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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1. Introduction The study of degradation processes during operation is a scientific basis for improving existing materials and developing new ones, which is necessary to improve the design characteristics and ensure the necessary level of safety of any technical objects as it described by Botvina (2008), Smirnov et al. (2003), McEvily (2002), Kablov (2015). Accordingly, the study of changes in structural state of the metal of long maintained thermally-loaded sections of gas turbine units (GTU) is an actual scientific and applied problem, which is required for optimizing the composition and technological processes of obtaining heat-resistant alloy, and to assess residual life and identify the causes of accidents of an equipment made of this alloy as shown by Trzeszczy ń ski and Stanek (2013), Ray et al. (2000), Viswanathan (1989), Tarasenko et al. (2008). The most intense operational effects are experienced by parts and elements of the hot path of gas turbines, which are subjected to a wide range of different types of loads (thermal, vibration, bending, corrosion, erosion, etc.). The serviceability of hot path parts mainly determines the reliability of power plants which failures are particularly dangerous for regions with a cold climate. Meanwhile, the operating practice shows a very unreliable work of hot parts of turbines in the North. The high level of loads and significant amplitudes of their fluctuations due to changes in the parameters of the outdoor air (upon this it is often necessary to increase the temperature of a working body) impose particularly hard requirements for the materials of resource-determining parts of gas turbine units operating in the cryolithozone. Non-stationary modes of operation increase the load on these parts and accelerate irreversible structural, phase and physical-chemical changes in metal leading to their destruction. Breakdowns can be caused not only by design and technological reasons, but also by errors in operation (for example, over temperature). It should be noted that the influence of climatic operating conditions, including outdoor air temperature, on the operation of gas turbine installations, has not been practically studied. Meanwhile, as noted above, the modern concepts for the development of heat-resistant alloy provide for an adapted optimization of their compositions and production technologies taking into account the operating conditions. These materials must have a high resistance to the development of degradation processes, that is, they must have a high structural and phase stability. In this regard, it is important to know how during long-term operation at high temperatures, heat-resistant alloys are softened and destroyed. The purpose of this work is to study the high-temperature degradation of metal and the nature of operational destruction of the hot path parts (heat insert, working blades) of gas turbine units operating in the North. 2. Objects, research methods and the equipment used In this paper, cases of destruction of a sheet article (heat insert) of the combustion chamber and two rotor blades of stationary gas turbine installations operating in Yakutia are investigated. All objects were collapsed long before the exhaust of assigned resource, which is several tens of thousands of hours. Fragments of the heat insert (Fig. 1, a) have undergone macrodeformation of various degrees, their surface shows signs of exposure to temperatures exceeding the maximum permissible for this alloy: typical dark areas without metallic luster ("overheating spots"). In the root of the working blade #1 cracks appeared in less than 10,000 hours of operation in the area of the ridge that passes into the root (Fig. 1, b). The feather of the rotor blade #2 collapsed after ≈ 10,000 hours of operation; on its surface there are traces of blows, apparently made by fragments of the neighboring blade (Fig. 1, c). “Spectroport-F” and “Foundry-master UVR” spectrometers were used to determine the chemical composition of metal samples of the studied objects. The stereoscopic microscope “Stemi 2000C” was used for shooting macro fractures. The microstructure was analyzed using the “Neophot-32” and “Axio Observer D1m” metallographic microscopes, JEOL JSM-6480LV electron-scan microscope, and JEM-200CX † transmission microscope. Hardness by Brinell measurements were performed with “Heckert” hardness tester; the microhardness values were measured using PMT-3M microhardness tester.

1 Metal analysis on a transmission electron microscope was performed at the Center for electron microscopy of the Institute of Metal Physics of Ural department RAS, Ekaterinburg.