Magnetic Barkhausen Emissions: A Tool to Study Creep Damage in Steel

Abstract

Slow plastic deformation at elevated temperature (0.5 to 0.7 of the melting temperature) and stress (much lesser than the yield strength) is called creep. In the creep process, cumulative change of microstructure, nucleation and growth of cavities at the grain boundaries, their subsequent linkage to form micro-cracks and their propagation takes place until failure of the material. The failures occur with no significant change in the macroscopic appearance of the material and therefore appear to be spontaneous. As magnetic Barkhausen emission (MBE) is very sensitive to microstructural changes in steel, the technique is used in the present study for evaluation of creep behaviour of Cr-Mo steel. The aim of the present study is to find the feasibility of using Barkhausen emissions technique as a tool for structural health monitoring of high temperature components used for extended period of service. Magnetic Hysteresis Loop (MHL) and Magnetic Barkhausen Emissions (MBE) techniques were used to study the creep behavior of Modified 9Cr- 1 Mo steel at 600°C1125 MPa. Magnetic hardening was observed in the primary stage of creep damage revealed by decrease in RMS voltage of the MBE and increase in coercivity and remanence. Magnetic softning was observed in the secondary stage of creep damage with the increase in RMS voltage of the MBE and decrease in coercivity in the material. In the tertiary stage of creep damage magnetic hardening was observed by increase in coercivity and the rate of increase in RMS voltage decreased.