Rayleigh Surface wave based non linear ultrasound to assess effect of precipitation hardening during tempering in P92 steel

This study presents the generation and variation of second harmonic of Rayleigh surface wave due to the change in precipitation morphology during tempering of P92 steel. The specimens from P92 steel were normalized at 1075 °C and then tempered in the temperature range of 745-835 °C at a step of 30 °C for 3hrs followed by air cooling. Non linear ultrasonic (NLU) parameter which is defined as the ratio of the amplitude of second harmonic to the square of the amplitude of the transmitted signal frequency was measured at each temperature and was correlated with the changes in precipitates number density, volume fraction and dislocation density. It was observed that the measured non linearity parameter is sensitive to the precipitate coherency strain. Similar trend in the variation of mechanical properties and NLU parameter with the tempering temperature suggests that this technique can be useful in monitoring material strengthening due to the precipitation hardening and precipitation-dislocations interactions.