Magnetic Detection of Microstructural Change in Power Plant Steels

Power plant components are expected to withstand service at high temperature and pressure for thirty years or more. One of the main failure mechanisms under these conditions is creep. The steel compositions and heat treatments for this application are chosen to confer microstructural stability and creep resistance. Nevertheless, gradual microstructural changes, which eventually degrade the creep properties, occur during the long service life. Conservative design lives are used in power plant, and it is often found that components can be used safely beyond the original design life. However, to benefit from this requires reliable monitoring methods. One such technique involves relating the microstructural state to measurable magnetic properties. Magnetic domain walls interact energetically with microstructural features such as grain boundaries, carbides and dislocations, and are ‘pinned’ in place at these sites until a sufficiently large field is applied to free them. When this occurs, the sudden change in magnetisation as the walls move can be detected as a voltage signal (Barkhausen noise). Previous work has suggested that grain boundaries and carbide particles in power plant steels act as pinning sites with characteristic strengths and strength distributions. In this study, the concept of pinning site strength distributions was used to develop a model for the variation of the Barkhausen noise signal with applied field. This gave a good fit to published data. The modelling parameters characterising pinning site strengths showed good correlations with grain and carbide particle sizes. New Barkhausen noise data were obtained from tempered power plant steel samples for further model testing. The Orientation Imaging Microscopy (OIM) technique was used to investigate the grain orientations and grain boundary properties in the steel and their possible role in Barkhausen noise behaviour. The model again fitted the data well, and a clear relationship could be seen between the pinning strength parameter and the severity of tempering (as expressed by the Larson-Miller tempering parameter) to which the steel was subjected. The experimental results suggest that the Barkhausen noise characteristics of the steels investigated depend strongly on the strain at grain boundaries. As tempering progresses and the grain boundary dislocation density falls, the pinning strength of the grain boundaries also decreases. A clear difference in Barkhausen noise response could be seen between a 2$\frac{1}{4}$Cr1Mo traditional power-plant steel and an 11Cr1Mo steel designed for superior heat resistance. A study of an oxide dispersion strengthened ferrous alloy, in which the microstructure undergoes dramatic coarsening on recrystallisation, was used to investigate further the effects of grain boundaries and particles on Barkhausen noise. The findings from these experiments supported the conclusion that grain boundary strain reduction gave large changes in the observed Barkhausen noise