Nondestructive evaluation of creep damage in power-plant steam generators and piping by magnetic measurements

Magnetic hysteresis measurements have been used to evaluate creep damage in power plant weldments. This method relies on the sensitivity of the magnetic properties of steels, such as coercivity, remanence and hysteresis loss, to microstructural changes occurring during creep. During high temperature creep there is a significant change in microstructure such as the formation of voids, dislocation networks and grain boundary cavities. The evolution of these defects during creep affects the magnetic properties by changing the impedance to magnetic domain wall motion and also by introducing internal demagnetizing fields associated with cavities. The present paper discusses results obtained from on-site inspection of creep damaged Cr-Mo steel welds at two thermal power plants. One of the objectives of this research was to establish whether there were any trends in the magnetic properties as a result of creep damage which could be used later as part of a more comprehensive screening procedure for monitoring the progress of creep damage.