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A18.pdf

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Wear 342-343 (2015) 92–99

Contents lists available at ScienceDirect

Wear

journal homepage: www.elsevier.com/locate/wear

Improving cavitation erosion resistance of austenitic stainless steel in liquid sodium by hardfacing – comparison of Ni and Co based deposits

B.K. Sreedhar a,n, S.K. Albert b, A.B. Pandit c

a Fast Reactor Technology Group (FRTG), Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, India b Materials Technology Division, Metallurgy and Materials Group, IGCAR, Kalpakkam, India

c Department of Chemical Engineering, Institute of Chemical Technology (ICT), Mumbai, India

article info

Article history:

Received 27 April 2015 Received in revised form

30 July 2015

Accepted 13 August 2015 Available online 21 August 2015

Keywords:

Cavitation erosion Sodium Hardfacing

SS 316L Colmonoy5 Stellite6

1. Introduction

Cavitation occurs in a hydraulic system when the static pres- sure at any point in the flow field drops below the vapor pressure of the liquid at the operating temperature. The resulting vapor cavities are transported by the flowing liquid and collapse when the pressure recovers producing extremely high localized pressure and temperature spikes [1–4]. Collapse of these cavities adjacent to metal boundaries causes severe pitting of the metal surface and thus affects the life of the equipment. Cavitation can be prevented by improved hydraulic design of plant and equipment resulting in increased available energy and reduced equipment requirement. It can also be prevented through the use of materials and coatings with high resistance to cavitation thus permitting operation with some degree of cavitation. This is often necessary because a cavi- tation free environment is often economically unrealizable.

1.1. Mechanism of cavitation damage

Rayleigh [5] in his seminal paper proposed the idea of material damage due to shock waves resulting from the symmetrical

n Corrresponding author.

E-mail address: bksd@igcar.gov.in (B.K. Sreedhar).

http://dx.doi.org/10.1016/j.wear.2015.08.009

0043-1648/& 2015 Published by Elsevier B.V.

& 2015 Published by Elsevier B.V.

collapse of individual empty or vapor filled spherical bubbles, at constant pressure during the collapse process, in an inviscid, incompressible liquid. This work was extended by Plesset, Poritsky and others to include the effects of internal pressure of gas in the bubble and the effects of liquid properties like surface tension and viscosity to give the now famous Rayleigh–Plesset equation for the collapse pressure of a single bubble. An alternative damage mechanism that has been proposed in cases where extremely high shock waves are not plausible is that due to microjets [1,6], of diameter ranging from few microns to several hundred microns, which are expected to occur when collapsing bubbles are distorted by pressure gradients or when they are located adjacent to solid boundaries resulting in the movement of high velocity liquid microjets through the cavity. The damaging phenomenon is thus characterized by high pressures and temperatures existing in localized regions (of few microns to hundreds of microns) over very short periods of the order of microseconds. In reality, the resulting shock wave or micro jet is due to the collapse of a cluster of bubbles/cavities with the collapsing cavities in the periphery serving to reinforce those at the center [7]. The surface is therefore subjected to repeated mechanical loading at high frequency. If the stresses generated are higher than the elastic limit this can result in permanent deformation; however, if the stresses are less than the elastic limit then failure can occur by fatigue. The capacity of the material to absorb the energy from bubble collapse without

abstract

Cavitation affects the performance of hydraulic machinery and also results in erosion damage. Although the damage produced in sodium is more intense than that in water, it is uneconomical to design hydraulics to totally avoid cavitation. The designer is left with the choices of improving hydraulic design and/or using materials/coatings with good resistance to cavitation. Susceptibility to cavitation is eval- uated for two different hardfaced coatings, viz. Co-based Stellite6s alloy coatings and Ni-based Col- monoy5s coatings, and the results compared with that for 316L austenitic stainless steel. Study reveals that Stellite6 alloy coating is more resistant to cavitation than Colmonoy5 in liquid sodium. However, the cavitation resistance of Colmonoy5 coating is better than that of austenitic stainless steel 316L, the substrate material on which these alloys are deposited. Results are explained based on the differences in the stacking fault energy and fracture toughness of the materials.

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