S K Tiwari

Cavitation Erosion in Hydraulic Turbine Components and Mitigation by Coatings: Current Status and Future Needs

Cavitation erosion is a frequently observed phenomenon in underwater engineering materials and is the primary reason for component failure. The damage due to cavitation erosion is not yet fully understood, as it is influenced by several parameters, such as hydrodynamics, component design, environment, and material chemistry. This article gives an overview of the current state of understanding of cavitation erosion of materials used in hydroturbines, coatings and coating methodologies for combating cavitation erosion, and methods to characterize cavitation erosion. No single material property fully characterizes the resistance to cavitation erosion. The combination of ultimate resilience, hardness, and toughness rather may be useful to estimate the cavitation erosion resistance of material. Improved hydrodynamic design and appropriate surface engineering practices reduce damage due to cavitation erosion. The coatings suggested for combating the cavitation erosion encompasses carbides (WC Cr2C3, Cr3C2, 20CrC-80WC), cermets of different compositions (e.g., 56W2C/Ni/Cr, 41WC/Ni/Cr/Co), intermetallic composites, intermetallic matrix composites with TiC reinforcement, composite nitrides such as TiAlN and elastomers. A few of them have also been used commercially. Thermal spraying, arc plasma spraying, and high velocity oxy-fuel (HVOF) processes have been used commercially to apply the coatings. Boronizing, laser surface hardening and cladding, chemical vapor deposition, physical vapor deposition, and plasma nitriding have been tried for surface treatments at laboratory levels and have shown promise to be used on actual components.

Influence of laser scanning speed on nitrided Ti6Al4V surface

ABSTRACT The influence of laser scan speed during nitriding in the range 30–150 cm min−1 on the physical properties, microstructure and corrosion behaviour of Ti6Al4V was studied. The surface energy of nitrided Ti6Al4V remained almost constant up to the scan speed of 60 cm min−1 followed by an increase in the maximum value at 80 cm min−1. A gradual drop in the microhardness of the coating from the outermost surface was noticed for specimens nitrided up to 80 cm min−1; an abrupt decrease, however, was found at a scan speed ≥100 cm min−1. The laser nitriding produced dendritic microstructure; an exponential decay in the secondary dendrite arm spacing (SDAS) with the increase in laser scan speed was revealed. The SDAS produced was smaller at the interphase and centre of the coating, compared to that at outermost surface. The passivation behaviour of Ti6Al4V at lower potentials shifted to the smaller current side after laser nitriding.

Cladding of Tungsten Carbide and Stellite Using High Power Diode Laser to Improve the Surface Properties of Stainless Steel

Surface engineering is one of the most viable methods, in addition to development of new alloys and equipment design, to minimize degradation due to cavitation erosion, and corrosion. Laser surface cladding is relatively a newer engineering technique to produce metallurgically bonded coating for industrial applications due to its inherent benefits. Present paper reports the results obtained on the laser cladding of stainless steel with tungsten carbide (WC) and stellite alloy powder using high power diode laser (HPDL) at various laser parameters. Cladded specimens were characterized for erosion, and corrosion resistance. Both WC and stellite cladding have increased the erosion resistance of stainless steels. WC cladding was found to reduce the corrosion resistance of steel while stellite showed it to increase significantly.