Abstract
We have investigated effects of long-term thermal aging at 550°C up to 10000 h on major-loop coercivity, hysteresis scaling of minor loops, and first-order reversal curves (FORCs) for low-alloy pressure vessel steels with low and high Ni contents. While major-loop coercivity and minor-loop coefficient of the scaling exhibit a gradual decrease with aging for high-Ni steel, those for low-Ni one are very weakly dependent on aging time. On the other hand, we found that FORC distribution becomes steep along both axes of interaction and switching fields and the peak shifts toward a lower switching field for both steels. Considering that there is no significant development of nanoscale precipitates during the aging as revealed with small-angle neutron scattering experiments, a relaxation of lattice strain in a matrix, possibly associated with diffusion of Ni atoms, may dominate magnetic properties at 550°C.
Highlights
A development of nondestructive evaluation techniques including a magnetic method for irradiation hardening in reactor pressure vessel steels is an important issue for long-term operation of nuclear power plants
In addition to hysteresis scaling method using symmetrical minor loops, that we extensively investigated due to its low measurement field useful for a possible application to nondestructive evaluation technique,[6,7] we focused on first-order reversal curves (FORCs); they are one kind of asymmetrical minor loops and give several aElectronic mail: koba@iwate-u.ac.jp 2158-3226/2017/7(5)/056002/6
We found that after aging up to 10 kh, FORC distributions for both VVER440 and VVER1000 become steep along both axes of interaction and switching fields and the peaks shift toward a lower Hc
Summary
A development of nondestructive evaluation techniques including a magnetic method for irradiation hardening in reactor pressure vessel steels is an important issue for long-term operation of nuclear power plants. The steels are exposed to neutron irradiation and high temperatures of 290◦C, resulting in the formation of nanoscale defects.[1,2] Recent magnetic investigations revealed a change of magnetic hysteresis during neutron irradiation, the detailed mechanism was not fully understood, primarily due to combined effects of irradiation effects (defect formation) and thermal embrittlement (recovery).[3,4,5] Investigations of steels subjected to thermal aging may give an insight into thermal embrittlement, and into irradiation effect by comparing data for aged and irradiated steels. We here investigate an effect of long-term thermal aging on magnetic hysteresis for VVER440 and high-Ni VVER1000 low-alloy pressure vessel steels. In addition to hysteresis scaling method using symmetrical minor loops, that we extensively investigated due to its low measurement field useful for a possible application to nondestructive evaluation technique,[6,7] we focused on first-order reversal curves (FORCs); they are one kind of asymmetrical minor loops and give several
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