Liquid impact erosion mechanism and theoretical impact stress analysis in TiN-coated steam turbine blade materials
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I. INTRODUCTION
EROSION of steam turbine blades, especially the last low pressure ones, in nuclear power plants has been a significant problem throughout the history of steam turbines in terms of turbine performance, efficiency, safety, and durability. This erosion was ascribed to the high impact pressure generated at the water/blade interface by collision of water drops in steam with high speed rotating blades.[1,2,3] Many studies have been made, therefore, on shielding of leading edges of blades, where damage is most significant, with more erosion-resistant material.[4–7] Stellite 6B is a good example of material commonly used for shielding. However, this shield material gets eroded and detached from the blade body if subjected to wet steam for a long time; therefore, a more erosion-resistant material is needed. In this study, in order to improve the erosion resistance of the blade material, attempts were made to coat with a high impedance (5density 3 sound speed) material, which could lower the impact stress based on Springer’s model.[8] TiN was selected as the coating material, because it has been widely used as a hard coating because of its high hardness, chemical stability, low friction coefficient, and good oxidation and corrosion resistance. TiN coating achieved by the ion plating process has been known as the most effective coating technique for improving mechanical properties.[9] In this study, the mechanisms and the erosion resistance of TiN-coated blade materials (12Cr steel and Stellite 6B) were studied and the effects of the TiN coating on the impact MIN KU LEE, formerly Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is Postdoctoral Student, Department of Material Science and Mechanical Engineering, Duke University, Durham, NC 27706. WHUNG WHOE KIM, Principal Researcher, and CHANG KYU RHEE, Senior Researcher, Advanced Nuclear Materials Department, and WON JONG LEE, Professor, Department of Materials Science and Engineering, are with the Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea. Manuscript submitted March 30, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
stresses generated in the blade materials were investigated by using Blowers’ impact model.[10] II. EXPERIMENTAL A. Liquid Impact Erosion Test The samples prepared for erosion test were 12Cr steel, Stellite 6B, TiN-coated 12Cr steel, and TiN-coated Stellite 6B. All samples were 1.5-mm-thick discs with a diameter of 15 or 25 mm. They were mirror polished to 1-mm Al2O3 powder. TiN coating was applied using deposition conditions, which were most effective in improving mechanical properties.[11] These deposition parameters are described in Table I. The whirl arm type[12] and the disk rotary type[13,14] erosion testers were widely used elsewhere, but their tests have had some inherent disadvantages such as limitation in attainable speeds, high power consumption, and vibrations due to unbalancing. Hence, an erosion tester was designed that could attain
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