Y. Kamada

Relationship between mechanical and magnetic properties in cold rolled low carbon steel

Structure-sensitive properties of minor hysteresis loops have been compared with Vickers hardness and ductile-brittle transition temperature (DBTT) obtained by Charpy impact test for cold rolled low carbon steel. Minor loops were measured with increasing magnetic field amplitude, step by step, and were analyzed in connection with the rolling reduction. We found that minor-loop coefficients deduced from relations between minor-loop parameters increase with increasing rolling reduction and have a simple relationship with both mechanical properties, Vickers hardness, and DBTT. We also found that these coefficients have a linear relation with coercive force obtained by the major loop. The present analysis method using minor loops is highly effective for nondestructive evaluation of ductile-brittle transition, in terms of the high sensitivity to lattice defects as well as low measurement field.

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.

Low-field magnetic characterization of ferromagnets using a minor-loop scaling law

A scaling power law relating hysteresis loss and remanence in minor hysteresis loops is proposed for the low-field magnetic characterization of ferromagnetic materials. We demonstrate that the law holds true for very low to an intermediate level of magnetization, associated with a universal exponent of ≈1.35, irrespective of types of ferromagnets and temperature, unlike the Steinmetz law with limited applicability. The coefficient in the scaling law shows almost the same behavior as coercivity and can be used for the evaluation of the magnetic quality of materials for which the Steinmetz law cannot be applied and/or low measurement field is necessary.

Detection of martensite transformation in high temperature compressively deformed austenitic stainless steel by magnetic NDE technique

The present work demonstrates that a magnetic non-destructive evaluation technique can be useful for detecting the presence and extent of ferromagnetic α′ martensitic phase in high temperature deformed 304 austenitic stainless steel.A good correlation between the martensitic transformation and magnetic parameters; saturation magnetization, coercive force and magnetic susceptibility have been obtained. Saturation magnetization was increased depending on the volume percentage of α′ martensite transformation. The volume percentage of α′ martensite was found to be dependent on the temperature and level of plastic strain. At temperatures below 623 K, martensitic transformation was detected after deformation of 10 to 40% plastic strain. A massive increase in α′ martensite phase was observed in the specimen deformed at RT to 40% plastic strain.Compressive deformation at RT formed thermodynamically more stable long and broad shape of martensite. But as the temperature of deformation increases lath shape gradually converted into needle shape.Coercive force was exclusively connected with size, shape and spatial distribution of martensite. For specimens deformed at 523 K coercive force were much higher than those of the specimens deformed at RT. Coercive force decreased remarkably at temperatures above 623 K. Formation of ferromagnetic α′ martensite in a paramagnetic matrix was also accompanied by an increase in magnetic susceptibility. Low magnetic susceptibility at temperatures above 623 K was due to disappearance of martensitic phase.

Feasibility Study of Application of MFL to Monitoring of Wall Thinning Under Reinforcing Plates in Nuclear Power Plants

This paper discusses a feasibility of magnetic flux leakage (MFL) method for estimation of wall thinning on pipes under reinforcing plates in nuclear power plants. The layered steel plates having a slit on its underlayer were prepared and the estimation of slit size by MFL method was examined. A single magnetic yoke is fixed on a layered specimen, and then a magnetic field sensor scans the leakage flux between the yoke legs for condition monitoring. The single magnetic yoke with a field sensor fixed at the center of the yoke is also scanned over layered specimens. It is clarified that estimation of the size of slit fabricated on underlayer of the layered specimen using profile of leakage field is possible: this indicates monitoring conditions of wall thinning to be occurred under reinforcing plates will be possible by MFL methods. Additionally, the effect of an air gap between yoke and first layer and a gap between first and under layer were also investigated experimentally.

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...

Superparamagnetic property and high microwave absorption performance of FeAl@(Al, Fe)2O3 nanoparticles induced by surface oxidation

Surface modification is an effective way to induce new magnetic phenomena in nanostructured materials. Herein, FeAl nanoparticles (NPs) with a mean diameter of 38 nm are produced by the hydrogen plasma-metal reaction (HPMR) approach. Via the subsequent passivation process, an oxide layer is generated outside the FeAl NPs as the result of surface oxidation. The 3 nm-thick amorphous-like oxide layer consists mainly of Al2O3 together with a small amount of Fe2O3. An Fe-enriched zone is created between the oxide layer and the FeAl core due to the much higher diffusion rate of Al than Fe towards the particle surface during the passivation process, which can be explained by the Kirkendall effect. The FeAl@(Al, Fe)2O3 NPs surprisingly display a superparamagnetic property with a blocking temperature (TB) of 250 K and a saturation magnetization of 36 emu g−1 at 4.2 K. They also exhibit high microwave absorption performance with a minimum reflection loss (RL) value of −22.6 dB at a thickness of 1.7 mm, and a broad absorption bandwidth of 8.3 GHz corresponding to the RL below −10 dB. Formation of the oxide layer plays a dominant role in inducing the superparamagnetic property and high microwave absorption performance in FeAl@(Al, Fe)2O3 NPs. Specific core@shell NPs may open a new way to tune the magnetic and electromagnetic properties of metallic nanomaterials through surface modification.

Magnetic properties of α′ martensite in austenitic stainless steel studied by a minor-loop scaling law

We study the scaling behavior of magnetic minor hysteresis loops in strain-induced ferromagnetic α′ martensites in an austenitic 316-type stainless steel. A scaling relationship between the hysteresis loss and the remanence, with a power law exponent of approximately 1.35, was found irrespective of the volume fraction of the α′ martensites as well as temperature. The coefficient of the power law largely decreases with volume fraction, whereas it increases with a decrease in temperature and exhibits a kink at around 40 K, close to the Neel temperature of an austenitic γ′ phase. The behavior of the coefficient was interpreted from the viewpoint of the morphology and exchange interaction of α′ martensites.

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.