A high sensitivity ultrasonic testing system for inspection of the weld seam of ERW pipes was developed. The factors that affect the quality of the weld seam were investigated using an ultrasonic C-scan method with a focused probe and samples sliced from weld seams. As the result, it was found that a scattered-type penetrator consisting of micro oxides is a key factor in the quality of the weld seam. Absorbed energy in the Charpy impact test can be evaluated by the ultrasonic echo amplitude with the optimized focused beam size (about 1mm2) to detect the scattered-type penetrator.
In order to evaluate the density of the scattered-type penetrator in weld seams with the optimum focused beam size for pipe, a point focused beam tandem method was developed by applying the ultrasonic phased array technique. The sensitivity of the developed method is 20dB higher for a standard artificial through drilled hole whose diameter is 1.6mm. A precise seam tracking system was also developed for application of the point focused beam tandem method to the actual ERW pipe manufacturing process. Since the allowance for applying the focused beam to the weld seam is very narrow, i.e., about 1mm, a circumference multi-point simultaneous receiving technique and thermal image-type seam detection technique were developed. The developed ultrasonic testing system has been in operation at the 24″ ERW mill at East Japan Works (Keihin District) of JFE Steel Corporation since March 2011. The combination of the ultrasonic testing system and an oxide control technique now contributes to production of high-performance, high-quality ERW pipe “Mighty Seam®” for use in frigid environments.