Analysis of the possibility of creep damage detection in T24 heat resistant steel with the help of magnetic nondestestructive testing methods

The paper presents the result of an analysis of applicability of various electromagnetic methods of nondestructive evaluation for creep damage detection in a novel heat resistant steel - T24 grade. Two sample sets, cut out from membrane wall tubes, were investigated – the as-delivered one and another exploited for 36,000 h in a power plant. There are described results of magnetic hysteresis loops B(H), Barkhausen noise (BN) and magnetoacoustic emission (MAE) signals measurements. Changes of the B(H) loops shape are observed mainly in the “knee” regions. The BN signal is practically unaffected by exploitation. The most strongly changing signal is the MAE signal, yet the change is not very easy to quantify as it concerns signal envelopes shape not overall intensity. The paper describes various possible signal parameters that can be used. The best one seems to be the ratio of the total pulse count for a quarter of magnetisation (demagnetisation) to the one for the magnetisation half-period. The important issue is in that case a proper choice of the threshold level in order to obtain good sensitivity to creep damage level and reasonably low stochastic pulse count scatter.