Ultrasonic noise relaxation for evaluating thermal aging embrittlement of duplex stainless steels

A noncontact ultrasonic method is presented to evaluate the thermal aging embrittlement of the cast duplex stainless steels, which uses the shear-wave backscattering noise detected by an electromagnetic acoustic transducer (EMAT). Duplex stainless steel is a highly damping material, and the pulse-echo measurement for the velocity and attenuation is unavailable. High damping comes from the scattering at boundaries between the austenitic and ferritic phases. But, since little energy is absorbed in the material, the elastic waves impinged by an EMAT last in the sample for a long period (in the order of 10 ms) and are received by the same EMAT as a slowly decaying backscattering noise. The relaxation time coefficient is calculated by integrating the digitized noise signal and is correlated with the aging time. It clearly discriminates four duplex stainless steels aged for 0, 300, 1000, and 3000 h at 673K. The noise decays in a shorter time as the aging period increases. The difference of the noise relaxation rate is interpreted by the phase decomposition of ferrite into Cr-rich alpha' phase and Fe-rich alpha phase as supported by transmission electron microscopy (TEM).