A metallurgical approach to improved cavitation-erosion resistance
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A Metallurgical Approach to Improved Cavitation-Erosion Resistance R.H. Richman and W.P. McNaughton Although cavitation erosion in hydraulic systems is an old problem, the damage mechanism that culminates in material loss was not known with certainty until recently. An investigation is described that aimed at clarifying the damage mechanism in cavitation erosion and applying that knowledge to make hydraulic equipment more resistant to cavitation. Strong correlations were established between cyclic deformation (fatigue) parameters and cavitation-erosion rates. This identification facilitated the search for more resistant materials. Finite element modeling confirmed that localized impacts on metal surfaces produce fatiguelike deformations and damage accumulation. Among available materials, near-equiatomic alloys of nickel and titanium are anomalously resistant to low-cycle fatigue and thus should be very resistant to cavitation erosion. Experiments confirmed the expected erosion resistance. Building large machines entirely out of NiTi is impractical, however, and a way of selectively cladding common constructional materials is required. Because NiTi has not been fusion welded successfully to other alloys, explosive bonding of thin NiTi plates to structural steel was investigated. Excellent welds were achieved, and the erosion resistance of the resulting clads has been demonstrated. Comparisons are made to other erosion processes and to other erosion-resistant materials, and some applications to hydraulic devices are suggested.
IKeywords
erosion cavitation, explosive bonding, fatigue, NiTi
I
1. Introduction
a result of single impulses or impacts. That is, damage accumulates over thousands of impacts before a particle is dislodged (Ref 2, 3). Several investigators had attributed the failure mode specifically to fatigue (Ref 4-6), and evidence of fatigue can be deduced from experiments (Ref 7-10). The basis for the present investigation was that if cavitation erosion is fatiguelike, then the factors contributing to fatigue resistance of materials (Ref 11, 12) should offer clear direction for achieving improved cavitation-erosion resistance.
Material removal by erosion imposes limitations on technologies in many industries. Although recent developments in materials design and fabrication have opened abundant opportunities for tailoring materials to specific applications, mitigation of erosion damage through materials improvements and surface treatments has been hampered because the damage mechanism in cavitation erosion was unknown and thus it has not been possible to specify the materials properties that ensure good erosion resistance. Machines subject to cavitation erosion (and closely related liquid-droplet erosion) include boiler feed pumps, valves, recirculation pumps in pressurized-water reactor (PWR) systems, hydroturbine runners and guide vanes, last-stage blades in steam turbines, and ship propellers. Although proper design is a necessary condition for long lives of equipment exposed to c
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