Magnetic Barkhausen Noise Measurements Research Articles

Nondestructive microstructural characterization of austempered ductile iron

Abstract Austempered ductile iron (ADI) has been preferred in a wide range of applications due its unique combination of high strength, good ductility, wear resistance and fracture toughness together with lower cost and lower density compared to steels. Magnetic Barkhausen noise (MBN) measurement offers a better alternative to traditional characterization techniques by being fast and non-destructive. A simple linear regression using only one single independent variable cannot correlate the MBN with the microstructure of ADI, since its microstructure is multi-component. Multiple linear regression analysis (MLRA) was used to build a model that uses the characteristic features of microstructural constituents as input parameters to predict the MBN. For that purpose, Cu-Ni-Mo alloyed ductile iron samples austempered between 325 and 400 °C and for 45–180 min duration were used. The results show that MBN is most sensitive to the size and shape of acicular ferrite and retained austenite. Moreover, MBN is almost insensitive to the size, morphology and volume fraction of graphite particles. This indicates that retained austenite pins the domain walls more effectively than the graphite particles. Considering the results MLRA, MBN technique can be used to characterize the ausferritic microstructure of ADI.

Grinding Burn Detection via Magnetic Barkhausen Noise Analysis Independently of Induction Hardened Depth

The electromagnetic technique based on magnetic Barkhausen noise (MBN) can be used to control the quality of ball screw shafts non-destructively, although identifying any slight grinding burns independently of induction-hardened depth remains a challenge. The capacity to detect slight grinding burns was studied using a set of ball screw shafts manufactured by means of different induction hardening treatments and different grinding conditions (some of them under abnormal conditions for the purpose of generating grinding burns), and MBN measurements were taken in the whole group of ball screw shafts. Additionally, some of them were tested using two different MBN systems in order to better understand the effect of the slight grinding burns, while Vickers microhardness and nanohardness measurements were taken in selected samples. To detect the grinding burns (both slight anddata intense) with varying depths of the hardened layer, a multiparametric analysis of the MBN signal is proposed using the main parameters of the MBN two-peak envelope. At first, the samples are classified into groups depending on their hardened layer depth, estimated using the intensity of the magnetic field measured on the first peak (H1) parameter, and the threshold functions of two parameters (the minimum amplitude between the peaks of the MBN envelope (MIN) and the amplitude of the second peak (P2)) are then determined to detect the slight grinding burns for the different groups.

Multifunctional Induction Coil Sensor for Evaluation of Carbon Content in Carbon Steel

Carbon steel has proven to be an important structural and functional material that plays an irreplaceable role in the worldwide economy. The influence of carbon on the mechanical and magnetic properties of the steel is well understood. Thus, the precise knowledge of the amount of carbon content in steel is crucial. Magnetic Barkhausen noise (MBN), magnetic hysteresis loop (MHL), and impedance measurements are reliable tools to assess carbon content. In this work, a multifunctional induction coil sensor used for MBN, MHL, and impedance measurements is designed and optimized. A multifunctional measurement system using the optimized induction coil is employed to measure MBN, MHL, and impedance signals. The parabolic dependence of the maximum value of MBN envelope on carbon content in steel is theoretically analyzed and experimentally verified. Coercive field and remanence from MHL measurements as well as the maximum impedance value are found to be proportional to carbon content and their dependence is explained with analytical simulations.

Characterization of the Fatigue Behaviour of Low Carbon Steels by Means of Temperature and Micromagnetic Measurements

Investigations on low carbon (non- and low-alloy) steels were conducted in form of load increase tests (LIT) and constant amplitude tests (CAT) to find the correlation among material behaviour, mechanical load, and the type of NDT method. With the help of preprogrammed load-free sequences, the thermal impact on magnetic Barkhausen noise (MBN) measurement can be avoided, so that the cyclic deformation properties of material responses can be interpreted more precisely. The results indicate differences between the change in temperature and the MBN-derived variable during LITs and CATs regarding the demonstration of the incubation stage and the cyclic hardening behaviour.

Magnetic properties and Barkhausen noise evolution during FeSiCuNbB nanocrystalline material aging

The use of compact electrical systems in aircraft applications and the automotive industry is promoted to increase energy efficiency. The reduction in mass and volume of electrical systems exposes magnetic components to high temperatures and degrades their properties over time. This paper investigated the thermal aging of different grades of FeSiCuNbB nanocrystalline materials. Aging evolution was analyzed by monitoring macroscopic magnetic properties. Furthermore, magnetic Barkhausen noise measurements were performed to investigate the domain pattern microstructure and domain wall movements during aging.

Innovative X-ray diffraction and micromagnetic approaches for reliable residual stress assessment in deep rolled and microfinished AISI 4140 components

The residual stress state in the subsurface is a key element of surface integrity. It is well known to have a significant impact on a component's properties in terms of fatigue behavior and resistance to wear and corrosion. For this reason, adjusting residual stresses during manufacturing is a major challenge in modern production engineering, to improve and ensure a component's fatigue strength. In this context, hydrostatic deep rolling of the workpiece surface using adapted parameters enables the targeted induction of compressive residual stresses into subsurface layers. Due to specific properties regarding subsurface and topography for functional components in tribological applications, a further machining operation by microfinishing following deep rolling seems to be purposeful. In particular with regard to the production of components exposed to periodic load changes when used, the process combination can enable a substitution of the typically required conventional subsurface zone hardening. With the aim of economical process design, the corresponding parts can be manufactured with significantly reduced time and costs. Efficient and well-founded methods for monitoring the resulting influence on the subsurface zone properties are essential for a reproducible and target-oriented process design. The prevailing method for the non-destructive assessment of residual stresses in both academia and industry is X-ray diffractometry using the sin2 ψ-method. However, this method is time-intensive and requires complex instrumentation. Thus, efforts have been undertaken in past decades to develop alternative methods for the efficient and reliable characterization of residual stresses. In this research, the applicability of the cos α-method in X-ray diffractometry and a micromagnetic approach for residual stress assessment was investigated, analyzing deep rolled and microfinished AISI 4140 specimen conditions. In addition to the diffractometric and micromagnetic measurements, metallographic and topographic analyses of machined surfaces were carried out. Deep rolling was found to induce significant compressive residual stresses of up to −1100 MPa. After microfinishing of the deep rolled surfaces, favorable compressive residual stresses remain in the subsurface, reaching approximately up to −750 MPa. Based on this, the production of tailored surfaces with respect to a suitable combination of topography and subsurface is possible. For all surface states investigated, a good agreement between the two approaches in X-ray diffraction was found. Magnetic Barkhausen noise (MBN) measurements prove to be well applicable for an efficient and holistic assessment of surface integrity in the subsurface of deep rolled and microfinished AISI 4140 specimens.

Design of advanced micro-magnetic sensors for various applications

The state of the microstructures determines both the mechanical properties (hardness, yield strength, elongation, etc.) and micro-magnetic properties (magnetic Barkhausen noise (MBN), incremental permeability (IP), etc.). The non-destructive nature of the micro-magnetic testing technology facilitate it be a valuable tool for quality control in the manufacturing of ferromagnetic components. In the implementation of micro-magnetic testing, the functions and the geometry of the sensors need be customized for specific target properties and components. In this paper, four types of advanced or multifunctional sensors are presented and applied for micro-magnetic testing. First, a multifunctional sensor is developed for flat surface. Unique intermittent magnetizing method using superimposed magnetic fields of high and low frequencies enable the sensor to measure the magnetic Barkhausen noise, incremental permeability, eddy currents, tangential magnetic field and main flux. Second, to conduct magnetic Barkhausen noise testing at the inner corner of V-shape bending plate, a miniaturized sensor with U-shape roller magnetizer is designed to perfect contact with the corner region. Third, read head of a magnetic tape player is revised to act as high-lateral-resolution (up to 500μm) sensor for magnetic Barkhausen noise measurement in ferromagnetic film. Fourth, a slender probe is developed and embedded into an anti-bulking fixture that is clamped onto the flat specimen. With the slender probe together with the special fixture, the dependency of the magnetic Barkhausen noise on the compressive and tensile stress is first experimentally observed in a single flat specimen under continuous load.

Comparison of longitudinal critically refracted ultrasonic waves and magnetic Barkhausen noise for tensile stress measurement

Longitudinal critically refracted (LCR) ultrasonic waves and magnetic Barkhausen noise (MBN) are two commonly used methods for measuring the residual stress of steel. Both of these methods have distinct advantages and disadvantages that need to be taken into consideration when selecting the most appropriate technique for measuring residual stress under specific circumstances. This issue is experimentally investigated in this paper by comparing the linearity, sensitivity and repeatability of these two measurement techniques for evaluating the stress within Q345 steel samples subjected to different levels of applied tensile stress. The time-of-flight (TOF) of LCR waves is demonstrated to conform almost perfectly with the applied stress in a linear relationship, while a similarly perfect confirmation is obtained for the root mean square (RMS) voltage of MBN signals to a quadratic curve. The results demonstrate that the RMS voltage of MBN signals is much more sensitive than the TOF of LCR waves to variations in surface stress, where the application of a 240 MPa tensile stress increases the RMS voltage of MBN signals by about 140% relative to that of the unstressed sample, while the TOF of LCR waves increases by less than 0.15% under corresponding conditions. Multiple testing of the same specimen demonstrates that MBN measurements more closely coincide than those of LCR waves. Therefore, MBN exhibits superior repeatability to LCR waves.

Mechanical and magnetic properties of TRIP690 steel strengthened by strain-induced martensite

The sensitivity of the induced magnetic signals to microstructures makes it possible to characterize nondestructively the mechanical properties of ferromagnetic materials. In this study, utilizing the hysteresis loop measurement and the magnetic Barkhausen noise measurement, the correlation between the mechanical and the magnetic properties of low-alloy multiphase transformation-induced plasticity (TRIP) steel after plastic deformation was investigated, then the strength of the TRIP steel after plastic deformation was characterized. The results showed that, as the strain level increased from 0 to 0.2, the martensite volume fraction of TRIP690 steel increased from 0.78% to 3.64% which contributed to the enhancement of mechanical properties and the change of magnetic properties, and in particular, hardness and tensile strength had linearly increased by approximately 42% and 20%, respectively. Meanwhile, the peak value of the magnetic Barkhausen noise signal (MBNS) increased linearly with the increasing strain levels, the coercivity increased rapidly and then slowly, while the susceptibility decreased exponentially. By theoretically deriving the effects of the ferromagnetic phase volume fraction, dislocation density and residual stress of the TRIP690 steel on susceptibility and coercivity, the variation of the MBNSs under the influence of phase transformation, dislocation and residual stress was analyzed. The above non-destructive magnetic technique makes it possible to evaluate the mechanical response of the TRIP steel after plastic deformation, which allows it to be further applied to the quality evaluation of steel parts in stamping, such as body-in-white for the automotive industry.

Microstructure analysis of single-lip deep hole drilled bores by electron backscatter diffraction and magnetic Barkhausen noise

The surface integrity is crucial for the capability of machined parts; therefore, a detailed characterization of the obtained microstructure is very important for the assessment of machining processes. In this paper, the microstructure of single-lip deep drilled specimen, especially the formation of white etching layers (WEL), is considered by microscopic and micromagnetic investigations. Due to its asymmetrical nature, the single-lip deep hole drilling process induces severe plastic deformations in the bores surface. These severe deformations lead to a desirable fine-grained microstructure and the induction of beneficial compressive residual stresses. When too hard drilling parameters are chosen, the formation of a WEL can occur. These layers tend to be brittle and have tensile compressive stresses. The microstructures resulting from varied process parameters are characterized by light and electron microscopy as well with electron backscatter diffraction. These findings were correlated to the non-destructive magnetic Barkhausen noise measurement with a tailored bore sensor. This correlation enables an efficient and reliable evaluation of the process-induced microstructure and associated surface integrity.

On the use of differential permeability and magnetic Barkhausen Noise Measurements for Magnetic NDT Applications

Mapping microstructural to macroscopic magnetic properties is challenging yet necessary for the development of novel non-destructive testing techniques based on magnetic measurements. The macroscopic properties of a material reflect the underlying magnetization processes and the microstructural configuration as a result of the history of the material. Magnetic nondestructive evaluation techniques are needed for the health monitoring in critical structures and the quality monitoring of materials used in components, such as transformers and motors. In this work, we assess the usability of metrics related to AC magnetometry and magnetic Barkhausen Noise through measurements on several series of samples of various grain sizes and levels of plastic deformation. AC magnetometry is preferred to magnetic Barkhausen Noise as it provides more repeatable and reliable metrics than mBN which suffers from higher uncertainty. Also, differential permeability curves are preferred to hysteresis loops because they can be obtained directly through ac magnetometry and provide several types of information, such as the peak value, the coercivity, the overall shape, the emergence and location of multiple peaks which are related to stresses. In the case of stress-dependent magnetization processes, both differential permeability and mBN results are consistent, regardless of the way strain is applied, and corroborate results previously reported. However, permeability measurements are inconclusive on the effect of grain size on magnetization processes while the mBN voltage increases monotonically with decreasing grain size, in both types of samples. The mBN energy associated with each applied field step is proposed as a complementary measure for the assessment of the contribution of the irreversible processes in a material where the permeability measurements are inconclusive. Finally, the experimental observations have been used to set up micromagnetic calculations by modeling a plastically deformed material as a soft matrix with grains enclosed by boundaries with high transverse anisotropy which act as pinning centres and prevent further rotation of the magnetization and its alignment with the applied field thus reproducing the typical phenomenological features of plastically strained materials.

Evaluation of Thermal Damage Impact on Microstructure and Properties of Carburized AISI 9310 Gear Steel Grade by Destructive and Non-Destructive Testing Methods

Advanced aircraft gearboxes operate under high mechanical loads. Currently, aircraft gears are manufactured from chromium–nickel–molybdenum steel grades such as AISI 9310 or Pyrowear 53. The major causes of gear failure are wear and fatigue cracking. As the crack initiation occurs predominantly on the component surface, the gears are routinely subjected to surface hardening processes such as low-pressure carburizing and case hardening. The gears are manufactured in a multiple operation process, in which teeth grinding is a crucial step. Selection of improper grinding conditions can lead to local heat concentration and creation of grinding burns, which are small areas where microstructure and properties changes are induced by high temperature generated during grinding. Their presence can lead to significant reduction of gear durability. Therefore destructive and non-destructive (NDT) quality-control methods such as chemical etching or magnetic Barkhausen noise (MBN) measurements are applied to detect the grinding burns. In the area of a grinding burn, effects related to the over-tempering or re-hardening of the carburized case may occur. In this paper, the results of the studies on the characterization of microstructure changes caused by local heating performed to simulate grinding burns are presented. The areas with the over-tempering and re-hardening effects typical for grinding burns were formed by laser surface heating of carburized AISI 9310 steel. Analyses of the microstructure, residual stresses, retained austenite content, and non-destructive testing by the MBN method were performed. The correlation between the MBN value and the properties of the modified surface layer was identified. It was also found that the re-hardened areas had similar characteristics of changes in the Barkhausen noise intensity, despite the significant differences in the width of the overheated zone, which depended on the laser-heating process conditions.

Magnetic Barkhausen Noise Technique for Early-Stage Fatigue Prediction in Martensitic Stainless-Steel Samples

Abstract Martensitic grade stainless-steel is generally used to manufacture steam turbine blades in power plants. The material degradation of those turbine blades, due to fatigue, will induce unexpected equipment damage. Fatigue cracks, too small to be detected, can grow severely in the next operating cycle and may cause failure before the next inspection opportunity. Therefore, a nondestructive electromagnetic technique, which is sensitive to microstructure changes in the material, is needed to provide a means to estimate the specimen’s fatigue life. To tackle these challenges, this paper presents a novel magnetic Barkhausen noise (MBN) technique for garnering information relating to the material microstructure changes under test. The MBN signals are analyzed in time as well as frequency domain to infer material information that are influenced by the samples’ material state. Principal component analysis (PCA) is applied to reduce the dimensionality of feature data and extract higher order features. Afterward, probabilistic neural network (PNN) classifies the sample based on the percentage fatigue life to discover the most correlated MBN features to indicate the remaining fatigue life. Furthermore, one criticism of MBN is its poor repeatability and stability, therefore, analysis of variance (ANOVA) is carried out to analyze the uncertainty associated with MBN measurements. The feasibility of MBN technique is investigated in detecting early-stage fatigue, which is associated with plastic deformation in ferromagnetic metallic structures. Experimental results demonstrate that the magnetic Barkhausen noise technique is a promising candidate for characterizing.

Analysis of Surface Roughness Influence in Non-Destructive Magnetic Measurements Applied to Reactor Pressure Vessel Steels

The influence of surface roughness on magnetic measurements of Reactor Pressure Vessel Steels was investigated by applying two types of magnetic, non-destructive measurement on nuclear reactor pressure vessel steel samples: magnetic adaptive testing (MAT) and magnetic Barkhausen noise measurement (MBN). The surface roughness was modified by primary and secondary machine cutting forces. Different settings of machine cutting produced different surface conditions. It was found that for both measurements a monotonic correlation was found to exist between magnetic parameters and surface roughness. Results of the MAT measurements found that the correlation depends on the speed (i.e., on the applied slew rate) of the magnetizing current. In a similar fashion, results from the MBN method show good agreement with MAT, where the response diminishes with an increase in surface roughness. The results show the importance of accounting for surface condition in the interpretation of results of non-destructive magnetic testing.

Correlation of magnetic field and stress-induced magnetic domain reorientation with Barkhausen Noise

This paper investigates magnetic field and stress-induced magnetic domain reorientation and its correlation with Magnetic Barkhausen Noise (MBN). The magnetic domain dynamics and MBN of grain-oriented transverse and longitudinal electrical steels are studied by using time-resolved magneto-optical imaging. Time and frequency-domain characteristics of MBN signal are investigated for evaluation of stress state and magnetocrystalline anisotropy. Transitions between magnetic domain states with differing tensile stress and magnetic anisotropy directly influence the MBN signal. Its shown that 180° domain wall (DW) reorganization and DW-type conversion generate peaks in the MBN measurement for longitudinal and transverse electrical steel, respectively. The magnetic domain imaging reveals complicated domain arrangement processes in transverse electrical steel. The results provide a physical insight into bulk micromagnetic phenomena.

Directional magnetic Barkhausen noise measurement using the magnetic needle probe method

A directional local magnetic measurement technique as example of measuring the angular dependence of the magnetic Barkhausen noise in electrical and construction steels at typical quasi-static frequency by the magnetic needle probe (MNP) method is presented. The directional measurement configuration of the MNP method is typical of a half-turn search coil sensor coupled to high gain analogue amplification and high order filtration stages, owing to its very weak induced electromotive force. The method exploits the MBNenergy(H) hysteresis cycle (time integration of the square of the MBN voltage signal as a function of the tangent excitation field H) and brings forth stable indicators possibly related to the Magnetocrystalline Anisotropy Energy (MAE). Experimental tests performed on grain-oriented Fe-Si and low carbon steel specimens, and comparison with the conventional search coil measurement results were worth the validation of the MNP method. These results put to the front the good directional selectivity of the MNP method over the conventional MBN sensors adversely affected by the transverse fields at the material surface. Eventually, the printed magnetic needle probe (PMNP) sensor was relatedly described to provide non-invasive directional measurements for continuous structural health monitoring.

Case depth evaluation of induction-hardened camshaft by using magnetic Barkhausen noise (MBN) method

ABSTRACT Magnetic Barkhausen noise (MBN) method is one of the non-destructive evaluating (NDE) techniques used in industry to monitor the quality of ferromagnetic products during manufacture. In this article, case depth evaluation of the camshaft lobes by this means after induction hardening is described. A routine industrial monitoring practice is found to have limitation to evaluate the thickness of this process-hardened layer. With the aid of metallography on selected samples, this uppermost layer is found to have one, or more than one microconstituents. This infers that each type possesses different physical properties in response to the MBN measurement. Consequently, the interpretation of the MBN signal/data for case depth evaluation is not straight-forward. From metallography, a qualified component should have a uniform layer of martensite with grains ≤ 50 µm and the thickness around 3.0–5.0 mm. This gives the magnetoelastic parameter (i.e. mp) in a range of 20–70 in industrial MBN measurement. The mp outside this range corresponds to either a non-martensitic type or a martensitic type with grains > 50 µm. In fact, the characteristic features of a Barkhausen burst like peak intensity, width and position can be used to categorise different microstructural conditions. Then, the case depth of the qualified components, or the thickness of the qualified martensite, can be estimated. Statistical regression decision tree model helps to divide this qualified group into three sub-groups between 3.0 and 6.0 mm, and each can be identified by the decision criteria based on the specific ranges of the mp reading, the RMS of peak intensity and the peak position. In the end, a physical model is used to show how the difference of microstructures is influencing the magnetic flux, and thus the mp. Nevertheless, more information is needed to improve the model for this application.

Use of a Micromagnetic Nondestructive Test in the Evaluation of the α’-Martensitic Transformation Generated in the Mechanical Fatigue Process of the AISI 304L Stainless Steel

The detection of the strain-induced martensitic transformation with the magnetic Barkhausen noise (MBN) was investigated in the high-cycle fatigue (HCF) of the AISI 304L stainless steel at different points of the surface of an hourglass-type specimen. Load-controlled fatigue tests with loads below and above the fatigue limit were performed. Each fatigue test was interrupted at certain numbers of cycles, prior to fracture, and MBN measurements were performed without applied loading. Comparing with the initial condition of the steel under study, no large changes in the MBN measurements were detected in the specimens subjected to loads near the fatigue limit, i.e., no $\alpha '$ -martensitic transformation was detected. In the specimen subjected to the greatest load, it was possible to detect an uncommon distribution of the MBN signals, which are correlated with the strain-induced $\alpha '$ -martensite and were verified with optical microscopy and X-ray diffraction (XRD). The results show that the MBN is a sensitive technique to detect the strain-induced $\alpha '$ -martensite in HCF tests, which can be used as an indirect parameter to show the fatigue damage of materials.

Magnetic Barkhausen Noise Measurements to Assess Temper Embrittlement in HY-80 Steels

Magnetic Barkhausen noise (MBN) can result from abrupt motion of domain walls due to their interaction with pinning sites during magnetization of ferromagnetic steel. However, understanding domain structure interactions with pinning site density is limited. In this article, the effect of density of pinning sites within grains of HY80 steel, on generation of MBN, was investigated. Pinning site density was modified by a heat treatment that produces temper embrittlement. Temper embrittlement refers to the reduction of fracture toughness of alloy steels when heated or slowly cooled through the embrittling temperature range and arises due to migration of impurity elements to grain boundaries. Samples, machined from a casting, were held at a constant temperature (525 °C) for different holding times, to generate varying degrees of temper embrittlement. MBN signal response was observed to decrease exponentially as a function of holding time with decay constants (relaxation time) in the range of 160-190 h. A reduction in the MBN signal along each sample's easy axis was attributed to different densities of impurity elements, which act as pinning sites for domain wall movement within grains. Samples were also characterized using a scanning electron microscope (SEM), hardness testing, and impact toughness testing. Hardness was observed to increase with holding time and MBN energy decreased linearly with increasing hardness. Furthermore, a linear relationship between impact energy and MBN energy was observed. This implies that MBN signal analysis can be used as an indirect measure of the change in material properties of the sample.

Detection of surface cracks in ferromagnetic materials by C-scan mapping of residual stresses using Barkhausen emissions

Surface cracks can develop in components due to residual stresses, fatigue, stress corrosion cracking, corrosion fatigue, etc, during service exposure. Different non-destructive testing (NDT) methods are employed to detect and monitor such cracks. Magnetic Barkhausen Noise (MBN) analysis is one such technique that is used for in situ examination of microstructural anomalies or stress patterns. In the present work, we study the applicability of MBN for the detection of surface cracks. A part through surface crack was created by controlled fatigue loading of a martensitic stainless steel plate. The surface of the sample was scanned for BN emissions in incremental steps parallel and perpendicular to the crack. Measurements of MBN signal strength were recorded and assessed. Localized peaks in the MBN values observed whilst scanning sample surface can reveal the presence of flaws. Furthermore, the remnant stress pattern ahead of the crack tip as well as in the wake of the crack can get reflected in the measured MBN values. The observations carried out show that the surface scan carried out with MBN measurements can be a good non-destructive method for in situ NDT to detect and characterize surface cracks.