Barkhausen Emission Research Articles

Evaluation of post-weld heat treatment behavior in modified 9Cr–1Mo steel weldment by magnetic Barkhausen emission

The butt joint of two sight tubes of modified 9Cr–1Mo steel is welded by TIG technique. Subsequently, the post-weld heat treatment (PWHT) of the tubes has been carried out at 650, 700 and 750 °C temperatures for 3 h. Magnetic Barkhausen emission (MBE) measurements are performed in fusion zone, heat affected zone (HAZ) and base metal region of as-welded and all the post-weld heat treated samples. In each weldment, the variation of the MBE signal emanated from fusion zone, HAZ and base metal has been examined. It is found that the RMS voltage of MBE signal increases with PWHT temperature. The conversion of retained austenite to alpha ferrite during PWHT makes the fusion region as a softer zone in the weldment. The variation of magnetic parameters with different PWHT is correlated with hardness and microstructures. This study can be used as an effective tool in evaluation of PWHT parameters in modified 9Cr–1Mo weld joint.

Magnetic Evaluation of Residual Stresses and Structure Transformations Induced in Soft Steel after Turning

This paper deals with the stress and temperature induced microstructure transformations in a soft bearing steel 100Cr6 after turning operation. Investigation is carried out through a technique based on Barkhausen e ect. Evaluation of surface integrity via structure transformations, X-ray di raction as well as microhardness inspection is carried out as well. Surface integrity is investigated with the respect to the variable tool wears and consequent mechanical and thermal loads applied on the inspected surfaces. Di erent surface microstructures caused by the mechanical and associated thermal aspects of the surface formation are discussed and correlated with the parameters obtained from the measured Barkhausen emissions.

Damage evaluation of fossil fuelled power plant boiler tubes through magnetic Barkhausen technique

The failure mechanism of two economizer tubes of a fossil fuelled 260MW power plant has been investigated through the magnetic Barkhausen emission (MBE) technique along with the conventional and established materials characterization and evaluation techniques (like microstructures, mechanical properties and X-ray diffraction). The longer service exposer period results in the breaking of ferrite–pearlite structure and spheroidization lamellar cementite in ferrite matrix, associating with high magnetic r.m.s. voltage for former structure. The failure mechanism was governed by the cold end corrosion process with the presence of sulfates, and the subsequent erosion through hard quartz particles leads to a localized thinning of outer surface. Accordingly, the hoop stress was operative in tube walls and failed through fish-mouth opening. The MBE r.m.s. voltage response and pulse height distribution throw light on microstructural changes with duration of service exposure as well as leaked and non-leaked regions of economizer tubes.

Optimization of sensor design for Barkhausen noise measurement using finite element analysis

The effects of design parameters for optimizing the performance of sensors for magnetic Barkhausen emission measurement are presented. This study was performed using finite element analysis. The design parameters investigated include core material, core-tip curvature, core length, and pole spacing. Considering a combination of permeability and saturation magnetization, iron was selected as the core material among other materials investigated. Although a flat core-tip would result in higher magnetic flux concentration in the test specimen, a curved core-tip is preferred. The sensor-to-specimen coupling is thereby improved especially for materials with different surface geometries. Smaller pole spacing resulted in higher flux concentration.

Study of microstress state of P91 steel using complementary mechanical Barkhausen, magnetoacoustic emission, and X-ray diffraction techniques

The paper deals with assessment of microstress state of martensite P91 steel using three complementary techniques: mechanical Barkhausen emission, magnetoacoustic emission (MAE), and X-ray diffraction (XRD) profile analysis. Magnetic coercivity Hc and microstructure were investigated with inductive magnetometry and magnetic force microscopy (MFM), respectively. Internal stress level of P91 steel was modified by heat treatment. Steel samples were austenitized, quenched, and then tempered at three temperatures (720 °C, 750 °C, and 780 °C) during increasing time (from 15 min up to 240 min). The microstrain level εi was evaluated using Williamson–Hall method. It was revealed that during tempering microstrain systematically decreases from εi = 2.5 × 10−3 for as quenched state down to εi = 0.3 × 10−3 for well tempered samples. Both mechanical hardness (Vicker's HV) and magnetic hardness (coercivity) decrease almost linearly with decreasing microstrain while the MAE and MBE intensities strongly increase. Tempering leads to evident shift of the MeBN intensity maximum recorded for the first load towards lower applied strain values and to increase of MAE intensity. This indicates that the microstress state deduced by magnetic techniques is correlated with microstrains evaluated with XRD technique.

Barkhausen spectroscopy: Non-destructive characterization of magnetic materials as a function of depth

In this study, we conceptually divided a ferromagnetic specimen into layers along its depth. For each layer, we derived a non-linear integral equation that describes the attenuation with frequency and distance of magnetic Barkhausen emissions coming from that layer. We postulate that the Barkhausen spectrum measured at the surface by an induction coil can be expressed as the sum of the individual layer spectra. We show how a non-linear least squares algorithm can be used to recover the properties in individual layers. These are related to stress using an extension to the theory of ferromagnetic hysteresis. We found that the quality of the fit is influenced by the sensitivity of the ferromagnetic material to strain, as well as by the sensor-specimen coupling. The proposed method can be used for the non-destructive characterization of stress as a function of depth in magnetic materials.

Comparison of four NDT methods for indication of reactor steel degradation by high fluences of neutron irradiation

Results of three magnetic nondestructive methods, Magnetic Barkhausen Emission (MBE), magnetic minor loops Power Scaling Laws (PSL) and Magnetic Adaptive Testing (MAT), and of one reference mechanical measurement, Vickers Hardness (HV), applied on the same series of neutron heavily irradiated nuclear reactor pressure vessel steel materials, were normalized and presented here for the purpose of their straightforward quantitative mutual comparison. It is uncommon to carry out different round-robin testing on irradiated materials, and if not answering all open questions, the comparison alone justifies this paper. The assessment methods were all based on ferromagnetism, although each of them used a different aspect of it. The presented comparison yielded a justified recommendation of the most reliable nondestructive method for indication of the reactor steel irradiation hardening and embrittlement effects. The A533 type B Class 1 steel (JRQ), and the base (15Kh2MFA) and welding (10KhMFT) steels for the WWER 440-type Russian reactors were used for the investigations. The samples were irradiated by high-energy neutrons (>1MeV) with up to 11.9×1019n/cm2 fluences. From all the applied measurements, the results of MAT produced the most satisfactory correlation with independently measured ductile-brittle-transition temperature (DBTT) values of the steel. The other two magnetic methods showed a weaker correlation with DBTT, but some other aspects and information could be assessed by them. As MAT and MBE were sensitive to uncontrolled fluctuation of surface quality of the steel, contact-less ways of testing and more conveniently shaped irradiated nuclear pressure vessel steel samples were suggested for future measurements.

Investigation of magnetic, mechanical and microfailure behavior of ARMCO-type low carbon steel corroded in 3.5% NaCl-aqueous solution

The corrosion effects of 3.5% NaCl aqueous solution on the tensile mechanical, micromagnetic Barkhausen emission (MBE) and microfailure behavior of low carbon steel were investigated. At first it was found that this steel shows a dramatic and consistent decrease of its ductility with time of corrosion. The ductility reduction as well as microfailure features obtained by Scanning Electron Microscopy (SEM)-fractographic analysis yields an acceptable evidence for the occurrence of the Hydrogen Environmental Embrittlement (HEE) phenomenon during the corrosion process.During tensile straining the MBE-signal increases monotically at an average constant rate towards necking point. This rate shows tendency to increase with corrosion time fact which helped to establish the existence of a corrosion-induced micromagnetic hardening rate effect. It was shown that the main contribution to this effect should be attributed primarily to the combined effect of the HEE and dislocation multiplication and to a lower extent to the formed magnetite (Fe3O4) surface layers. Necking plasticity and MBE were correlated on the basis of combined stress bias and microvoid/microcracking-induced creation and rearangement of magnetic domains.

A New Method for Obtaining Stress-Depth Calibration Profiles for Non-Destructive Evaluation Using a Frequency-Dependent Model of Barkhausen Emissions

This study presents the development of a non-destructive method of detecting stress as a function of depth, useful for inspecting steel structures and components without the need to calibrate against x-ray diffraction data. A new frequency-dependent model for Barkhausen emissions based on the attenuation of emission with frequency and distance is used to extract depth-dependent stress information. Controlled, uniform stresses are induced in an ASTM A36 steel specimen, which are then used as a reference to obtain stress-voltage calibration profiles. An inversion process can then be employed to assess specimens of unknown stress states, by using the previously calculated profiles. The slope of the calibration profiles is found to vary with depth, and a simple computer algorithm may be used to extract stresses at different depths by using an averaging method.

Residual Stress Analysis in Surface Mechanical Attrition Treated (SMAT) Iron and Steel Component Materials by Magnetic Barkhausen Emission Technique

Surface plastic deformation processes like shot peening (SP) have been extensively used by engineering industries to introduce compressive stresses at the surface to achieve superior mechanical properties (especially fatigue and wear resistance). A new family of surface severe plastic deformation processes (S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> PD) using high-energy balls, widely known as the Surface Mechanical Attrition Treatment (SMAT), is gaining prominence. The investigation addresses the analysis of residual stresses in SMAT-exposed pure iron and SA 516 Grade 70 pressure vessel steel components by Magnetic Barkhausen emission (MBE) technique which is a potential nondestructive evaluation tool. The stresses corresponding to optimized band filter frequencies in MBE technique exhibit good correlation with the X-ray-diffraction based residual stress measurement data. The stresses correlate at a filter frequency of 100-250 kHz in the case of pure iron and 100-300 kHz in the case of SA 516 Grade 70 pressure vessel steel. Depth profiling by chemical etching showed that the nature of stresses changed from compressive to tensile. Microhardness measurements revealed an exponential decrease of hardness with depth.

Mapping Stress as a Function of Depth at the Surface of Steel Structures Using a Frequency Dependent Magnetic Barkhausen Noise Technique

Profiling of stress as a function of depth is an important tool for nondestructive evaluation and can be used to prevent catastrophic failures in structures. In this work, the underlying theory of a new model for the depth profiling of stress in ferromagnetic structures based on the magnetic Barkhausen method is investigated and commented upon. In the model, a ferromagnetic structure is divided into layers of different stress states. By analysis of the constituent equations it was found that the measured Barkhausen voltage increases as the frequency span increases, for both the one- and the two layer cases. It was also found that two layers with the same amplitude of Barkhausen emission at the origin can be approximated as one layer, provided that the upper and lower depths, as well as the frequency range are identical.

Modeling the effects of nanosized precipitates on magnetic hysteresis and Barkhausen effect signal

A model has been developed for describing the effects of randomly distributed precipitates on magnetic hysteresis and Barkhausen emissions based on a hysteretic-stochastic process model of domain wall dynamics. The pinning of magnetic domain walls by spherical precipitates is described in terms of the pinning strength, which is proportional to the number density and cross-sectional area of the precipitates, and of the interaction range, which depends on the particle spacing and grain size. The model was used to simulate hysteresis loops and Barkhausen effect signals of a series of thermally aged FeCu samples with different number densities and sizes of Cu-rich precipitates. The linear dependence of the coercivity on the sample hardness was reproduced in the simulations. The rms values and pulse height distributions of the measured and modeled Barkhausen signals show similar dependence on the aging time, which can be interpreted by considering the effects of varying the precipitate size and spacing on the strength and interaction range of domain wall pinning.

Experimental Verification of the Linear Relationship Between Stress and the Reciprocal of the Peak Barkhausen Voltage in ASTM A36 Steel

This study presents an experimental validation of a model theory for determining the relationship between a nondestructive measurement parameter and a property of interest. It was found that the reciprocal of the peak envelope amplitude of the Barkhausen emission voltage follows a linear relationship with stress. A linear relationship between stress and the reciprocal of the root mean square voltage was also obtained. These observations represent an important step towards improving the use of Barkhausen signals for magnetic nondestructive evaluation of stress as a function of depth in ferromagnetic load bearing structures.

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.

A New Method for Evaluation of Mechanical Stress Using the Reciprocal Amplitude of Magnetic Barkhausen Noise

This paper describes a method for detecting stress in the surface of magnetic materials, such as steels, based on the measurement of magnetic Barkhausen emissions. The paper presents a simple practical linear calibration curve that can be used to determine stress, and also provides a theoretical explanation of the reason for such a relationship. The influence of elastic tensile and compressive stresses of various magnitudes on the magnetic Barkhausen emissions was studied, with the objective of developing a technique for quantitative measurements of surface stress in machined steels. The peak amplitude of the Barkhausen emissions was found to correlate with both residual and applied stress, showing a clear rising trend for transition from compressive to tensile stress. The relationship between stress dependence of the maximum differential anhysteretic susceptibility and Barkhausen peak amplitude was observed. Plots of reciprocal values 1/χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> and 1/V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MBN max</sub> against stress showed a linear relationship providing a convenient method for detecting stress levels in near-surface regions from Barkhausen measurements.

Magnetic Nondestructive Evaluation of Creep Behavior in Water-Quenched Modified 9Cr-1Mo Steel

Water-quenched modified 9Cr-1Mo steel was creep tested in the laboratory at a temperature of 600 °C and 125 MPa stress. Magnetic Barkhausen emissions (MBE) measurements were carried out by interrupting the test at different lengths of time. Creep damage in such steel was observed by an increase in root mean square (RMS) voltage of the MBE. The magnetic softening was corroborated with the decrease in pinning density in the material for the coarsening of carbides (M23C6) and formation of massive phases (Fe2Mo), which comes at the expense of a large number of finer carbides. Before failure, the rate of increase in RMS voltage of the MBE decreased due to the demagnetizing field offered by the massive phases. The microstructural analysis was carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) study.

Magnetic measurements for evaluation of radiation damage on nuclear reactor materials

It is important for the plant lifetime management to estimate the loss of toughness and other effects, e.g. increase of yield strength and hardness, caused by irradiation. This paper deals with the use of magnetic measurements to determine the irradiation effects on nuclear reactor structural materials. Three types of nuclear reactor materials were studied. Samples of the materials were irradiated with different neutron fluences. The Magnetic Barkhausen Emission was measured using stabilised flux mode, i.e., control of the magnetic flux within the sample to compensate for leakage and variations on the flux. The samples were carefully identified according to position and the cutting direction within the steel forging block. Anisotropy effect due to sample cutting direction was observed and it masks the magnetic signal results induced by the irradiation effects. The results on the specimens cut in the same direction shows correlation with irradiation induced material hardening and its dependence on fluence. Different materials show different hardening levels. Barkhausen emission results were correlated to neutron fluence taking into account the cutting direction.

Evaluation of tempering behaviour in modified 9Cr–1Mo steel by magnetic non-destructive techniques

Modified 9Cr–1Mo steels were normalized at 1050 °C/30 min and tempered over a wide range of temperatures to observe the effect of tempering temperature on the material properties. The material revealed mechanical softening when tempered at temperature less than the lower critical temperature (∼810 °C: Ac 1). Beyond Ac 1 temperature the materials strength increased drastically due to the formation of fresh martensite. Magnetic Hysteresis Loop (MHL) and Magnetic Barkhausen Emissions (MBE) techniques were used to evaluate the magnetic properties of the materials with tempering. Magnetic softening was observed by tempering the material below the Ac 1 temperature where a decrease in coercivity and an increase in RMS voltage of the MBE were found. Tempering beyond Ac 1 magnetic hardening was observed by the increase in coercivity and the decrease in RMS voltage of MBE. Such results revealed that magnetic techniques could be a better tool for the evaluation of tempering of modified 9Cr–1Mo steel.

Study of high-temperature ageing and creep on bainitic 5Cr–0.5Mo steel by magnetic NDE techniques

5Cr–0.5Mo steel having initial bainitic microstructure has been aged at 600 °C for 5000 h and subjected to creep at 600 °C/60 MPa. At different lengths of time the tests were interrupted for magnetic measurement using magnetic Barkhausen emissions (MBE) and magnetic hysteresis loop (MHL) techniques. Composition, morphology and distribution of carbides were studied using scanning electron microscopy (SEM). Magnetic softness was observed in the material up to 1600 h of ageing due to the decrease in pinning density for the coarsening of carbides by accumulation of large number of finer carbides. At higher ageing periods the demagnetizing field offered by the large massive carbides causes magnetic hardening. During creep test since the material has been normalized at 50 h of creep test also the structural change in creep test is accelerated, the magnetic softening for the interstitial carbon migration to the grain boundary and stress relaxation has removed in such early periods of ageing. When the carbides started coarsening the number density of the carbides decreased and inter-carbide distance increased which enhances the magnetic softening in the material in the secondary stage of creep. Formation of massive carbides and micro voids at the late tertiary stage of creep offers demagnetizing field causes magnetic hardening before failure of the material.

Magnetic properties evaluation of ageing behaviour in water-quenched 5Cr–0.5Mo steel

Magnetic Barkhausen emissions and magnetic hysteresis measurements were carried out on water-quenched 5Cr–0.5Mo steel subjected to ageing at 600 °C up to 5000 h. During initial ageing, this steel exhibited magnetic softening, which was attributed to relaxation of quenching stress in the material as well as decrease in dislocation density and migration of interstitial carbon atoms towards the grain boundary. Further ageing resulted in magnetic hardening owing to the restricted movement of the domain wall by the precipitation of carbides such as M3C2, M2C, M7C3 where M stands for Fe, Cr or a combination of them. At longer ageing periods, magnetic behaviour was affected by a change in the composition and morphology of the carbides. Massive M23C6 types of carbides were formed during longer periods of ageing. The coarsening of carbides decreased the pinning density for the domain wall motion and affected the magnetic properties of the steel. The effect of demagnetizing field from voids and non-magnetic massive carbides also affected the magnetic behaviour. Magnetic behaviour and Vickers hardness measurements during ageing have been effectively supported by microstructural evaluations suggesting the capability of the magnetic techniques for assessment of damage during ageing in high temperature 5Cr–0.5Mo steel components.