Abstract
In high-chromium ferrite heat-resistant steel pipe welds, which are often used in ultra-supercritical pressure boilers in Japan, a large number of Type IV creep voids occur in the region deep beneath the outer surface of the pipe and then develop into cracks. When a crack is detected, the main piping of the power plant is judged to have zero remaining life. Since the damage occur in deep areas of the material and there is no non-destructive evaluation (NDE) method for dense creep voids prior to crack formation, NDE methods cannot be used for plant-maintenance planning. The authors selected the water-immersion acoustic-imaging method, which has the highest measurement resolution among the ultrasonic-measurement methods that can be expected to be applied to actual equipment, and searched for a measurement method that greatly exceeds the existing measurement resolution. And we focused on the reception characteristics of ultra-high-focused acoustic lens. For that purpose, we designed an aspherical, angled acoustic lens that can be applied to the welded part of the actual piping. We also developed a new measurement system that can mechanically scan this probe with high accuracy. As a result, using the developed measurement system, changes in the scattered wave amplitude could be observed depending on the presence or absence of Type IV creep void density in the deep region of the material, which led to the evaluation of the void density area.
