Magnetic Hysteresis Loop Technique as a NDE Tool for the Evaluation of Microstructure and Mechanical Properties of 2.25Cr-1Mo Steel

This study intends to understand the effect of short-term overheating on microstructure modifications, and variation in mechanical and magnetic properties of boiler tubes. It is based on the hypothesis that short-term overheating on boiler tubes leads to microstructural changes degrading their mechanical properties, thus resulting in their failure. As part of this study, fresh 2.25Cr-1Mo boiler tube samples were heat treated in the range of 700-950 . Magnetic hysteresis loop (MHL) measurements were carried for the non-destructive evaluation (NDE) of the mechanical properties that get altered due to microstructural modifications. For comparison, MHL was also carried out on a service-exposed boiler tube, which had failed due to short-term overheating. The magnetic parameters were correlated with the change in microstructure and micro-hardness of the samples. A decrease in coercivity, remanence, maximum induction and micro hardness were found at the lower soaking temperatures, due to: easy magnetic domain wall and dislocation motions for the stress relaxation; annihilation and recovery of dislocations; increase in inter carbide distance; and the decrease in the number density of carbides for the coarsening of carbides. A subsequent increase in coercivity, remanence and maximum inductions along with hardness are due to the nucleation and growth of fresh bainitic phase, obstructing the magnetic domain wall and dislocation motion. A drastic decrease in coercivity for the service-exposed tube is due to the transformation of MX type carbides to type and accumulation of such carbides at the grain boundaries along with the decrease in number density of the carbides. The presence of scale has less effect on the coercivity, but its demagnetizing effect largely decreases the remanence and maximum induction. The results of the study show that the MHL can be a suitable NDE technique for the evaluation of change in microstructure and degradation of mechanical properties in power plant steels.