Study on the applicability of the measurements of magnetoelastic properties for a nondestructive evaluation of thermally induced microstructure changes in the P91 grade steel

Abstract

The paper presents the results of the investigation of the influence of duration and temperature of tempering process on the magnetoacoustic properties of P91 tempered martensite ferritic steel. During tempering of martensitic steels the dislocation density, which initially is very high, systematically decreases what is confirmed both by analysis of the width of X-ray diffraction peaks and hardness measurements. Magnetic hysteresis loops undergo a systematic change resulting in the monotonous decrease of magnetic “hardness” of the investigated samples. The influence of the tempering time on the Barkhausen noise (BN) and magnetoacoustic emission (MAE) signals intensity was investigated in order to verify the possibility of application of those methods for a nondestructive monitoring of heat treatment quality. Barkhausen noise signal envelopes change their shape very strongly during the initial stages of tempering, yet the overall signal intensity is weakly affected. As for the magnetoacoustic emission a very strong increase of its intensity is observed for almost all the tempering times and temperatures. It was concluded that the MAE signal intensity is a suitable parameter for the evaluation of tempering time, as for the BN signal the envelope shape analysis would be necessary for such a purpose.