Cavitation Erosion Behavior of High Strength Steels

Abstract

SUMMARY Cavitation erosion is still one of the most important degradation modes in hydraulic turbine runners. Part of researches in this field focuses on finding new materials or coatings which exhibit high resistance to this phenomenon. In terms of purely mechanical properties, quenched and tempered high strength steels turn out to be a good alternative to the commonly used martensitic stainless steels (13Cr-4Ni). In the present study, the cavitation erosion behavior of ASTM A514 high strength steels in a laminated form and in welded form is investigated. This paper states a two side approach which has been used to get an advanced understanding of this kind of steel behavior regarding cavitation phenomenon. First, the overall behavior is determined using a cavitating liquid jet facility according to a modified ASTM G134-95 standard to simulate aggressive flow conditions and determine the cavitation erosion resistance. Moreover, 20 kHz vibratory test according ASTM G32-10 standard has been performed to study as thoroughly as possible the incubation period to get evidence of the degradation mechanism during the incubation stage. In both cases, mass losses and erosion rate have been followed during the exposure time. Furthermore, a particular effort has been implemented to determine the evolution of surface damages in terms of pitting, surface cracking and material removal. For this, 3-D optical profilometry technique as well as scanning electron microscopy has been used to link the microstructure to the degradation mechanisms at each stage. The results showed that the cavitation erosion behavior is similar for base and welded materials in aggressive conditions whereas it is possible to observe a difference in smoother conditions. Actually a longer incubation time has been determined for welded form due to bainite structure. In both case the degradation mechanism suggests that the erosion was mainly controlled by ductile fracture.