Jun-Hwa Hong

Evaluation of thermal recovery of neutron-irradiated SA508-3 steel using magnetic property measurements

The Vickers microhardness and magnetic properties have been used to investigate irradiation effects and thermal recovery characteristics of neutron-irradiated SA508-3 reactor pressure vessel steel specimens irradiated to a neutron fluence of 5.5×1017 n cm-2 (E>1 MeV) at 70°C. Two recovery stages were identified from the hardness results during isochronal annealing and the mechanism responsible for the two stages was explained using the results of Barkhausen noise on the basis of the interaction between radiation-induced defects and the magnetic domain wall. The neutron irradiation caused the coercivity to decrease, whereas the maximum magnetic induction increased. Barkhausen noise parameters associated with the domain-wall motion were decreased by neutron irradiation and recovered with subsequent heat treatments. From the sensitive changes in the Barkhausen noise upon annealing heat treatment, it is suggested that the Barkhausen noise measurements may be used as a useful tool for monitoring the early stage of the thermal recovery behaviour of neutron-irradiated reactor pressure vessel steels.

The Effect of BCC-Cu Precipitation on Magnetism in Thermally Aged Fe-1wt.%Cu Model Alloys

Magnetic properties of thermally aged Fe-1wt.%Cu alloys were investigated. With increasing aging time, coercive force gradually increased and Barkhausen signal peak voltage decreased. Both behaviors suggested that Bloch wall movement was hindered by Cu precipitates and the pinning was strengthened with increasing precipitate size. On the other hand, at the initial precipitation stage where the precipitate size is much smaller than the Bloch wall width, the inclination of the hysteresis curve greatly changed, but recovered with further thermal aging. The peculiar inclination behavior would be associated with the nucleation of coherent bcc-Cu precipitates

A study on the radiation damage and recovery of neutron irradiated vessel steel using magnetic Barkhausen noise

The radiation damage and thermal recovery characteristic of neutron irradiated SA508-3 reactor pressure vessel steel specimens have been investigated. Two recovery stages were identified from the results of hardness measurements during isochronal annealing and the mechanism responsible for the two stages was explained by using the results of Barkhausen noise measurement on the basis of the interaction between radiation induced defects and the magnetic domain wall. The coercivity was not changed by neutron irradiation, whereas the maximum magnetic induction increased. Barkhausen noise parameters associated with the domain wall motion were decreased by neutron irradiation and recovered with subsequent heat treatments.

Changes in magnetic properties of neutron irradiated RPV steel

Conventional magnetic parameters and Barkhausen noise have been measured in pressure vessel steel samples both as-received and irradiated with doses of up to 10/sup 18/ n/cm/sup 2/. The conventional magnetic parameters, i.e. coercive force, remanence and maximum induction did not change significantly with irradiation, whereas the Barkhausen noise amplitude and energy during a magnetization cycle decreased markedly with irradiation dose. A three stage variation of Barkhausen noise with neutron dose was observed in the present work, namely an initial decrease, a near plateau and rapid decrease. The three stage variation with neutron dose is in qualitative agreement with computer simulations of the radiation damage process performed by Beeler (1965). The hardness also varied in three stages in a reverse manner with transition at the same doses.

Microstructural dependence of Barkhausen noise and magnetic relaxation in the weld HAZ of an RPV steel

Magnetic Barkhausen noise and permeability spectra have been measured to characterize different microstructure regions such as coarse-grain region, fine-grain region, intercritical structure (composed of tempered martensite and bainite) within the heat-affected zone (HAZ) of SA508-3 steel weldments using simulated HAZ microstructure sample. The intercritical region and coarse-grained region can be distinguished from the BNE and relaxation frequency. The BNE was decreased in the martensite regions and increased in the bainite regions by the post-weld heat treatment (PWHT). The change of relaxation frequency also showed similar trends, but the rate of change was less than that of BNE. The behavior of BNE and permeability spectra on the corresponding microstructure can be explained in terms of carbide morphology and residual stress related with domain wall motion.

Evaluation of irradiation effects of 16 MeV proton-irradiated 12Cr1MoV steel by small punch(SP) tests

Recently, interest in small-scale specimens for testing irradiated materials has arisen in conjunction with the need to develop materials for fusion reactor materials and to study irradiation effects using an ion irradiation facility. Several attempts have been made to evaluate material property changes due to irradiation using a small specimen technique. The SP (small punch) test is an example of small-scale specimen test techniques, originally developed by Baik et al. to estimate DBTT (ductile-to-brittle transition temperature) using broken standard CVN (Charpy 5-notch) specimens. The objective of the present study is to evaluate 16 MeV proton irradiation effects on a fusion reactor candidate material in terms of changes in energy up to failure and J[sub IC] fracture toughness (SP J[sub IC]) by using a SP test technique and a J[sub IC] - [bar [epsilon]][sub qf] relationship. It has been known that protons at 16MeV accurately simulate the 14 MeV neutron-damage energy spectrum over most of the PKA energy range.

Effect of microstructures on magnetic Barkhausen noise level in the weld HAZ of an RPV steel

Microstructural state and magnetic Barkhausen noise (BN) level have been correlated in the weld heat-affected zone (HAZ) of a pressure vessel steel. The BN level increased with increasing size of carbide, and the tempered bainite structure showed higher BN signal than the tempered martensite. The results indicated that heat-treated materials may result in microstructurally different domain wall pinning obstacles at different thermal cycles.

Irradiation effects on magnetic properties in neutron and proton irradiated reactor pressure vessel steel

The effects of neutron and proton dose on the magnetic properties of a reactor pressure vessel (RPV) steel were investigated. The coercivity and maximum induction increased in two stages with respect to neutron dose, being nearly constant up to a dose of 1.5/spl times/10/sup -7/ dpa, followed by a rapid increase up to a dose of 1.5/spl times/10/sup -5/ dpa. The coercivity and maximum induction in the proton irradiated specimens also showed a two stage variation with respect to proton dose, namely a rapid increase up to a dose of 0.2/spl times/10/sup -2/ dpa, then a decrease up to 1.2/spl times/10/sup -2/ dpa. The Barkhausen noise (BN) amplitude in neutron irradiated specimens also varied in two stages in a reverse manner, the transition at the same dose of 1.5/spl times/10/sup -7/ dpa. The BN amplitude in proton irradiated specimens decreased by 60% up to 0.2/spl times/10/sup -2/ dpa followed by an increase up to 1.2/spl times/10/sup -2/ dpa. The results were in good accord with the one dimensional domain wall model considering the density of defects and wall energy.

Estimating unbalance mass of vertical axis washing machine by measuring current ripple of PMSM motor

In this study, first, an equivalent circuit of a permanent-magnet synchronous motor (PMSM) and a dynamic model of a washing machine are derived; second, a method for deriving the unbalance mass from the ripple of the current flowing through the electric motor is suggested; and finally experimental verification of the results are conducted.

Performance improvement of an automotive alternator by heat treatment

In this paper, heat treatment (HT) is proposed to maximize magnetic property recovering strain and stress due to recrystallization. To find the optimal point of HT, two HT are performed at two different temperatures. This HT study showed that both magnetization and core loss characteristics are greatly improved which enables application to alternator performance test. Two alternator models are fabricated with the HT stator cores and used for performance test showing 5% output increase and 0 .4% VDA efficiency improvement. Audible noise test is also performed to verify deterioration due to HT.