Comparison of acoustic and hydrodynamic cavitation: Material point of view

Abstract

This study investigated the difference in mechanical response of the martensitic stainless steel X3CrNiMo13-4/S41500/CA6 NM QT780 between hydrodynamic and acoustic cavitation erosion. The results show that acoustic cavitation erosion generates small pits at a high temporal frequency on the material, while hydrodynamic cavitation erosion produces larger pits at a lower frequency. Acoustic cavitation erosion tests have been performed using a 20 kHz ultrasonic horn located at 500 μm in front of a specimen. This experimental setup, known as an indirect method, is inspired from the ASTM G32 standard. Hydrodynamic cavitation erosion tests were conducted with classic experimental conditions of a PREVERO device: a cavitation number of 0.87 corresponding to a flow velocity of 90 m s−1 and an upstream pressure of 40 bars. In addition, for a given exposure time, the percentage of surface covered by the pits is smaller for acoustic cavitation than for hydrodynamic cavitation. Three successive steps have been identified during the damage process: persistent slip bands (PSB) first appear on the surface, cracks initiate and propagate at the PSB locations and nonmetallic interfaces, and finally, parts of the matter are torn off. A careful time examination of the same small area of the exposed sample surface by scanning electron microscopy reveals that acoustic cavitation is faster to initiate damage than hydrodynamic cavitation.