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

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.