Title of Invention	A METHOD OF PRODUCING SILT EROSION AND CORROSION RESISTANT COATINGS
Abstract	A method of producing silt erosion and corrosion resistant coatings comprising the steps of supplying compressed oxygen to combustion chamber to produce a high spraying velocity and flame temperature, feeding a predetermined quantity of a carbide/metallic powder into the high temperature flame and deposition on the component to be coated.

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FIELD OF THE INVENTION: This relates to a method of producing silt erosion and corrosion resistant coatings. This invention further relates to a method of producing silt erosion and corrosion resistant coatings using liquid fuel based high velocity oxy fuel (HVOF) spraying system. The silt and corrosion resistant coating has been obtained by controlling the spraying parameters and automatic manipulation of the components as well as HVOF gun. BACKGROUND OF THE INVENTION: Silt erosion in hydroelectric power stations and machinery is a serious problem. This is more aggressive in India, especially in the Himalayan region. This is mainly due to excessive silt content having particle size in excess of 90um (ASTM 170 mesh). During monsoon it becomes impossible to control the silt content passing through the turbine. It increases from 2500 to 10,000 ppm and size exceeds 1000um. The concentration of quartz in Indian silt is also very high (-90%). This is responsible for very quick erosion. The components, which are exposed to erosion in existing power stations are guide vanes, labyrinth seals, runner blades, lower rings and top covers and guide vane bushings. These are made of corrosion-resistant martensitic (13Cr-4Ni & 12Cr steels), austenitic (18Cri-8Ni steel) and manganese steels (1.5 Mn steel). All these steels are considerably less resistant to silt erosion, and require suitable protective coatings. If unattended, the erosion of critical components such as guide vanes, labyrinth seals, runner blades, guide rings etc. can lead to loss of turbine efficiency as high as 5-10%. This causes a heavy revenue loss for all the silt affected hydro power stations. The coatings are generally applied by plasma, flame, wire and twin wire arc spraying techniques. To overcome the silt erosion of hydro turbine power station, a number of techniques such as weld deposit of hard materials, plasma spraying of hard ceramic coating, metallic coating by flame spray and plasma nitriding are used very commonly. However, these cannot solve the problem of silt damage occurring on the hydro machinery. In some cases the worn out hydro machinery is replaced with new machinery. The main disadvantages of the present thermally sprayed coatings applied by plasma, flame, twin wire arc spraying techniques and HVOF coatings are the lack of controlled movement of spraying guns as well as movement of symmetrical and unsymmetrical hydro turbine components. The components such as guide vanes, runner blades, labyrinth seal, top cover and lower ring require precise movement to control the uniformity of the HVOF coating thickness, porosity, hardness etc, which otherwise uncontrolled generally leads to excessive erosion. At present, these coatings are either applied manually or with partially controlled movement either of the job or of the HVOF gun. The coating powder generally used in the HVOF system for this application are tungsten carbide in combination with Ni, Co, Cr as a binder in combination or alone. The other disadvantage of conventional techniques is that these do not adequately solve silt erosion problem. Ceramic coatings applied by plasma spraying are brittle in nature and debond while operating hydro machinery under high stress abrasive conditions. Disadvantages of plasma nitriding and hard weld deposits are that these lead to excessive distortion and warpages of' the hydro components. OBJECTS OF THE INVENTION: It is therefore an object of this invention to propose a method of producing silt erosion and corrosion resistant coatings, which is generally applicable to hydroturbines and pumps. It is a further object of this invention to propose a method of producing silt erosion and corrosion resistant coatings, which is dense, oxide free and has limited voids. Another object of this invention is to propose a method of producing silt erosion and corrosion resistant coatings, which used improved HVOF spraying parameters. Yet another object of this invention to propose a method of producing silt erosion and corrosion resistant coatings, which has improved surface roughness. DESCRIPTION OF THE INVENTION: This invention relates to a method of producing silt erosion and corrosion resistant coatings, comprising the steps of supplying compressed oxygen to high velocity spraying chamber to produce a high spraying velocity, feeding a predetermined quantity of a carbide/metallic powder into the high temperature flame and deposition on the component to be coated. In accordance with this invention is provided a method for providing silt erosion and corrosion resistant cdatings, HVOF spraying is used especially for materials with melting point below 3000K. It shows advantages in density and bond strength making it attractive for many silt erosion and corrosion resistance applications. Its high coating quality results from the use of a hot combination- driven high-speed gas jet for thermal spraying. These coatings have environmental advantages compares to chemically electrochemically formed coatings. Tungsten and chromium carbide powders are used in the HVOF spraying system. These are used to produce dense, high hardness and excellent silt erosion resistance coatings generally to combat the erosion and corrosion at high as well as room temperature. In applications where abrasive or erosive silt erosion resistance is of primary importance, Tungsten carbide-cobalt-chromium. (WC-Co) with and without nickel or chrome is used. Tungsten carbide-cobalt (WC-Co-Cr) powders are preferred when high corrosion resistance at room temperature is needed. The abrasive and erosive silt erosion resistance depends upon oxides, pores and the phase transformation occurring during spraying. The coating powders generally used in HVOF spraying are WC+Co, Cr3C2+NiCr, WC+Cr3C2+NiCr, WC+Co+Cr. High velocity oxy fuel sprayed coatings are commonly applied by HP/HVOF JP-5000, DS-100, Met jet ll/lll, OSU, Metco Diamond jet and unique coat AC HVAF systems. These systems are based on liquid as well as gaseous fuel and oxygen/air. The liquid fuel injection systems are preferred due to their advantages of producing dense and high hardness coatings. The HVOF coatings are generally based on the combustion of liquid fuel such as kerosene and gaseous fuel such as hydrogen, propane or natural gas and oxygen. The HVOF spraying system is improved by controlling Indian aviation fuel/oxygen flow rates and thereby producing a controlled thermal energy which is converted into kinetic energy resulting in proper melting and accelerating the powders to produce a silt erosion and corrosion resistant coating. As Indian aviation fuel has more aromatics, the parameters have been modified suitably. The oxygen flow rate employed is upto 1000 Lpm, the fuel flow rate is upto 25 Lph and the carrier gas flow rate is upto 50Lpm. The melted powder is accelerated to a velocity of 700 to 800 m/sec for deposition on the surface of the component to be coated. The flame temperature is increased through Indian aviation fuel injection and combusting with commercial oxygen to get medium temperature of carbide particles upto 1800°C. The spraying gas velocity generated through combustion of compressed oxygen and fuel is above 2500m/sec. The combustion pressure is employed upto 0.95 MPa, the spray distance is maintained upto 450mm and the spray angle is upto 90°C. The length of the barrel of the plasma gun is upto 200 mm. In accordance with an embodiment of this invention tungsten carbide-cobalt- chromium powder is used where cobalt is upto 12%, chromium upto 6%, the balance being made up of tungsten carbide. In accordance with a further embodiment of this invention, the roughness of the surface to be coated is increased, before applying the coating. As the adhesion of coatings is directly related to the roughness of the surface and it is controlled by the type of grit blasting machine, blasting pressure, angle, distance, time and grit blasting nozzle. The grit blasting is carried out with a pressure blaster having nozzle upto 8 mm dia using blasting pressure upto 0.6 MPa and alumina grit of size 12-16 mesh. The grit-blasting stand off distance is upto 500 mm so that optimum surfaces finish is obtained. The surface finish achieved on the components is upto 15 micron Ra. The HVOF coatings obtained according to the process of the invention was evaluated for its silt erosion resistance behavior. This was tested by using a facility which was designed in house and fabricated by considering the high- stress abrasive wear of hydro turbines components. Water mixed with sand was pumped through a known gap between a rotating disc and the housing where hydrofoils of required shape and sizes were fixed. The coated samples were mounted at required radial locations. The sand slurry was accelerated and impinged on these hydrofoils, causing hydro-abrasion. By choosing different radii, the linear velocity and the centrifugal force, and hence the intensity of abrasion, can be varied. By regulating the sand and water flow rates the silt concentration was kept constant. The sand was continuously fed at the rate of 37g/min, which ensures continuous replacement of worn-out sand particles. Velocity of the sand-laden water was of the order of 66.3 m/s, corresponding to a maximum acceleration up to 18000 g. In principle, such high accelerations are reached only in extremely high-head and high-speed turbines. This test facility was designed in such a way that silt concentration can be varied from 1.5 kg/m3 to 10 kg/m3. In actual hydro power stations, such values of silt concentration may occur only during a few days in a year. Thus the abrasive wear results from this test facility, for a test duration of just a few hours, can be considered to simulate the abrasive wear of the turbine components of the actual power stations during the whole year. Test Conditions: Erodent type Mineral sand of hardness 1100 HV Silt Concentration upto 2350 ppm Water velocity upto 66.3 m/s Chamber pressure upto 330 mm of water column Specimen type and size Hydrofoils scaled down to 1/10 of actual] hydro turbine blade and also round samples. Additionally the coatings have been tested for high stress abrasive wear by applying a load of 50 N and using rubber wheel (as per ASTM G-65). Different quantities of erodent were used to get 100 to 1000 revolutions of rubber wheel. The test parameters adopted are given below: Erodent type Mineral sand of hardness of 1100 HV Size of erodent 180-250 microns Erodent flow rate upto 5.5 g/s Load applied 50 N Sample size 75 x 25 x 6 Test duration 600 revolutions Hardness of rubber wheel Shore 70 The test results are given in the Figures 1 and 2 of the accompanying drawings and also in Table 1. The HVOF spraying parameters take into accounts the distance variation, which generally occurs in case of runner blades and guide vanes. Fig 1 shows the simulated silt erosion test results of HVOF coating and Fig 2 shows the effect of particle impact energy on erosion rate. HVOF coatings applied by controlled parameters have been proven on the symmetrical top cover and lower ring weighting 16 ton & 12 ton respectively and on the unsymmetrical guidevanes. These coatings do not debond while erosion testing under high stress abrasive conditions. The coatings were done by automatic movement of the component and HVOF gun. The components were mounted on a job rotator and the HVOF gun on the boom. The components, which were rotated at controlled speed to get a uniform coating thickness of 250 micron. Other components such as guide vanes and runner blades were also coated in similar way. TABLE-1: Abrasion resistance of HVOF coatings at different distance and angles (as per ASTM G-65 using mineral sand 60 mesh and applying a load of 50N) WE CLAIM: 1. A method of producing silt erosion and corrosion resistant coatings comprising the steps of supplying compressed oxygen to combustion chamber to produce a high spraying velocity and flame temperature, feeding a predetermined quantity of a carbide/metallic powder into the high temperature flame and deposition on the component to be coated. 2. The method as claimed in claim 1, wherein the flame temperature is increased by injecting fuel into the spraying chamber. 3. The method as claimed in claim 1, wherein the coating powder used is such as tungsten carbide in combination at least one with metal selected from nickel, cobalt, chromium, as a binder. 4. The method as claimed in claim 1, wherein the fuel used is selected from liquid fuel such as kerosene and gaseous fuel such as hydrogen, propane or natural gas and oxygen. 5. The method as claimed in claim 1 and 4, wherein the fuel flow rate is upto 25 Lph. 6. The method as claimed in claim 1 and 4, wherein the oxygen flow rate is upto 1000 Lpm. 7. The method as claimed in claim 1, wherein a carrier gas is used with a flow rate of upto 50 Lpm. 8. The method as claimed in claim 1, wherein a combustion pressure of upto 0.95 MPa is used. 9. The method as claimed in claim 1, wherein the coating powder is maintained at a temperature of upto 1800°C 10. The method as claimed in claim 1, wherein the spraying gas velocity is upto 2500 m/s 11. The method as claimed in claim 1, wherein the roughness of the surface to be coated is increased before applying the coating. 12. The method as claimed in claim 11, wherein the roughness is increased by pressure blasting. 13. The method as claimed in claim 11, wherein the pressure blasting employed is preferably grit blasting with alumina grits of 12 to 16 mesh size. 14. The method as claimed in claim 11, wherein the blasting is carried out at a pressure of upto 0.6 MPa. 15. A method of producing silt erosion and corrosion resistant coatings substantially as herein described. 1. A method of producing silt erosion and corrosion resistant coatings comprising the steps of supplying compressed oxygen to combustion chamber to produce a high spraying velocity and flame temperature, feeding a predetermined quantity of a carbide/metallic powder into the high temperature flame and deposition on the component to be coated.

Documents:

235-KOL-2005-ABSTRACT 1.1.pdf

235-kol-2005-abstract.pdf

235-KOL-2005-AMANDED PAGES OF SPECIFICATION.pdf

235-KOL-2005-CLAIMS 1.1.pdf

235-kol-2005-claims.pdf

235-KOL-2005-CORRESPONDENCE 1.2.pdf

235-KOL-2005-CORRESPONDENCE-1.1.pdf

235-kol-2005-correspondence.pdf

235-KOL-2005-DESCRIPTION (COMPLETE) 1.1.pdf

235-kol-2005-description (complete).pdf

235-kol-2005-drawings.pdf

235-KOL-2005-EXAMINATION REPORT REPLY RECIEVED.pdf

235-KOL-2005-FORM 1 1.1.pdf

235-kol-2005-form 1.pdf

235-kol-2005-form 18.pdf

235-KOL-2005-FORM 2 1.1.pdf

235-kol-2005-form 2.pdf

235-kol-2005-form 3.pdf

235-KOL-2005-OTHERS 1.1.pdf

235-KOL-2005-PA.pdf

235-kol-2005-specification.pdf