Characterizing microstructural variability in P91 steel using nonlinear ultrasonic Rayleigh waves

A significant portion of failures in grade 91-92 steels have been attributed to poor fabrication that results from a high production variability and repair/management activities such as post weld heat treatment. This can result in microstructures which are detrimental to creep and fatigue properties of the structure. The objective of this article is to use Rayleigh wave nonlinear ultrasonics to differentiate and classify the different microstructures. Eight different microstructures were chosen for testing with as-received, gradual and gross degradation in the microstructure. The nonlinear 2nd and 3rd harmonics were measured for each microstructural condition and used for analysis. The destructive analysis included hardness measurement and microscopy to get the average grain sizes. The hardness was used to calculate the dislocation density, which was further correlated with the 2nd and 3rd harmonic nonlinear parameters. The results suggest that the 3rd harmonic is better at classifying the microstructures compared to the 2nd harmonic or the linear Rayleigh wave velocity. Conjectures were developed destructive and nondestructive results.