Vibratory Stress Relief and Vibratory Weld Conditioning of Flux cored arc welded CA6NM steel

ASTM A743 CA6NM steel is used in the manufacturing of hydraulic turbines components. Multipass welding is commonly used for their fabrication or repairing. In this work, two different vibratory welding procedures were studied: vibration applied during welding (VWC) and vibration applied after welding (VSR). Results have shown that in both conditions, CA6NM steel presented a martensitic microstructure, in which the VSR welded joint presented column-shaped packets and fine martensite delineating the individual beads, while VWC joint presented grain refinement. Heat affected zones (HAZ) presented δ-phase in small amounts for both conditions in the regions which reached higher temperatures. VSR and VWC conditions presented similar behavior in terms of hardness, HAZ hardness values being close to those of the weld metal, except for the root regions, where higher values were obtained. Charpy-V results showed that HAZs presented higher impact values than those of the weld metal. The low impact values of the weld metal were attributed to presence of inclusions from the welding electrode.

Slurry erosion performance study of HVFS sprayed Ni-20Al2O3 and Ni-15Al2O3-5TiO2 coatings under hydro accelerated conditions

Purpose This paper aims to cover all the aspects of development, investigation and analysis phases to evaluate the slurry erosion performance of test coatings. The powders having composition of Ni-20Al2O3 and Ni-15Al2O3-5TiO2 were deposited on CA6NM grade turbine steel by using high velocity flame spray (HVFS) technique. The characterization of the coatings was done with the help of SEM/EDS and XRD techniques. Various properties such as micro-hardness and bonding strength of the coatings were also evaluated. Thereafter, these coatings were subjected to an indigenously developed high speed slurry erosion tester at different levels of rotational speed, erodent particle size and slurry concentration. The effect of these parameters on the erosion behavior of coatings was also evaluated. The slurry erosion tests and SEM of the eroded surfaces revealed remarkable improvement in slurry erosion resistance of Ni-15Al2O3-5TiO2 coating in comparison with Ni-20Al2O3 coating. Design/methodology/approach Two different compositions of HVFS coating were developed onto CA6NM steel. Subsequently, these coatings were evaluated by means of mechanical and microstructural characterization. Further, slurry erosion testing was done to analyze the erosive wear behavior of developed coatings. Findings The coatings were successfully developed by HVFS process. Cross-sectional microscopic analysis of sprayed coatings revealed a continuous and defect-free contact between substrate and coating. Ni-15Al2O3-5TiO2 coating showed higher value of bond strength in comparison with Ni-20Al2O3 coating. Under all the testing conditions, Ni-15Al2O3-5TiO2 coatings showed higher resistance to slurry erosion in comparison with Ni-20Al2O3 coatings. Rotational speed, average particle size of erodent and slurry concentration were found to have proportional effect on specific mass loss of coatings. The mixed behavior (brittle as well as ductile) of the material removal mechanism was observed for the coatings. Originality/value From the literature review, it was found that researchers have documented the various studies on Ni-Al2O3, Ni-TiO2 and Al2O3-TiO2 coatings. No one has ascertained the synergetic effect of Alumina and Titania on the slurry erosion performance of Nickel-based coating. In view of this, the authors have developed Ni-Al2O3 and Ni-Al2O3-TiO2 coatings, and an attempt has been made to compare their mechanical, microstructural and slurry erosion characteristics.

SOLID PARTICLE EROSION RESISTANCE OF HVAF-SPRAYED WC-10Co-4Cr COATING ON CA6NM STEEL

In the present investigation, WC-10Co-4Cr coating was deposited by high velocity air-fuel (HVAF) process on CA6NM hydro turbine steel. A detailed microstructural and phase compositional study was carried out on the coating. Mechanical properties of the coating were also evaluated. WC-10Co-4Cr coating showed a homogeneous, well-bonded structure with low porosity, which is mainly attributed to less decarburization of WC. Erosion resistance of the coating was evaluated by air jet erosion tester at three different impingement angles (30[Formula: see text], 60[Formula: see text] and 90[Formula: see text]) for 35 and 70[Formula: see text]m/s impact velocities. The FESEM micrographs were taken, before and after erosion tests, to determine the erosion mechanism. The test results revealed that the coating protects the substrate at 30[Formula: see text], 60[Formula: see text] and 90[Formula: see text] impingement angles. At 70[Formula: see text]m/s impact velocity, uncoated and coated steel showed higher cumulative volume loss than in the case of 35[Formula: see text]m/s impact velocity. It was observed that uncoated steel showed a ductile behavior during erosion and WC-10Co-4Cr coating showed mixed (ductile and brittle) mode of fracture during erosion.

Effect of Impingement Angle on Erosion Resistance of HVOF Sprayed WC-10Co-4Cr Coating on CA6NM Steel

In the present investigation, WC–10Co–4Cr coating was deposited by high velocity oxy-fuel (HVOF) process on CA6NM hydro turbine steel to improve its erosion resistance. The coating was characterized in term of crossectional microstructure, phase, microhardness and fracture toughness using a field emission scanning electron microscope (FESEM), X-ray diffractometer and microhardness tester respectively. Solid particle erosion resistance of the substrate and coating were evaluated by air jet erosion tester at two different impingement angles (30° and 90°). Coating microstructure has shown a homogeneous and well-bonded laminar morphology. The microhardness of the coating was observed more than three times higher than CA6NM substrate. This resulted in significant improvement in erosion resistance of coated CA6NM steel at both impingement angles.

Improving Erosion Resistance of Hydroturbine Steel Using Friction Stir Processing

In the present work, the slurry erosion behavior of friction stir processed (FSPed) hydroturbine steel (CA6NM) was investigated. For comparison, the erosion performance of unprocessed CA6NM steel was evaluated under similar conditions. Friction stir processing (FSP) is a microstructural refinement tool which is useful in enhancing the bulk and surface properties of materials. An in-depth characterization of both steels was done using an optical microscope (OM), a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), the electron backscatter diffraction (EBSD) technique, and micro- and nano-indentation techniques. The FSP of the steel helped in reducing the erosion rates by 50% to 60%, depending upon the impingement angle. The improved performance of the FSPed steel in comparison to unprocessed steel was attributed to microstructural refinement, which increased the hardness and yield strength. At an oblique impingement angle, plowing, along with microcutting, was observed to be the dominant erosion mechanism. At a normal impingement angle, the material removal process was controlled by the platelet mechanism of erosion. A modified form of the mathematical model for predicting the erosion rates of the ductile materials, proposed by authors earlier, was also presented. This modified model based upon the theory of plasticity was able to predict the erosion rates with an accuracy of ±20%.

Erosive Wear Study of HVOF Spray Cr3C2–NiCr Coated CA6NM Turbine Steel

Degradation of surfaces of hydroturbine components caused by impact of abrasive particles carried by flowing water is a serious issue. To counteract the same, surface modification of turbine materials by the application of protective coatings is gaining popularity these days. In this work, Cr3C2–NiCr coating was deposited on CA6NM turbine steel by the HVOF spray process and studied with regard to its performance under different slurry erosion conditions. The effect of three parameters, namely average particle size of slurry particles, speed (rpm), and slurry concentration on slurry erosion of this coating material, was studied by using a high speed erosion test rig. The analysis of the surfaces of the samples before and after slurry erosion tests was done by using SEM. The HVOF sprayed Cr3C2–NiCr coating showed very good performance under slurry erosion in comparison with uncoated CA6NM steel.