Abstract
In the present investigation, studies were done to measure the acoustic nonlinearity by second harmonic generation of ultrasonic wave, known as non-linear ultrasonic (NLU), due to materials damage during creep in P92 steel. Creep rupture behaviour of this steel was carried out at a temperature of 600°C and stress level of 160MPa. Non linearity parameter (β) which is defined as the ratio of the amplitude of second harmonic to the square of the amplitude of the transmitted signal frequency of ultrasonic wave was evaluated. About two order change in non linearity parameter was observed between as tempered and creep ruptured samples. The influence of microstructure on the nonlinear wave propagation was analyzed. Based on metallographic studies, it was revealed that value of β in the material increases due to a rising of the precipitation area fraction, disappearance of prior austenite grain boundary, coarsening of precipitates, as well as formation of new precipitates like Laves phase. The nonlinear ultrasonic wave exhibits the potential for the creep damage monitoring of in-service components.
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