TITLE:
A novel feed stock material composition and High Velocity Oxy Fuel (HVOF) process for
preventing high temperature erosion in T22 and T91 materials.
FIELD OF INVENTION
The present invention relates to a High Velocity Oxy Fuel (HVOF) thermal spray process, its
parameters and consumables for providing erosion resistant surface coating for components
working under high temperatures. More particularly this invention relates to the optimisation of
chemical composition for the mechanically blended feedstock powder to prevent high
temperature erosion of heat transfer surfaces of supercritical and Advanced Ultra-Supercritical
Steam Generators involving T22 and T91 tubes working at maximum surface temperatures up to
650 °C and environment temperature up to 1600 °C.
BACKGROUND OF THE INVENTION
Thermal power plants firing high ash containing coals face problems of ash deposits on heat
transfer surfaces, which reduce useful heat transfer to the working medium. Cleaning of these
deposits is carried out online using high pressure steam jet injected almost parallel to the
evaporator tube panels. Due lack of visibility / instrumentation, these cleaning systems are
operated ritually without understanding the need for cleaning. Hence, when the steam jet is
passed over a clean surface, it erodes the evaporator tube panels.

Similarly, in the tangential firing system, the primary air and pulverized coal mixer is pumped as
a jet through the burner nozzles at each corner at different elevations of the furnace such that the
streams of coal and air are projected along a line tangent to a small circle formed in a horizontal
plane at the center of the furnace. Due to operational deviations, misalignments and due to
damages in burner nozzles, the expanded or deflected coal air jets hit and erode the tubes in the
corners adjacent to the burner nozzles.
When the thickness of such eroded tube is less than the minimum required, the tube fails leading
to Steam Generator shut down and results in loss of power generation and revenue. These two
modes of erosion happen in high temperature furnace of the Steam Generators. The very high
steam parameters of supercritical, ultra and Advanced Ultra-Supercritical (AUSC) steam
generators require tubes of high grade materials like T22 and T91 for the high temperature
erosion prone furnace water walls. As such no reliable process and materials have been
developed for providing high temperature erosion resistant coatings for this applications. Hence,
there is a need to develop a thermal spray coating of erosion resistant materials suitable for tube
materials of the high temperature Steam Generator furnace zones.
Japanese patent JP2185961 describes the method to improve the corrosion and erosion
resistance of a steel tube for boiler tubes coated with self-fluxing alloy such as metal sensitive
fraction (MSF)Ni l-45orMSF Co 1 standardized by JIS by previously forming an Al coated

layer on the steel tube. The proposed self-alloying coating is implemented on a steel tube coated
with Al and a self-fluxing alloy containing 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 by flame spraymg method and fused
on resulting Al coated layer for boiler tubes of carbon steel.
Japanese parent JP10030897 discloses an overlaying welding process which overlays the Ni
alloy in longitudinal direction with specified thickness and bending process, which bends the
steel tubes in U shape. The described composition includes 15 to 20wt % of Cr, 9 to 15wt % of
Mo and the rest of Ni. The high temperature steel tube includes a serpentine-formed steel tube
having the coating layer in an area of contact with a high temperature gas out of straight running
parts and or curved parts of the high temperature steel tube.
Chinese paten, CM814852 relates to the protection of surfaces in power station Steam
Generator pipelines by welding a layer of harder alloy on a cleaned metal matrix with self-
protection metal core wire with weld current of 180-220A, voltage of 18-22V, the welded metal
matrix is sprayed with sand to eliminate the oxidized skins, rust and the old coating and expose
the metalic brightness then a compound iayer of anti-erosion and anti-abrasion is sprayed on the
matrix by an arc spraying method.

Canadian patent CA1067354 discuss a method for boiler tube coating in which the boiler tubes
and/or integrated panels of steel boiler tubes, ate provided with a fused overlay of a corrosion
and erosion resistant coating comprised of a refractory hard component.
US patent 7141110 discloses an improved method for developing an erosion resistance coating
for hydroelectric turbine. The erosion resistant coating composition includes restructured
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 process are suited for
hydroelectric turbine coatings.
US patent US7066242 discloses a method of prevention of erosion of Steam Generator tube,
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 overling 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 ho, combustion gas containing particulates
This is an external protection and is no, an Integral part of the boiler tube. Hence, there is a
possibility of reduction in heat transfer due to the shading effect provided by the shield.

Indian patent IN269234 discloses an improved method for thermal spraying of boiler
components to withstand for a long duration against erosion resistance. The tubular boiler panels
are exposed to a high temperature, which significantly needs to be coated with high erosion
resistant coatings. A wire arc spraying by utilising Nano grade flux core wires is adapted with a
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 prior art search indicates that no work has yet been done for protecting the T22 and T91
tubes working at high surface temperatures up to 650 °C in a dusty environment temperature of
up to 1600 °C against erosion.
OBJECTS OF THE INVENTION
An object of the present invention is to propose a feedstock powder composition.
Another object of the present invention is to propose a method for improving the erosion
resistance of T22 and T91 materials.
Still another object of the present invention is to propose suitable operating parameters for
HVOF surface coating process to provide erosion resistant coating of the proposed erosion
resistant materials over T22 and T91 tubes.

SUMMARY OF THE INVENTION
This invention relates to a feed stock composition comprising:
a blend of 20 % CrC-25NiCr (20% Ni, 10% C and the remaining being chromium) and the
remaining compositions (80 %) are NiCrFeSiB (73.9/14.5/4.3/4.3/3)
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 shows the process flow for the erosion resistance coating
Fig. 2 shows the microhardness of as coated HVOF coatings over T22 samples
Fig. 3 shows the microhardness of as coated HVOF coatings over T91 samples.
Fig. 4 shows the microhardness of HVOF coatings over T22 samples after 20 thermal cycling up
to 550 °C
Fig. 5 shows the microhardness of HVOF coatings over T91 samples after 20 thermal cycling up
to 650 °C
Fig 6 shows the microhardness of HVOF coatings over T22 samples steady state thermal
exposure at 550 °C for 125 hrs
Fig 7 shows the trend of microhardness of HVOF coatings over T91 samples steady state thermal
exposure at 650 °C for 125 hrs
Fig 8 shows the weight loss of coatings with Jet Velocity: 150 m/s, Erodent feed rate: 0.5 g/min
for T22 at 550°C

Fig 9 shows the weight loss of coatings with Jet Velocity: 150 m/s, Erodent feed rate: 0.5 g/min
for T91 at 650 °C
Fig 10 shows the weight loss of coatings with Jet Velocity: 25 m/s, Erodent feed rate: 2 g/min for
T22 at 550 °C
Fig 11 shows the weight loss of coatings with Jet Velocity: 25 m/s, Erodent feed rate: 2 g/min for
T91 at 650 °C.
DETAILED DESCRTPTION OF A PREFFERED EMBODIMENT OF THE
PRESENT INVENTION
The present invention proposes a method for improving the erosion resistance of T22 and T91
materials working with surface temperatures up to 650 °C in a dusty environment temperature of
up to 1600 °C.
HVOF thermal spray coating with varying parameters and metal powder blends were carried out
over test specimens of SA213 T22 and SA213 T91 and evaluated for their high temperature
erosion behavior. The primary characteristics of porosity, hardness and bond strength of the
coated surface were evaluated for all the coated samples. The erosion resistance of HVOF
sprayed coatings was evaluated in hot erosion tester. Considering lower porosity, almost equal

surface finish and bond strength, higher hardness and lower weight loss in high temperature
erosion testing, HVOF coating of NiCrFeSiB-20% (CrC-25%NiCr) with optimised parameters
are proposed for both T22 and T91 tubes and materials to withstand high temperature erosion.
The process flow for the high temperature erosion resistance High Velocity Oxy-Fuel (HVOF)
thermal spray coating is shown in Fig. 1. X% of NiCrFeSiB (73.9/14.5/4.3/4.3/3) powder is
mechanically blended with (100-X)% of CrC-25%NiCr to form new composition of NiCrFeSiB-
X% (CrC-25%NiCr). The surface of the T22 and T91 samples are 2 µ blasted to achieve a
surface finish of 8-10 µ HVOF thermal spray coating of NiCrFeSiB-X% (CrC-25%NiCr)-(100-
X)% on the tube surface with the optimized process parameters shown below is carried out:

The coated surfaces are tested as detailed below:
• Measuring the Vickers microhardness using 300 g load applied to the indenter with a dwell
time of 10S under the following conditions:
a. As coated
b. After thermal cycling up to the rated service temperature for about 20 cycles.
c. After steady state exposure at the service temperature for about 125 hrs

Considering functional requirement, the hardness has to be more than 750 HV0.3,
• Carrying out adhesion Test
• Carrying out erosion test at the required service temperature with A12O3 as erodent
material with jet velocity of 150 m/s and erodent feed rate of 0.5 g/min for simulating
steam wall blowing.
• Carrying out erosion test at the required service temperature was carried out with A12O3
as erodent material with jet velocity of 25 m/s and erodent feed rate of 2 g/min for
simulating coal airflow near burners.
If the above tests indicate any deviation in erosion resistance, rectifications of the coatings shall
be carried out.
Fig. 2 to Fig. 11 show comparative results of various tests carried out on coatings.

WE CLAIM
1. A feed stock composition comprising:
a blend of 20 % CrC-25NiCr (20% Ni, 10% C and the remaining being chromium) and the
remaining compositions (80 %) are NiCrFeSiB (73.9/14.5/4.3/4.3/3)
2. A method for improving erosion resistance of steam generates comprising:
preparing a blend of NiCrFeSiB, CrC and NiCr,
subjecting the surface to be coated to grit blasting
coating the surface by HVOF thermal spray using the blend mixture,
testing the anti-erosion coating.
3. The method as claimed in claim 1, wherein the surface to be coated is SA213 T22 & SA213
T91 tube materials.