Microstructural degradation assessment in pressure vessel steel by harmonic generation technique

The harmonic generation technique has been used to characterize the isothermal degradation of pressure vessel steel. The isothermal degradation was conducted at 630degC with forged 2.25Cr-1Mo steel. The variation in the normalized ultrasonic nonlinearity parameter (β/β0) was interpreted as having resulted from microstructural evolution, which was supported by the results of electron microscopy and X-ray diffraction, in addition to Vickers hardness and ductile-brittle transition temperature. The normalized ultrasonic nonlinearity parameter increased progressively with the isothermal degradation time due to the increase in the volume fraction of equilibrium M6C carbide and the variation in the lattice parameter of M23C6 carbide. It was found that the nonlinearity parameter was very sensitive to the microstructure during the isothermal degradation of 2.25Cr-1Mo steel. The harmonic generation technique has the potential to assess microstructural changes due to isothermal degradation. (author)