaks in a continuous casting defect echo. Where there is neither bottom echo nor casting defect echo, the flaw detection sensitivity shall be improved to observe whether there are casting defects with inclined reflection surface and incident sound beam.