Evaluation of Cavitation Erosion Behavior of Commercial Steel Grades Used in the Design of Fluid Machinery
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INTRODUCTION
CAVITATION is the repeated formation and violent collapse of bubbles containing vapor and/or gas in a liquid caused by periodic tensile stresses imposed onto the liquid phase. Cavitation can be induced by high-frequency vibrations (acoustic cavitation) or by sudden pressure variation in the flow (hydrodynamic cavitation). In the latter, at a critical pressure, cavitation can be initiated by the localized phase change (partial liquefaction), from vapor to liquid, e.g., by a high-velocity working gas as in scroll expander systems.[1,2] When cavitation bubbles implode close to a surface, powerful micro-jets of velocity in the range of 300 to 1000 m/s with hydrodynamic impact pressures of more than 1 GPa are produced.[3,4] These micro-jets in combination with the pressure waves emitted during the implosion of cavitation bubbles promote the formation of incubation pits of various sizes and shapes across solid surfaces.[5–7] The cyclic nature of the surface stress leads to the coalescence of the incubation pits and consequently to the typical cavitation erosion damage (i.e., formation of deep craters and cracks).[5,8] Cavitation erosion is the major failure mechanism of many fluid machinery components, being responsible I. TZANAKIS is with Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University London, Uxbridge, Middlesex, UB8 3PH, UK, and also with Oxford Brookes University, Faculty of Technology, Design and Environment, Oxford, OX33 1HX, UK. Contact e-mail: [email protected] L. BOLZONI is with the Faculty of Science and Engineering, Waikato University, Hamilton 3240, New Zealand. D.G. ESKIN is with Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University London, and also with Tomsk State University, Tomsk, 634050 Russia. M. HADFIELD is with the Department of the Design and Engineering, Bournemouth University, Poole, BH12 5BB, UK. Manuscript submitted June 24, 2016. Article published online March 6, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
for high maintenance costs. Therefore, cavitation erosion rate and resistance are important parameters required for designing hydraulic parts and estimating their in-service performance. This is especially true when cavitation cannot be avoided due to design limitations, such as within scroll expander systems where the working fluid transfers energy to the rotor. Thus, a proper selection of materials for critical components with high cavitation erosion resistance is a necessity. Moreover, the damaged surface of eroded components may induce perturbations of the fluid flow; and the overall pressure ratio as well as the operating efficiency can be significantly decreased.[9,10] The most common method to simulate short-period (incubation pits) and long-period (craters formation) cavitation erosion at the laboratory scale is by using high-frequency sound waves generated by an ultrasonic device. Ultrasonic vibrations are introduced into the liquid by a sonotrode (ultrasonic horn). The sonotrode tip is immersed in a given liqui
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