CC BY
41
DOI 10.5862/JEST.249.10 УДК 669.295: 669.018
N.A. Krylov, M.A. Skotnikova, G.V. Tsvetkova, G.V. Ivanova
influence of structure and phase composition of the material of steam turbine blades from titanium alloys on their resistance to erosive destruction
Н.А. Крылов, М.А. Скотникова, Г.В. Цветкова, Г.В. Иванова
влияние структуры и фазового состава материала лопаток паровых турбин из титановых сплавов
на их устойчивость к эрозионному разрушению
Titanium alloys keep a leading position in manufacturing steam turbine blades. The unique combination of high specific strength and fracture toughness, corrosion resistance, polymorphism, a tendency to phase transformations and high resistance to shock loading determined their wide application in power engineering. However, power engineering is still facing a problem of erosion damage of steam turbine blades as a result of a drop impact of vapor particles at speeds of 150...600 m / s. The complication in solving this problem is that it is difficult to establish a link between wear and structural-phase composition of the surface of turbine blades. In this paper, using optical metallography, electron microscopy, ray analysis and X-ray microspectral analysis, we developed a method for studying structural and phase transformations in the material of steam turbine blades of titanium alloy VT6 after technological treatments in different modes. An attempt was made to establish a link between resistance to erosion destruction and structural and phase composition of titanium alloys.
STEAM TURBINE; STRUCTURAL AND PHASE TRANSFORMATIONS; OPTICAL METALLOGRAPHY; ELECTRON MICROSCOPY; X-RAY MICROSPECTRAL ANALYSIS.
Титановые сплавы сохраняют лидирующее положение в производстве лопаток паровых турбин. Уникальное сочетание высокой удельной прочности и вязкости разрушения, коррозионной стойкости, полиморфизм, склонность к фазовым превращениям и высокая устойчивость к ударной нагрузке определили их широкое применение в энергетике. Тем не менее в энергетическом машиностроении до сих пор существует проблема эрозионного повреждения лопаток паровых турбин в результате каплеударного воздействия частиц пара со скоростями 150—600 м / с. Сложность решения этой проблемы заключается в том, что трудно установить связь между износом и структурно-фазовым составом поверхности лопаток турбин. В данной работе с использованием оптической металлографии, электронной микроскопии, рентгеноструктурного анализа и рентгеновского микроспектрального анализа разработан метод исследования для изучения структурных и фазовых превращений в материале паровых лопаток турбин из титанового сплава ВТ6, после технологической обработки на различных режимах. Предпринята попытка установить связь между устойчивостью к эрозионному разрушению и структурно-фазовым составом титановых сплавов.
ПАРОВЫЕ ТУРБИНЫ; СТРУКТУРНЫЕ И ФАЗОВЫЕ ПРЕВРАЩЕНИЯ; ОПТИЧЕСКАЯ МЕТАЛЛОГРАФИЯ; ЭЛЕКТРОННАЯ МИКРОСКОПИЯ; МИКРО-РЕНТГЕНОСПЕКТРАЛЬ-НЫЙ АНАЛИЗ.
Introduction
Titanium alloys keep a leading position in the creation of steam turbine blades. The unique combination of high specific strength and fracture
toughness, corrosion resistance, polymorphism, a tendency to phase transformations and high resistance to shock loading defined their wide application in power engineering (for example see fig. 1,a, b)) [1, 2].
10
9 8
S 7 "=
J 5
to 4
f
? J £
I 2
CL
1
11 Cu
— Stainless / j, J
Mo/ *
10j 10* 1»' 10* 101 Velocity deformation, s '
J2® 1600 HOOO
Yield strength, , MPa.
Fig. 1. Dependence of spall strength from velocity deformation for different metals (a); dependence of fracture toughness from yield strength for titanium alloys (b)
However, the power engineering still has a problem of erosion damage of steam turbine blades as a result of a drop impact of vapor particles at a speed of 150—600 m / s. The difficulty of solving the problem is that difficult to establish a link between wear and structural-phase composition of the surface of turbine blades (the typical example of erosion destruction of steam turbine blades is shown in fig. 2).
Fig. 2. Example of erosion destruction of steam turbine blades
Two-phase titanium alloys found wide application in turbine construction due to opinion [3], that the greatest resistance to effect of exposure steam drops should be two-phase alloys with low internal stresses and high plastic properties, during which time of stress there exist phase transformations strengthening the subject material.
At effect of exposure steam drops, field sizes of a stressing are commensurable with sizes of structural components, and redistribution of internal stresses between them is impossible. The role of individual durability, intensity, a chemical composition and phase transformations in separate structural compo-
nents therefore increases. To solve this problem it is necessary find the relation between the wear and structural-phase state of the surface and axial layers of blades material.
The object of research
The object of research has been samples of steam turbine blades of the two-phase titanium alloy VT6 (Ti-6Al-4V) average strength after deformation in b area and final deformed by stamping in areas a+p next to the reduced temperatures T50 and T70, corresponding to the first and second technology. The temperature of final transition of an alloy in b- region (T ) was made 1015 0C.
As follows from earlier carried out works [4—8], on the basis of complex research of mechanisms of formation and decomposition nonequilibrium P(a)-and a(P)- phases, redistribution between them of alloying elements, physical and mechanical properties of the deformed titanium alloys of a different alloying, and also the established laws, the generalized kinetic diagram, fig. 3, has been constructed. Thus we took into account not only a temperature of heating absolute (T), but also produced the identical contents of high-temperature p-phases (Tp). We established, that the more nonequilibrum P(a)- and a(P)-solid solutions contained the same alloying elements; their polystage decomposition occurred at lower temperatures, and for greater time. It is shown, that titanium blanks possess high technological properties in a temperature-time interval two-phase (a+P)- region at temperatures identical (50 %) contents high-temperature a- and p- phases (T50) and near to a temperature of transition in single-phase p- region (T75).
customizations of dislocation. The second technology is seen less of number curved extinction contours, indicate the presence of internal stresses.
0,5 1 2 5 10 20
Time, h r s.
100 200 500
Fig. 3. Generalized kinetic diagram of structure and phase transformation in titanium alloys
The product possesses material with high operational properties after an isothermal exposure near to temperature T . In these cases a- and phase components appear enriched, the as a- and stabilizing alloying elements, in these processes of a polygoniza-tion that causes increase of plasticity characteristics and allows crack origin of alloys to develop.
Thus, construction of serial curves of increase of the contents high-temperature phases depending on a temperature of heating (a method of hardening), will allow us to estimate the resulted temperatures of heating and to develop the scientifically-grounded modes of technological treatment of titanium blanks [6].
Structure and the Phase Composition
Both stamping technology provides a bimodal structure a-phase globular (aI) and platelet-shaped (aI I), divided by layers of | — phases (fig. 4,a, b). Electron microscope photos (fig. 4,c, d) shows that the boundaries of phase components (aI, aII, |II) for the first technology much better relaxation, due to
Fig. 4. The microstructure after stamping on the first
(a) and second (b) technologies; the electron microscope structure after stamping on the first (c) and second (d) to technologies
As seen from the table 1, width plate of an - phase by the first technology in 3 times and interlayers of Pjj -phase in 8 times greater than by the second technology. This evidence different temperatures and cooling rates. By the first technology, at low-temperature is formed structure similar to the Widmanstat-ten. Such structure, in a number of sources, has a higher fatigue strength than the martensite-like by the second technology [9]. In materials of both blades with bimodal structure, the size lamellar (P + an) - a component was more size globular a: — phases.
Distribution of Alloying Elements
The research results obtained by X-ray microspectral analysis of the contents of alloying elements (aluminum, vanadium, titanium, iron) in separate phase components shows in table 2.
Table 1
The particles size of phase components of metal blades, fabricated on two technologies
Phase components Size of particles of phases after the first technology, ^m Size of particles of phases after the second technology, ^.m
ai 15 12
aii 2,2 0,7
ß„ 0,8 0,1
«ii+ß,, 22 18
Table 2
The contents of alloying elements in phase components of metal blades, fabricated on two technologies
Concentration after the first technology, Concentration after the second technology,
Phase components weight % weight %
Al V Al V
ai aii 7,30 1,35 6,97 2,04
6,01 3,60 5,68 4,21
ßii 4,14 9,64 4,61 8,28
As seen from the table for both technologies, the secondary plates of a - phase have a chemical composition that is comparable to the average composition of the alloy Ti-6AL-4V. The globular primary a: - phase, compared with the plate a - phase contains
less than 2,2% vanadium and more than 1,3% aluminum. Thin layers of secondary P: - phase have been
enriched by vanadium (P-stabilizer). In the material by the first technology P - phase contains vanadium more at 1,3% and aluminum less at 0,5%, than the second technology.
Such distribution alloying elements provides relative softness of layers P - phase and higher perfor-
mance as compared with the second technology, in which inverse ratio of alloying elements [10]. On the contrary, globalizes primary a: - phases in blades material fabricated on the first technology, in comparison with the second, contained a vanadium less on 0,6 % and aluminum more on 1,3 % (weight).
The fig. 5 shows a typical distribution of the alloying elements with step 0,5 microns. It is seen, that is most non-uniform in a material the vanadium is distributed, being focused in p - phase. Its concentration changes from 1 up to 20 %. Aluminum is distributed more similarly, its concentration changes from 3 up to 7 %.
0 20 40 É0 80 100
Disturb mrftnl
Fig. 5. The microstructure and distribution of alloying elements in structural components of material blades from alloy VT6.
Microhardness of phase components
In Table 3 are shown the data of statistical treatment of results of measurements microhardness of separate phase components. It is seen that the microhardness of the material by the second technology is higher than the first, which confirms the presence of internal stresses. It is necessary noted that the first technology makes less the difference between the hardness of globular and lamellar structure.
The blades material fabricated on both technologies had a bimodal structure. Probably, the strength balance aI- and (an + PI) — structural fashions can guarantee high serviceability of a material under loa-ding. Apparently from table 3, in blades materials fabricated on the first technology, in comparison with the second, the smaller difference in hardness between globular and lamellar structures, which made 551 and 644 MPa, accordingly has been achieved.
On fig. 6 correlation dependences of results of measurements of microhardness on a chemical compound of separate phases (ap an, Pn) in a condition of delivery and in a martensite phase after hard-enings from different temperatures are submitted. It is seen that the microhardness of plates an-phase and the globules of a:-phase increases with them a and b stabilizing elements. It should be noted that the microhardness increases with growth of a-stabi-lizer, probably due to the formation a1 - martensite and decreases with increasing amount of p stabilizing elements, leading to the formation a11 - martensite. This agrees with what was seen earlier in the material by the first technology in comparison with the second technology is formed p-phase with a large content of p-stabilizing elements. This agrees with the figure, such a distribution of vanadium, gives less microhardness and greater softness of layers p -phase.
Table 3
Microhardness of phase components of blades metal fabricated on two technologies
Phase components Microhardness of particles of phases after the first technology, MPa Microhardness of particles of phases after the second technology, MPa
ai 3550 3664
aii 3830 4091
Pii 2584 3000
aii +pii 2999 3020
Fig. 6. Dependence of results of a rating of microhardness on a chemical compound of separate phase components (ap an, p martensite)
alloy VT6
Conclusion
As results of research have shown, blades materials of steam turbines from alloy VT6 fabricated by final stamping under two technological circuits had bimodal structure, in which the share lamellar (an + Pn)-structures (50—70 %) prevailed of a share globular aj- structures (30—50 %). The blades material fabricated on the first technology, in comparison with the second, possessed wider layers "soft" p - the phases, enriched same it b-stabilizers. At the same time, this material contained higher concentration of aluminum in primary globalizes a:- phases, that pro-vided strength balance (close microhardness) structural components.
It is known, that at cyclic loadings in regular intervals distributed soft faltering layers p - phases, transiting on the contour of rather solid secondary a - phases and of strength balance globalizes a:-phases, complicating premature localization of
plastic deformation and origin of a crack in separate phases. And at the stage of distribution, the crack is more difficult to increase the length in (an + an)-lamellar structure as it is always braked by soft layers of bjj- phase and is compelled to change the trajectory, bending around globalized particles a:- phases. Thus operational properties of product material increase [9—11].
Feature of effect of exposure steam drops loadings is not only recurrence and cyclicity of the enclosed stress, but also its dynamism. Through of short duration of influence, internal stresses have insufficient time to be redistributed, there is localization of the big loadings in small microvolumes, in separate structural and phase components of material. The suc-
cessful combination of structure, chemical compound and properties, abilities to resist the microshock influence of these components, determines durability of products.
Researched alloy VT6 concerning martensite class, in stable condition contains 10—18 % p- phases, which at sharp cooling turns in a1- or a11- martensite. However, p ^ a11- transformation can take place and at room temperature. It is known, that plastic deformation accelerates decomposition enriched b- stabilizers of b- solid solution with education a11 - phases, as a result of a high level of internal stresses [12]. The subsequent ageing at temperatures 450—500 0C, results in its decomposition and education a1 + (P) - phases [13]. Presence of phase transformation a11 ^ a1 + (P) results in significant strengthening of an alloy.
It is possible to believe, that in result of effect blows by pair, in soft enriched of vanadium, wide, regular intervals distributed p- layers there is an accumulation of defects of crystal structure and internal stresses. Transition deformed p- phases in a non-equilibrum condition, causes phase p ^ a11 — transformation, accompanying with local frictional [14], and as consequence, — the subsequent ageing strengthening of microvolumes of alloy in result a11 ^ a1 + (p) — transformations. According to generalized kinetic diagram shown in fig. 3, the more layers of p- phase contain quantity isomorphic p- stabilizing elements, those at lower temperatures and for a longer time will be p ^ a11- takes transformation (more completely). Thus, the most important factor is strengthening of borders between solid particles of a- phases which will be increase terms of operation of a finished product, due to increase the resistance to steam drop impact.
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СВЕДЕНИЯ ОБ АВТОРАХ/AUTHORS
KRYLOV Nikolai A. — Peter the Great St. Petersburg Polytechnic University. 29 Politechnicheskaya St., St. Petersburg, 195251, Russia. E-mail: [email protected]
КРылОВ Николай Александрович — кандидат технических наук ведущий научный сотрудник Санкт-Петербургского политехнического университета Петра Великого. 195251, Россия, г. Санкт-Петербург, Политехническая ул., 29. E-mail: [email protected]
SKOTNIKOVA Margarita A. - Peter the Great St. Petersburg Polytechnic University. 29 Politechnicheskaya St., St. Petersburg, 195251, Russia. E-mail: [email protected]
СКОТНИКОВА Маргарита Александровна — доктор технических наук заведующая кафедрой Санкт-Петербургского политехнического университета Петра Великого. 195251, Россия, г. Санкт-Петербург, Политехническая ул., 29. E-mail: [email protected]
TSVETKOVA Galina V. - Peter the Great St. Petersburg Polytechnic University. 29 Politechnicheskaya St., St. Petersburg, 195251, Russia. E-mail: [email protected]
цВЕТКОВА Галина Викторовна — кандидат технических наук доцент Санкт-Петербургского политехнического университета Петра Великого. 195251, Россия, г. Санкт-Петербург, Политехническая ул., 29. E-mail: [email protected]
IVANOVA GalinaV. — Peter the Great St. Petersburg Polytechnic University. 29 Politechnicheskaya St., St. Petersburg, 195251, Russia. E-mail: [email protected]
ИВАНОВА Галина Валерьевна — заведующая лабораторией Санкт-Петербургского политехнического университета Петра Великого. 195251, Россия, г. Санкт-Петербург, Политехническая ул., 29. E-mail: [email protected]
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