PSI - Issue 30

S.P. Yakovleva et al. / Procedia Structural Integrity 30 (2020) 201–208 Yakovleva S. P. et al. / Structural Integrity Procedia 00 (2020) 000–000

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Instability and decay of the strengthening γ '-phase during continuous operation at high temperatures, its transformation, the formation of recrystallized grains, as well as the grid of boundaries between small grains caused fluctuations in hardness by Brinell values at various sections of a heat insert in the range of 160-400 НВ . Significant differences in the hardness parameter are caused not only by the heterogeneity of the metal structure, but also by the formation of discontinuities. Figure 2, d illustrates the output of high-temperature damage to the level of occurrence of intergranular vuggs and microcracks, as well as the presence of internal porosity in the thickness of a sheet. Such signs are peculiar to the development of a creep failure which occurs due to the origin and subsequent union of pores in the inner layers (for example, fatigue damage is initiated in the near-surface zones). This equates to the state that the softening process of dilute alloys is controlled by a recrystallization mechanism. Thus, the material of the degraded heat insert has an insufficient resistance to the development of high-temperature degradation during prolonged dynamic contact with the gas environment of the combustion chamber of a gas turbine installation operating in extreme climatic conditions of the North (the main reason: with a small number of alloying elements added into the alloy, a small amount of unstable strengthening γ '-phase was formed under these operating conditions). Changes in the material structure also caused a distortion of a heat insert. 3.3. The destruction of the working blades of a gas turbine unit . The studied cast blades are made of the complex alloyed corrosion-resistant alloy CHS-70V, the chemical composition of which is shown in Table 2. To ensure phase stability and heat resistance at long exposures, such alloys are characterized by a higher (up to 12-16 %) chromium content as pointed by Reed (2008), Donachie et al. (2002). ≤ 1 Blade #1 is at the stage of development of thermal fatigue cracks that originated on the surface of depressions between the ridges of the root, that is, in the places of stress concentration (Fig. 1, b and Fig. 3). It can be seen (Fig. 3, a ) that initially there were single cracks which then grew and united by breaking the bridges between them. The root section of blades usually operates at relatively low temperatures which are not sufficient for significant structural changes running. However, as shown in Fig. 3 c, d , the root material is characterized by an uneven distribution of strengthening phases: there are accumulations of the γ '-phase, as well as areas depleted by it. There is also a noticeable decoration of grain boundaries with intermetallide and carbide particles (shown by arrows in Fig. 3, d ). The microhardness of the matrix, free from extractions, is ≈ 3600 MPa; intragrain areas with particles of the γ '- phase is about 4000 MPa; microhardness of the zone of intergrain boundaries is ≈ 4400 MPa. The initial microhardness of the tail part of the blades made of CHS-70V alloy is usually equal to about 4900 MPa inside the grain and up to 5100 at the grain boundaries. The decrease in the metal microhardness of the destructed blade and the uneven distribution of the hardening phase indicate its partial dissolution in the matrix and coagulation due to the accelerated development of diffusion processes under the influence of unexpectedly high temperatures. Obviously, the operation of the gas turbine unit could not avoid overload conditions, in particular due to the difficulties of functioning in the extremely cold region. During operation at elevated temperatures, the alloy was softened due to the dissolution of intermetallide particles. In addition, heat transfer loads cause more damage to the grain boundaries than to the internal grain volumes. It is known that the fatigue life of heat-resistant alloys is controlled by the transition from transcrystalline to intergranular destruction, when porosity is accumulated at grain boundaries weakened by coarse particles as shown by Skelton (1988), Getsov (2010). As can be seen in Fig. 3 c, d , the crack propagation in the blade root goes along the intergranular boundaries and in the adjacent zone with thermal-fatigue damages. If the regulated operating conditions are observed, the grain boundaries can maintain their strength for a long time. Thus, the structural changes of the blade metal indicate a deviation of the temperature conditions of operation. Since the initial micro fractures occurred in the surface layer of the blade after a short time of operation, it is obvious that the application of a strengthening coating would increase its serviceability. Table 2. Chemical composition of metal working blades of GTU (base - nickel) Material C Cr W Mo Co Al Nb Ti B Fe CHS-70V 0.10 14.4 5.6 3.5 10.6 3.5 0.25 4.4 0.35