Effect of different stages of tensile deformation on micromagnetic parameters in high-strength, low-alloy steel

Abstract

The influence of tensile deformation on the magnetic Barkhausen emissions (MBE) and hysteresis loop has been studied in a high-strength, low-alloy steel (HSLA) and its weldment. The magnetic measurements were made both in loaded and unloaded conditions for different stress levels. The rootmean-square (RMS) voltage of the MBE has been used for analysis. This study shows that the preyield and postyield deformation can be identified from the change in the the MBE profile. The initial elastic deformation showed a linear increase in the MBE level in the loaded condition, and the MBE level remained constant in the unloaded condition. The microplastic yielding, well below the macroyield stress, significantly reduces the MBE, indicating the operation of grain-boundary dislocation sources below the macroyield stress. This is indicated by the slow increase in the MBE level in the loaded condition and the decrease in the MBE level in the unloaded condition. The macroyielding resulted in a significant increase in the MBE level in the loaded condition and, more clearly, in the unloaded condition. The increase in the MBE level during macroyielding has been attributed to the grain rotation phenomenon, in order to maintain the boundary integrity between adjacent grains, which would preferentially align the magnetic domains along the stress direction. With progressive plastic deformation, the MBE level remains more or less constant in the loaded condition and decreases linearly with strain in the unloaded condition. This constant MBE behavior in the loaded condition is attributed to the opposing effect of applied tensile stress (which tends to align the domains along the stress direction) and the dislocation density (which reduces the domain-wall displacement). The decreasing MBE in the unloaded condition is attributed to the combined effect of compressive residual stress at the surface layers (which tends to align the domain wall perpendicular to the stress direction) and the dislocation density. This study shows that MBE during tensile deformation can be classified into four stages: (1) perfectly elastic, (2) microplastic yielding, (3) macroyielding, and (4) progressive plastic deformation. A multimagnetic parameter approach, combining the hysteresis loop and MBE, has been suggested to evaluate the residual stresses.