FIELD OF INVENTION:
This invention relates to a method for coating hydro turbine to protect it from
degradation due to silt erosion under silted water condition.
BACKGROUND OF THE INVENTION:
The hydro turbine components which are worn out due to silt and cavatation
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 nickel or chromium is used. WC-Co-Cr powders are
preferred when high corrosion and erosion 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. This has a further
reference to the earlier patent applications 040014RD and 080060RD on HVOF
coating related process.
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 major cause of erosion is the silt which comes along with the water. The
silt comprising hard quartz (hardness on Moh scale is 7) particles of size ranging
from 10 to 250 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.

Attempts are being made to reduce the damage caused by silt erosion either by
reducing the particle velocity, controlling their size and concentration, or by using
HVOF cermet coatings. HVOF cermet coatings of hard carbide phase (WC)
embedded in ductile matrix; typically coating system consisting of WC-Co, WC-
Co-Cr, WC-Ni-Cr and FeCrA1Y-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 gate
valves.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a method for coating hydro turbine with
HVOF spray system;
Another object of this invention is to propose a method for coating hydro turbine
with HVOF to improve the erosion resistance;
Another object of this invention is to propose a method of improving the erosion
resistance of HVOF spray coating for hydro turbine and thereby enhancing the
hydro-turbine component life.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a method for coating hydro turbine to
protect it from degradation comprising:
preparing the surface for grit blasting coating the surface with HVOF coating
followed by laser surface treatment and
subjecting the surface to the step of grinding of the coating.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig.1: Effect of laser treatment and surface grinding on surface micro-hardness of
HVOF coated samples.
Fig. 2: Effect of laser treatment and surface grinding on erosive wear resistance
of WC-CoCr based coating.
DETAILED DESCRIPTION OF THE INVENTION:
Power generation by hydro turbine is one of the major sources in Northern 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
power. 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, subsequent diode laser treatment and surface
grinding was carried out with a diamond abrasive wheel. Surface grinding were
carried out to a depth of 100 microns. These samples were subjected to slurry
erosion test. It was found that when a WC-CoCr based HVOF coated surface is
laser treated at 1 .OkW power in argon atmosphere and surface ground to a depth
of 50 to 100 microns thickness it gives better erosion resistance than as-sprayed
coating. Approximately 70% increase in erosion resistance was observed
compared to as-sprayed sample. High erosion resistance was due to increase in
coating hardness from 1106 to 2047 Hv (Vickers Hardness).
In our study, Metjet-ll system from Metallization, UK was used for HVOF coating.
This is based on liquid fuel (Kerosene of aviation grade) and oxygen gas. The

HVOF spray gun was mounted on a 6 axis Kuka 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 400 microns for all the samples. To see the effect of grinding,
three sets of samples were made. The details of the samples are given in Table-
1. After the coating, the coated samples were subjected to laser treatment with
argon shielding. These samples were further ground after laser treatment using
diamond abrasive wheel upto a depth of 100 microns. The grinding was carried
out with water soluble coolant and the final surface finish for all the samples
were maintained around 0.2 urn.
Table-1: The details of the sample made for the study

After grinding the surface Microhardness was measured using Vickers hardness
at a load 300 gms and the hardness result is shown in Table -2. 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 laser treatment followed by grinding. If it is compared with the as-
coated sample then there is almost 82% increase in hardness can be observed
over the as-coated sample (Hv 1106). The hardness after different level of
grinding can be seen from the Fig. 1.

Table-2: Hardness of coatings after and before surface grinding

Slurry erosion was carried out in a slurry erosion test rig. In this test 200 gms of
silica sand is mixed in 10 litres of water. The slurry is pumped and pass through
a 6 mm nozzles and hit the samples at 90 degree angle. This test is carried out
for one hour duration (test parameters given in Table-3). The weight of the
sample before and after the test is taken to find out the volume loss of the
sample. The volume loss of the coating was calculated by taking the coating
density of 13.8 g/cm3.
The erosive wear rate of the HVOF coated samples with different treatment
(laser and grinding) is given in Fig 2. From the figure (Fig. 2), it can be
understood that the wear rate of as-coated sample is higher than the ground
sample. Volume loss (wear rate) of as coated sample is 3.7 mm3, laser treated
sample is 3.15 mm3 and that of ground samples is around 1.1 mm3, i.e. by
surface grinding, the coating gets more wear resistant than as-coated samples.
The increase in wear resistance can be attributed to the increase in micro-
hardness of the coating because the wear rate is directly proportional with the
hardness of the material being subjected to wear.

Table-3: Slurry erosion test condition

The silt erosion resistance is improved by using combined process of HVOF
coating, laser treatment and surface grinding. Combined process of HVOF
coating, laser treatment and surface grinding increase the micro-hardness of the
coating by 82%. Combined process of HVOF coating, laser treatment and
surface grinding increase the silt erosion resistance of the coating by 70%.

WE CLAIM:
1. A method for coating hydro turbine components to protect it from degradation
comprising:
preparing the surface by grit blasting for HVOF coating followed by laser surface
treatment and subjecting the surface to grinding of the coating.
2. The method as claimed in claim 1, wherein the grit blasting is performed using
12-16 mesh alumina grit powders.
3. The method as claimed in claim 1, wherein HVOF spray coating is deposited
using the erosion resistant coating powder up to a thickness of 400 urn.
4. The method as claimed in claim 1, wherein the laser surface treatment is done
in controlled atmosphere.
5. The method as claimed in claim 1, wherein the surface grinding of coating is
done to a depth of 50 to 100um across the surface.
6. The method as claimed in claim 1, wherein the surface grinding of HVOF
coating is done using abrasive wheel.

A method for coating hydro turbine to protect it from degradation comprising:
preparing the surface for grit blasting for HVOF coating followed by laser surface
treatment in controlled atmosphere and subjecting the surface for surface
grinding of the coating, the combinations of these processes improve significantly
surface micro-hardness of the coating as well as slurry erosion resistance of the
HVOF coating.