Slurry erosion comparison of d gun sprayed stellite-6, cr3 c2-25nicr coatings-2

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME
210
SLURRY EROSION COMPARISON OF D-GUN SPRAYED STELLITE-
6, Cr3C2-25NiCr COATINGS AND SUBSTRATE 13Cr4Ni UNDER
HYDRO ACCELERATED CONDITION
Gurpreet Singh1
and Sanjeev Bhandari2
1
M.Tech, Department of Mechanical Engineering,
Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Punjab-140407, India
2
Assistant Professor, Department of Mechanical Engineering,
Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Punjab-140407, India
ABSTRACT
Degradation of under water parts in hydro turbines is serious issue mainly in the
North India. So it’s a challenge to develop new more erosion resistant materials. In the
present study, slurry erosion performance of detonation gun (D-gun) spray ceramic coatings
(Stellite-6 and Cr3C2-25NiCr) on 13Cr4Ni stainless steel has been investigated. Attempt has
been made to study the comparison between coatings and substrate steel under a particular set
of parameters (concentration, average particle size and rotational speed) in hydro accelerated
condition. Commercially available silica sand is used as an abrasive media. All
experimentation is done in High Speed Erosion Test rig. Comparison is made on two
different angles i.e. 30° and 90°. At 30°, Stellite-6 coating performed better in comparison
with Cr3C2-25NiCr coating and substrate steel specimen. On the other hand substrate steel
specimen performed better at 90° than the coatings.
Index Terms - Slurry erosion, High speed erosion tester, D-gun Spraying, Ceramic coatings.
1. INTRODUCTION
Hydro power plants which are located on the Himalayan Rivers have had to face
high silt content in the water passing through the turbines causing the large amount of
damage to various under water components of turbines like runner, guide vanes, needles and
seats of Pelton-type turbines. Water contains Quartz, Tourmaline, Garnet, Zircon, etc of
Hardness 7 on mho scale [1]. These sediments are formed due to the fragmentation of rocks,
erosion of land and land sliding because of heavy rains during the monsoon period in the
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ISSN 0976 - 6480 (Print)
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Himalayan region of India [2]. Because of this kind of environment erosion of turbine
components occurs [3]. Erosion due to the impact of hard particles is common phenomenon
of underwater parts in turbines [4]. A lot of investigations have been done by the researchers
to identify the factors responsible for the slurry erosion behavior of turbine materials due to
the water containing silt. It has been shown that slurry erosion at oblique angle can generate a
rougher surface when compared with that at normal incident. Also at oblique angles it can
render a greater susceptibility to pitting during erosion than at normal incident. Thus
Individual erosion events at oblique angles are thus expected to be more destructive than
those at normal incident [5]. Ductile materials during erosion are considered to loose material
through a cutting and ploughing mechanism at a low impact angle [6]. On the other hand,
cracking, fragmentation and removal of flakes is common phenomenon of erosion in brittle
materials [7].
1.1 Detonation Spray Coating
To improve the surface performance and durability of engineering components
which expose to different forms of wear such as abrasion, erosion and corrosion Thermal
Spray techniques are a versatile means of developing a large variety of coatings/protective
layers [8–10]. Detonation gun (D-gun) spray process is a thermal spray coating process,
which provides an extremely low porosity, good adhesive strength, coating surface with
compressive residual stresses, low oxide contents and high intersplat strength [11, 12]. The
D-gun spray process involves the impingement of powdered materials with the supersonic
speed through a water-cooled barrel on the surface of substrate. The two phase mixture of
coating are heated to plasticity and impinges against a target substrate, where the high
temperature, high velocity coating particles bond into the surface of substrate and a
mechanical interlocking and microscopic welding may take place [13]. Ceramic materials are
now commonly employed in the form of coatings to resist wear. Due to high melting point of
the ceramic powders which require a high temperature jet to get deformed during coating
formation, these coatings are usually deposited by atmospheric plasma spraying [14].
However, plasma sprayed coatings are possess more porous and brittle nature than high
velocity thermal-sprayed coatings [15, 16]. On the other hand, due to close interlamellar
contacts and small porosity, the high-velocity combustion spraying techniques provides
greater hardness [17, 18]. This is why several efforts have been undertaken to use these high-
velocity spray techniques like to spray oxides D-gun spraying is mainly used [18].
2. EXPERIMENTATION
2.1 Material
CA6NM steel containing 13% Cr and 4% Ni (also known as 13/4) is being used in
fabrication of hydro turbine underwater parts. Chemical composition of 13/4 stainless steel is
given in table 1. Rectangular specimens of 10 mm x 10 mm were prepared.
Table 1. Chemical composition of 13/4 stainless steel (wt %)
Steel C Si Mn Cr Ni N S Cu Co P Mo Fe
13/4 0.06 0.74 1.16 13.14 3.9 -- 0.014 0.088 0.035 0.015 0.61 Bal.

212
2.1 Deposition of Coatings
Commercially available powder of Stellite-6 and Cr3C2-25NiCr are coated on
13Cr4Ni stainless steel using D-Gun process available with SVX Powder M Surface
Engineering Pvt. Ltd., Noida, India.
2.2 Slurry Erosion Testing
A high speed erosion tester (DUCOM TR401, Bangalore make) was used to study
the slurry erosion behavior of 13Cr4Ni stainless steel specimens. The tester shown in Figure
1 consists of various components such as slurry abrasion chamber, slurry tank, rotor, control
panel, 3 phase induction motor.
Figure 1 Experimental setup of High Speed Erosion Tester (DUCOM TR401)
In slurry abrasion chamber test specimens and slurry is enclosed and specimens are
rotated. Slurry tank is a cylindrical vessel made up of stainless steel, in which the slurry is
prepared to the required concentration. There are three inlet and three outlet pipes between
slurry abrasion chamber and slurry tank for re-circulation of slurry. Due to rotation of rotor
vacuum is created in the slurry abrasion chamber and therefore slurry from cylindrical vessel
through inlet pipes to chamber and used slurry leaves the chamber and enters the tank from
bottom side through outlet pipes, thus ensuring continuous re-circulation of slurry. This high
speed erosion tester is capable of creating accelerated hydro conditions to simulate the
erosion of standard test specimens with water containing abrasive particles of controlled size
and composition (slurry). Main advantages of this rig is that at a time, 12 specimens may be
tested, thus ensuring zero tolerance to change of experimental conditions during comparison
of slurry erosion testing of different specimens under similar experimental conditions. Only
cylindrical samples can be tested in this tester.

213
For placing rectangular specimen in this tester we needed to design a specimen
holder in which we can accommodate rectangular specimen. After two different designs we
design the specimen holder shown in Fig 2.
Figure 2 Specimen holders for rectangular specimens
T 304 Stainless Steel Square bar was selected for the fabrication of specimen holders
as it is ideal for all applications where greater strength and superior corrosion resistance is
required. 304 Stainless Square has a durable dull, mill finish that is widely used for all types
of fabrication projects that are exposed to the elements - chemical, acidic, fresh water, and
salt water environments. Specifications, application and Mechanical Properties of T304
stainless steel are mention in table 2.
Table 2 Specifications, application and Mechanical Properties of T304 stainless steel
Specifications of T304 ASTM A276, QQS-763, T304/304L, non-polished finish.
Applications Frame work, braces, supports, shafts, axels, marine, etc.
Workability Easy to Weld, Moderate Cutting, Forming and Machining.
Mechanical Properties Brinell = 170,
Tensile Strength = 505 MPa,
Yield Strength = 215 MPa,
Nonmagnetic
Effect of average particle size, concentration (ppm) and velocity has been studied by
several researchers and showed that these are significant factors, which can affect the erosion
phenomenon. To study this, commercially used silica sand is used as slurry medium as silica
is found to be main constituent of slurry as is evident from the literature; the slurry is found to
be consisting of SiO2, Al2O3, CaO, and MgO in hydro power plant in northern India [4,19].
Slurry concentrations having average particle sizes of 300 µm were prepared to
simulate the test in more accelerated conditions. Tests were carried under concentration of
10000 ppm, with erodent particle size 300 µm and rotation speed of 3800 rpm to study the
slurry erosion performance of Stellite-6 and Cr3C2-25NiCr coated 13Cr4Ni and 13Cr4Ni
stainless steel which were placed at 2 different angles i.e 30° and 90°. These two angles were

214
select to analyze the performance of specimen when sand particles strike the surface
tangentially and normally.
The interaction of slurry particles with rectangular specimen in slurry chamber is
shown in Figure 3. A typical erosion test cycle began with mounting of specimen holder at
the proper place at correct angle. Then specimen was placed in the holders in the slurry
chamber. After fixing the specimens in holders every time chamber was made air tight by
tightening the nuts at four places so that proper vacuum can be created. The water was filled
in stainless steel water tank and silica sand of appropriate particle size was added to the water
for preparing required concentration. Then rotational speed was set and test was started. After
completing the slurry erosion testing cycle of 1 h, specimens were removed from the rotor
assembly, brushed gently and cleaned with acetone to remove attached sand particles if any.
The specimens were weighed before and after each slurry erosion cycle. The loss in mass of
each specimen was recorded with the help of precision micro weighing scale having an
accuracy of 0.1 mg. As erosion is a surface phenomenon so the peripheral surface area of
each specimen was calculated by measuring the length and width at two places, taking their
mean for getting average length and width of specimen with the help of digital vernier caliper
of least count 0.01 mm.
Figure 3 Interaction of slurry particles with rectangular specimen in slurry chamber

215
Specific mass loss was calculated using
Specific mass loss = mass loss (g) X 106
/ Exposed surface area (m2
)
The slurry erosion process was repeated for six cycles for each of the samples The
results have been plotted as cumulative mass loss per unit area in (g/m2
) versus time of
exposure (h) to ascertain the kinetics of slurry erosion behavior of coatings and substrate.
3. RESULTS AND DISCUSSION
Figure 4 shows cumulative weight loss per unit area (g/m2
) versus time (h) graph of
bared 13Cr4Ni stainless steel and Stellite-6 coated 13Cr4Ni at 30° under a set of parameters
of concentration of 10000 ppm, erodent particle size 300 µ and rotation speed of 3800 rpm.
From graph it can be observed that as the time increases the cumulative weight loss per unit
area increases for both materials. Also it can be seen that after a run of 6 hours overall
specific weight loss for 13Cr4Ni is 1897.43 g/m2
and for Stellite-6 coating it is 345.31 g/m2
.
It means specific weight loss for 13Cr4Ni is 5.5 times the specific weight loss of Stellite-6.
Figure 4 Comparision between cumulative mass loss per unit area of 13Cr4Ni and Stellite-6
at 30°
bared 13Cr4Ni stainless steel and Stellite-6 coated 13Cr4Ni at 90° under same set of
parameters. It can be observed that as time progresses cumulative weight loss per unit area
also increases. After a run of 6 hours overall specific weight loss for 13Cr4Ni is 172.87 g/m2
. It means specific weight loss for Stellite-6 is 2.43
times the 13Cr4Ni.

216
In Figure 6 there is a comparison between 13Cr4Ni and Cr3C2-25NiCr at 30° on
cumulative mass loss per unit area versus time graph. With the increase in time weight loss
per unit area increases for both. After 6 hour run overall specific weight loss for 13Cr4Ni is
1897.43 g/m2
. Specific weight loss for 13Cr4Ni is
1.35 times specific weight loss for Stellite-6.
Figure 5 Comparision between cumulative mass loss per unit area of 13Cr4Ni and Stellite-6
at 90°
Figure 6 Comparision between cumulative mass loss per unit area of 13Cr4Ni and Cr3C2-
25NiCr at 30°

217
The cumulative specific mass loss curves of the 13Cr4Ni and Cr3C2-25NiCr coated
13Cr4Ni steel at 90° for a total duration of 6-h slurry erosion testing are shown in Figure 7
for same set of parameters. It can be observed from the graph that the overall specific mass
losses are remarkably different. The maximum specific weight loss at the end of 6 hour of
slurry erosion testing for the bared 13Cr4Ni and Cr3C2-25NiCr coating is 968.57 and 172.87
g/m2
, respectively. So it can be observed coating is eroded 5.6 times than bared steel.
Figure 7 Comparision between cumulative mass loss per unit area of 13Cr4Ni and Cr3C2-
25NiCr at 90°
Figure 8 depicts the comparision chart between cumulative mass loss per unit area of
Stellite-6 and Cr3C2-25NiCr at 30° and in Figure 9 Stellite-6 and Cr3C2-25NiCr at 90 ° for a
total duration of 6-h slurry erosion testing with same set of parametes. At 30° the maximum
specific mass loss at the end of 6 hour run is 345.31 and 1399.19 g/m2
for Stellite-6 and
Cr3C2-25NiCr respectively. At 90° the maximum specific mass loss at the end of 6 hour run
is 420.82 and 968.57 g/m2
for Stellite-6 and Cr3C2-25NiCr respectively. It can be observed
that from Figure 8 and 9 that maximum specific weight loss for Stellite-6 is less than that of
Cr3C2-25NiCr. Maximum specific weight loss after 6 hour for Cr3C2-25NiCr is 4.05 times
than Stellite-6 at 30° while at 90° maximum specific weight loss for Cr3C2-25NiCr is 2.3
times than Stellite-6.

218
Figure 8 Comparision between cumulative mass loss per unit area of Stellite-6 and Cr3C2-
25NiCr at 30°
Figure 9 Comparision between cumulative mass loss per unit area of Stellite-6 and Cr3C2-
25NiCr at 90°

219
bared 13Cr4Ni stainless steel 30° and 90° under same set of parameters. It can be seen from
the graph that that 13Cr4Ni get more eroded at 30° than at 90° approximately 11 times. As
the maximum weight loss for 13Cr4Ni at 30° is 1897.43 g/m2
and at 90° it is 172.87 g/m2
after 6 hour run in erosion tester.
Figure 10 Comparision between cumulative mass loss per unit area of 13Cr4Ni at 30° and
90°
Cumulative mass loss per unit area of Stellite-6 at 30° and 90° is shown in Figure 11.
From the graph it can be seen that Stellite-6 performed better at 30° as the maximum weight
loss for Stellite-6 at 30° is less than the Stellite-6 at 90°. The maximum weight loss for
Stellite-6 at 30° is 345.31 g/m2
and at 90° it is 420.82 g/m2
.
In Figure 12 there is a comparison between cumulative mass loss per unit area of
Cr3C2-25NiCr at 30° and 90°. More erosion of Cr3C2-25NiCr takes place at 30°. The
maximum weight loss for Cr3C2-25NiCr is 1399.19 and 968.57 g/m2
at 30° and 90°
respectively after a 6 hour run in slury erosion chamber of high speed tester. The ratio of
maximum weight loss at 30° to 90° comes out 1.44.

220
Figure 11 Comparision between cumulative mass loss per unit area of Stellite-6 at 30° and
90°
Figure 12 Comparision between cumulative mass loss per unit area of Cr3C2-25NiCr at 30°
and 90°

221
4. CONCLUSIONS
Due to higher hardness, Stellite-6 coating performed better at 30° than uncoated
13Cr4Ni while substrate13Cr4Ni steel (due to high toughness) showed better slurry
resistance than Stellite-6 coating at 90°.
Cr3C2-25NiCr coating was found to be more erosion resistant at 30° than bared
13Cr4Ni but not at 90°. At 90° substrate 13Cr4Ni steel showed far much better
perforamnce than Cr3C2-25NiCr coating.
While comparing both coatings it was found that Stellite-6 coating ( due to higher
toughness) is more resistant to slurry erosion than Cr3C2-25NiCr coating at 30° as
well as at 90°.
Uncoated 13Cr4Ni steel and Cr3C2-25NiCr coating showed a better performance to
slurry erosion at 90° than at 30° while Stelite-6 coating was better at 90° when
compraing their performances separately at two different angles.
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