Nonlinear ultrasonic characterization of thermal degradation in ferritic 2.25Cr–1Mo steel

The thermal degradation of ferritic 2.25Cr–1Mo steel was characterized using the nonlinear ultrasonic technique for the purpose of structural health monitoring (SHM). An accelerated heat treatment was performed in order to simulate differing levels of thermal degradation. The variation in the ultrasonic nonlinearity was interpreted regarding the microstructural evolution during thermal degradation. The normalized ultrasonic nonlinearity parameter (β/β0) monotonically increased due to the precipitation of equilibrium M6C carbide and the variation in the lattice parameter of M23C6 carbide. Ultrasonic nonlinearity was found to be strongly sensitive to the carbide behavior during thermal degradation of ferritic 2.25Cr–1Mo steel.