Magnetic Barkhausen Emission Technique Research Articles

Characterization of High Temperature (873 K) Low Cycle Fatigue Damage in Modified 9Cr–1Mo Steel Using Magnetic Barkhausen Emission Technique

Modified 9Cr–1Mo (P91) ferritic steel is the steam generator structural material for prototype fast breeder reactor. During the start-ups and shutdowns, the material is subjected to conditions leading to low cycle fatigue damage. Detection of initiation of the fatigue damage is required to prevent the catastrophic failure in components of the steam generator. The present study reports the non-destructive characterization of substructural and microstructural changes occurring during low cycle fatigue (LCF) of modified 9Cr–1Mo base metal and weld joint at 873 K, using MBE measurements. MBE studies have been carried out on the samples at various levels of fatigue life by interrupting the fatigue test. MBE RMS voltage has been measured at different locations in the gauge portion of the samples to map the damage profile. The systematic changes in the MBE revealed various stages of progressive LCF damage starting from initial softening to final failure in the ferritic steel.

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.

Magnetic Barkhausen emission measurements for evaluation of material properties in gears

The magnetic Barkhausen emission (MBE) technique is considered as a potential non-destructive evaluation (NDE) method for assessing material properties in case-hardened gears. For gear manufacturing industries, the evaluation of material properties such as tempering induced changes in hardness level and the associated microstuctural changes, variations in case-depth of hardened layer, grinding process induced alteration in microstructure, hardness and residual stress distributions are of primary importance in order to ensure the quality of gears during service. This paper presents an overview of different research and developmental studies carried out at Design Unit, Newcastle University on the MBE technique suitable for different applications in the evaluation of quality of gears.

Magnetic Barkhausen emission technique for evaluation of residual stress alteration by grinding in case-carburised En36 steel

The effect of residual stress changes caused by grinding on the magnetic Barkhausen emission (MBE) has been studied in case-carburised En36 steel. Both high and low frequency MBE measurements were made on specimens with case-depth of 0.55 and 0.8 mm subjected to varying level of grinding damage. The high frequency MBE profile shows single peak while the low frequency MBE profile shows two peaks. The first peak is not affected by grinding damages. But, the second peak increases and shifts to lower field systematically indicating the changes in microstructure and residual stresses. Both high and low frequency MBE measurements have been correlated to residual stress (RS)-depth profile measured using X-ray diffraction method. The high frequency MBE indicates the changes in surface RS, but does not indicate changes in RS at depths >10 μm. The low frequency MBE profile indicates the changes in RS below the surface.

Magnetic Barkhausen emission technique for detecting the overstressing during bending fatigue in case-carburised En36 steel

The effect of bending fatigue at different maximum stress levels on the magnetic Barkhausen emission (MBE) has been studied in case-carburised En36 steel specimens. The low frequency MBE profile has been measured after unloading the specimen at different number of fatigue cycles. It has been found that, beyond 1000 MPa, the MBE peak height decreases just after few thousand cycles and the percentage reduction in the MBE peak increases with maximum bending stress level. The reduction in MBE peak at lower stresses (<1400 MPa) is attributed mainly to the effect of residual stresses becoming more compressive below the surface due to the application prior tensile stress. At higher stresses (1500 MPa), the variation in the MBE peak also indicates the effect of cyclic hardening and softening with progressive fatigue cycles. The MBE profiles measured after monotonic loading and unloading with different maximum stress levels also show similar reduction in the MBE peak with increase in pre-stress level. However, at higher stresses (>1400 MPa), the cyclic loading shows larger reduction in the MBE peak than the monotonic loading. This is attributed to the effect of cyclic microplasticity induced enhancement of dislocation density in addition to the residual stress modification. This study clearly shows the MBE technique can be used to detect the maximum stress level seen by the specimen beyond 1000 MPa. Any overstressing of this case-carburised steel beyond the fatigue limit of 1150 MPa can be easily detected from the percentage reduction in the MBE peak. Since the crack propagation stage is insignificant in these hard steels, the detection of any overstressing using the MBE technique would be very useful in assessing and preventing the impending catastrophic failure.

Erratum to: ?Evaluation of applied and residual stresses in case-carburised En36 steel subjected to bending using the magnetic Barkhausen emission technique$rdquo; 6Acta Mater 52 (2004) 1927?19369

Erratum to: ?Evaluation of applied and residual stresses in case-carburised En36 steel subjected to bending using the magnetic Barkhausen emission technique$rdquo; 6Acta Mater 52 (2004) 1927?19369

Evaluation of applied and residual stresses in case-carburised En36 steel subjected to bending using the magnetic Barkhausen emission technique

The effect of applied and residual stresses on the magnetic Barkhausen emission (MBE) profile has been studied in case-carburised En36 steel specimens with 0.65 and 0.95 mm case-depth. The initial MBE profile shows two peaks in lower case-depth specimen and only single peak in higher case-depth specimen. The MBE peaks gradually increase in height with applied tensile stress. Under compression, the first peak in lower case-depth specimen disappears and the dominant second peak decreases in both case-depth specimens. However, after unloading from higher stress levels, the MBE profiles show different behaviour. In lower case-depth specimen, on tensile side, the first peak disappears and the second peak decreases with pre-stress level. On compressive side, the first peak increases while the second peak decreases with pre-stress level. In higher case-depth specimen, on both tensile and compressive sides, the MBE peak decreases with pre-stress, but to different extent. These variations in MBE peaks are correlated with residual stress redistribution in the near-surface and subsurface regions.

Characterisation of Microstructures, Deformation, and Fatigue Damage in Different Steels Using Magnetic Barkhausen Emission Technique

The characterization of microstructures, mechanical properties, deformation, damage initiation, and growth by Non-Destructive Evaluation (NDE) techniques is assuming a vital role in various industries because of the growing awareness of the benefits that can be derived by using NDE techniques for assessing the performance of various components. NDE is widely applied for assessment of material degradation, where investment in new plants is not cost-effective and safe operational life of existing plants needs to be extended. In recent years, various advanced NDE techniques have been successfully employed for characterization of defects and microstructural features such as grain size, texture, nucleation and growth of second phases, assessment of tensile, creep and fatigue properties, deformation, and damage. With the advent of fracture mechanics concepts, microstructure, defects as well as stresses must be quantitatively characterized to have reliable and fail-safe materials and components. Any alteration in the microstructure, which reduces the life or performance, should be predicted sufficiently in advance in order to ensure safe, reliable, and economic operation of the components. This is possible when one realizes that the interaction of the nondestructive probing medium with the material depends on the substructural/microstructural features such as point defects, dislocations, voids, micro and macro cracks, secondary phases, texture, residual stress, etc. In this paper, use of Magnetic Barkhausen Emission technique for characterization of microstructures, deformation, and fatigue damage in different steels would be discussed. When a ferromagnetic material is subjected to a varying magnetic field, the magnetic flux densidy during magnetization varies in discrete steps as the magnetic domain walls have to overcome various types of obstacles during their movement. The discrete changes in magnetization induce electric voltage pulses in pickup coil placed near the surface of ferromagnetic materials. This noise like voltage pulses were, firstly observed by Barkhausen in 1919 and this phenomenon is named as magnetic Barkhausen Noise (MBN) or magnetic Barkhausen emission (MBE). Based on the two stage magnetization process modeled by the authors, the size of the grain/lath and carbides in ferritic steels could be correlated with MBE parameters. The influence of morphology of cementite (lamellar and spheroidized) on the MBE behaviour has been understood. Various stages of tensile deformation, viz. (i) perfectly elastic, (ii) micro-plastic yielding, (iii) macroyielding, and (iv) progressive plastic deformation in ferritic steels could be identified using the MBE parameters. The hardening depth and its quality with respect to the microstructure in induction hardened specimens could also be established using the MBE technique. MBE is found to be highly sensitive in detection of stress induced martensite in stainless steels with metastable austenite phase. Different stages of fatigue damage in 9Cr-1Mo steel, viz. (i) cyclic hardening, (ii) softening, (iii) saturation, and (iv) crack initiation could be identified using the MBE parameters.

Evaluation of induction hardened case depth through microstructural characterisation using magnetic Barkhausen emission technique

The influence on the hysteresis loop and the magnetic Barkhausen emission (MBE) of microstructure within the case of different induction hardened carbon steel shafts has been studied. The hysteresis loop shows a distortion with a sudden reduction in the rate of magnetisation (dB/dH ) before approaching the maximum magnetic flux density indicating surface hardening. The systematic changes in the MBE profile for different voltages applied during induction heating indicate the microstructural variations within the case. A single peak MBE profile for a fully martensitic structure gradually changes into two peaks on reducing the induction hardening voltage indicating the formation of an additional soft ferrite phase within the case. The systematic changes in the two MBE peak heights indicate the synergistic decrease in the volume fraction of martensite and the increase in the volume fraction of ferrite phase within the case due to reduction in the induction hardening voltage. The changes in the MBE profile for different case depth specimens are more prominent than the hysteresis loop. This study shows that the MBE alone gives better insight in evaluating the induction hardened components (having case depth ≲1·5 mm), since the height and position of the two MBE peaks are directly influenced by the volume fraction and composition of hard and soft phases within the case. In general, this study reveals the high sensitivity of the MBE technique to the finer microstructural changes due to surface heat treatment in ferritic steels.

An assessment of low cycle fatigue damage using magnetic Barkhausen emission in 9Cr–1Mo ferritic steel

A non-destructive, magnetic Barkhausen emission (MBE) technique has been used to assess various stages of low cycle fatigue (LCF) damage in 9Cr–1Mo ferritic steel. The initial decrease in the MBE peak height in the early stage of LCF cycling indicates the cyclic hardening stage, in which the formation of dislocation tangles reduces the mean free path of the domain wall movement. The increase in the MBE level again on further cycling indicates the progressive cyclic softening stage where the rearrangement of dislocation tangles into cells enhances the domain wall movement. The unaltered behaviour of MBE on continued cycling shows the saturation stage where the stabilization of dislocation substructure maintains the MBE level. Finally, a sharp increase in the MBE peak value identifies surface crack initiation and propagation, which is ascribed to the movement of additional reverse domains produced at the crack surfaces. This study establishes that the MBE technique can be used to assess the progressive degradation in the fatigue life of the ferritic steel components.