Stress response of Barkhausen noise and coercive force in 9Ni steel

The stress response curves of barkhausen noise, coercive force and lattice parameter of ratained austenite are measured for 9Ni steel in as-received condition (quenched and tempered plate), in tempered states following annealing for 1 to 243 h at 590°C, and in a coarse-grained state after annealing at 1200°C. The stress response is strong in all cases, but depends on the content of retained austenite, exhibiting the strongest response at 3 to 5 vol% austenite. The total range of retained austenite content was from 1.5 to 11.1%. The coercive force rises from about 500 to 850 A/m at temperings from 1 to 243 h, and contains a major contribution from dislocation of the fresh martensite formed furing cooling. The figures for austenite content and the lattice parameter measurements reveal that the austenite begins to lose stability after a few of tempering.     

Governing conditions of repeatable Barkhausen noise response

The paper is devoted to the establishment of experimental conditions, which ensure the repeatability of magnetic Barkhausen noise testing in practice. For this task, the measurements were performed on open flat samples using different experimental configurations, including: different magnetization frequencies, sampling rates, and filter cut-off frequencies; using a sample-wrapped coil and using attached pick-up coils of various dimensions, with different lift-offs of a single yoke magnet and of the attached coil. The sample magnetization was controlled by a vertical array of three Hall sensors; their readings were extrapolated to the sample surface to precisely define its field. After analysis of the results, a scheme for an optimized sensor with a controlled field waveform was suggested to improve the measurement repeatability. The important issues of signal processing and parameter applicability were also discussed in detail.     

Modeling of magnetic Barkhausen noise in single and dual easy axis systems in steel

Angular dependent magnetic Barkhausen noise (MBN) signals measured on plate steel, and on the inside and outside surfaces of sections of seam welded and spiral welded 2% Mn steel pipe are modeled by considering a system of dipole moments. The relative orientation of dipole moments is fixed within the material, but their magnitude grows in the presence of an applied field. Growth of the moments is proportional to the magnetic field projected along a particular moment axis. A single easy axis material consists of an isotropically aligned population of moments, giving the background, upon which is superimposed a population of moments with relative orientations that result in a net moment within the sample. A dual easy axis system is proposed to consist of: (i) a second population of moments with orientations resulting in a net moment with orientation different from that of the first and (ii) interactions, possibly quadrupolar in nature, that occur between the individual moments of each population. The model is used to explain differences in the angular-dependent MBN signal between the seam welded pipe, with a single easy axis, and the spiral welded pipe, with a dual easy axis. The source of the dual easy axis system in the spiral welded pipe, which is different on the two pipe surfaces, is considered in terms of the asymmetric manufacturing processes, relative to the pipe axis, applied during its production.     

Effect of anisotropic microstructure on magnetic Barkhausen noise in cold rolled low carbon steel

We have measured the orientation and depth dependence of the magnetic Barkhausen noise (MBN) in an industrial pipeline steel sheet. Thin samples were cut out from the sheet cross section at different depths, parallel and transverse to the rolling direction (rolling and transverse samples). The integrated MBN energy has shown no depth dependence for transverse samples. On the other hand, the rolling samples have shown larger MBN noise energy and strong depth dependence.The microstructure of the samples was studied by optical and electron microscopy, parameters related to the shape and the quantity of carbon precipitates and grains were extracted by image processing techniques. The strong depth and orientation dependence of the MBN have been explained by changes of the grain and precipitate microstructure.     

Barkhausen noise studies in amorphous materials at vanishing anisotropies

In this work, Barkhausen noise measurements are shown for the amorphous ribbons Metglas 2605TCA and Fe_73.5Cu₁Nb₃Si_18.5B₄ FINEMET material which is the amorphous precursor of the well-known nanocrystalline material, under different stress, annealing and magnetizing frequency conditions. These parameters were varied in search for different dynamical regimes of the complex system composed by the set of domain walls interacting with a reservoir of defects.     

Barkhausen effect in magnetic thin films: Experimental noise spectra

In this paper Barkhausen noise spectra are shown that were measured on thin uniaxial 83-17 Ni?Fe films and these spectra were compared with the known magnetic behaviour of these films. On these types of sample the Barkhausen effect had been investigated by Lambeck [1], but their Barkhausen noise spectra have not been investigated so far. The film thicknessd _m in our experiments ranged between 400 and 2400 Å. In this range a change occurs in the dynamics of the magnetization behaviour caused by transitions of the type of domain wall. The results presented here show the very strong dependence of the Barkhausen noise spectra on the type of domain wall. Films with Bloch walls always show a frequency dependence off ^?1.7 in the higher frequency range. Samples in the thickness range where the transition occurs from the Néel wall via the cross-tie wall to the Bloch wall, have exponents between ?1 and ?2. The frequencyf _c above which the measured noise intensity begins to decrease varies very much for the different films. The curve off _c versus film thicknessd _m has the same form as the curve of the domain wall mobilitym versusd _m.     

Magnetic Barkhausen emission in lightly deformed AISI 1070 steel

The Magnetic Barkhausen Noise (MBN) technique can evaluate both micro- and macro-residual stresses, and provides indication about the relevance of contribution of these different stress components. MBN measurements were performed in AISI 1070 steel sheet samples, where different strains were applied. The Barkhausen emission is also analyzed when two different sheets, deformed and non-deformed, are evaluated together. This study is useful to understand the effect of a deformed region near the surface on MBN. The low permeability of the deformed region affects MBN, and if the deformed region is below the surface the magnetic Barkhausen signal increases.     

Novel processing of Barkhausen noise signal for assessment of residual stress in surface ground components exhibiting poor magnetic response

The Barkhausen Noise Analysis (BNA) technique has been utilised to assess surface integrity of steels. But the BNA technique is not very successful in evaluating surface integrity of ground steels that exhibit poor micro-magnetic response. A new approach has been proposed for the processing of BN signal and two newly proposed parameters, namely ‘count’ and ‘event’, have been shown to correlate linearly with the residual stress upon grinding, with judicious choice of user defined ‘threshold’, even when the micro-magnetic response of the work material is poor. In the present study, residual stress induced upon conventional plunge surface grinding of hardened bearing steel has been investigated along with unhardened bearing steel for benchmarking. Moreover, similar correlation has been established, when primarily compressive stress is induced upon high speed grinding using cBN wheel with moderately deep cut suppressing the micro-magnetic response from the ground medium carbon steel as the work material.     

Evaluation of surface decarburization depth by magnetic Barkhausen noise technique

Industrially unfavorable process of steel surface decarburization was induced by annealing in air. Two methods of after-anneal surface treatment were used: an acid pickling and a sand blasting. The obtained decarburized layers were examined by optical microscope, wave dispersive spectrometer, and surface X-ray diffraction method. Magnetic Barkhausen noise technique was tested for applicability of non-destructive characterization of the decarburized layer depth. A newly introduced parameter, Barkhausen noise coercivity, was proposed for practical use due to its sensitivity to decarburization and stability to measurement conditions. Other magnetic parameters, e.g. number of Barkhausen noise counts, were found to be sensitive to the compressive residual stress caused by the sand blasting.     

Application of the group pulse processes in the theory of Barkhausen noise

In the paper the 2nd-order periodic group pulse process is considered a statistical model of the Barkhausen voltage. Using this model an analytical formula for the power spectrum is derived and further specified to fit the measured spectrograms of Barkhausen noise. The results are interpreted with respect to the generating physical mechanism and examples of several numerical values are given.     

Exactly solvable model of avalanches dynamics for Barkhausen crackling noise

We review the present state of understanding of the Barkhausen effect in soft ferromagnetic materials. Barkhausen noise (BN) is generated by the discontinuous motion of magnetic domains as they interact with impurities and defects. BN is one of the many examples of crackling noise, arising in a variety of contexts with remarkably similar features, and occurring when a system responds in a jerky manner to a smooth external forcing. Among all crackling system, we focus on BN, where a complete and consistent picture emerges thanks to an exactly solvable model of avalanche dynamics, known as the ABBM model, which ultimately describes the system in terms of a Langevin equation for the velocity of the avalanche front. Despite its simplicity, the ABBM model is able to accurately reproduce the phenomenology observed in the experiments on a large class of magnetic materials, as long as universal properties are involved. To complete the picture and to understand the long-standing discrepancy between the ABBM theory and the experiments, which otherwise agree exceptionally well, consisting of the puzzling asymmetric shape of the noise pulses, microscopic details must be taken into account, namely the effects of eddy current retardation. These effects can be incorporated in the model, and result, to a first-order approximation, in a negative effective mass associated with the wall. The progress made in understanding BN is potentially relevant for other crackling systems: on the one hand, the ABBM model turns out to be a paradigmatic model for the universal behaviour of avalanche dynamics; on the other hand, the microscopic explanation of the asymmetry in the noise pulses suggests that inertial effects may also be at the origin of pulses asymmetry observed in other crackling systems.     

Study of the surface structure of tempered carbon steel

A CEMS and XMS study of carbon steel has shown that the microstructure of the bulk material as well as the surface depends on the temperature at which the steel was tempered. Ion milling of the surface results in no change to the surface microstructure which corresponds closely to the bulk properties. However, after polishing the surface with 3μm diamond, the surface composition is changed to a large extent. Polishing decreases the amount of retained austenite and also the fraction of interstitial carbon within the austenite on the surface of the non-tempered steel. A related increase in the amount of surface martensite is observed. The effects of polishing extends to depths of over 1000 Å. Both X- and θ-carbides are present in each of the tempered steels and only the amount of θ-carbide is seen to increase as the samples are tempered to decrease hardness. Tempering to higher temperatures decreases the martensite content and a corresponding increase is seen in the θ-carbide only. Polishing also removes a large feaction of the carbide from the surface and this may account for the anomalously high wear resistance previously determined for the steel.     

Distortions of the statistical distribution of Barkhausen noise measured by magneto-optical Kerr effect

In magneto-optical Kerr measurements of the Barkhausen noise, a magnetization jump ΔM due to a domain reversal produces a variation ΔI of the intensity of a laser beam reflected by the sample, which is the physical quantity actually measured. Due to the non-uniform beam intensity profile, the magnitude of ΔI depends both on ΔM and on its position on the laser spot. This could distort the statistical distribution p(ΔI) of the measured ΔI with respect to the true distribution p(ΔM) of the magnetization jumps ΔM. In this work the exact relationship between the two distributions is derived in a general form, which will be applied to some possible beam profiles. It will be shown that in most cases the usual Gaussian beam produces a negligible statistical distortion. Moreover, for small ΔI the noise of the experimental setup can also distort the statistical distribution p(ΔI), by erroneously rejecting small ΔI as noise. This effect has been calculated for white noise, and it will be shown that it is relatively small but not totally negligible as the measured ΔI approaches the detection limit.     

Mössbauer spectroscopy analysis on a tempered martensitic 9% Cr steel

We report an analysis by means of Mössbauer spectroscopy, on a 9% Cr steel submitted to a tempering process at 780°C. Spectra of several samples with different tempering times obtained at room temperature are fitted, and a study on existent phases is made. From the former analysis, we infer the existence of different neighbors of the Fe atom governed by the concentration of alloying elements. In particular, we analyze the behavior of Cr, as the main substitutional atom. An additional measurement of samples at low velocities is made, aiming to reveal the existence of precipitated carbides. Finally a comparison between samples is performed, in order to obtain a detailed study of the effect of tempering time on hyperfine parameters.     

Effect of hardness and composition gradients on Barkhausen emission in case hardened steel

The effect of gradients in hardness, structure and composition in the surface layers on magnetic Barkhausen noise (MBN) was investigated in two widely used low alloy steels. One material was case hardened by induction hardening. The other was case carburised. Similar hardness profiles were produced by the two treatments. It was found that the variation in structure and hardness through the case in the induction-hardened steel had a minor effect on the MBN profile. In contrast, the inhomogeneity in the case-carburised material showed up clearly. This took the form of a broadened or two-peak MBN profile. When the surface layer containing the case was removed by etching, the broadened profile was replaced by a profile with a single peak. It is concluded that the shape and position of the MBN profile is significantly affected when a gradient in microstructure is induced by a gradient in carbon content. On the other hand, a gradient in microstructure induced by heat treatment with a constant carbon level has much less affect on the MBN profile for the induction-hardened steel.     

Nonlinearity associated with the motion of a Bloch wall-onset of a chaotic regime and analysis of the Barkhausen noise

The noise V(t) generated by the displacement of a magnetic domain wall is investigated. As the velocity of the wall increases. the trajectories in the (V, V) plane exhibit a transition from quasi-periodic to chaotic motion.     

Barkhausen noise and domain structure dynamics in Si-Fe at different points of the magnetization curve

In the present relationship between Barkhausen noise and domain structure evolution along the hysteresis loop in 3% Si–Fe has been investigated. The noise power spectrum has been measured vs. induction during cyclic magnetization of the material at a very low constant rate. The obtained results show that the behavior of the noise power along the magnetization curve can be related to the character of the domain structure rearrangements taking place at different levels of induction. Such a connection is reflected in a relationship between the noise power and the shape of the hysteresis loop. The measured noise power spectra are interpreted through the use of a suitable model. This makes possible to connect the shape of the magnetization curve to the microscopic parameters characterizing the dynamic of the domain wall during a large Barkhausen jump.     

Substructural and carbide transformations during plastic deformation in tempered chromium-nickel martensitic steel

The evolution of the defect and carbide subsystems has been studied during the plastic deformation of chromium nickel steel with the structure of tempered martensite. A correlation has been established between the substructural transformation and the change during plastic deformation. The nondislocation shear resistance has been estimated.Tomsk Civil Engineering Institute. Translated from Izvestlya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 25–32, December, 1992.