Nondestructive evaluation of degraded structural materials by micromagnetic technique

Abstract

The variations of magnetic properties and Barkhausen noise (BN) characteristics on the microstructural evolution, induced by isothermal annealing and isochronal tempering of 12\% CrMoV steel and cold rolling of ASTM A 508 reactor pressure vessel (RPV) steel, have been investigated to evaluate degraded structural materials nondestructively. In the isothermally annealed specimens, a rapid decrease in coercive force(Hc), remanence (Br), hysteresis loss (Wh) and hardness took place in accordance with the release of internal stresses from supersaturated martensite after annealing for only 20 minutes at 923 K. The induced noise voltage picked up by the BN sensor is proportional to the pinning force of the domain walls by defects and the sweep-out volume by domain wall in the course of irreversible displacement. BN energy is correspondingly related with three stages of microstructural evolution such as the recovery of strain energy, the increase of precipitates size as a result of Ostwald ripening and the annihilation of dislocation density during isothermal annealing. In the isochronally tempered specimens, a secondary hardening characteristic appears with an increase to a peak of 47.5 HRC, typically after 1 hr at the tempering temperature of 773 K, followed by overaging. The first effect of tempering is a slight increase in the structure-sensitive magnetic properties of Hc, Br and Wh, regardless of austenitizing temperatures, followed by a quite rapid magnetic softening with a minimum at about 1023 K. Then, the magnetic properties increase abruptly from 1073 K by passing the Curie temperature of 1041 K for pure iron. The variation of the BN energy with tempering temperature exhibits an inverse relation with that of structure-sensitive magnetic properties. In the plastically deformed ASTM A 508 RPV steel, BN parameters of BN energy, BN counts and BN peak amplitude, measured by surface type BN sensor, decrease with plastic deformation due to the hindrance of domain wa...