Brief overview of the steps involved in ultrasonic testing for non-destructive inspection

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The leak detection system of ultrasonic detectors differs from specific gas sensors, which are limited to the particular gases they are designed to detect; instead, it uses sound for detection. By scanning with an ultrasonic detector leak detection system, a leakage sound can be heard through the headphones or changes in the digital signal can be observed. The closer to the leak point, the more obvious it is. 1 Principle of ultrasonic testing: Ultrasonic flaw detection is a non-destructive testing method that utilizes the differences in the acoustic properties of materials and their defects to examine internal defects in materials, by analyzing the reflections of ultrasonic waves and the changes in energy associated with their penetration time. In pulse-echo method, longitudinal waves are used for vertical flaw detection, while transverse waves are used for oblique flaw detection. Pulse reflection testing includes longitudinal wave testing and transverse wave testing. On the oscilloscope screen of the ultrasonic instrument, the horizontal axis represents the propagation time of the sound waves, while the vertical axis indicates the amplitude of the echo signal. For the same homogeneous medium, the propagation time of pulse waves is proportional to the distance traveled. Therefore, the presence of defects can be determined by the appearance of defect echo signals ； The distance of the defect from the detection surface can also be determined by the position where the echo signal appears, thereby enabling defect localization ； The equivalent size of a defect is determined by the echo amplitude. 2 Characteristics of ultrasonic waves: 1) The ultrasonic wave beam can be focused in a specific direction and travels in a straight line through the medium, exhibiting good directivity. 2) As ultrasound propagates through a medium, it undergoes attenuation and scattering. 3) Ultrasonic waves undergo reflection, refraction, and wave pattern conversion at the interface between different media. By utilizing these properties, the reflected waves coming back from the defect interface can be obtained, thereby enabling the detection of defects. 4) The energy of ultrasonic waves is much greater than that of sound waves. 5) Ultrasonic waves suffer little transmission loss in solids, allowing for a large detection depth. However, ultrasonic waves are reflected and refracted at heterogeneous interfaces, and in particular, they cannot pass through the gas-solid interface. If the metal contains defects such as pores, cracks, delamination (with gas within these defects) or inclusions, ultrasonic waves will be reflected entirely or partially when they reach the interface between the metal and these defects. The reflected ultrasonic waves are detected by the probe, processed by the circuits inside the instrument, and waveforms of different heights and at certain intervals are displayed on the instrument’s fluorescent screen. The depth, position, and shape of defects in the workpiece can be determined based on the characteristics of the waveform changes. 6) Disadvantages: The advantages of ultrasonic testing include the ability to detect thick components, high sensitivity, fast speed, low cost, and no harm to humans; it also enables the localization and quantification of defects. Ultrasonic flaw detection does not provide an intuitive visualization of defects; it is technically challenging and easily influenced by both subjective and objective factors. Additionally, the inspection results are difficult to preserve. Ultrasonic testing requires a smooth working surface, and only experienced inspectors can identify the types of defects. It is also suitable for inspecting parts with greater thickness. All these factors contribute to the limitations of ultrasonic flaw detection. 3 Ultrasonic Testing Steps 3.1 Preparation before testing ① Familiarize yourself with the workpiece to be tested (workpiece name, material, specifications, groove type, welding method, heat treatment status, surface condition of the workpiece, testing standards, acceptable quality level, testing ratio, etc.) ； ②Select the instrument and probes (based on standard requirements and site conditions, determine the flaw detector, probes, test blocks, scanning scale, detection sensitivity, and detection method). ③ Calibration of the instrument (when the instrument is first put into use, measure its horizontal and vertical linearity.) ) ④ Probe calibration (carry out calibration for the leading edge, refractive angle, main beam deviation, sensitivity margin, and resolution.) ) ⑤ Instrument adjustment (the time baseline scale can be adjusted proportionally to represent the horizontal distance, depth, or sound range of the pulse echo. ) ⑥ Adjustment of sensitivity (the sensitivity is verified using reference blocks or other equivalent test blocks. ) 3.2, Inspection Procedures ① Inspection of the base material: Before inspection, the wall thickness of the tube should be measured, with a measurement taken at least every 90° to serve as a reference during the inspection. Adjust the secondary bottom wave at the defect-free area to the full scale of the fluorescent screen as the detection sensitivity ； ②Inspection of welded joints: The scanning sensitivity should be no lower than that of the evaluation line (EL line); the scanning speed of the probe should not exceed 150 mm/s, and when scanning, there should be at least 10% overlap between consecutive movements of the probe. 3.3 Test Results and Rating: The rating is determined based on the nature and severity of the defects, as well as the indicated length, in accordance with relevant standards. 3.4 Conduct calibration and re-inspection of instruments and equipment. 3.5 Issue a test report. Note: For welded joints with defects exceeding the specified limits, both the areas that require repair and the regions affected during the repair process must be re-inspected under the original inspection conditions. 4 Standard methods for non-destructive ultrasonic testing of metal materials: JB/T 4730.3-2005 – Non-destructive testing of pressure equipment: Ultrasonic testing; GB/T 11345-1989 – Methods for manual ultrasonic testing of steel welds and classification of test results; CB/T 3559-2011 – Ultrasonic testing procedures and quality classification for ship steel welds