Magnetic Barkhausen Emission Analysis for Assessment of Microstructures and Damage

Application of magnetic Barkhausen emission (MBE) analysis for assessment of microstructures and damage in various materials including carbon steel, Cr-Mo ferritic steels, 17-4 PH steel and metastable austenitic stainless steel has been highlighted in this paper. Thermally induced microstructural changes in Cr-Mo steels have been correlated with MBE, based on a two stage magnetization process model. The MBE parameters have also been used to characterise different stages of tensile deformation and to assess tensile strength, Charpy impact energy, quality of induction hardening process, progress of carburization in reformer tubes and fatigue damage. Introduction: 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. Fracture mechanics based analysis of component integrity requires quantitatively characterization of microstructure, defects as well as stresses. 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 prediction is possible with NDE techniques, when it is realised 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 and residual stress. In this paper, use of MBE technique for characterization of microstructures, deformation and fatigue damage in different steels is discussed. Assessment of Grain/ Lath Size and Carbide Size Associated with Thermal Ageing in Ferritic Steels: During thermal aging of ferritic steel components operating at higher temperatures, the dislocation density, size and distribution of laths / grains and second phase precipitates would vary in a synergistic manner. Using MBE parameters, a two-stage magnetization process has been proposed in sufficiently tempered microstructures, considering the grain boundaries and the carbide precipitates as the two major types of obstacles to the domain wall movement. The model has been used to characterise the tempered microstructures on the MBE behaviour in carbon steel, 2.25Cr-1Mo steel and 9Cr-1Mo steels [1]. The variation in the RMS voltage of the MBE with. current applied to the electromagnetic yoke systematically changes from a single peak to two peaks with increase in tempering time in carbon steel and Cr-Mo steels. Figures 1(a, b) typically show the results obtained for 0.3% carbon steel [1]. Based on the two-stage magnetization process model proposed, the MBE peak 1 has been attributed to influence of grain boundaries and the peak 2 to that of carbides [1, 2]. As per this model, the grain boundaries and precipitates would act as obstacles to the domain wall movement in different field ranges and two peaks in the MBE profile have been observed for these steels in sufficiently tempered condition. The systematic changes in the height and position of these two MBE peaks at different stages of tempering have been explained based on the variations in the lath/grain size and type and size of carbides [1, 2]. Since the formation and growth of reverse domains would become more and more easy with the increase in grain size due to large demagnetizing field at the grain boundaries and hence, the MBE peak 1 position would shift to lower magnetic field as, shown in Fig. 1(a, b). Similarly, with increasing tempering time, the carbide size increases. The carbides with higher size offer more resistance to domain wall movement and hence the field corresponding to peak 2 shifts to higher filed, as shown in Fig. 1(a, b). Excellent correlations between positions of MBE peaks 1 and 2 and microstructural parameters (grain/lath size and carbide size) have been obtained, as shown in Figs. 2(a, b). These results indicate that the MBE technique can be used to evaluate the changes in the grain size and precipitate size in long-term aged ferritic steels. However, this is difficult in short-term aged conditions, where the dislocation density is high, and the lath and precipitate sizes are very small.