CC BY
12
УДК 624.21 DOI: 10.30977/BUL.2219-5548.2020.88.1.37
STRENGTHENING OF THE WORKING EDGES OF THE BLADES OF STEAM TURBINES
fflushkova D. B., Grinchenko E. D., Kostina L. L. Kharkiv National Automobile and Highway University
Abstract. The working conditions of the blades of steam turbines require increased hardness of the input edges and resistance to impact erosion, high corrosion resistance.
In order to increase the service life of the blades, the inlet edges are quenched by high-frequency currents. The hardened layer was formed by electrospark alloying with T15K6 alloy and 15X11MFSh steel. The use of the widely used T15K6 alloy as a hardened electrode is limited due to the presence of cobalt, as an element forming, as a result of activation, long-lived isotopes that reduce the erosion resistance of the blades. Ultra high speed heating and cooling, pulsed effects of temperatures and pressures result lead to the fact that the composition of the alloyed layer can significantly differ from the composition of the starting materials. The microstructure, microhardness, and thickness of the deposited layer were investigated. The hardened surface was monitored by external inspection, bending tests, microhardness measurements, and microstructure studies. The advantages of 15X11MFSh steel for hardening the input edges of the working blades of steam turbines are substantiated. Keywords: electrospark alloying, electrode, deposited layer, microstructure, microhardness, hardening.
Introduction
Modern issues of creation and modernization of power equipment put forward high demands for improving the indicators of economy and reliability of turbines. Erosion destruction of the working blades leading edges of low-pressure aft stages of heavy steam turbines becomes one of the main factors determining the functional ability and efficiency of a blade system and a turbine as a whole. A blade system, especially blading of low-pressure rotors, as the most complex and highly loaded part of a turbine, largely determines the entire unit reliability.
Analysis of publications
The blade of the aft stage of low-pressure cylinder determines the threshold for maximum turbine power. Creation of a blade system represents a complex task in the field of strength, gas dynamics, vibration, anti-erosion protection with obligatory blades optimization in bench conditions. [1].
One of the most important issues is development of effective methods to protect blade systems from erosion with simultaneous reducing of mechanical losses caused by humidity.
Presence of a liquid phase in the working medium of steam turbines causes additional energy losses in stages and erosive wear of the flow range elements. Different density of the components of working medium leads to a significant mismatch between the liquid and vapor phases velocities and, as a consequence, to a
complex pattern of motion of many drop flows, in some cases resulting in a sharp concentration of coarse moisture in various locations of the steam part.
Effectiveness of currently developed methods of the stages erosion protection is mainly determined by a particular manufacturer's own experience of implementing anti-erosion measures [2].
Despite the wide experience of creating various active and passive methods of anti-erosion protection gathered to the present days all over the world, cases of serious damage to the working blades of the aft stages of steam turbines due to the erosion-hazardous dropping moisture in the steam part occurrence can be observed.
In order to increase the blades service lifetime, the blades leading edges are protected in various ways, among which are: high-frequency current hardening, electrospark alloying with solid alloys based on W, Ti carbides, welding of stellite plates, ion implantation with TiN formation on the coating surface, subsonic and supersonic plasma spraying of wear-resistant coatings etc.
Each method application has its limitations: it is technically difficult to harden the radius transition from a blade airfoil to a desk-type bandage by high-frequency currents; utilization of the widely used T15K6 alloy as a strengthening electrode for blades operating in primary circuit turbines is unacceptable because of the presence of cobalt element, which in result of
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bichuk xhafly, bun. 88, 2020, t. i
activation forms long-lived isotopes [3]; the method of ion implantation involves usage of expensive vacuum equipment [4].
Figure 1. Erosive wear of K 220-44 turbine blade leading edge (17520 hours running)
Resulting from topicality of the issue of increasing the erosion resistance of steam turbine blades, works to improve the available technologies for protecting the leading edges of working blades of the low pressure aft stages were executed.
The purpose and objectives of the study
The subject of consideration in the present academic paper constitutes investigation of the possibility of using 15X11MO-ffl steel as a material for a strengthening electrode while performing the electrospark alloying of a leading edge of steam turbine working blade in order to protect it against the impact of water droplet erosion during operation in phase transition zone.
The comparative results of conducted experiments on hardening of 15X11MO-ffl steel for blades of steam turbines with electrospark alloying with T15K6 alloy and 15X11MO-ffl steel are given herein.
Objective
The subject of consideration in the present academic paper constitutes investigation of the possibility of using 15X11MO-ffl steel as a material for a strengthening electrode while performing the electrospark alloying of a leading edge of steam turbine working blade in order to protect it against the impact of water droplet erosion during operation in phase transition zone.
The comparative results of conducted experiments on hardening of 15X11MO-ffl steel for blades of steam turbines with electrospark alloying with T15K6 alloy and 15X11MO-ffl steel are given herein.
Statement of basic materials
Special requirements constitute subject to the coatings designed for aft stages working blades of heavy steam turbines, among which are: droplet erosion resistance, continuity and adhesion under the operational loads action, absence of a negative effect of the coating formation parameters on the blade material mechanical properties, high corrosion properties, linear expansion coefficient close to the one that the main blade metal has.
In order to perform the test, the samples from working blades of 15X11MO-ffl steel were strengthened by the method of electric spark alloying (ESA) with two materials: the traditional T15K6 alloy and pioneer used to harden the leading edges of blades 15X11M$-ffl steel identical to the blade material.
The method of electrospark alloying is based on the phenomenon of electrical erosion of materials during a spark discharge in a gaseous medium, polar transit of erosion products onto a layer of altered structure and an alloy. As a result of electrical breakdown of the interelectrode spacing arises a spark discharge in which the electron flux leads to a local heating of the electrode (anode). Mixing of cathode and anode material under the influence of significant thermal loads occurs on the cathode surface, which promotes the formation of high adhesion between the substrate and generated layers.
The works on hardening of the samples were executed on ЭHП8А model of electrospark installation in mode No. 7 (pulse current amplitude value I = 175+10 A, pulse energy Epulse = 3.15 J, pulse duration time tpulse = 1000 ms, frequency at 600 Hz).
The study was carried out on samples from blade blank parts of 15X11MO-ffl steel, manufactured by forged method and heat-treated to a hardness of 271HB. The samples material has the following mechanical properties:
Fig. 2. The sample of blade strengthened with T15K6 alloy by the method of electrospark alloying
Fig. 3. A sample of a blade strengthened with 15X11MO steel by the method of electro-spark alloying
Microstructure of the samples parent metal constitutes a sorbitol with preservation of martensitic planes orientation. Microstructure of the samples features uniformity, grains of different etchability can be observed within the struc-
Table 1 - Mechanical
ture, size of needles corresponds to 7-8 points of State Standard of Ukraine 8233-56 (Fig. 4). Hardened layers' surface is studied. Figures 5 and 6 show SEM images of the samples' surface at magnifications at x50 and x1000. Appears that the shown surface is very heterogeneous; strengthening is performed unevenly due to the hardening pulse discreteness. Undulation of the sample's surface of hardened layer performed by T15K6 alloy is in 2.8 times coarser than the one performed by 15X11MO steel (43.9 and 15.3 ^m respectively).
Determination of chemical elements on the surface of the layers is executed by XPS spec-troscopy method.
properties of the blade
00,2, MPa Om, MPa Ss, % % KCU, J/cm2 HB, MPa
Testing results 669 827 20 58 116 2710
108.020.03-82 Industrial Standard requirements 666,4813,4 >814 >13 >40 >39,2 2480-2850
Fig. 5. Surface topography of hardened layer performed by T15K6 alloy,x50
Fig. 6. Surface topography of hardened layer performed by 15X11 M<t>.\ 1000
wide stall
1000 300 (500 4QC 200 0
Binding Energy (eV) CasaXPS (This stiirigcanbe editedmCesaXPS.DEFiPiintFoeiNoSe.trt)
Fig. 7. Speculative analysis of elements in the layer hardened by T15K6. XP spectrum of 2 keV Ar+ observed in 5 minutes after etching
0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200
Fig. 8. Dispensing of elements in the layer hardened by T15K6
Table 2 - Content of elements in the sample hardened by T15K6
Element Series Mass, % Weight, % Inaccuracy, %
Ti K 8,30 10,75 0,77
Cr K 11,87 14,16 1,01
Fe K 62,24 69,15 4,17
W L 17,60 5,94 5,80
Fe, Cr, W, Si, S and N are revealed; in XP spectrum a carbon peak is expressed and the manganese peak is clearly defined. Each metal
element is in an oxidized state. Manganese and zinc are found exclusively as traces.
Aide sesrt
Etching interval, s
Fig. 9. Speculative analysis of elements in the layer hardened by 15X11MO steel. XP spectrum of 2 keV Ar+ observed in 5 minutes after etching
Fig. 10. Speculative analysis of elements in the layer hardened by 15X11MO steel. ISS spectrum data on the sample was collected by He+ ions of 800 eV without etching
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bichuk xhafly, bun. 88, 2020, t. i
30-1 79 -78 -777675
18 -171615 141312
Etching interval, s
-i—■—i—*—i—■—i—■—f—■—i—■—i—•
0 200 400 600 800 10001200
Fig. 11. Dispensing of elements in the layer hardened by 15X11MO steel Table 3 - Content of elements in the sample hardened by 15X11MO steel
Element Series Mass, % Weight, % Inaccuracy, %
Si K 2,67 5,09 0,26
Cr K 20,09 20,69 1,29
V K 1,19 1,25 0,31
Mn K 3,49 3,40 0,58
Fe K 72,55 69,56 4,47
Fig. 12. Electrospark alloying layer performed by T15K6, a - x100; b - x1000
The surface layer showed a high content of chromium, namely up to 20 %.
Metallographic examination of the layers was executed. The surface hardened layer on both samples has an explicit dendritic structure. XDR states that a significant amount of the aphase and the austenite phase are present in the layer hardened by T15K6; the composition includes tungsten carbide, titanium and cobalt. aphase, austenite phase, insignificant amount of titanium carbide are observed in the layer strengthened by 15X11MO.
Figures 10 and 11 show photo illustrations of the layers at 50-fold and 1000-fold magnifications. The surface of the samples is rough, in-homogeneous.
Measurement of the hardened layer thickness was conducted in sections made according to the specimens' cross-sectional plane.
The surface hardened layer features heterogeneity through the layer's thickness, but in cases of hardening by T15K6 alloy and 15X11MO^ steel the average thickness values practically coincide (Fig. 14).
iilS^I
mm m w
' * * «1 % 1
' ' / '« -¿J - <
f' r ' -r , f _____• * -
Fig. 13. Electrospark alloying layer performed by 15X11MO a - x100; b - x1000
Investigation of the strengthened layer microstructure has showed that the structure is homogeneous, practically non-etching. In some places, single pores are found. In the surface layer of the parent metal a light-etching diffusion zone of the
800
p 700
1 600
I
500
It *
400
300
if
200
100
electrode material through the sample depthward and a dark-etching zone of sub-hardening are observed under the influence of high temperatures. In some places, pores are found.
0,08
E0,07 E
A06 it), 05
4H
So,04
w
J 0,03 P0,02
Fig. 14. Histograms of the average values of the hardened layer thickness: 1 - T15K6 alloy, 2 - 15X11MO-m steel
Figure 15 shows histograms of the micro-hardness measurements of the test samples.
As follows from the histograms presented above, the microhardness for hardening by T15K6 alloy and 15X11M$-m steel is practically the same in all zones.
A full-scale experiment on K220-44 turbine was conducted. Operation of the blades hardened at the leading edge by 15X11MO steel for 2 years in a slightly alkaline medium at pH 9.8 has showen satisfactory results, as demonstrated in Fig. 16 and 17.
720 /|M 700
1*600 t500
J400
S 300
SJOO
100
11
= 700
2 soo ®soo
Et>
s
- 400 b
£ 300 ■ i£
E 200 100
= 700
k 600
71.
, 500
in
3
| 400 300 ® 200 100
Fig. 15. Histograms of the microhardness measurements of the test samples strengthened with T15K6 alloy (1) and 15X11MO-m steel (2): a - hardened layer; b - transition (diffusion) zone; c - heat-affected zone (~0,05mm from the «parent metal - hardened layer» edge); d - heat-affected zone (~0,1mm from the «parent metal - hardened layer» edge)
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Вюник ХНАДУ, вип. 88, 2020, т. I
Fig. 16. Blades hardened with T15K6 alloy by electrospark alloying method
Fig. 17. Blades hardened with 15X11MO steel by electrospark alloying method
Conclusions
1. 15X11MO material identical to steel of which the working blades of steam turbines are manufactured is advantageous for the erosion-resistant protection formation, which results in cost avoidance in electrospark alloying electrodes material purchasing.
2. The roughness of the layer made by 15X11MO steel is lower, than of the one performed with T15K6 hard alloy metal.
3. The average thickness of the surface layers performed in the same modes, both with the T15K6 alloy and with 15X11MO-m steel, practically coincides.
4. Microhardness on the surface of the hardened layer is higher while using 15X11MO-ffl steel than when using T15K6 hard alloy metal; microhardness of the transition zone, the heat-affected zone at different distances from the «parent metal - hardened layer» edge practically do not differ.
5. On the basis of conducted researches it is possible to recommend replacement of the applied strengthening electrode from T15K6 alloy on 15X11MO-ffl steel for protection from water droplet erosion of leading edges of steam turbines working blades.
Reference
1. Subbotin V. H., Levchenko E. V., Shvetsov V. L. Steam turbines for thermal electric power stations of «Turboatom» Publicly-Traded Company. Energy and thermotechnical processes and equipment. 2009. № 3.
2. Shubenko A. L., Kovalskii A. E. Droplet impingement erosion of blade systems of steam turbines. Predicting modeling and protection methods, Energy and thermotechnical processes and equipment. 2017. № 7.
3. Technical Guides «РТМ 108.020.15-86». Metals for turbines and heat-exchange equipment of nuclear power plants.
4. Beliakov A. V., Shapin V. I., Horbachev A. N. Practice of electrospark coatings formation for hardening and repair of blading system of steam turbines flowing part of thermal and nuclear power plants. Ivanovo State Power University Reporter. 2008. № 4.
Hlushkova Diana - Doctor of Technical Sciences, Head of the Department of Technology of Metals and Materials Science, tel. 097-481-15-93, diana@khadi.kharkov.ua
Grinchenko E.D. - engineer of the Department of Technology of Metals and Materials Science, tel.: +038-70737-29, diana@khadi.kharkov.ua Kostina Lyudmila - of the Department of Technology of Metals and Materials Science тел.: 066-150-89-72, kostina4991@gmail. com
Змщнення вхщних крайок робочих лопаток парових турбш
Анотаця. Умови роботи лопаток парових турбш вимагають тдвищеног твердостi exidHUX крайок, ^wm^i проти ударного ерозтного руйнування, а також високог корозтног стт-костх. З метою збыьшення строку служби лопаток exidHi кромки тддають загартовуванню струмами високог частоти. Змщнений шар фор-мують електроюкровим легуванням сплавом Т15К6 i сталлю 15Х11МФШ. Як змщнюваний електрод використовують сплав Т15К6, що е обмеженим вна^док наявностi кобальту як елемента, що утворюе в результатi активацп iзотоnи, ям живуть тривалий час та знижують ерозтну сттюсть лопаток. Надвисока швид-юсть нагрiвання й охолодження, iмпульсний вплив високих температур i тисюв призводять до того, що склад легованого шару може значно вiдрiзнятися вiд складу виxiдниx матерiалiв. Була до^джена мжроструктура, мтротверд^ть i товщина наплавленого шару. Контроль змщненог
поверхт здшснювали зовнштм оглядом, випро-буваннями на вигин, вимiрюваннями мкротвер-достi, вивченням мкроструктури. Обтрунтовано переваги сталi 15Х11МФШ для змщнення вхiдних кромок робочих лопаток парових турбiн. Ключовi слова: електроiскрове легування, елек-трод, наплавлений шар, мiкроструктура, мiкро-твердiсть, змщнення.
Глушкова Дiана Борийвна - д.т.н., завщувач кафедри технологи металiв та матерiалознавства, тел.: 097-481-15-93, diana@khadi.kharkov.ua Костша Людмила Леонадвна - к.т.н., доцент кафедри технологи металiв та матерiалознавства, тел.: 066-150-89-72, kostina4991@gmail.com Гршченко Олена Дмитрiвна - iнженер кафедри технологи металiв та матерiалознавства, тел.: +038-707-37-29, diana@khadi.kharkov.ua
Упрочнение входных кромок рабочих лопаток паровых турбин
Аннотация. Условия работы лопаток паровых турбин требуют повышенной твердости входных кромок, стойкости против ударного эрозионного разрушения, а также высокой коррозионной стойкости.
С целью увеличения срока службы лопаток входные кромки подвергают закалке токами высокой частоты. Упрочненный слой формировали электроискровым легированием сплавом Т15К6 и сталью 15Х11МФШ. Использование в качестве упрочняемого электрода широко применяемого
сплава Т15К6 ограничено вследствие наличия кобальта как элемента, образующего в результате активации долгоживущие изотопы, которые снижают эрозионную стойкость лопаток. Сверхвысокая скорость нагрева и охлаждения, импульсное воздействие высоких температур и давлений приводят к тому, что состав легированного слоя может значительно отличаться от состава исходных материалов. Исследовали микроструктуру, микротвердость и толщину наплавленного слоя. Контроль упрочненной поверхности осуществляли внешним осмотром, испытаниями на изгиб, замерами микротвердости, изучением микроструктуры. Обоснованы преимущества стали 15Х11МФШ для упрочнения входных кромок рабочих лопаток паровых турбин.
Ключевые слова: электроискровое легирование, электрод, наплавленный слой, микроструктура, микротвердость, упрочнение.
Глушкова Диана Борисовна - д.т.н., заведующий кафедры технологии металлов и материаловедения, тел.: 097-481-15-93, diana@khadi.kharkov.ua
Костина Людмила Леонидовна - к.т.н., доцент кафедры технологии металлов и материаловедения, тел.: 066-150-89-72, kostina4991@gmail.com Гринченко Елена Дмитриевна - инженер кафедры технологии металлов и материаловедения, тел.: +038-707-37-29, diana@khadi.kharkov.ua
