Effects of high strain rate deformation on magnetic hysteresis in high tensile steels Effects of high strain rate deformation on magnetic hysteresis in high tensile steels Ryo Morita, Satoru Kobayashi[.]
Effects of high-strain-rate deformation on magnetic hysteresis in high-tensile steels , Ryo Morita, Satoru Kobayashi , Akindele G Odeshi, Jerzy A Szpunar, Kodai Miura, and Yasuhiro Kamada Citation: AIP Advances 6, 055903 (2016); doi: 10.1063/1.4942824 View online: http://dx.doi.org/10.1063/1.4942824 View Table of Contents: http://aip.scitation.org/toc/adv/6/5 Published by the American Institute of Physics AIP ADVANCES 6, 055903 (2016) Effects of high-strain-rate deformation on magnetic hysteresis in high-tensile steels Ryo Morita,1 Satoru Kobayashi,1,a Akindele G Odeshi,2 Jerzy A Szpunar,2 Kodai Miura,1 and Yasuhiro Kamada1 Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, Ueda 4-3-5, Morioka 020-8551, Japan Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A9, Canada (Presented 15 January 2016; received November 2015; accepted 24 November 2015; published online 22 February 2016) We have studied a relationship between magnetic hysteresis and microstructures on high-tensile AISI 4340 steels after impact loading with a strain rate up to 2100 s−1 We find that coercivity, and minor-loop coefficient which is deduced from a power-law scaling between minor-loop parameters increase with strain rate, show a maximum at around a strain rate of 1400 s−1, followed by a decrease at a higher strain rate, associated with magnetic anisotropy with respect to impact direction These observations are explained from the viewpoints of heat generation and heterogeneous microstructures characteristic to steels subjected to high strain rate deformation C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4942824] I INTRODUCTION Structural steels with high resistance to impact loading are widely used for automobile parts and aircraft structural materials because of minimizing driver’s or passenger’s injuries during a crash Generally, high-strain-rate deformation under impact loading results in heterogeneous microstructures, associated with the formation of adiabatic shear band, i.e narrow path with strain localization.1,2 Although understanding of detailed mechanism of microstructural and structure-sensitiveproperty changes during impact loading are crucial for improvement of steel quality, effects of high-strain-rate deformation on magnetism have not been studied The magnetic method using hysteresis loops is one of useful nondestructive evaluation methods to investigate the formation of microstructural defects in ferromagnetic materials.3 Since magnetic domain walls interact with lattice defects through magnetoelastic coupling, their movement is largely disturbed by the defects, which is reflected in a shape of magnetic hysteresis loops In this study, we report results of magnetic hysteresis measurements on high-tensile steels before and after impact loading, where the correlation with microstructure and Vickers hardness is examined II EXPERIMENTAL PROCEDURE Cylindrical specimens, 9.55 mm in diameter and 10.55 mm long, of high strength low-alloy AISI 4340 steels were used The chemical compositions is listed in Table I The specimens which were solution-treated at 850◦C for 30 min., followed by oil quenching, were subjected to compressive loading using split Hopkinson pressure bar.1 The samples with five different strain rates up to 2100 s−1 were prepared Their microstructure was investigated using a JEOL JSM-7001F field a Electronic mail: koba@iwate-u.ac.jp 2158-3226/2016/6(5)/055903/5 6, 055903-1 © Author(s) 2016 055903-2 Morita et al AIP Advances 6, 055903 (2016) TABLE I Chemical compositions of AISI 4340 steel (wt%) C 0.40 Ni Cr Mo Mn Si Fe 1.80 0.80 0.25 0.75 0.25 Bal emission scanning electron microscope (FE-SEM) The Vickers hardness was measured with the standard indentation technique The applied load was 300 gf and 30 indents were taken and averaged for each sample Magnetic measurements were performed at room temperature for disk-shaped sample with dimensions of mm in diameter and mm in thickness The sample surface was polished to optical quality by sanding to get smooth and flat surface, and then electrochemically polished with a nitric acid/acetic acid solution to minimize residual stress, caused by the mechanical polishing A setup of our magnetic measurement system is schematically shown in Fig A Fe yoke wound with 500-turn excitation and 200-turn pick-up coil was attached to the disk to form a closed magnetic circuit A voltage waveform generated in a function generator was applied to a bipolar power supply, which converts voltage to current and amplifies it The current was applied to the exciting coil to generate a cyclic magnetic field and magnetize the sample The magnetic field within the sample was obtained from the voltage across a ohm resistance connected to the exciting coil in series The induced voltage of the pickup coil was integrated in order to obtain a magnetic flux within the sample A set of minor B-H loops with various amplitudes of a cyclic field, Ha up to 60 kA/m, was measured by step-by-step increasing Ha, keeping the sweep rate of dH/dt = 14 kA/m/s A typical example of minor B-H loops is given in Fig 3(c) The B − H loop with Ha = 60 kA/m was assumed to be the major loop in this study Measurements of B-H loops were performed, varying an angle, θ, between magnetization and loading direction When θ=0, the magnetization direction corresponds to loading one Before each measurement, a demagnetized state was achieved by changing the field amplitude from 60 kA/m to zero with a decaying alternating field with a frequency of Hz Here, reproducibility of data for our measurement setup should be noted To check the reproducibility, measurements of B-H loops were typically repeated five times and the obtained data were averaged We found that obtained magnetic properties are strongly influenced by quality of the magnetic contact between the sample surface and the yoke faces, and the results are not completely reproduced A standard deviation of coercivity and a minor-loop property was approximately % of their mean value at a maximum As will be shown later in Fig 4, this deviation is smaller than a variation of the magnetic properties with θ due to their anisotropy and did not alter our conclusions III RESULTS AND DISCUSSION Figure shows results of SEM observation before and after impact loading For as-received sample, it contains fine equiaxed grains with a few micron size After impact loading, grains FIG A setup of our magnetic measurement system 055903-3 Morita et al AIP Advances 6, 055903 (2016) FIG SEM images for as-received sample [(a)] and deformed one with a strain rate (SR) of 1100 s−1 [(b)] and 2100 s−1 [(c)] become finer and elongated as shown in Figs 2(b) and 2(c) Particularly, for sample with a highest strain rate of 2100 s−1, elongated grains are partially aligned along the direction perpendicular to impact loading direction as shown in Fig 2(c) Such inhomogeneous microstructure was typically observed for steels after high speed deformation.1,2 To see changes of magnetic properties due to impact loading, we first pay attention to major loops Figure 3(a) shows major B − H loops obtained before and after impact loading with a strain rate of 1100 s−1 Here, the magnetization direction is perpendicular to the direction of impact loading, i.e θ = 90 deg Figure 3(b) shows major B − H loops for θ = and 90 deg for a strain rate of 1100 s−1 One can see that a change of the major loop after impact loading and its dependence on magnetization direction is very small As is seen in the insets in Fig 3(a) and 3(b), the coercivity Hc at which the magnetization becomes zero seems to slightly increase, after the impact loading or when the magnetization direction is perpendicular to the direction of impact loading, i.e θ = 90 deg Fig 4(a) summarizes Hc as a function of θ, obtained before and after impact loading For as-received sample, Hc is very weakly dependent on θ and almost isotropic On the other hand, Hc of the deformed samples exhibits a large value, when the magnetization direction is nearly perpendicular to the direction of impact loading Such anisotropy in Hc was seen for other deformed samples with different strain rate This implies the formation of texture due to high strain-rate deformation, in which magnetic easy axis such as [100] crystal axis is preferentially oriented along the direction of impact loading Moreover, we found that absolute value of Hc strongly depends on strain rate Fig 5(b) shows Hc averaged over all the measuring data for different θ With increasing strain rate, Hc monotonically increases, maximizes at around a strain rate of 1400 s−1, and then falls into a value, which is FIG (a) B − H loops obtained before and after impact loading with a strain rate (SR) of 1100 s−1 The magnetization direction is along the direction of impact loading, i.e θ = 90 deg (b) B − H loops for sample deformed with a strain rate of 1100 s−1, taken at θ = and 90 deg between directions of magnetization and impact loading The insets in (a) and (b) show their enlargement around zero field (c) A typical example of a set of minor loops with different field amplitudes The inset shows the definition of parameters of a minor loop 055903-4 Morita et al AIP Advances 6, 055903 (2016) FIG (a) coercivity Hc and (b) minor-loop coefficient WF0 as a function of θ, obtained for as-received sample, and samples after impact loading with a strain rate (SR) of 1100 and 2100 s−1 nearly the same as that of the as-received sample The increase of Hc is ∼ 16% of that of as-received sample Generally, coercivity increases when density of lattice defects such as dislocation increases since lattice defects act as obstacles that pin domain walls Therefore, a proportional relationship exists between Vickers hardness and coercivity.4 However, although as shown in Fig 5(a) Vickers hardness increases monotonically, coercivity sharply decreases at a higher strain rate During very high-strain-rate deformation, heat is locally generated and sample temperature may rise up to 1200 K, because the majority of the plastic energy is converted into heat in such adiabatic condition.5 This generated heat leads to recovery and the formation of non-uniform dislocation structure.1,2 Since Hc is very sensitive to recovery process compared with mechanical properties,6 the decrease of Hc can be due to the heat generated at very high strain rate Finally, we present results of minor B − H loop measurements, whose properties are very useful for nondestructive evaluation of material’s quality because of very low measurement field.4,7 Figure 3(c) shows a typical example of minor loops with different field amplitude Ha For each minor loop, parameters of hysteresis loss WF and maximum flux density Ba∗ shown in the inset of Fig 3(c) were determined and a minor-loop coefficient WF0 was obtained using a power law, given by ( ∗ ) nF ∗ Ba WF = WF , (1) Bs FIG Strain-rate dependence of (a) Vickers hardness, (b) coercivity Hc and minor-loop coefficient WF0 Here, Hc and WF0 are values averaged over data obtained for different θ The error bars in (b) originate from scattering of data at each θ as well as the variation of Hc and WF0 with θ, but the latter mainly influences their size in particular; while the error of Hc and WF0 for the as-received sample is ∼ % of the averaged value, that for samples after impact loading has a larger value of 6−11 % for Hc and 4−9 % for WF0 due to their anisotropy shown in Fig 055903-5 Morita et al AIP Advances 6, 055903 (2016) where Bs is a normalization constant and was assumed to be 1.8 T in this study nF is a scaling exponent and was approximately 1.76 for the present steels, irrespective of strain rate and θ Figure 4(b) shows WF0 as a function of θ, obtained before and after impact loading We also show their average value as a function of strain rate in Fig 5(b) We found that the overall behavior is very similar to that of Ha as is evident from the reduction of WF0 at a high strain rate Here, it should be noted that the data for Ba∗ = 0.08−1.2 T were used for the present analysis, but almost the same results can be obtained by using data below Ba∗ = 0.5 T, which corresponds to the maximum field of ∼ kA/m These results indicate that the information about microstructures which can be deduced from coercivity measurements can also be obtained with low measurement field by analyzing minor loops This feature may be useful for such as in situ integrity assessment of materials subjected to mechanical impact, where use of portable and compact measuring device are desirable IV CONCLUSION We have investigated magnetic hysteresis properties of high-strength low-alloy steel AISI 4340, subjected to high strain-rate deformation Results of major-loop measurements showed that coercivity maximizes when the magnetization direction is nearly perpendicular to the direction of impact loading This would be related with inhomogeneous microstructure, associated with the formation of elongated grains which partially orient along the direction perpendicular to the loading direction We also found that coercivity monotonically increases at a low strain rate, but it decreases at higher strain rate above 1400 s−1 Similar behavior was observed for a minor-loop coefficient, obtained from a power-law scaling between parameters of minor B − H loops with different field amplitudes The strain-rate dependence on magnetic properties can be explained by competing microstructural effects; the formation of finer grains and recovery due to a rise of sample temperature during adiabatic impact loading Systematic microstructural investigations by x-ray texture measurements and electron-back scattering diffraction technique are planned in order to clarify the detailed mechanism of magnetic 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different strain rate This implies the formation of texture due to high strain- rate deformation, in which magnetic. ..AIP ADVANCES 6, 055903 (2016) Effects of high- strain- rate deformation on magnetic hysteresis in high- tensile steels Ryo Morita,1 Satoru Kobayashi,1,a Akindele G Odeshi,2 Jerzy A Szpunar,2