Katsuyuki Ara

In situ magnetic measurements under neutron radiation in Fe metal and low carbon steel

Magnetic minor hysteresis loops of low carbon steel and Fe metal have been measured during neutron radiation at 563K in a 50MW nuclear reactor. For investigation of nucleation mechanism of copper precipitates and dislocation loops during neutron radiation, special attention was paid to minor-loop coefficients, which are deduced from simple relations between minor-loop parameters and are very sensitive to lattice defects such as dislocations, copper precipitates, and grain boundaries. We found that with increasing neutron fluence, the minor-loop coefficients of low carbon steel sharply increase and show a maximum at the fluence of 1×1019cm−2, followed by a slow decrease. The appearance of the maximum suggests the presence of two mechanisms of internal stress; while copper precipitates and dislocation loops in the matrix make the internal stress increase, those grown in the vicinity of dislocations compensate the internal stress of dislocations so as to minimize the elastic energy. On the other hand, the mi...

Effect of Microstructure Changes on Barkhausen Noise Properties and Hysteresis Loop in Cold Rolled Low Carbon Steel

Low carbon steel specimens cold rolled at ratios of 0-40% have been examined comprehensively by means of magnetic Barkhausen noise (MBN) method and a physical parameter obtained from a hysteresis loop, and their microstructures were studied by a transmission electron microscope. The behaviors of MBN and coercive force with reduction ratio were discussed in relation to microstructure changes. The MBN energy rises rapidly with cold rolling below 10% reduction, and saturates at higher reduction ratio. The peak in averaged rms voltage exhibited the same behavior as that of the MBN energy, whereas the coercive force and the magnetizing current when the averaged rms showed a peak increased monotonically with increasing reduction ratio. These phenomena are attributed to the combined effects of cell texture and dislocation density.

Magnetic measurement to evaluate material properties of ferromagnetic structural steels with planar coils

Nondestructive measurement experiments were carried out with a planar coil to evaluate changes of material properties of ferromagnetic structural steels. Examined steel were of A533B that is a low-alloy steel and of SUS410 that is a martensitic stainless steel. The planar coil has two windings; one is of primary for excitation and the other secondary for induction of output voltage. The coil was placed on a test plate with a magnetic yoke for application of a bias dc magnetic field, and excited with a constant current of 25 Hz. Then the output voltages were measured while slowly changing the bias field by excitation of the magnetic yoke with a triangular-wave form current of 0.005 Hz. Changes of output voltages with different test plates were correlated with their mechanical and magnetic properties. The correlation is so good that this measuring method could be applied to nondestructive evaluation of material degradation in ferromagnetic structural steels.

Formation of magnetic grating on steel plates by electron/laser beam irradiation

The steels examined were a ferromagnetic carbon steel, S35C (equivalent to AISI 1035), and a nonmagnetic austenitic stainless steel, SUS 304 (equivalent to AISI 304). The gratings formed were magnetized in a 5.4-kOe field and the profile of the vertical component of magnetic flux leakage from each magnetized grating was measured. Changes of microstructure and mechanical strength of the beam-irradiated portions of the steel plates were also investigated. The measurements of the leakage flux profiles revealed the existence of remanent magnetism in the gratings. Metallographic and mechanical-hardness examinations showed that the original mixed ferrite/pearlite phase in the S35C steel was changed to martensite by beam irradiation. This increased its mechanical hardness. It was also determined that small grains of the ferrimagnetic ferrite phase were precipitated in the base of the nonmagnetic austenitic phase in the SUS 304. >

Analysis of minor hysteresis loops in plastically deformed low carbon steel

We have measured minor hysteresis loops with increasing magnetic-field amplitude Ha step by step and have analyzed them in connection with lattice defects such as dislocations in deformed low carbon steel. We define several magnetic parameters in the minor loops: pseudocoercive force HC*, pseudoremanence MR*, pseudosusceptibility at pseudocoercive force χH*, pseudohysteresis loss WF*, and pseudoremanence work WR*. We find several simple relations between the pseudomagnetic properties, namely, MR*∕Ma*=9∕10 and 3∕5, and WR*∕WF*=1∕6 and 1∕8, before and after plastic deformation, respectively. These relations are due to the similarity of minor loops. Six magnetic coefficients that are sensitive to lattice defects and are independent of Ha, as well as of the magnetic field, are obtained from the pseudomagnetic properties. These coefficients are effective parameters for nondestructive evaluation of degradation before the initiation of cracking. The minor-loop method has several advantages for nondestructive eva...

Characteristics of Barkhausen Noise Properties and Hysteresis Loop on Tensile Stressed Rolled Steels

The rolled steels for welded structure applied tensile stress have been examined by means of magnetic Barkhausen noise (MBN) method and of a physical parameter obtained from a hysteresis loop. The behaviors of MBN parameters and coercive force with tensile stress were discussed in relation to microstructure changes. There is no change in MBN parameters and coercive force below yield strength. The coercive force rises rapidly with tensile stress above yield strength. On the other hand, the rms voltage and the peak in averaged rms voltage take a maximum around yield strength and then decreases. The magnetomotive force at peak in the averaged rms voltage shows a minimum around yield strength. These phenomena are attributed to the combined effects of cell texture and dislocation density. In addition, the behaviors of MBN parameters around yield strength may be reflected by the localized changes in strain field due to the formation of dislocation tangles.

The effect of temperature on laws of minor hysteresis loops in nickel single crystals with compressive deformation

The temperature dependence of minor hysteresis loops of compressively deformed nickel single crystals has been investigated in a wide temperature range below the Curie temperature of 628 K. There exist power-law relations between the field-dependent parameters of minor-loops, and their exponents are independent of both temperature and strain after the compressive deformation. These observations indicate the presence of universal power laws in minor hysteresis loops. The minor-loop coefficients of the power laws show a similar temperature dependence of the coercive force, which is quantitatively related to the dislocation density. These properties of minor hysteresis loops are useful for the accurate and quantitative nondestructive evaluation of age degradation in ferromagnetic materials.

Neutron irradiation effects in Fe–Cu–Ni–Mn model alloys studied by measurements of magnetic minor hysteresis loops

We have measured magnetic minor hysteresis loops of neutron-irradiated Fe–Cu–Ni–Mn model alloys with variable combinations of Cu and Ni contents. It was revealed that minor-loop coefficients which are in proportion to internal stress decrease after neutron irradiation to a fluence of 0.44 × 1019 n cm−2 for all alloys. The decrease of the coefficients strongly depends on both Cu and Ni contents and is enhanced for alloys with high Cu and high Ni contents, in particular. The decrease of the coefficients during neutron irradiation was explained as due to preferential formation of Cu rich and/or Mn-Ni rich precipitates on dislocations which reduces internal stress of dislocations.

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

Feasibility study of magnetic flux leakage method for condition monitoring of wall thinning on tube

This paper discusses a feasibility of magnetic flux leakage ( MFL) method for estimation of wall thinning on tube in nuclear power plant. A single magnetic yoke is fixed on the s pecimen, and then a magnetic field sensor scans the leakage field between the yoke legs. The single magnetic yoke with gau ss meter fixed at the center of the yoke is also scanned over specimens having a slit. From both FEM analysis and experimental measurement, it is clarified that estimation of the size of slit fabricated in the specimen using profile of leakage field is possible; this indicates monitoring conditions of wall t hinning will be possible by MFL method.