|Title of Invention||
A PROCESS OF IMPROVING ABRASIVE AND EROSIVE RESISTANCE OF HIGH VELOCITY OXY FUEL (HVOF) SPRAY COATING AND COATED PARTS THEREOF
|Abstract||ABSTRACT A Process of Improving Abrasive and Erosive Resistance of High Velocity Oxy Fuel (HVOF) Spray Coating And Coated Parts Thereof. This invention relates to a process of improving abrasive and erosion resistance of high velocity oxy fuel (HVOF) spray coating and coated parts thereof subjected to high abrasive and erosive conditions such as slit erosion comprising the steps of surface preparation of machine parts on grit blasting using 12-16 mesh alumina grit powders coating upto 500 microns thickness on the machine part inside an acoustic chamber on employing liquid fuel and oxygen gas supplied HVOF spray system carried out by HVOF sparay gun mounted on 6 axis ABB robot, the coated parts then were surface grinded upto 100 micron to 300 micron; followed by characterize test evaluation of substantial increase in hardness and decrease in wear rate of the grinded coating upto 200 micron surface grinding. The invention relates also to the improve erosive resistant coated products resulted from the process steps. Fig.-1|
FIELD OF THE INVENTION
This invention relates to a novel method of High Velocity Oxy Fuel (HVOF) spray
coating followed by surface grindng. More specifically the present invention
relates to combating silt and cavitation erosion of the hydro turbine components,
which are prone to erosion.
BACKGROUND OF THE INVENTION
Degradation of Indian hydro turbine components such as spears / needles, guide
vanes, lower ring, top cover, labyrinth seals, etc. is a serious problem. This is
mainly due to presence of excessive silt in water during monsoon. In hydro
turbines there are basically two type of erosion. They are silt and cavitation
erosion. The silt comprising hard quartz (hardness on Moh scale is 7) particles of
size ranging from 10 to 200 microns striking on the exposed surfaces of the
hydro components. This problem is particularly acute during rainy season when
excessive land slides on to the riverbed cause the number of silt particles to
increase in excess of 5000 ppm. This kind of erosion on the components is called
silt erosion. The second type of erosion is cavitation. If the pressure of a fluid
drops below its vapour pressure due to an increase in its velocity or otherwise,
vapour bubbles tend to form. These bubbles grow their size at low pressure and
when they come in contact with regions of high pressure, i.e. on the runner
surface or draft tube, etc. they collapse and implode. Over a period of time,
small pits are developed on the surface of the component exposed to repetitive
collapse of the bubbles thereby causing erosion of the base material. This
phenomenon is called cavitation.
Attempts are being made to reduce the above damage caused by silt erosion
either by reducing the particle velocity, controlling their size and concentration,
or by using HVOF cermet coatings. HVOF cermet of hard carbide phase (WC)
embedded in ductile matrix; typically coating system consisting of WC-Co, WC-
Co-Cr, WC-Ni-Cr and FeCrAIY-Cr3C2 are being used in different industries.
Tungsten carbide (WC) based powders are widely used in the HVOF spraying
system because of their high wear and corrosion resistance. These are used to
produce dense, high hardness and excellent wear resistance coatings generally
to combat the erosion and corrosion occurring in hydro power plants and pumps.
The present invention in the above retrospective has proposed to develop a
process of increasing the wear resistance of the components subjected to high
abrasive conditions during their use in adverse conditions on increasing the wear
resistance of carbide coatings on the components put up in high abrasive
Power generation by hydro turbine is one of the major sources in Nothern India.
These Hydro turbines operate under silted water condition. Due to high silt
content, the hydro turbine components are eroded severely. This erosion leads
to break down of the hydro power house and that lead to huge loss of electrical
In the existing art the hydro turbine components which are worn out due to silt
and cavitation erosion are coated by HVOF (high velocity oxy fuel) spray process
with WC based powder. The hardness of the coating is 1150 to 1200 Hv. The
coating is highly dense compare to other thermal spray techniques like plasma
spray. In applications where abrasive or erosive wear resistance is of primary
importance, Wc-Co with and without chromium is used. Wc-Co-Cr powders are
preferred when high corrosion resistance is needed. The abrasive and erosive
wear resistance also depends upon oxides, pores, and the phase transformation
occurring during spraying. High velocity oxy fuel sprayed coatings are commonly
applied by HP/HVOF JP-5000, DS-100, Met jet II, Diamond jet and
Praxair 2000 HVAF (High velocity air fuel) systems. These systems are based on
liquid as well as gaseous fuel and oxygen/air.
DESCRIPTION OF THE INVENTION
The present invention has been developed on finding effect of surface grinding
of WC-Co based HVOF coating on considering the extent of damage specifically
caused in the hydroturbine components employed in the Himalayan region.
According to the invention main objective is to improve the erosion resistance of
carbide based HVOF coating on machine components subjected to adverse
erosive conditions by surface grinding of coating.
Another objective of the invention is to improve the erosion resistance of WC-Co
based HVOF coating by surface grinding of hydroturbine components coated with
A still another objective of the invention is to increase the wear resistance of any
alloy coating and coated parts thereof on subjecting the coating to abrasion.
Yet another objective of the invention is to decrease the wear rate of the coating
and thus enhancing life of the coating and coated parts thereof by abrasive
wheel grinding of the coating.
In order to improve the performance of the hydro turbine components, high
velocity oxy fuel (HVOF) coating has been applied and it has given good result.
For further improvement, surface grinding was carried out with a diamond
abrasive wheel. Different thicknesses of surface grinding were carried out, to see
the behaviour in erosion resistance. It was found that when a WC-CoCr based
HVOF coated surface is ground 200 microns thick it gives better erosion
resistance than as-coated surface. Approximately 30% increase in erosion
resistance was observed compare to as-coated sample. High erosion
resistance was due to increase in coating hardness from 1106 to 1472 Hv
(Vickers Hardness). This hardness increase was due to increase in
residual stress of the coating after surface grinding.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The present invention will be better understood from the following
illustrative embodiment with reference to drawing in which:
Figure 1 represents effect of surface grinding on hardness of HVOF
coated samples, and
Figure 2 represents effects of surface grinding on erosive wear resistance
of WC-Co-Cr based coating.
DETAILED DESCRIPTION OF INVENTION
Metjet-II system from Metallization, UK was used for HVOF. This is
based on liquid fuel (Kerosesne of aviation grade) and oxygen gas. The
HVOF spray gun was mounted on a 6-axis ABB robot for carrying out the
HVOF coating. This coating was carried out inside an acoustic chamber
because the HVOF coating process produces very heavy noise. The
coating thickness was maintained around 500 microns for all the
samples. To see the effect of grinding, four sets of samples were made.
The details of the samples are given in Table -1. After the coated
samples were ground using diamond abrasive wheel and the grinding
parameters are maintained as given in Table-2 on the samples tested
according to table 1. The grinding was carried out with water soluble
coolant and the final surface roughness for all the samples were
maintained around 0.2 µm.
Microhardness (At 300 grams load) result on testing the samples on above
parameter maintenance is shown in Table-3. For comparison, the hardness was
also measured on an AISi 410 stainless steel plate. The hardness values show
that there is a substantial increase in hardness of HVOF coating after grinding.
/The increase in hardness was highest when the grinding thickness was 200
microns (Hv 1472) If it is compared with the as-coated sample then three is
almost 33% increase in, hardness which is observed over the as-coated sample
(Hv 1106). The trend of hardness after different level of grinding is shown in Fig.
1. The hardness after 300 microns grinding decreased to 1348 Hv. Such increase
in hardness of coating is due to increase in residual stress imparted on the
coating after surface grinding.
The erosive wear rate of the HVOF coated samples with different grinding thicknesses
decreases upto 200 micron, and thereafter increases as given in Fig. 2. From the said
figure, it is observed that the wear rate of as coated sample is higher than the ground
sample. Volume loss (wear rate) of as coated sample is 1.74 mm3 and that of ground
samples are around 0.62 mm3, from which it is concluded that by surface grinding, the
coating gets more wear resistant and enhancing life of the coating and thus the coated
parts than as-coated samples. The increase in wear resistance is attributed to the increase
in microhardness of the coating because the wear rate is directly proportional with the
hardness of the material being subjected to wear. The volume loss of the coating was
calculated by taking the coating density of 13.8 g/cm3.
Though the present invention has been narrated with an exemplary embodiment the said
invention should not be read and construed in a restrictive manner as some alterations,
modifications and adaptations are possible in respect of various coatings of alloy or
carbides on various machine parts subjected to adverse abrasive and erosive conditions
within the scope and limit of the invention as defined in the appended claims.
1. A process for surface grinding of high velocity oxy fuel spray coating
-preparing a coating substrate by grit blasting using hard alumina grit,
-depositing tungsten carbide coating by high velocity oxy fuel spray to a
thickness of 500 microns, and
-subjecting the coated surface to the step of surface grinding using a
2. The process as claimed in claim 1 wherein the HVOF spray coating
was carried through a conventional Metjet-II system for metallization.
3. The process as claimed in claim 1 wherein the surface grinding of the
coating was carried out through diamond abrasive wheel.
4. The process as claimed in the preceding claims wherein grinding was
carried out with water soluble coolant and the final surface roughness
for all the samples were maintained around 0.2µm.
5. The process as claimed in claim 1 wherein the coating of carbides or
an alloy is provided through HVOF spray coating.
6. The process as claimed in claim 1 wherein the coating is of WC-Co-Cr
7. The process as claimed in the preceding claims wherein the hardness
of coating after and before surface grinding of hydro turbine components
was found to be of highest hardness of grinded coating as Hv 1472 to the
grinded thickness of 200 microns, which is 33% increase in hardness
over the as coated sample of Hv 1106.
8. The process as claimed in the preceding claims wherein erosive wear
rate of HVOF coated samples with different grinding thickness decreases,
upto 200 micron, and thereafter increases.
9. An improved abrasive and erosive resistant coated products produced
by the process as claimed in the preceding claims wherein increase of
hardness and decrease of wear rate upto 200 micron surface grinding is
due to increase in residual stress of the coating of the coated
hydroturbine components, after surface grinding.
A Process of Improving Abrasive and Erosive Resistance of High Velocity Oxy
Fuel (HVOF) Spray Coating And Coated Parts Thereof.
This invention relates to a process of improving abrasive and erosion resistance
of high velocity oxy fuel (HVOF) spray coating and coated parts thereof
subjected to high abrasive and erosive conditions such as slit erosion comprising
the steps of surface preparation of machine parts on grit blasting using 12-16
mesh alumina grit powders coating upto 500 microns thickness on the machine
part inside an acoustic chamber on employing liquid fuel and oxygen gas
supplied HVOF spray system carried out by HVOF sparay gun mounted on 6 axis
ABB robot, the coated parts then were surface grinded upto 100 micron to 300
micron; followed by characterize test evaluation of substantial increase in
hardness and decrease in wear rate of the grinded coating upto 200 micron
surface grinding. The invention relates also to the improve erosive resistant
coated products resulted from the process steps.
|Indian Patent Application Number||1732/KOL/2007|
|PG Journal Number||20/2012|
|Date of Filing||26-Dec-2007|
|Name of Patentee||BHARAT HEAVY ELECTRICALS LIMITED|
|Applicant Address||REGIONAL OPERATIONS DIVISION (ROD), PLOT NO: 9/1, DJ BLOCK 3RD FLOOR, KARUNAMOYEE, SALT LAKE CITY, KOLKATA-700091, HAVING ITS REGISTERED OFFICE AT BHEL HOUSE, SIRI FORT, NEW DELHI- 110049|
|PCT International Classification Number||C21D9/00|
|PCT International Application Number||N/A|
|PCT International Filing date|