Ultrasonic and Nanoindentation Evaluation of SAF 2507 Aging: Elastic Moduli, Attenuation, and Microstructure

Super duplex stainless steels are highly recommended for corrosive environments. However, detrimental phases tend to precipitate in the austenite-ferrite interfaces at certain temperatures. In this study, the elastic moduli of ten specimens of SAF 2507 that underwent isothermal heat treatment at 900 ºC for different amounts of time were determined using the ultrasonic non-destructive technique. These results were validated by nanoindentation, confirming the accuracy of the ultrasonic inspection. An investigation of microstructural changes, employing optical microscopy and X-ray diffraction, enabled elucidation of how these modifications influence acoustic wave attenuation and velocity, as well as the elastic modulus. As the ferrite content decreases and the sigma phase content increases, the attenuation coefficients tend to decay, while the elastic modulus increases until a treatment time of 45 min and remains stable for higher aging times. Additionally, a robust correlation between shear wave velocity and the elastic modulus was identified.

Keywords:
Super duplex; nondestructive testing; nanoindentation; ultrasonics; metallography