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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. [ABSTRACT FROM AUTHOR]

The flow field in a steam turbine main stop valve bypass valve (MSVBV) has been investigated by means of CFD simulations. Because the entire flow to the turbine during start ups is carried by the MSVBV it is subject to serious solid particle erosion problems and requires frequent replacement to avoid the catastrophic damage which can occurred when the MSVBV skirt eroded through causing large pieces of metal to be carried directly into the turbine. For some of the most important geometric parameters of the MSVBV, design recommendation have been made. [ABSTRACT FROM AUTHOR]

This study presents the failure analysis of the 37.5 MW gas turbine third stage buckets made of Udimet 500 (U-500) superalloy. The buckets experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely: the commensurate loss of a load bearing cross-sectional area of the bucket tip shroud by the conversion of metal to the corrosion product (scale) and related increase in stress; the influence of the tip shroud microstructure, e.g. a presence of equiaxed and columnar grains, their distribution and orientation; evidence of the transgranular fracture initiation and intergranular creep mechanism fracture propagation. Finally, the most probable mechanism for bucket damage, conclusions and recommendations to prevent failures of the buckets are presented. [ABSTRACT FROM AUTHOR]