A study on detection of micro-cracks in the dissimilar metal weld through ultrasound infrared thermography

• Through hot spots obtained from each 0–90° angle, this study could judge the location of defects.
• When more than one micro-crack existed nearby, hot spots appeared as a wide and single defect.
• When vibration was applied to the outside of a pipe, much clearer thermographic images could be detected from the inside.
• It was found that IGSCC micro-crack defects existing in PWSCC can be detected in the initial stage through ultrasound infrared thermography.

This study was conducted to investigate a possibility of detecting stress corrosion crack defects in a pipe welded with dissimilar metals (STS304 and SA106 Gr. b) through infrared ultrasound thermography and lock-in phase method. The ultrasound generator was set as 250 W in output and 19.8 kHz in frequency. With experiment results, this study could detect, cracks located inside the dissimilar metal weld pipe through lock-in infrared thermography and compare thermography images obtained from both the inside and the outside when the ultrasound vibration was applied to the outer part of the pipe. Besides, after cutting off the pipe in the axial direction, this study conducted PT inspection. As a result, it was found there existed more than a single crack in a certain range inside the pipe, which made hot spots appear in a wide range on the thermography image. Moreover, through ultrasound infrared thermography and lock-in phase method this study verified the possibility of detecting micro-sized shattered cracks through ultrasound thermography, which were not easy to detect with the existing techniques.