H. Singh

Parametric Study of Slurry-Erosion of Hydroturbine Steels with and without Detonation Gun Spray Coatings using Taguchi Technique

WC-Co-Cr coatings were deposited on some hydroturbine 13Cr4Ni and 16Cr5Ni steels by the detonation-gun spray process. An in-depth characterization of the as-sprayed coating was done using X-ray diffraction (XRD) and scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS) techniques. Microhardness and porosity measurements were also made. The coating was found to have a typical splat-like morphology with some indications of unmelted carbide particles. The XRD results showed the presence of WC as the primary phase along with W2C and Co6W6C as secondary phases. Furthermore, the slurry erosion behavior of the coatings was investigated to ascertain the usefulness of the coatings to reduce the slurry erosion of the steels. The effect of four operating factors viz. the velocity, impact angle, concentration, and particle size on the slurry erosion of coated and bare steels has been studied using a high-speed jet-type test rig. The sand used as an erodent was collected from a power plant to replicate the actual turbine conditions. It has been observed that the given cermet coating can enhance the erosion resistance of the steel. Velocity was found to be the most significant factor affecting the erosion behavior of the coating, whereas it was the erodent particle size in the case of uncoated steel. As evidenced from the SEM images, the platelet mechanism of erosion seemed to be the prominent one, causing the removal of material from the surface of the steel, whereas for the coating, the formation and interlinking of cracks resulted in the removal of material.

Design and Development of High-Velocity Slurry Erosion Test Rig Using CFD

Slurry erosion (SE) is commonly observed in almost all kinds of components and machineries involved in fluid (liquid) transfer and delivery. During design and development phase of these components, test rigs are usually required to evaluate their performance; however, only few detailed designs of test rigs are available for SE investigations. Among the existing designs of SE test rigs, most of them belong to rotary type. In the present study, design of a new type of SE test rig has been proposed, which is simpler in construction and working. This newly designed test rig could possibly eliminate some of the limitations (velocity-concentration interdependence and lack of acceleration distance) found in the existing set-ups. Calibration of the test rig was done for jet velocity and erodent concentration. Commissioning of the rig was undertaken by evaluating the effect of operating parameters (concentration and impingement angle) on the erosion rates of aluminum and cast iron. Results show that the rig was able to capture the traditional responses of ductile and brittle erosion behaviors being observed for these materials. Repeatability of the test rig was ensured, and the results were found to be within the acceptable error limits.

Identifying Erosion Mechanism: A Novel Approach

Understanding the erosion mechanism is a key to improve the performance of material subjected to erosive condition. Capability to predict the erosion mechanism could prove to be useful tool. In this work, a parameter named “erosion mechanism identifier,” ξ, is proposed to predict the erosion mechanism in materials. Suitability of ξ in predicting erosion mechanism of ductile and brittle materials was evaluated using the data reported in the literature. It was observed that ξ is able to predict the erosion mechanism for both categories of material. The predictability of ξ was not restrained by different operating conditions.

Effect of zirconium addition in HVOF sprayed Ni–20Cr coating

Abstract High velocity oxyfuel spraying was used to deposit Ni–20Cr coating on an ASTM-SA213-T22 boiler steel with and without 1%Zr addition. High temperature oxidation behaviour of both coatings was evaluated under cyclic thermal loading conditions at an elevated temperature of 900°C. Mass change data were measured to formulate the kinetics of oxidation for the specimens. The exposed specimens were characterised by X-ray diffraction and field emission scanning electron microscopy/energy dispersive spectroscopy. It was observed that mass loss was reduced by 46% after the Zr addition in the commercially available Ni–20Cr coating. The formation of Zr rich pegs in the blended coating might have resulted in beneficial results.

Synthesis and deposition of Ni–20Cr powder using cold spraying

Abstract Chromium nanoparticles were first synthesised using planetary ball mill in wet environment. The particle size was found to be nearly 60 nm, which was confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and particle size analyser techniques. Subsequently, Ni–20Cr alloy powder was synthesised by mechanical alloying of micrometre sized Ni (80 wt-%) and presynthesised Cr (20 wt-%) nanosized powders. X-ray diffraction revealed the solid solubility of Cr in Ni after ball milling for 20 h. The synthesised Ni–20Cr powder was subsequently deposited on SA 516 by cold spraying process. The as sprayed coatings were characterised by SEM/EDS, XRD, microhardness and porosity analyses. A nanostructured Ni–20Cr coating could be developed with an average thickness of 225 μm and microhardness of 335 HV having an apparent porosity of <1·5%.

Evaluation and Development of Economically Viable Coatings for Erosion Protection of Hydroturbines

Hydrotubines do face regular exposure to cavitating environment, causing damage to guide vanes, impeller, nozzle, buckets and spear. Quite a large number of surface modification techniques have been employed to counter such a deleterious effect. Among these large numbers of techniques, thermal spray coatings has been mostly focused and studied. In the present work one such technique has been opted for study. Selection of coating technique and composition was made keeping their economical viability in mind. Hardness and toughness have been identified as significant factors effecting cavitation erosion of materials. In-depth analysis of erosion mechanism of coated and un-coated samples was undertaken using SEM/EDS, micro-hardness tester and XRD. Erosion mechanism of steel was mainly plastic deformation, whereas in case of coating, (i) lack of adhesion between splats (ii) lower toughness of alumina (iii) loosely bonded un-melted alumina particles, were dominant factor controlling the erosion mechanism of coating.

X-Ray Diffraction Study of Cold Sprayed Ni-20Cr and Ni-50Cr Coatings on Boiler

X-ray diffraction (XRD) is a versatile, non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of materials. In this work Ni-20Cr and Ni-50Cr coatings were deposited on two boiler steels namely T22 and SA 516 steel. The measurement of residual stresses of these cold sprayed coatings was done with the help of X-ray diffraction technique. This paper discussed the XRD study of the as-sprayed coatings. Further the XRD technique was used to study the uncoated and coated steels after cyclic exposure to air, molten salt [Na2SO4-60%V2O5], and actual boiler environments. The results obtained from the XRD analysis have been shown. The weight change results showed that the coated steels performed better than their uncoated counterparts which might be attributed to the formation of protective phases.

Friction Stir Processing of Mild Steel to Enhance its Surface Hardness

Friction stir processing (FSP) is a material processing technique recognized to enhance the mechanical properties of the material owing to micro-structural refinement. In the present study, the feasibility of the use of FSP for processing of mild steel has been explored. Using FSP, the micro-structural refinement of the steel surface was obtained, resulting in an average grain sizes of the order of 100 nm as confirmed by atomic force microscopy (AFM). Along with grain size refinement, an evolution of various phases was also studied with the help of X-ray diffraction (XRD). It was observed that ferrite was the only phase present in both processed and unprocessed steel.FSP was found to be useful to enhance the micro-hardness of the steel by 50 to 80%, in comparison with that of the un-processed steel.

Extraction and transport behaviour of tripodal receptor: selective recovery of Ni2+ and processing into nickel nanoparticles

Abstract A tripodal receptor bearing the S,N donor sites was investigated for the extraction and transportation of metal ions. The receptor showed selectivity towards nickel ion as it could extract 87% of nickel ion from aqueous medium. Subsequently, nickel nanoparticles were synthesised by reduction of Ni2+ with NaBH4 at an optimised set of process conditions.