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

The paper presents the resuIts of the investigation of the influence of duration and temperature oftempering process on the magnetoacoustic properties of P91 tempered martensite ferritic steeI. Duringtempering of martensitic steels the dislocation density. which initially is very high. systematicallydecreases what is confirmed both by analysis of the width of X-ray diffraction peaks and hardnessmeasurements. Magnetic hysteresis loops undergo a systematic change resulting in the monotonousdecrease of magnetic ''hardness'' of the investigated sampIes. The influence of the tempering time onthe Barkhausen noise (BN) and magnetoacoustic emission (MAE)signals intensity was investigated inorder to verify the possibility of application of those methods for a nondestructive monitoring of heattreatment quality. Barkhausen noise signal envelopes change their shape very strongly during theinitial stages of tempering. yet the overall signal intensity is weakly affected. As for the magnetoacousticemission a very strong increase of its intensity is observed for almost all the tempering timesand temperatures. It was concluded that the MAEsignal intensity is a suitable parameter for theevaluation of tempering time. as for the BNsignal the envelope shape analysis would be necessary forsuch a purpose.

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