Ultrasonic detection by diffractive time difference method:Time Of Flight Diffraction，Short for TOFD. The basic principle of the detector is to use two wide-band narrow pulse probes, one for transmitting and one for receiving, which are symmetrically arranged relative to the weld center line. The transmitting probe generates the unfocused longitudinal wave beam, which is incident into the inspected workpiece at a certain angle. Some of the beams propagate along the near surface and are received by the receiving probe, and some of the beams are received by the probe after being reflected by the bottom surface. The receiving probe determines the position and height of the defect by receiving the diffraction signal of the defect tip and its time difference. TOFD technology has the advantages of good reliability, high quantitative accuracy, simple and fast accurate measurement and detection. The most commonly used non parallel scanning can be operated by only one person, and the probe can only move along both sides of the weld without sawtooth scanning. It has high detection efficiency and low operation cost. TOFD is widely used in pressure pipeline, pressure vessel and other chemical industries.