Abstract
Type IV creep voids are creep voids that originate inside the weld zones of high-chromium steel piping. The voids occur in main piping structures for ultra-supercritical pressure coal-fired power generation equipment. The damage of the high-chromium material will start with the increase of creep voids density and grows to the micro-cracks and at last grows to the macro-cracks to relate for the rupture accidents. Since these damage considered serious in power generation industry, the mechanism of the Type IV creep damage have been studied all over the world. Tough conventional nondestructive inspection methods, including advanced phased-array ultrasonic system, applied for the damage detection of piping structure with deep-zone creep voids, any methods cannot satisfy the resolution to estimate the creep damaged at the periodic inspection of power plant. Thus, new nondestructive evaluation techniques to detect the microstructural creep damage of concentration of creep voids on grain boundaries have been required. The authors focused on water immersion acoustic imaging technique, which has the highest spatial resolution among the commercial ultrasonic inspection systems, to estimate initial stage of Type IV creep void. We developed a new measurement system with designing a high-focusing aspherical angle acoustic lens, and with its scanning pitch modified with high precision. As a result, we have discovered that the new measurement system which we have developed can lead to the evaluation of Type IV creep void density in deep areas of the material.
