Thermally Sprayed Coatings for High Temperature Applications in

Thermally Sprayed Coatings for High Temperature Applications in
Power Industry
František Zahálka1, a, Michaela Kašparová2,b and Šárka Houdková3,c
1
ŠKODA VÝZKUM s.r.o, Tylova 1/57, Plzeň, 316 00, Czech Republic
2
ŠKODA VÝZKUM s.r.o, Tylova 1/57, Plzeň, 316 00, Czech Republic
3
ŠKODA VÝZKUM s.r.o, Tylova 1/57, Plzeň, 316 00, Czech Republic
a
[email protected], [email protected],
c
[email protected]
Keywords: coating, thermal spraying, HVOF, crack, high-temperature, corrosion,
Abstract.
The paper is focused on the evaluation of thermally sprayed coatings, prepared using the
HP/HVOF JP-5000 spraying equipment. The spraying parameters for all coatings´ materials were
optimized to obtain a satisfactory microstructure and mechanical properties of the coatings. Testing
at high-temperature and in corrosive environments was used for the application in a power industry.
Corrosion resistance of the coatings was tested in steam at temperature up to 550°C and pressure at
25 MPa. Further, a thermal shock resistance of the coatings deposited on the P91 and 15 128 steel
substrates was measured using the SMITWELD apparatus.
Evaluation based on the SEM observation of the microstructure and changes in mechanical
properties of the coatings´ materials after the tests was used to identify material degradation, the
presence of cracks or other significant changes in the coatings microstructure. The results show a
dependence of the cracks presence in a coating on the substrate material in the used experimental
condition. Nevertheless some coatings were excellently resistant to corrosion and cracking
independently of the substrate material.
Introduction
In power industry, especially in steam turbines many components are made with different
surface treatments (e.g. nitriding or hard chrome plating) to enhance their wear resistance and
decrease a friction coefficient. However, these types of a surface treatment have not met
requirements for some new types of turbines with higher service parameters of the steam
temperature.
However regarding the fact that some of these parts are used in the turbine system´s steam
regulation section there are justified requirements for many test of the coatings before their
application in real turbines. The application process consists of many steps from the optimization of
the spraying parameters for each coating material to reach the best quality microstructure and
mechanical properties to a special test focused on the evaluation of the coatings´ behavior close to
service conditions.
In this work nine experimental spray powders were sprayed using the HP/HVOF JP-5000
equipment. These materials were selected on the basis of a literature survey focused on the power
industry. Two methods applied in ŚKODA VYZKUM s.r.o. were used for the coatings´ deposition
process optimization to find out optimal physical and mechanical properties of the coatings. The
methods based on a statistical principle – Design of Experiment DoE [1,2] and objective
comparison were performed. Samples for corrosion resistance and thermal shock resistance were
sprayed onto two different substrate materials which are commonly used for many turbines parts´
production. Chemical composition of the 15.128 (CrMoV) and P92 substrate materials is mentioned
in [3].
Experiments
The starting composition of the spray powders used in this work is given in Table 1. A typical
nominal spray powder particle size was 15-45 μm. All powders are commercially available. The
coatings were sprayed onto a grit-blasted steel made of the 15 128 and P92 substrates using the
parameters which were previously optimized and are currently used in ŠKODA VÝZKUM s.r.o. in
the JP-5000 (TAFA, USA) spraying equipment.
Table 1 Chemical composition of spraying powders
Powder
Cr3C2-35%NiCr
Cr
%
C(total)
%
Ni
%
Si
%
bal.
8,06
27,92
Cr3C2-25%CoNiCrAlY
69,29
10,09
7,4
WC-20%CrC-7%Ni
WC-17%Co*/NiSF
(50/50)
18,15
6,74
6,85
14,5
0,72
CoMoCrSi
8,34
CoCrWCSi
29,5
CoNiCrAlY
21
NiCrMoWFe
15,4
0,13
bal.
0,5
NiCrFeMoCoTi
18,87
0,01
53,24
0,1
Ti
%
Mo
%
W
%
Co
%
Y
%
8,91
0,11
Ο
%
B
%
Fe
%
Mn
%
V
%
Al
%
0,11
bal.
bal.
4,37
0,01
0,15
2,35
1,2
2,4
1,4
0,96
16,4
bal.
32
3,04
0,04
0,7
0,61
8
0,03
3,9
0,12
3,85
0,15
bal.
4,5
0,97
3,2
bal.
29,79
16,1
1,91
0,04
0,01
0,004
2,8
1,2
17,9
0,02
0,4
0,48
* Co = 16,40%, WC=Balance
Coatings microstructures were studied using optical and SEM micrographs of metallographically
prepared cross-sections. Hardness testing was performed using the Vickers microhardness tester
using 300g load.
For high temperature cycling of the sprayed coatings the SMITWELD TTU 2002 machine was
used in ŠKODA VÝZKUM s.r.o. laboratories, all samples were heated applying electrical
resistance. Electric heating affects only a short line between the contact points. Material heating was
very fast, around 15 °C/s. The heat stresses acting on peripheral parts of samples were negligible.
The temperature changed linearly without pauses during testing between the limits 200 and 660 °C
at rate 15°C/s. One temperature cycle was 60 seconds and for each material 50 temperature cycles
were applied.
The corrosion tests experiments were carried out in the Laboratory of surface analysis of VŠCHT
Praha, Czech Republic. Corrosion tests were performed in the Super Critical Water (SCW)
corrosion autoclave. Experimental conditions of the test are summarized in Table 2. The thickness
of the coatings was between 300-400μm but higher thickness on edges can occur due to the
complicated deposition because the deposition on the whole surface of the sample was requested for
the test. Coatings were used for the test at as-sprayed conditions. Porosity was measured and
calculated applying the image analysis. Additional testing, e.g. EDX, RXD etc. of the mentioned
coatings are described in [3].
Table 2 Corrosion test conditions
Exposition
temperature [°C]
Time of
exposition
[h]
Supplying water
Water
conductibility
[µS/cm]
Pressure
[MPa]
550°C
150
Demineralized water with dissolved oxygen in
balance with atmosphere 8ppm
2<
24,5
Results and discussion
All coatings after spraying using the optimized parameters show relatively homogenous
microstructure, good adhesion to the substrate and porosity below 3%. However, these properties
are slightly different depending on the material of the coating.
The results of the cycling at temperature 600°C show dependence of the crack resistance on the
substrate material for the coatings based on the CoNiCrAlY, CoCrWCSi super alloys and the CrCCoNiCrAlY composite coating. Other coatings do not show any presence of the cracks depending
on the type of the substrate. However, the experimental set up of this test significantly exceeded the
calculated values for service conditions, where the change of temperature 280°C takes 1 hour, and
therefore a good resistance for coatings with cracks can also be assumed. This has to be confirmed
by an additional test with the parameters close to the service conditions. Calculated maximum
dilatation of the substrate during the test was 0,057 mm. On the other side the superpositions of the
temperature and mechanical stresses also have to be taken into account.
Fig. 1 OM analysis of the CoNiCrAlY coating after high temperature cycling, cross-section of
the coating deposited on the P92 substrate with a visible transversal crack in the coating (left) and
on the 15 128 substrate without the crack presence in the coating at the same conditions (right)
Results of the hardness measurements are shown in Table 3. Measurements were performed in
three regions, in the region without temperature changes (right), in the middle of the area affected
by the temperature cycling (left) and in the region with the maximum temperature changes. A
visible increasing in the hardness changes in different affected zones for some coatings can be seen.
In the SEM micrographs, there are visible differences in microstructure between the basic and high
temperature cycling coatings for some type of coatings which are in good correlation with the
hardness measurements.
Table 3 Hardness measurements of the heat affected zones of the coating after thermal cycling
test with max. temperature 600°C
Coating
Cr3C2-35%NiCr
Non affected zone on the edge
HV0,3
729
Between zones
HV0,3
750
Max. heat affected zone
HV0,3
730
Cr3C2-25%CoNiCrAlY
950
1132
1089
WC-20%CrC-7%Ni
1008
1248
1270
WC-17%Co*/NiSF (50/50)
774
922
935
CoMoCrSi
688
763
801
CoCrWCSi
643
687
653
CoNiCrAlY
631
613
637
NiCrMoWFe
534
529
532
NiCrFeMoCoTi
378
452
469
Results of corrosion resistance tests are given in Table 4. The results are shown for selected
materials with the P92 and 15 128 substrate materials. For the selected coatings was after their
exposition and drying in all cases measured positive mass gain. No debris or coating spalling,
characterizing a critical damage due to the corrosion were observed for the selected coating.
Furthermore, no dependence of the corrosion rate depending on the substrate type was observed.
Surface characterization by the optical microscopy after the corrosion test is shown in Fig. 2.
Table 4 Weight changes after the corrosion test, 1 coatings deposited on the P92 substrate, 4 –
coatings deposited on the 15 128 substrate
Coating - substrate
m0[g]
m1(g)
Δm (g)
Cr3C2-35%NiCr - 1
Cr3C2-35%NiCr - 4
NiCrFeMoCoTi - 1
NiCrFeMoCoTi - 4
NiCrMoWFe - 1
NiCrMoWFe - 4
28.5839
27.9911
31.2457
28.0356
28.4672
29.8180
28.662
28.073
31.272
28.061
28.499
29.853
0.0785
0.0818
0.0258
0.0251
0.0314
0.0353
Fig. 2 Surface of the NiCrFeMoCoTi coating before and after the corrosion test, increased corrosion
on the edge of the sample
Summary
All thermally sprayed coatings deposited applying the HP/HVOF technology and using the JP-5000
gun with the optimized spraying parameters developed in ŠKODA VÝZKUM s.r.o. show enhanced
mechanical properties in comparison with the coatings deposited with the parameters supplied by
manufacturer. The coatings show very low dependence of crack and corrosion resistance on the type
of substrates, which are widely used in the power industry for selected parts of turbines, although
undefined factors in service conditions must be taken into account as well. That is why many
additional tests have to be performed before applying the selected thermally sprayed coatings into
real systems.
Acknowledgements
This paper was prepared with the support of the national project MPO FT-TA5/072. The Authors
also would like to thank Mr. P. Sajdl (Department of Power engineering, University of ChemicalTechnology in Prague, Czech Republic) for performance of the corrosion tests and Mr. A Shorný
(ŠKODA VÝZKUM s.r.o) for performance of the high temperature cycling tests.
References
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[3]
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