A Non-invasive Technique to Estimate the Onset of Creep Strength Deterioration

A material when exposed to high temperature under a constant load for a prolonged period, it experiences deformation due to Creep. The strength of precipitation hardening materials reduces when exposed to high temperature under load due to the coarsening of precipitates and their incoherency with the matrix. Growth of precipitates beyond a critical size introduces nonlinearity in the propagation of elastic waves through a material. This nonlinearity in the elastic waves can be measured by a parameter known as nonlinear ultrasonic (NLU) parameter which is proportional to the ratio of the amplitude of 2nd harmonic to the square of the amplitude of the fundamental frequency of the sinusoidal wave propagated through the material. This paper addresses the effect of generation and growth of precipitates and their sizes on the changes in NLU parameter in P92 steel, a prime candidate material for power plant, exposed at temperature 650 °C under a load of 120 MPa for a duration till the material ruptures due to creep. Two different mode of experiments under the same condition; multiple samples with single interruption (MSSI) and single sample with multiple interruptions (SSMI), were designed to establish NLU technique to assess the critical stage of the said material. It was observed that in both the test modes when the changes in the NLU parameter were plotted as a function of strain, the peak value of the NLU parameter corresponds to the onset of deterioration in the mechanical strength of the material. The results were verified through microstructural characterization and mechanical test.