Title of Invention	AN IMPROVED METHOD OF THERMAL SPRAYING OF BOILERS AND A DEVICE FOR THE SAME
Abstract	To overcome these difficulties of thermal spraying of boiler components such as burner panel, steam cooled wall tubes, etc. and to improve the life of the boiler tube against erosion, it is proposed that the wire arc spraying by making use of nano grade flux core wires. By this method, the portions of the tubular boiler panels exposed to high temperature will be coated by high erosion resistant coatings. Accordingly, there is provided a process for twin wire arc spraying with the standard equipment available in the market. The erosion resistant coating composition includes amorphous matrix structure containing starburst shaped boride ((FeCrMoW)2B) and carbide (FeCrMoW)23C6) crystallites with sizes ranging from 60 to 140 nm.

Title of Invention

AN IMPROVED METHOD OF THERMAL SPRAYING OF BOILERS AND A DEVICE FOR THE SAME

Abstract

To overcome these difficulties of thermal spraying of boiler components such as burner panel, steam cooled wall tubes, etc. and to improve the life of the boiler tube against erosion, it is proposed that the wire arc spraying by making use of nano grade flux core wires. By this method, the portions of the tubular boiler panels exposed to high temperature will be coated by high erosion resistant coatings. Accordingly, there is provided a process for twin wire arc spraying with the standard equipment available in the market. The erosion resistant coating composition includes amorphous matrix structure containing starburst shaped boride ((FeCrMoW)2B) and carbide (FeCrMoW)23C6) crystallites with sizes ranging from 60 to 140 nm.

Full Text	FIELD OF INVENTION The invention relates to an improved method of thermal spraying of boilers. The invention further relates to a coating composition for carrying-out the improved method of thermal spraying of boilers. The invention also relates to a device for implementing the improved method of thermal spraying of boilers. BACKGROUND OF INVENTION Erosion-resistant coatings are frequently used on fossil boiler components such as burner panels, wall blower areas, steam cooled walls, economizer tubes and bends, inlet header of low temperature superheater (LTSH) etc, including various other components of the system that are prone to erosion due to flying ash. Erosion of these components generally occurs by impingement of fly ash having highly erosive constituents. Existing base materials for fossil boiler components such as carbon and low alloy steels do not have adequate erosion resistance under these conditions. For example, coal fired boiler components such as economizers, LTSH when exposed to fly ash have been found to undergo significant erosion due to the Indian coal containing as high as 45% ash. This problem is severe in India, where boilers are heavily depended on coal or lignite as the fuel. The severe erosion that damages the boiler components causing frequent maintenance related shutdowns, loss of generation and the need to replace various components on a regular basis. Erosive high temperature wear in boilers is one of the main causes of downtime and one of the principle engineering problems in these installations. As a result of the combustion process, the oxide particles produced have much higher melting point than the boiler operating temperature. Thus, these particles remain intact and then get entrained into the exhaust gas resulting in a constant bombardment of particulate matter. Maintenance costs for replacing worn tubes are very high and the downtime associated with unscheduled breakdowns caused by the failure of exchange tubes is a source of lost revenue. The environment is also very erosive due to a number of sources. While it is possible to provide additional protection shields by way of covering to the erosion prone areas, the shields can not be used at the areas where higher flue gas temperature exists for example, wall blower, burner panel etc because they get warped or even burnt. At low temperature areas, if installation of the shields is not proper, these shields get displaced exposing the surface for erosion and sometimes even shift the erosion location. In order to avoid erosion problems in power stations during the shut down, generally, the tubes are either replaced, or an weld overlay of carbon steel by SMAW technique is attempted which is quite cumbersome process in practice as it takes more time and more labour and even the tube might get damaged. Further these weld overlay or replacement of tubular panels is not totally effective during prolonged exposure to erosion. Current erosion resistant coatings are usually applied by thermal spray techniques, such as high velocity oxy-fuel (HVOF) and weld overlay by SMAW of carbon steel for various boiler components. Of all the different prior art deposition processes, HVOF yield the most dense erosion resistant coatings and as such, is generally preferred for forming erosion resistant coatings. Even though the HVOF yields coatings with high residual stress, it however, limits the coating thickness to about 300 microns in thickness. Further, because of the gas constituents used in the HVOF process, and the resulting particle temperature and velocity, the so-formed coatings generally contain some degree of decarburization, which significantly reduces the coating erosion resistance. Further, there is a need for large amounts of LPG gas and oxygen for carrying out the coating and making them available at site is a major problem besides safety issues. The time taken for the coating is also very high. Japanese patent JP2185961 discloses a method to improve the corrosion and erosion resistances of a steel tube for a boiler coated with a self-fluxing alloy such as MSF Ni 1-45 or MSF Co 1 standardized by JIS by previously forming an Al coated layer on the steel tube. The outer surface of a steel tube is coated with Al and a self-fluxing alloy contg. Ni-Cr-B-Si-C-Fe-Co, Ni-Cr-B-Si-C-Fe-Co-Cu, Ni-Cr- B-Si-C-Fe- Co-Mo-Cu or Ni-Cr-B-Si-C-Fe-Co-Mo-W is flame sprayed and fused on the resulting Al coated layer to obtain a steel tube for a boiler. Japanese patent JP10030897 describes an overlay welding process which overlays the Ni alloy in a desired area in the longitudinal direction with a specified thickness and a bending process which bends the steel tubes in U shape. The composition of the Ni alloy includes 15 to 20wt.% of Cr, 9 to 15wt.% of Mo and the rest of Ni. The high temperature steel tube includes the serpentine-formed steel tube having the coating layer in an area in contact with a high temperature gas out of straight running parts and or curved parts of the high temperature steel tube. Chinese patent CN1858293 relates to surface processing material technology, in particular, a nano grade modified wear resistant and erosion resistant arc sprayed wire material. The wire material has coating of Fe or Ni, and powdered core comprising high carbon ferrochrome 45-60 wt%, Ni 10-20 wt%, ferrosilicon 4-10 wt%, ferroboron 5010 wt%, Cr3C2, TiB2 or TiC 10-15 wt%, micron level TiB2 5-15 wt%, nanometer level AI2O3 5-15 wt%, and optional wollastonite 1-3 wt%, ZrO2 2-5 wt% and TiO2 5-15 wt%. The material is used mainly for coating the surface of blower blade, tube wall in power station boiler etc. and the coated layer can increase the surface life of the said parts. Chinese patent CN1814852 discloses an invention related to surface engineering field used in the protection of power station boiler pipelines including welding a layer of hardness alloy on a cleaned metal matrix with self protection metal core wire, in which, the weld current is 180-220A, voltage is 18-22V, the welded metal matrix is sprayed with sand to eliminate the oxidized skins, rust and the old coating and expose the metallic brightness then a compound layer of anti- erosion and anti-abrasion is sprayed on the matrix by an arc spraying method. Canadian patent CA1067354 discloses a method of boiler tube coating wherein steel tubes or pipes, for example, steel boiler tubes and/or integrated panels of steel boiler tubes, are provided with a fused overlay of a corrosion and erosion resistant coating comprised of a refractory hard component, e.g. tungsten carbide, dispersed through a corrosion resistant matrix alloy. US patent 7,141,110 discloses an improved erosion resistant coatings for hydro electric turbine component. The erosion resistant coating composition includes nanostructured grains of tungsten carbide (WC) and/or submicron sized grains of WC embedded into a cobalt chromium (CoCr) binder matrix. A high velocity air fuel thermal spray process (HVAF) is used to create thick coatings in excess of about 500 microns with high percentages of primary carbide for longer life better erosion resistant coatings. These materials and processes are especially suited for hydroelectric turbine components. US patent US7066242 discloses a method of prevention of erosion of boiler tubes. The refractory shield assembly comprises: a semi-circular, elongate, metal shield; a plurality of spaced apart anchors protruding from the front surface of the shield; a layer of abrasion-resistant refractory material overlying the surface and extending between and engaging the anchors, whereby the refractory material is held on the shield by the anchors; and means, such as clamps, for securing the shield on a boiler tube. The refractory shield assembly functions to protect the underlying boiler tube from erosion by a stream of hot combustion gas containing particulates. Accordingly, there remains a need in the art for quicker and improved coating methods and coating compositions that provide effective protection against erosion resistance, such as is required for boiler components such as burner panels, steam cooled walls, etc. Improved coating methods and/or coating compositions on regions of boiler components desirably need coatings with a combination of high erosion resistance, low residual stresses, and higher thickness to provide a coating with long life and high erosion resistance under Indian operating conditions having coal with high ash content. OBJECTS OF THE INVENTION It is therefore an object of the invention to propose an improved method of thermal spraying for boilers. Another object of the invention is to propose an improved method of thermal spraying for boilers in which wire arc spraying of burner panel using nano grade wire, which is manifold productive, is adapted. A still another object of the invention is to propose an improved method of thermal spraying for boilers which can be done faster on site-condition without the requirement of complicated work handling devices. A further object of the invention is to propose an improved method of thermal spraying for boilers which provides coatings with a combination of high erosion resistance, low residual stresses, and higher thickness. Yet another object of the invention is to propose a device for implementing an improved method of thermal spraying of boilers. A still further object of the invention is to propose a coating composition to carry- out the improved method of thermal spraying for boilers. These and other objects and advantages of the invention will be apparent from the ensuing description. At the outset of the description, which follows, it is to be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment and the teachings of the invention are not intended to be taken restrictively. SUMMARY OF THE INVENTION To overcome these difficulties of thermal spraying of boiler components such as burner panel, steam cooled wall tubes, etc. and to improve the life of the boiler tube against erosion, it is proposed that the wire arc spraying by making use of nano grade flux core wires. By this method, the portions of the tubular boiler panels exposed to high temperature will be coated by high erosion resistant coatings. Accordingly, there is provided a process for twin wire arc spraying with the standard equipment available in the market. The erosion resistant coating composition includes amorphous matrix structure containing starburst shaped boride ((FeCrMoW)2B) and carbide (FeCrMoW)23C6) crystallites with sizes ranging from 60 to 140 nm. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1- Schematically shows a prior art process of providing a protective plate over the tube for erosion protection including wire arc coating. Figure 2- shows an embodiment of the device for implementing an improved method of thermal spraying of boilers according to the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Thermal spray coatings is an alternate approach to the wire arc coating that offers advantages because they allow in situ recoating of the boiler tubes with the additional ability to repair localized defects inside the boiler. The ability to recoat boilers during a scheduled outage is especially attractive because downtime can be minimized, which translates into significant operational cost savings. Wire arc spraying is a thermal spraying method which utilizes the heat from an electric arc to melt various metallic materials. In this method, two consumable wire electrodes (5) are fed automatically to the arc zone. The initiated arc between the wire electrodes (5) melts the wire tips. The fused metal is atomized by compressed air using an air nozzle (7), and thrown with high velocity onto a substrate (6), forming the coating (Fig. 2). Normally, a dc constant potential power source (9) is used to provide a voltage between 18 and 40V depending on the spray material. The voltage should be kept at the lowest level with regard to arc stability in order to produce smooth and dense coatings. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, protective shields over the tube. The erosion resistance coating on a boiler panel is shown in FIG. 2 is to be made with the least possible outlay by the process according to the invention. As per figure 2, the coating is completely made by making use of only twin wire arc thermal spray process using the nano grade wire of following chemistry. Wire Material Chemistry Chromium Molybdenum The development of nanoscale coatings can be achieved via processing through a solid/solid-state transformation, which can occur during wire arc spraying. This route to nanoscale coatings is enabled by developing alloys that readily form metallic glass structures at cooling rates in the range of thermal spray processes (i.e., 104 to 105 K/s) (Ref 1,2). During the devitrification step, the glass recursor, when heated to its crystallization temperature, readily transforms into a nanoscale composite structure. This refinement is due to the uniform nucleation and extremely high nucleation frequency during crystallization, resulting in little time for grain growth before impingement between neighboring grains. By this route, it is possible to develop very stable nanostructures that resist coarsening at elevated temperatures. The erosion resistant coating composition includes amorphous matrix structure containing starburst shaped boride ((FeCrMoW)2B) and carbide (FeCrMoW)23C6) crystallites with sizes ranging from 60 to 140 nm. The chief advantage of this approach, in contrast to other approaches outlined previously, is that the feedstock material cored wire for wire-arc spraying is physically identical to conventional feedstock, which eliminates the spraying problems normally associated with nanosized particulate materials and also bypasses their high cost. The ability to develop nanoscale microstructures with enhanced properties using conventional technology is revolutionary. Conventional power source for wire arc spraying can be used and flux core wire of nano grade wire is used. All the coatings are developed and applied by wire arc spraying. According to this embodiment, the wire arc spraying of nanograde wire is carried out as per the enclosed parameters in Table 1. In the laboratory test of the above wire arc sprayed tube exhibited the required hardness, reduced porosity and oxide inclusions and finally thermal spraying is being performed in the actual power plant site. The microstructure is composed of three phases, consisting of Fe, a carbide phase, tentatively identified as the (Fe-Cr-W-Mo)23C6 type phase, and a boride phase, tentatively identified as the (Fe-Cr-Mo-W)3B type phase. The microstructure is found to consist of ≈ 2/3 volume fraction of nanoscale complex carbide and complex boride phases intermixed with the Fe phase. Table -1 Typical Wire arc Spraying parameters During spraying, the visual examination reveals that there is no spalling, and bend testing of the coating in a 1.25 mm carbon steel sheet reveals that there is no cracking or spalling. The compelling properties observed result from the ability to develop nanoscale composite microstructures in air using conventional wire-arc spray guns. The ability to spray the above wire-arch coatings in the field has been demonstrated, because the coating is found to be easy to apply with a wide processing window and very good spray forgiveness, and, once coated, is easily recoated and repaired. While there are many different types of coating materials with different combinations of properties, all coatings can only provide protection to a component as long as they remain bonded to the component. The advantages of the invention can be seen, inter alia, in the fact that the wire arc spraying can be done by making use of simple wire arc spraying equipment without costly consumables required for HVOF process. Further, the old method of HVOF or protective shields on the erosion prone area requires lot of lead time in providing the protection and also costly. Moreover, this method will not be suitable for providing life of more than 2 years. We Claim: 1. An improved method of thermal spraying of boilers, comprising the steps of: - feeding at least two consumable wire electrodes to the arc zone; - initiating an arc between the at least two consumable wire electrodes; and - atomizing the fused material by using at least one air nozzle, and directing the fused materials at a high velocity onto a substrate forming a nanoscale coating. 2. The method as claimed in claim 1, wherein the nanoscale coating is formed by developing alloys that readily form metallic glass structures at cooling rates of 104 to 105 K/s. 3. The method as claimed in claim 1 or 2, wherein the glass recursor in a devitrification step is heated to its crystallization temperature to transform into a nanoscale composite structure. 4. The method as claimed in claim 1, wherein a voltage between 18 and 40V is supplied via a dc constant potential power source. 5. The method as claimed in any of the preceding claims, wherein the consumable wire electrodes arc selected with a wire material chemistry of chromium Molybdenum 6. An erosion resistant coating composition for thermal spraying of boilers, comprising amorphous matrix structure containing starburst shaped boride ((FecrMOW)2B), and Carbide ((FecrMOW)23C6) crystalites with sizes ranging from 60 to 140nm. 7. A device for thermal spraying of boilers, comprising: - a wire feeder (4) for automatic feeding of at least two consumable wire electrodes (5) to the arc zone; - means for (9) initiating an arc between the wire electrodes (5) causing the wire tips to melt in a wire melting zone (8); - an air nozzle (7) to direct the fused metal atomized by compressed air onto a substrate (6) forming the coating. 8. The device as claimed in claim 7, wherein the wire diameter, console air pressure, voltage, amperage, spray distance, and method selected respectively at 1.6mm, 8 bar, 33-35V, 180-240 amp, 125 mm, and manual 9. An improved method of thermal spraying of boilers as substantially described and illustrated herein with reference to the accompanying drawings. 10. A device for thermal spraying of boiler as substantially described and illustrated herein with reference to the accompanying drawings. To overcome these difficulties of thermal spraying of boiler components such as burner panel, steam cooled wall tubes, etc. and to improve the life of the boiler tube against erosion, it is proposed that the wire arc spraying by making use of nano grade flux core wires. By this method, the portions of the tubular boiler panels exposed to high temperature will be coated by high erosion resistant coatings. Accordingly, there is provided a process for twin wire arc spraying with the standard equipment available in the market. The erosion resistant coating composition includes amorphous matrix structure containing starburst shaped boride ((FeCrMoW)2B) and carbide (FeCrMoW)23C6) crystallites with sizes ranging from 60 to 140 nm.

Full Text

FIELD OF INVENTION
The invention relates to an improved method of thermal spraying of boilers. The
invention further relates to a coating composition for carrying-out the improved
method of thermal spraying of boilers. The invention also relates to a device for
implementing the improved method of thermal spraying of boilers.
BACKGROUND OF INVENTION
Erosion-resistant coatings are frequently used on fossil boiler components such
as burner panels, wall blower areas, steam cooled walls, economizer tubes and
bends, inlet header of low temperature superheater (LTSH) etc, including various
other components of the system that are prone to erosion due to flying ash.
Erosion of these components generally occurs by impingement of fly ash having
highly erosive constituents. Existing base materials for fossil boiler components
such as carbon and low alloy steels do not have adequate erosion resistance
under these conditions. For example, coal fired boiler components such as
economizers, LTSH when exposed to fly ash have been found to undergo
significant erosion due to the Indian coal containing as high as 45% ash. This
problem is severe in India, where boilers are heavily depended on coal or lignite
as the fuel. The severe erosion that damages the boiler components causing
frequent maintenance related shutdowns, loss of generation and the need to
replace various components on a regular basis.
Erosive high temperature wear in boilers is one of the main causes of downtime
and one of the principle engineering problems in these installations. As a result
of the combustion process, the oxide particles produced have much higher
melting point than the boiler operating temperature. Thus, these particles remain

intact and then get entrained into the exhaust gas resulting in a constant
bombardment of particulate matter. Maintenance costs for replacing worn tubes
are very high and the downtime associated with unscheduled breakdowns
caused by the failure of exchange tubes is a source of lost revenue. The
environment is also very erosive due to a number of sources.
While it is possible to provide additional protection shields by way of covering to
the erosion prone areas, the shields can not be used at the areas where higher
flue gas temperature exists for example, wall blower, burner panel etc because
they get warped or even burnt. At low temperature areas, if installation of the
shields is not proper, these shields get displaced exposing the surface for erosion
and sometimes even shift the erosion location.
In order to avoid erosion problems in power stations during the shut down,
generally, the tubes are either replaced, or an weld overlay of carbon steel by
SMAW technique is attempted which is quite cumbersome process in practice as
it takes more time and more labour and even the tube might get damaged.
Further these weld overlay or replacement of tubular panels is not totally
effective during prolonged exposure to erosion.
Current erosion resistant coatings are usually applied by thermal spray
techniques, such as high velocity oxy-fuel (HVOF) and weld overlay by SMAW of
carbon steel for various boiler components.
Of all the different prior art deposition processes, HVOF yield the most dense
erosion resistant coatings and as such, is generally preferred for forming erosion
resistant coatings. Even though the HVOF yields coatings with high residual

stress, it however, limits the coating thickness to about 300 microns in thickness.
Further, because of the gas constituents used in the HVOF process, and the
resulting particle temperature and velocity, the so-formed coatings generally
contain some degree of decarburization, which significantly reduces the coating
erosion resistance. Further, there is a need for large amounts of LPG gas and
oxygen for carrying out the coating and making them available at site is a major
problem besides safety issues. The time taken for the coating is also very high.
Japanese patent JP2185961 discloses a method to improve the corrosion and
erosion resistances of a steel tube for a boiler coated with a self-fluxing alloy
such as MSF Ni 1-45 or MSF Co 1 standardized by JIS by previously forming an Al
coated layer on the steel tube. The outer surface of a steel tube is coated with Al
and a self-fluxing alloy contg. Ni-Cr-B-Si-C-Fe-Co, Ni-Cr-B-Si-C-Fe-Co-Cu, Ni-Cr-
B-Si-C-Fe- Co-Mo-Cu or Ni-Cr-B-Si-C-Fe-Co-Mo-W is flame sprayed and fused on
the resulting Al coated layer to obtain a steel tube for a boiler.
Japanese patent JP10030897 describes an overlay welding process which
overlays the Ni alloy in a desired area in the longitudinal direction with a
specified thickness and a bending process which bends the steel tubes in U
shape. The composition of the Ni alloy includes 15 to 20wt.% of Cr, 9 to 15wt.%
of Mo and the rest of Ni. The high temperature steel tube includes the
serpentine-formed steel tube having the coating layer in an area in contact with
a high temperature gas out of straight running parts and or curved parts of the
high temperature steel tube.

Chinese patent CN1858293 relates to surface processing material technology, in
particular, a nano grade modified wear resistant and erosion resistant arc
sprayed wire material. The wire material has coating of Fe or Ni, and powdered
core comprising high carbon ferrochrome 45-60 wt%, Ni 10-20 wt%, ferrosilicon
4-10 wt%, ferroboron 5010 wt%, Cr3C2, TiB2 or TiC 10-15 wt%, micron level
TiB2 5-15 wt%, nanometer level AI2O3 5-15 wt%, and optional wollastonite 1-3
wt%, ZrO2 2-5 wt% and TiO2 5-15 wt%. The material is used mainly for coating
the surface of blower blade, tube wall in power station boiler etc. and the coated
layer can increase the surface life of the said parts.
Chinese patent CN1814852 discloses an invention related to surface engineering
field used in the protection of power station boiler pipelines including welding a
layer of hardness alloy on a cleaned metal matrix with self protection metal core
wire, in which, the weld current is 180-220A, voltage is 18-22V, the welded
metal matrix is sprayed with sand to eliminate the oxidized skins, rust and the
old coating and expose the metallic brightness then a compound layer of anti-
erosion and anti-abrasion is sprayed on the matrix by an arc spraying method.
Canadian patent CA1067354 discloses a method of boiler tube coating wherein
steel tubes or pipes, for example, steel boiler tubes and/or integrated panels of
steel boiler tubes, are provided with a fused overlay of a corrosion and erosion
resistant coating comprised of a refractory hard component, e.g. tungsten
carbide, dispersed through a corrosion resistant matrix alloy.

US patent 7,141,110 discloses an improved erosion resistant coatings for hydro
electric turbine component. The erosion resistant coating composition includes
nanostructured grains of tungsten carbide (WC) and/or submicron sized grains of
WC embedded into a cobalt chromium (CoCr) binder matrix. A high velocity air
fuel thermal spray process (HVAF) is used to create thick coatings in excess of
about 500 microns with high percentages of primary carbide for longer life better
erosion resistant coatings. These materials and processes are especially suited
for hydroelectric turbine components.
US patent US7066242 discloses a method of prevention of erosion of boiler
tubes. The refractory shield assembly comprises: a semi-circular, elongate, metal
shield; a plurality of spaced apart anchors protruding from the front surface of
the shield; a layer of abrasion-resistant refractory material overlying the surface
and extending between and engaging the anchors, whereby the refractory
material is held on the shield by the anchors; and means, such as clamps, for
securing the shield on a boiler tube. The refractory shield assembly functions to
protect the underlying boiler tube from erosion by a stream of hot combustion
gas containing particulates.
Accordingly, there remains a need in the art for quicker and improved coating
methods and coating compositions that provide effective protection against
erosion resistance, such as is required for boiler components such as burner
panels, steam cooled walls, etc. Improved coating methods and/or coating
compositions on regions of boiler components desirably need coatings with a
combination of high erosion resistance, low residual stresses, and higher
thickness to provide a coating with long life and high erosion resistance under
Indian operating conditions having coal with high ash content.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved method of
thermal spraying for boilers.
Another object of the invention is to propose an improved method of thermal
spraying for boilers in which wire arc spraying of burner panel using nano grade
wire, which is manifold productive, is adapted.
A still another object of the invention is to propose an improved method of
thermal spraying for boilers which can be done faster on site-condition without
the requirement of complicated work handling devices.
A further object of the invention is to propose an improved method of thermal
spraying for boilers which provides coatings with a combination of high erosion
resistance, low residual stresses, and higher thickness.
Yet another object of the invention is to propose a device for implementing an
improved method of thermal spraying of boilers.
A still further object of the invention is to propose a coating composition to carry-
out the improved method of thermal spraying for boilers.
These and other objects and advantages of the invention will be apparent from
the ensuing description.

At the outset of the description, which follows, it is to be understood that the
ensuing description only illustrates a particular form of this invention. However,
such a particular form is only an exemplary embodiment and the teachings of the
invention are not intended to be taken restrictively.
SUMMARY OF THE INVENTION
To overcome these difficulties of thermal spraying of boiler components such as
burner panel, steam cooled wall tubes, etc. and to improve the life of the boiler
tube against erosion, it is proposed that the wire arc spraying by making use of
nano grade flux core wires. By this method, the portions of the tubular boiler
panels exposed to high temperature will be coated by high erosion resistant
coatings. Accordingly, there is provided a process for twin wire arc spraying with
the standard equipment available in the market. The erosion resistant coating
composition includes amorphous matrix structure containing starburst shaped
boride ((FeCrMoW)2B) and carbide (FeCrMoW)23C6) crystallites with sizes
ranging from 60 to 140 nm.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1- Schematically shows a prior art process of providing a protective plate
over the tube for erosion protection including wire arc coating.
Figure 2- shows an embodiment of the device for implementing an improved
method of thermal spraying of boilers according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Thermal spray coatings is an alternate approach to the wire arc coating that
offers advantages because they allow in situ recoating of the boiler tubes with
the additional ability to repair localized defects inside the boiler. The ability to
recoat boilers during a scheduled outage is especially attractive because
downtime can be minimized, which translates into significant operational cost
savings.
Wire arc spraying is a thermal spraying method which utilizes the heat from an
electric arc to melt various metallic materials. In this method, two consumable
wire electrodes (5) are fed automatically to the arc zone. The initiated arc
between the wire electrodes (5) melts the wire tips. The fused metal is atomized
by compressed air using an air nozzle (7), and thrown with high velocity onto a
substrate (6), forming the coating (Fig. 2). Normally, a dc constant potential
power source (9) is used to provide a voltage between 18 and 40V depending on
the spray material. The voltage should be kept at the lowest level with regard to
arc stability in order to produce smooth and dense coatings.
Referring now to the drawings, wherein like reference numerals designate
identical or corresponding parts throughout the several views, protective shields
over the tube. The erosion resistance coating on a boiler panel is shown in FIG. 2
is to be made with the least possible outlay by the process according to the
invention. As per figure 2, the coating is completely made by making use of only
twin wire arc thermal spray process using the nano grade wire of following
chemistry.

Wire Material Chemistry
Chromium Molybdenum The development of nanoscale coatings can be achieved via processing through
a solid/solid-state transformation, which can occur during wire arc spraying. This
route to nanoscale coatings is enabled by developing alloys that readily form
metallic glass structures at cooling rates in the range of thermal spray processes
(i.e., 104 to 105 K/s) (Ref 1,2). During the devitrification step, the glass recursor,
when heated to its crystallization temperature, readily transforms into a
nanoscale composite structure. This refinement is due to the uniform nucleation
and extremely high nucleation frequency during crystallization, resulting in little
time for grain growth before impingement between neighboring grains. By this
route, it is possible to develop very stable nanostructures that resist coarsening
at elevated temperatures. The erosion resistant coating composition includes
amorphous matrix structure containing starburst shaped boride ((FeCrMoW)2B)
and carbide (FeCrMoW)23C6) crystallites with sizes ranging from 60 to 140 nm.
The chief advantage of this approach, in contrast to other approaches outlined
previously, is that the feedstock material cored wire for wire-arc spraying is
physically identical to conventional feedstock, which eliminates the spraying
problems normally associated with nanosized particulate materials and also
bypasses their high cost. The ability to develop nanoscale microstructures with
enhanced properties using conventional technology is revolutionary.

Conventional power source for wire arc spraying can be used and flux core wire
of nano grade wire is used. All the coatings are developed and applied by wire
arc spraying. According to this embodiment, the wire arc spraying of nanograde
wire is carried out as per the enclosed parameters in Table 1. In the laboratory
test of the above wire arc sprayed tube exhibited the required hardness, reduced
porosity and oxide inclusions and finally thermal spraying is being performed in
the actual power plant site. The microstructure is composed of three phases,
consisting of Fe, a carbide phase, tentatively identified as the (Fe-Cr-W-Mo)23C6
type phase, and a boride phase, tentatively identified as the (Fe-Cr-Mo-W)3B
type phase. The microstructure is found to consist of ≈ 2/3 volume fraction of
nanoscale complex carbide and complex boride phases intermixed with the Fe
phase.
Table -1 Typical Wire arc Spraying parameters

During spraying, the visual examination reveals that there is no spalling, and
bend testing of the coating in a 1.25 mm carbon steel sheet reveals that there is
no cracking or spalling.

The compelling properties observed result from the ability to develop nanoscale
composite microstructures in air using conventional wire-arc spray guns. The
ability to spray the above wire-arch coatings in the field has been demonstrated,
because the coating is found to be easy to apply with a wide processing window
and very good spray forgiveness, and, once coated, is easily recoated and
repaired. While there are many different types of coating materials with different
combinations of properties, all coatings can only provide protection to a
component as long as they remain bonded to the component.
The advantages of the invention can be seen, inter alia, in the fact that the wire
arc spraying can be done by making use of simple wire arc spraying equipment
without costly consumables required for HVOF process. Further, the old method
of HVOF or protective shields on the erosion prone area requires lot of lead time
in providing the protection and also costly. Moreover, this method will not be
suitable for providing life of more than 2 years.

We Claim:
1. An improved method of thermal spraying of boilers, comprising the steps
of:
- feeding at least two consumable wire electrodes to the arc zone;
- initiating an arc between the at least two consumable wire
electrodes; and
- atomizing the fused material by using at least one air nozzle, and
directing the fused materials at a high velocity onto a substrate
forming a nanoscale coating.

2. The method as claimed in claim 1, wherein the nanoscale coating is
formed by developing alloys that readily form metallic glass structures at
cooling rates of 104 to 105 K/s.
3. The method as claimed in claim 1 or 2, wherein the glass recursor in a
devitrification step is heated to its crystallization temperature to transform
into a nanoscale composite structure.
4. The method as claimed in claim 1, wherein a voltage between 18 and 40V
is supplied via a dc constant potential power source.
5. The method as claimed in any of the preceding claims, wherein the
consumable wire electrodes arc selected with a wire material chemistry of
chromium Molybdenum
6. An erosion resistant coating composition for thermal spraying of boilers,
comprising amorphous matrix structure containing starburst shaped boride
((FecrMOW)2B), and Carbide ((FecrMOW)23C6) crystalites with sizes
ranging from 60 to 140nm.
7. A device for thermal spraying of boilers, comprising:

- a wire feeder (4) for automatic feeding of at least two consumable
wire electrodes (5) to the arc zone;
- means for (9) initiating an arc between the wire electrodes (5)
causing the wire tips to melt in a wire melting zone (8);
- an air nozzle (7) to direct the fused metal atomized by compressed
air onto a substrate (6) forming the coating.

8. The device as claimed in claim 7, wherein the wire diameter, console air
pressure, voltage, amperage, spray distance, and method selected
respectively at 1.6mm, 8 bar, 33-35V, 180-240 amp, 125 mm, and manual
9. An improved method of thermal spraying of boilers as substantially
described and illustrated herein with reference to the accompanying
drawings.
10. A device for thermal spraying of boiler as substantially described and
illustrated herein with reference to the accompanying drawings.

To overcome these difficulties of thermal spraying of boiler components such as
burner panel, steam cooled wall tubes, etc. and to improve the life of the boiler tube against erosion, it is proposed that the wire arc spraying by making use of nano grade flux core wires. By this method, the portions of the tubular boiler panels exposed to high temperature will be coated by high erosion resistant coatings. Accordingly, there is provided a process for twin wire arc spraying with the standard equipment available in the market. The erosion resistant coating
composition includes amorphous matrix structure containing starburst shaped boride ((FeCrMoW)2B) and carbide (FeCrMoW)23C6) crystallites with sizes ranging from 60 to 140 nm.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=km/6Lg0Oo7ust6DKyJmSiw==&loc=wDBSZCsAt7zoiVrqcFJsRw==

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Patent Number

269234

Indian Patent Application Number

731/KOL/2008

PG Journal Number

42/2015

Publication Date

16-Oct-2015

Grant Date

12-Oct-2015

Date of Filing

17-Apr-2008

Name of Patentee

BHARAT HEAVY ELECTRICALS LIMITED

Applicant Address

REGIONAL OPERATIONS DIVISION (ROD), PLOT NO : 9/1, DJBLOCK 3RD FLOOR, KARUNAMOYEE, SALT LAKE CITY, KOLKATA - 700091, HAVING ITS REGISTERED OFFICE AT BHEL HOUSE, SIRI FORT, NEW DELHI – 110049

Inventors:

#	Inventor's Name	Inventor's Address
1	DR. KALIDASS ASOKKUMAR	BHARAT HEAVY ELECTRICALS LIMITED, TRICHY-620014
2	DR. KANHAIYALAL ROHIRA	BHARAT HEAVY ELECTRICALS LIMITED, TRICHY-620014
3	MR. SEENI THIYAGARAJAN	BHARAT HEAVY ELECTRICALS LIMITED, TRICHY-620014
4	MR. KRISHNARAO VENUGOPAL	BHARAT HEAVY ELECTRICALS LIMITED, CORPORATE R&D
5	MR. NELARATAN NAYAK	BHARAT HEAVY ELECTRICALS LIMITED, CORPORATE R&D
6	DR. ARASAN RAJA	BHARAT HEAVY ELECTRICALS LIMITED, TRICHY-620014

PCT International Classification Number

G01K11/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA