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Evaluation of microstructure and mechanical properties of a steam turbine casing after long-term service

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Author(s): J. Ćwiek

Journal: Journal of Achievements in Materials and Manufacturing Engineering
ISSN 1734-8412

Volume: 49;
Issue: 1;
Start page: 27;
Date: 2011;
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Keywords: Cracking | Structure degradation | Cr-Mo-V steels | Steam turbine casing

ABSTRACT
Purpose: of this paper is to reveal the microstructural changes in Cr-Mo and Cr-Mo-V cast steels steel exposed to long-term service at elevated temperatures. The paper presents results of research and failure analysis undertaken to determine failure causes of a steam turbine casing. After 130,000 hours of service the crack in a outer shell of the turbine casing was found.Design/methodology/approach: Following research were performed in order to determine causes of the casing failure: chemical analysis; microstructure examinations with the use of light microscope, scanning electron microscope (SEM); transmission electron microscopy (TEM); mechanical properties examinations using the Charpy impact test, and Vickers hardness test; fracture mode evaluation with SEM; the energy dispersive X-ray spectrometry (EDS).Findings: The cracking of the outer casing occurred due to various causes. The main cause was stress distribution and stress changes during service of the turbine. The microstructure of ferrite and bainite/perlite is more susceptible to cracking than tempered martensite. Carbides coagulation process occurs at ferrite grain boundaries which increased embrittlement. Big nonmetallic inclusions also contribute to brittleness of material.Research limitations/implications: The whole history of start-ups and shutdowns of the turbine during long term service has not been recorded. There was no possibility to take samples with fracture area. Thus, service conditions of investigated samples and material of cracking area were different.Practical implications: Useability of the method for assessing the current degradation based on analysis of carbides morphology was confirmed for Cr-Mo and Cr-Mo-V cast steels.Originality/value: Microstructure composed of ferrite and perlite/bainite is more liable for degradation processes, during long-term exploitation at elevated temperature, than microstructure of tempered martensite.
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Tangokurs Rapperswil-Jona