Magneto-acoustic emission for the characterisation of ferritic stainless steel microstructural state

Abstract

The role of residual stresses in the failure of metallic components and the need to determine such stresses is well recognised. Magneto-acoustic emission (MAE) is a relatively new non-destructive detection technique and its working principle is based on Barkhausen discontinuities or noise and magnetostriction when a ferromagnetic material is subjected to a varying magnetic field. MAE is being used to characterise the stress state of a ferritic stainless steel (AISI 430). Other stress measurement techniques; X-ray diffraction (XRD), magnetic Barkhausen noise (MBN) have also been used to confirm / support the results achieved using MAE. A new measurement parameter has been developed for stress characterisation called MAE absolute energy and has proved to be a useful quantitative method in MAE waveform measurement. I can't seem to get to grips with this last sentence. When you say stress characterisation, what do you mean? INTRODUCTIONHigh stress corrosion resistance and relatively low production costs are the main reasons for the increasing applications of AISI 430 ferritic stainless steels [1]. But problems such as loss of ductility and toughness when exposed to elevated temperatures, for instance during welding, is a main concern [2]. The heat of welding leads to grain coarsening in the heat effected zone and in the weld metal of ferritic stainless steels because they solidify directly from the liquid to the ferrite phase without any intermediate phase transformation [2][3]. The use of a non-destructive evaluation technique to characterise the post-welding microstructural state of a material, could be of use in determining the optimal levels of heat input and welding speeds.For characterisation, ferritic stainless steel (AISI 430) samples were plastically deformed and heat treated to various degrees and measured using Barkhausen sensing techniques. In a ferromagnetic material, Barkhausen noise (BN) is generated by the discontinuous movement of irreversible domains walls. This movement can be induced by applying a time varying magnetic field across the sample. This noise can be detected in the form of acoustic noise (electrical energy) or in the form of voltage pulses which are induced in a coil placed near the surface of the material. Magnetic Barkhausen noise (MBN) results from the reversible and irreversible displacement of 180º and non-180º domain walls, or by abrupt rotation of domain magnetisation vectors at higher magnetic fields. Acoustic Barkhausen noise, measured by magneto-acoustic

Prediction of the Mechanical Properties of P91 Steel by Means of Magneto-acoustic Emission and Acoustic Birefringence

“…In the case, when the stress exceeds a defined value, the dislocation segment breaks away from the points of weak fixation [24]. As a result, due to the dislocation structure modification, movement of the domain walls becomes less effective and the MAE intensity signal and its parameters Int(U a ) decrease monotonically [10].…”

“…Domain walls are pinned temporarily by microstructural barriers to disable their motion, and then they are released abruptly with an increase of the magnetic field. Grain boundaries, precipitates, dislocation tangles [9] and voids [10] are microstructural barriers hindering domain walls movement. It was also suggested that irreversible rotation of the domain through angles other than 180 • can also contribute to the MAE signal [11].…”

“…An advantage of the MAE method over the MBE method results from the depth of measurements. In the case of MAE it is dependent only on the ability to magnetise the investigated material and easily reaches 10 mm (or even more in special cases) while for MBE it is only 0-1 mm due to attenuation of the generated electromagnetic waves (in the kHz range) by eddy currents [10].…”

Assessment of Microhardness Profile in Grinding Using Barkhausen Noise Technique at Various Analysis Parameters

“…They also advocated use of micromagnetic technique for assessment of microhardness. In another study, O'Sullivan et al [23] observed decrease in BN (rms) with increase in microhardness while characterizing plastically deformed and heat treated AISI 430 ferritic stainless steel samples. Hardness increases with plastic deformation due to the increased dislocation density, which impedes magnetic domain walls movement as the magnetizing force is not enough to set the domain walls free, and reduced domain movement results in decreased BN activity.…”

“…Figures 3 and 4 show variation in microhardness of the ground sample along the depth from the edge of the ground surface for different downfeed undertaken. Typically, Vickers hardness increases with plastic deformation [21][22][23]. Thompson and Tanner [24] studied the effect of plastic deformation on Vickers hardness value and observed that Vickers hardness increases with plastic deformation when pearlitic steel bars were deformed to different degrees of tensile strain.…”