Slurry Erosion Characteristics and Erosive Wear Mechanisms of Co-Based and Ni-Based Coatings Formed by Laser Surface All
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INTRODUCTION
EROSION due to silt/slurry has a predominant effect on the efficiency and life of the hydroturbine in hydropower plants. The dynamic action of silt, which is running along the water that comes into contact with turbine components, is mainly responsible for the silt erosion.[1] Therefore, studies of silt properties, operating conditions, and substrate properties are essential in understanding the silt erosion operative mechanisms of hydroturbine components. The angle of impingement is one of the major test parameters that is associated with turbine operating conditions. Hydroturbine components are subjected to a wide range of impingement angles at different locations. Hence, an investigation of the erosion rate at different angles gives an idea of the maximum erosion rate that may occur in particular test conditions. These tests also indicate the mode of erosion for specific operating
R.C. SHIVAMURTHY and M. KAMARAJ are with the Department of Metallurgical and Materials Engineering (MME), Indian Institute of Technology Madras (IITM), Chennai-600036, India. R. NAGARAJAN is with the Department of Chemical Engineering, IITM, Chennai-600036, India. S.M. SHARIFF and G. PADMANABHAM are with the Centre for Laser Processing of Materials (CLPM), International Advanced Research Centre for Powder Metallurgy and Newer Materials (ARCI), Hyderabad500005, India. Contact e-mail: [email protected] Manuscript submitted August 25, 2008. Article published online November 4, 2009 470—VOLUME 41A, FEBRUARY 2010
conditions.[2] The erosion can be divided into two different modes. One is the ductile mode of erosion, in which the erosion rate is maximum at a low impingement angle. The other is the brittle mode of erosion, which occurs if the erosion rate approaches maximum at a normal impingement angle.[2,3] Tucker et al.[4] have shown that the slurry erosion resistance of mixed-particle laser-clad overlays is superior to that of precipitation-hardened alloys. Yarrapareddy et al.[5] demonstrated the superior slurry wear properties of nickel-tungsten functionally graded materials developed by laser cladding. Chattopadhyay[6] found excellent erosion resistance in a Stellite 6 plasmawelded overlay on cast-ferritic stainless steel (CA6NM) that is used as a turbine runner material. RiabkinaFishman and Zahavi[7] and Vilar[8] showed various approaches and advantages of laser surface alloying (LSA). The LSA (in which the surface chemical composition of a material is modified by adding a suitable proportion of alloying elements to the melt pool created with the laser beam) creates substantial savings in the consumption of strategic elements such as Cr, Ni, Mo, and Co, because less costly substrates can be surface alloyed for different applications. A high depth of alloying with low distortion can also be achieved.[9] Among hardfacing alloys, nickel-based Colmonoy 88 powder is a unique alloy that contains chromiumtungsten borides and carbides for maximum abrasion and corrosion resistance and is a relatively low-cost alloy.[10] The excellent prop
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