Non-destructive characterisation of N/Al level in P91 steels using electromagnetic sensors

Abstract

P91 steels with different N/Al ratios short term tempered or long term aged at a series of temperatures have been characterised using a multifrequency electromagnetic sensor. It was found that the low frequency inductance value is sensitive to the intralath MX precipitates that determine the mean free path for domain wall motion and hence the initial relative permeability of the steels. The electromagnetic sensor is capable of separating P91 steels with low N/Al ratio in the service entry or early service state. In contrast, the hardness measurements were found to be insensitive to the N/Al ratio for the short term tempered samples.

Electromagnetic evaluation of the microstructure of Grade 91 tubes/pipes

“…It has been shown that the multi-frequency EM sensors are able to detect the initial relative permeability and resistivity changes, resulting from microstructural changes in P9 and T22 power plant steels during service at high temperatures [16]. The developed laboratory-based EM sensor is also capable of differentiating P91 steels with different N:Al ratios in the short term tempered or long term aged condition, based on the principle that the N:Al ratio significantly affects the number density of intra-lath MX carbonitride precipitates [17]. These precipitates determine the mean free path to magnetic domain wall motion and hence the initial relative permeability of the steels and the EM signals.…”