Abstract
Superalloys are a group of alloys based on nickel, iron, or cobalt, which are used to operate at high temperatures (T> 540°C) and in situations involving very high stresses like in gas turbines, particularly in the manufacture of blades, nozzles, combustors, and discs. Besides keeping its high resistance to temperatures which may approach 85% of their melting temperature, these materials have excellent corrosion resistance and oxidation. However, after long service, these components undergo mechanical and microstructural degradation; the latter is considered a major cause for replacement of the main components of gas turbines. After certain operating time, these components are very expensive to replace, so the microstructural analysis is an important tool to determine the mode of microstructure degradation, residual lifetime estimation, and operating temperature and most important to determine the method of rehabilitation for extending its life. Microstructural analysis can avoid catastrophic failures and optimize the operating mode of the turbine. A case study is presented in this paper.
Highlights
Gas turbines blades are manufactured mainly with nickelbased and cobalt-based superalloys
Superalloys are a group of alloys based on nickel, iron, or cobalt, which are used to operate at high temperatures (T > 540∘C) and in situations involving very high stresses like in gas turbines, in the manufacture of blades, nozzles, combustors, and discs
Besides keeping its high resistance to temperatures which may approach 85% of their melting temperature, these materials have excellent corrosion resistance and oxidation. After long service, these components undergo mechanical and microstructural degradation; the latter is considered a major cause for replacement of the main components of gas turbines. These components are very expensive to replace, so the microstructural analysis is an important tool to determine the mode of microstructure degradation, residual lifetime estimation, and operating temperature and most important to determine the method of rehabilitation for extending its life
Summary
Gas turbines blades are manufactured mainly with nickelbased and cobalt-based superalloys. The major phases present in these superalloys are [3] as follows: gamma matrix (γ), Ni-based austenitic phase (FCC), usually containing a high percentage of solid solution elements such as Co, Cr, Mo, and W; gamma prime (γ), which is Ni3(Al, Ti) based intermetallic phase; Carbides, generally types M6C and M23C6 which tend to precipitate into grain boundaries; topologically closed packed (TCP) type phases, such as σ, μ, and Laves, which precipitate after prolonged high temperature service. These alloys can be classified into solid-solution hardened alloys and precipitation hardened alloys or gamma prime (γ) alloys. Such superalloys consist of an austenitic matrix (fcc) and a variety of precipitated phases such as primary carbides (M3C2, M7C3, and MC) and coarse carbides (M23C6) and GCP types phases (geometrically compact phases) such as γ and η (Ni3Al) and TCP (topologically close packed) type phases (σ, μ, R, or L) (Cr, Mo)x(Ni, Co)y [5]
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