Kwon-Sang Ryu

A new non-destructive method for estimating the remanent life of a turbine rotor steel by reversible magnetic permeability

Abstract We present a new magnetic and non-destructive procedure to evaluate the remanent life of 1Cr–1Mo–0.25V steel using the value of reversible magnetic permeability. The method is based on the existence of reversible magnetic permeability in the differential magnetization around the coercive force. The measurement principle is based on the foundation harmonics voltage induced in a coil using a lock-in amplifier tuned to a frequency of the exciting one. Results obtained for reversible magnetic permeability and Vickers hardness on the aged sample show that the peak interval of reversible magnetic permeability (PIRMP) and Vickers hardness decreases as aging time increases. A softening curve is obtained from the correlation between Vickers hardness and the PIRMP. This curve can be used as a non-destructive method to evaluate the remanent life of 1Cr–1Mo–0.25V steel.

Analysis of giant magneto-impedance in terms of anisotropy field in annealed amorphous Co66Fe4NiB14Si15 ribbons

Abstract The magneto-impedance (MI) profiles of Co66Fe4NiB14Si15 amorphous ribbons changed sensitively with the annealing temperature, reflecting the change in anisotropy field, which determines the field dependence of transverse permeability. The effective anisotropy field, evaluated from these profiles, were discussed in terms of the increase of magnetic softness and microstructural change due to the annealing.

Degradation Evaluation of Mechanical Property for Modified 9Cr-1Mo Steel by Reversible Permeability

The present work studies a nondestructive evaluation of the degradation of modified 9Cr-1Mo steel using a magnetic method based on the existence of the peaks of reversible permeability (RP) in the differential magnetization around the coercive force. The apparatus is based on detection of the voltage induced in a coil using a lock-in amplifier tuned to the frequency of the AC perturbing field. Results obtained for the reversible permeability and Vickers hardness on the aged samples showed the peak interval of reversible permeability (PIRP) and Vickers hardness decrease as aging time increased. The correlation between Vickes hardness and the PIRP could be used to evaluate degradation of modified 9Cr-1Mo steel.

Enhancement of permeability of Co-based amorphous alloy by two-step cooling method

Abstract The soft magnetic properties of Co-based amorphous alloy have been investigated as a function of cooling condition followed by high-temperature annealing. Furnace cooling to the Curie temperature and subsequent water quenching (F&W method) considerably increased the initial permeability by 60% and 100% higher than those of the furnace-cooled and water-quenched samples, respectively. The F&W sample also showed superior high-frequency characteristics. This phenomenon was analyzed in terms of internal thermal strees and the magnetic ordering. We confirmed the alloy annealed by F&W shows better shielding effectiveness

Detection of Magnetic Phase in the Steam Generator Tubes of NPP

Steam generator tube (SGT) in nuclear power plant is a boundary between primary side and secondary side, whose integrity is one of the most critical factors to nuclear safety (Roberts, 1981). The SGT is made of nickel based Inconel alloy, which is composed of 75% Ni, 16.5%Cr and 8.15%Fe. Inconel alloy has very little magnetic response due to its austenitic nickel base, and the domain walls move relatively unimpeded through the material. The SGT are continuously exposed to harsh environmental conditions including high temperatures, pressures, fluid flow rates and material interactions resulting in various types of degradation mechanism such as corrosion, pitting, denting and inter granular attack. Multifrequency eddy current inspection techniques are currently among the most widespread techniques for the rapid inspection of SGT in nuclear power industry. Although the eddy current test (ECT) technique is adopted widespread in the nuclear industry (Bakhtiari, 1999), it has the limitation to size the flaw accurately because the eddy current measure the impedance signified by the conductivity change associated with the volumetric change of flaws, where the permeability of flaw is considered unity. The EC test currently applied in the nonferrous materials having relative permeability 1 such as Inconel alloy because the magnetic permeability of magnetic materials severely limits the depth of penetration of induced eddy currents. Furthermore, the small magnetic phase having permeability variation inherent in SGT can cause spurious EC test results (Park et al., 2010). It is well known that some part of SGT change as a magnetic phase known as permeability variation clusters (PVC) under the conditions of high pressure and temperature which is the operating environments of nuclear power plant. The relative permeability of the magnetic phase is greater than 1, and with a number of ferromagnetic metals, a value of several thousand can be reached. Internal stresses caused by drawing, straightening, or similar work on the material, can give rise to severe fluctuations in the permeability (Takahashi, 2004). These fluctuations would always cause interference with the test signals. In order to eliminate this interference effect during testing, the ferromagnetic test piece is magnetized by a suitable device such as magnetized ECT probe. Recently, to eliminate ECT signal fluctuation, the magnetized probe with the built-in permanent magnet is being used in the SG tube inspection, because a strong magnetic field of this probe reduces the variation of magnetic permeability, which gains S/N ratio. If we can separate magnetic phase (MP) selectively from the flaws using magnetic sensor, the reliability of EC in SGT inspection will

Change in magnetic properties of a cold rolled and thermally aged Fe–Cu alloy

The variation in magnetic properties of a Fe–1%Cu model alloy due to a cold rolling and a thermal aging has been evaluated to simulate the radiation damage of reactor pressure vessel of nuclear power plant. The thermal aging was conducted at 500 °C with different aging times in series. The hysteresis loops, magnetic Barkhausen noise (BN) and Vickers microhardness were measured for prestrained, strained, and thermal aged samples. The coercivity increased by a plastic strain and decreased by thermal aging, The BN decreased in the prestrained and strained samples but large changes were observed in the strained sample. These results were interpreted in terms of the domain wall motion signified by a change in the mean free path associated with microinternal stress and copper rich precipitates.

Quantitative phase analysis of boron nitride nanotubes using Rietveld refinement

Crystalline boron nitride nanotubes (BNNTs) with an average outer diameter of about 40?nm and several micrometres in length were synthesized by chemical vapour deposition from boron and magnesium oxide. Rietveld refinement and 11B solid-state nuclear magnetic resonance (NMR) spectroscopy were employed to quantitatively determine the phase of synthesized BNNTs. The structural parameters for the BNNTs were determined by Rietveld refinement against powder x-ray diffraction data with a model based on the results of NMR analysis. The final weighted R-factor, Rwp, was 6.08% and the goodness-of-fit indicator, S (= Rwp/Re), was 1.52. The BNNTs consisted of hexagonal BN, rhombohedral BN and MgO with refined weight fractions of 76.3%, 21.2% and 2.5%, respectively.

Calculation of saturation magnetostriction from measured second harmonics in 3% silicon steel

The profiles of the second-order harmonics during B-H hysteresis loop have been measured as functions of the tensile stress and the angle /spl phi/ between H-field and [001] axis in 3% silicon steel. The observed harmonics profiles have been analyzed in terms of the nonlinear, asymmetric magnetic induction which reflects the domain reorientation under the field. The field interval of the second harmonics is related to the nucleation and annihilation fields and a measurement method for the magnetostriction is proposed using the harmonics profiles under tensile stress.

Magnetic field simulation of magnetic phase detection sensor for steam generator tube in nuclear power plants

Magnetic phases and defects are partly produced in steam generator tubes by stress and heat, because steam generator tubes in nuclear power plants are used under high temperature, high pressure, and radioactivity. The magnetic phases induce an error in the detection of the defects in steam generator tubes by the conventional eddy current method. So a new method is needed for detecting the magnetic phases in the steam generator tubes. We designed a new U-type yoke which has two kinds of coils and simulated the signal by the magnetic phases and defects in the Inconnel 600 tube.

Reactance simulation for the defects in steam generator tube with outside ferrite sludge

A magnetic sludge is partly produced around the tube sheet outside a steam generator due to stress and heat. The sludge with magnetite is one of the important factors affecting eddy current signals. It causes trouble for the safety of the steam generator tubes and is difficult to detect by conventional eddy current methods. A new type of probe is needed to detect the signals for the magnetic sludge. We designed a new U-type yoke which has two kinds of coils—a magnetizing coil and the other a detecting coil—and we simulated the signal induced by the ferromagnetic sludge in the Inconel 600 tube.