Characterization of thermal embrittlement in 2507 super duplex stainless steel using nonlinear acoustic effects

The structural integrity of components that operate at elevated temperatures can be compromised by the precipitation of second phases during long-term service. In industry, the 475 °C embrittlement phenomenon due to spinodal decomposition of the ferrite phase that results in the formation of Cr-rich α′ precipitates at a relatively low temperature has been a concern because of its adverse effects on ferritic, duplex and super duplex stainless steels; this precipitation causes a rapid decrease in the impact and corrosion resistance of these steels. Some of the precipitates have micro or nanometer dimensions, which makes them difficult to detect with conventional ultrasonic nondestructive evaluation (NDE) methods. In contrast to the conventional linear ultrasonic NDE, nonlinear ultrasound offers a unique advantage of high sensitivity to microstructural changes such as those due to the precipitation of second phases, fatigue damage, plastic deformation, and thermal damage in metallic alloys. In this research, ultrasonic measurements were conducted to determine the nonlinearity parameter (β) of thermally embrittled 2507 SDSS specimens. The β parameters of these specimens were measured by exploiting two different nonlinear effects: the acoustoelastic effect and second harmonic generation in Rayleigh waves. Experimental results indicate that nonlinear parameters obtained from these independent experiments are consistent and sensitive to the 475 °C embrittlement process.