FIELD OF THE INVENTION
This invention relates to analytical techniques for the high-temperature erosion
behaviour of coatings for finding out the deformation mechanics for new boiler
coating materials subjected to simulated boiler condition.
BACKGROUND OF THE INVENTION
Burning of coal in fossil fuel power plants generates a huge amount of fly ash,
which causes intense and localized erosive wear of power plants equipments. For
example, the erosive wear is the main cause of failures in boiler economizer
tubes. The erosion wear mechanisms involve the removal of material from a
given surface due to the impact of solid particles. This invention reveals the
erosion properties of coatings and identify possible applications in the boilers and
advanced wear protection on power plant components with a view to enhance
their life and reduce tube failures in such power plants.
Major premature tube failures are due to excessive fireside boiler tube erosion,
which is caused by the impact of flue gases including the cutting action and
abrasive wear by fly ash entrained in the flue gases. The most problematic,
hardest to predict and seemingly increasing is the erosion-caused failures. The
usable life of critical system components such as fans, burners, pulverizers, and
boiler tubes can be extended if the erosion-caused failures could be minimized.
Conventional coatings such as wire arc spray, HVOF and Chrome carbide are
known to be applied to effectively protect equipment against many of the
corrosion, erosion and wear problems faced by power equipment maintenance
teams.
It is known that erosive high temperature wear in boilers is one of the main
causes of downtime of the power plant. As a result of the combustion process,
the oxide particles are produced which have a much higher melting point than
the boiler operating temperature. Thus these particles remain intact and then
entrained in to the exhaust gas, resulting in a constant impact of the particulate
matter.
It is recognized in the art that fireside boiler tube erosion is caused by the
cutting action and abrasive wear of fly ash. In case of erosion, the mechanism
that involves are cutting wear and plastic deformation. The most problematic,
hardest to predict and seemingly increasing are erosion-caused failures. In
addition to the ploughing on coating surfaces, the cracking along splat interfaces
in the coating is clearly observed on the cross sectional microstructure.
In the recent years, the subject of remaining life prediction of boiler components
has drawn considerable attention in the power generation industry. The interest
in the issue of remaining life prediction originates from the necessity to avoid
costly forced outages, to extend the component life beyond the original design
life for economic reasons, and also from safety considerations. In power
Generation systems, a variety of structural components, e.g. Steam pipes,
turbine rotors and casings and super heater headers, typically operate at high
temperatures, where erosion deformation and rupture are the important
considerations. Thus, a life prediction methodology, which accounts for erosion,
is needed for these components.
Because of the long-term, high-temperature operation, material properties of an
in-service (or ex-service) steel are significantly different from those of
corresponding virgin steel, it is extremely important to develop material
properties of in-service and/or ex-service steels for life assessment of the steam
pipes of boilers.
US Patent no.: US1476775 teaches test apparatus for testing the erosion
resistance of a specimen. The apparatus has an enclosure containing in its test
chamber an abrasive medium. Also included is a spindle attached to the
enclosure. The spindle has an inside end adapted to hold the specimen. A motor
coupled to the spindle can spin the inside end at an adjustable angular speed
within the medium. Preferably, a thermal jacket can moderate temperature
within the spindle apparatus. The apparatus is used to spin the specimen
through the abrasive medium at a chosen speed and then measure the extent of
erosion.
US Patent Number 4,780,319 teaches the use of carboxylic acids incorporated as
a catalyst in poly(orthoester)s and other acid labile polymers such that upon
exposure to aqueous environments the acid catalyzes the erosion of the polymer
matrix. The rate of release of a drug substance incorporated into or surrounded
by the poly(orthoester) or other acid labile polymer can be controlled in that the
drug is released as the polymer is eroded in response to the catalytic action of
the acid incorporated therein.
US Patent No.3944826 A describes a method of producing ionized material for
mass analysis, in which ions are produced in a gaseous medium at high
temperature in a region at substantially atmospheric pressure; the material is
introduced into the medium, to be ionized, and the medium containing ions of
the material is fed at low pressure to a mass analyzer.
US patents 5837552 discloses surfaced-enhanced, analytical procedures wherein
a surfaced article includes a substrate surface, metal islands, a spacing/coupling
agent layer, and binding partner molecules which bond with work piece
molecules to be detected. A population of spaced apart metal islands are formed
on the substrate and have at least some interconnections formed between them.
A continuous layer coats the islands and all surfaces between the islands. The
continuous layer includes a coupling agent which immobilizes first binding
partner molecules. The first partner molecules bond to the coupling agent and
the second binding partner molecules bind to the first binding partner molecules
to allow detection of presence or concentration of the work piece binding partner
molecules.
US Patent Number: 4,860,224 discloses an apparatus for surface analysis
microscopy, having a number of analysis device are mounted on an ultra-high
vacuum chamber. The devices include a beam source of locally heating a
selected region of a specimen and a temperature-detector for monitoring the
heating of the selected region, as well as an electron gun and an analyzer for
detecting emission from a specimen region subjected to electron bombardment.
An ion gun may also be provided. The apparatus enables thermal microscopy of
a specimen to be carried out in conjunction with other surface analysis
techniques including, inter alia, scanning electron microscopy and Auger electron
microscopy, within a single apparatus and during a single experimental
operation. A novel configuration of cylindrical mirror analyser facilitates mounting
a multiplicity of analysis devices on the chamber in a compact manner for
studying a specimen at a single position.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention is to propose a method of
estimating the erosion deformation mechanism of coated boiler components
including life expectancy of the component.
Another object of the present invention is to propose a method of estimating the
erosion deformation mechanics of boiler materials including life expectancy of
the coated boiler component, in which eroded specimen of the coated boiler
components are prepared by NDT method.
A still another object of the present invention is to propose a method of
estimating the erosion deformation mechanics of coated boiler components
including life expectancy of the component, in which eroded specimen is
prepared from in- service component.
A further object of the present invention is to propose a method of estimating
the erosion deformation mechanics of coated boiler components including life
expectancy of the component, in which erosion testing is completed within a
substantially shorter time.
A still further object of the present invention is to propose a method of
estimating the erosion deformation mechanics of coated boiler components
including life expectancy of the component, which allows erosion test in different
coated specimens.
SUMMARY OF THE INVENTION
Accordingly, there is provided A method for metallurgical evaluation of eroded
coating samples comprising the steps of preparing a plurality sized specimen to
be subjected to erosion testing after cleaning in acetone; loading the preweighed
coated specimen in an angle sample holder of a High temperature air jet erosion
testing machine; Setting the testing temperature of the sample including erodent
and loading the selected preheated erodent; allowing a constant flow of the
erodent, the time of loading the preheated erodent (fly ash) being set; providing
a data acquisition system to record the reading for particular time, flow and
temperature of the erodent including the coated sample was connected;
characterising the eroded coated specimen, after being allowed to reach room
temperature, wherein the eroded coated tested sample is immersed in acetone
for ultrasonic cleaning and macro examination under a stereo microscope,
wherein the samples are mounted on a araldite base in its cross section, grinded
in a belt grinder, polished with emery sheets without damaging the coated layer;
wherein the mounted specimen is etched using etchant, and wherein said
specimen after micro preparation is examined under Scanning Electron
Microscope to reveal the eroded regions and to assess the level of damage.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a flow-diagram showing the analytical method of the
invention
Figure 2(a) - a specimen sample holder with 30°C angle;
Figure 2(b) - a specimen sample holder with 90°C angle;
Figure 3 - Shows the coated sample with the sample holder.
Figure 4 - Shows the diagram of the coated specimen
Figure 5 - Shows the diagram of the coated specimen holder for erosion test
Figure 6 - Shows the diagram of erosion test
Figure 7 - Shows the diagram of the coated specimen with erosion area
Figure 8 - Shows the diagram of the coated specimen with erosion spot
Figure 9 - Shows the specimen mounted on a araldite base the position of the
sample in the mounting - vertical
Figure 10 - Shows the eroded specimen after grinding
Figure 11 - Shows the area of eroded region, metallurgical^ evaluated
Figure 12 - Shows the eroded surface of the coated specimen with total
coating washed away.
DETAIL DESCRIPTION OF THE INVENTION
Testing process
Specimen
Base metal specimen with coating was prepared with uniform coating sample of
size 10mm x 10mm x 4mm with out damaging the coated layer and cleaned in
acetone and initial weight was taken.
Testing details
Specimen was kept in different sample holder (angle of impingement 10°, 20°
30°,60°. 90°) and kept in a high temperature air jet erosion testing machine.
Testing temperature, air temperature and sample temperature were set in the
range of 400°c- 800°c and maintained constant. Erodent, fly ash collected from
the power plant was sieved and used. Erodent flow or velocity was fixed with a
particular erodent selected for the study and impingement time was fixed. Once
erosion testing was over, the specimen was cooled to room temperature and
removed from the equipment and weight after erosion was taken Specimen
was prepared for macro examination Erosion tested coated sample was
immersed in acetone and ultrasonic cleaned.
Analysing technique
Macro examination of the eroded sample is examined under stereo microscope.
The specimen was examined under a scanning electron microscope on its eroded
surface. The specimen was mounted on a araldite base on its cross section. For
micro examination, the specimen was grinded upto the eroded region in a belt
grinder. Then the specimen was polished with emery sheets. Then it was etched
with the suitable etchant. SEM of the specimen after micro preparation was
examined. Under SEM to revealed the eroded regions-microstructure just below
the eroded area. The level of damage is assessed.
Metallurgical evaluation of the specimen
The coated samples taken after the erosion test were cleaned in acetone by
Ultrasonic cleaning process. Then the macro images of the eroded coated
samples were taken using Stereo Microscope. The width of the erosion in coated
samples was measured and compared with the different angle of impingement
and time of impingement. The macrostructures of the samples revealed the
eroded surface of the coated specimen. Then the samples were mounted on the
araldite base in its cross section to view the eroded spot in the coated specimen.
Then the samples were grinded with the emery sheets of 80 X very carefully, not
to remove coated material to just reach the erosion spot using the grinding
machine. Then the samples were polished using the emery sheets from 120X,
200X, 400X and 800X till polished surface was obtained. The polished eroded
coated specimen was etched with suitable etchant to get microstructure with
clear grain boundaries. Then the etched specimens were examined under the
Scanning Electron Microscope. Eroded regions in the coated specimens were
revealed from the SEM examination. Microscopy of eroded specimen (coated)
showed high density of cavities mostly of carbides. Micro hardness of the
specimen also showed the similar hardness of the base metal spot.
When coated samples with NiCrC subjected for erosion test at different angles,
30°, 60° and 90° and at different temperatures, 35°C, 400°C, 600 °C and at
800°C. It was observed that in all temperatures studied, at 30 ° shows high
erosion loss and with 90° shows less erosion loss. At room temperature 35°C
shows the almost no erosion loss. Similar trend is seen with other Tungsten
coated samples. But erosion loss is less with Tungsten carbide coated samples at
all temperatures and at all impingement angles studied.
Effect of impact angle was also studied at different temperature, Lesser the
impact angle, more the erosion weight loss at all temperature studied. As the
test temperature increases weight loss also increases. Similar trend is seen in
both coatings. But with Tungsten Carbide coating the erosion loss is very much
reduced. Experimental details and results with different coatings and
experimental details and results at different temperatures, experimental details
and results with different time and the effect of different time of discharge on
erosion rate were noted.
Erosion tests conducted at 400 °C with NiCrC coating as well as with Tungsten
carbide at different exposure time (min). It was observed that with 10 minutes
time duration shows high erosion rate, in both the coatings.
WE CLAIM
1. A method for metallurgical evaluation of eroded coating samples
comprising the steps of preparing a plurality sized specimen to be
subjected to erosion testing after cleaning in acetone; loading the
preweighed coated specimen in an angle sample holder of a High
temperature air jet erosion testing machine; Setting the testing
temperature of the sample including erodent and loading the selected
preheated erodent; allowing a constant flow of the erodent, the time of
loading the preheated erodent (fly ash) being set; providing a data
acquisition system to record the reading for particular time, flow and
temperature of the erodent including the coated sample was connected;
characterising the eroded coated specimen, after being allowed to reach
room temperature, wherein the eroded coated tested sample is immersed
in acetone for ultrasonic cleaning and macro examination under a stereo
microscope, wherein the samples are mounted on a araldite base in its
cross section, grinded in a belt grinder, polished with emery sheets
without damaging the coated layer; wherein the mounted specimen is
etched using etchant, and wherein said specimen after micro preparation
is examined under Scanning Electron Microscope to reveal the eroded
regions and to assess the level of damage.
2. A method as claimed in claim 1, wherein the sample tested is of size
25mm X 25mm X 5mm and thickness of coating 300 microns.
3. A method as claimed in claim 1, wherein the range of testing temperature
is 400°C to 800 °C.
4. A method as claimed in claim 1, wherein the range of testing temperature
of the pre weighed coated specimen is 400°C - 800°C.
5. A method as claimed in claim 1, wherein the range of erodent
impingement angle is 10°, 20°, 30°, 60° and 90° sample holder are used.
6. A method as claimed in claim 1, wherein the discharge rate the erodent,
fly ash is 30-50 m/s.
7. A method as claimed in claim 1, wherein the size of the erodent, fly ash is
5-50 microns.
8. A method as claimed in claim 1, wherein the time of testing temperature
is 5min, 10 min, 15 min, 20 min, 25 min and 30 min.
9. A method as claimed in claim 1, wherein the samples are polished with
emery sheets of 120, 200,400 and 800 grits.
10. A method as claimed in claim 1, wherein the eroded specimen is etched
with suitable enchant to get microstructure with clear grain boundaries.

ABSTRACT

The invention relates to a method for metallurgical evaluation of eroded coating
samples comprising the steps of preparing a plurality sized specimen to be
subjected to erosion testing after cleaning in acetone; loading the preweighed
coated specimen in an angle sample holder of a High temperature air jet erosion
testing machine; Setting the testing temperature of the sample including erodent
and loading the selected preheated erodent; allowing a constant flow of the
erodent, the time of loading the preheated erodent (fly ash) being set; providing
a data acquisition system to record the reading for particular time, flow and
temperature of the erodent including the coated sample was connected;
characterising the eroded coated specimen, after being allowed to reach room
temperature, wherein the eroded coated tested sample is immersed in acetone
for ultrasonic cleaning and macro examination under a stereo microscope,
wherein the samples are mounted on a araldite base in its cross section, grinded
in a belt grinder, polished with emery sheets without damaging the coated layer;
wherein the mounted specimen is etched using etchant, and wherein said
specimen after micro preparation is examined under Scanning Electron
Microscope to reveal the eroded regions and to assess the level of damage.