High sensitivity ultrasonic NDT technique for detecting creep damage at the early stage in power plant steels

Failures of welded main steam pipes in power station plants are related to cracks forming and propagating in the fine-grained zone, so called heat-affected zone (HAZ). Experience shows that Type IV cracks are indeed some of the most common causes of creep failure for low-alloy ferritic steel pipes and headers in power stations. Typically, crack damage will be characterised by high strain rates in the HAZ and local grain boundary separation, which leads to crack growth. In this paper, the high sensitivity technique for detecting creep damage (Type IV cracking), based on focused ultrasound was implemented. Signal processing algorithms were proposed that enable associating the properties of backscattered signals to the internal characteristics of the metal structure. As the result, the boundaries of the parent metal, HAZ and weld metal in an ultrasonic image can be indicated. The approach was validated on the samples exposed to different levels of creep damage and verified with metallographic analysis. It demonstrated that technique allowed detecting clouds of early stage creep damage.

Automated summary

Failures of welded main steam pipes in power plants are often caused by cracks in the heat-affected zone (HAZ), specifically Type IV cracks. This paper presents a high sensitivity technique using focused ultrasound for detecting creep damage, which allows for identifying the internal characteristics of the metal structure through signal processing algorithms. The technique was validated on creep-damaged samples and verified with metallographic analysis, demonstrating its ability to detect early stage creep damage.