Evaluation of sensitization in stainless steel 304 and 304L using nonlinear Rayleigh waves
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Austenitic stainless steels have a wide range of applications in the energy industry, but the corrosion resistance of these stainless steels can be reduced by sensitization, which is the formation of chromium carbide precipitates at grain boundaries, causing the formation of a zone of chromium depletion at the grain boundary. Since chromium is the primary alloying element that makes stainless steel corrosion resistant, this chromium depleted zone is susceptible to stress corrosion cracking. Sensitization occurs when the stainless steel is exposed to a high temperature for an extended time period. The objective of this research is to determine the sensitivity of nonlinear ultrasound to the presence of sensitization in austenitic stainless steels. This research uses nonlinear Rayleigh waves to quantitatively track the sensitization of SS304 and SS304L stainless steels as a function of holding time at 675 °C. The effect of the carbon contents of the alloys (SS304 versus SS304L) to the sensitization process and the measured nonlinearity parameters are investigated. An initial annealing of these specimens isolates the effect of just sensitization, removing the resence of cold work and residual stresses which can also affect the material nonlinearity. Complementary Electrochemical reactivation (EPR) measurements and microscopy are used to confirm the absence or presence of sensitization. These results show that the acoustic nonlinearity parameter is sensitive to the presence of chromium carbide precipitates. Furthermore, we investigated the sensitization process in the heat affected zone of a weld joint. The results show a dependency of the nonlinearity parameter to the distance from the weld joint.