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

• Autorzy: Ćwiek J.
• Języki publikacji: EN

Abstrakty

EN

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.

Słowa kluczowe

EN

cracking; structure degradation; Cr-Mo-V steels; steam turbine casing

• Wydawca: International OCSCO World Press
• Czasopismo: Journal of Achievements in Materials and Manufacturing Engineering
• Rocznik: 2011
• Tom: Vol. 49, nr 1
• Strony: 27--34
• Opis fizyczny: Bibliogr. 18 poz., rys., tab.

Twórcy

autor

Ćwiek J.

• Institute of Engineering Materials and Biomaterials, Silesian University of Technology, ul. Konarskiego 18a, 44-100 Gliwice, Poland

Bibliografia

• [1] S. Ghosh Chowdhury, N.K. Mukhopadhyay, G. Das, S.K. Das, D.K. Bhattacharya, Engineering Failure Analysis 4 (1998) 194-107.
• [2] R. Viswanathan, Damage mechanisms and life assessment of high temperature components, ASM International, Metals Park Ohio, USA, 1989.
• [3] W.M. Payten, T. Wei, K.U. Snowden, P. Bendeich, M. Lawa, D. Charman, Crack initiation and crack growth assessment of a high pressure steam chest, International Journal of Pressure Vessels and Piping 88/1 (2011) 34-44.
• [4] K.-S. Cheong, A.D. Karstensen, Integrity assessment of an embrittled steam turbine casing, International Journal of Pressure Vessels and Piping 86/4 (2009) 265-272.
• [5] M. Holzmann, L. Dlouhy, B. Vlach, J. Krumpos, Degradation of mechanical properties of Cr-Mo-V and Cr-Mo-V-W steam turbine rotors after long-term operation at elevated temperatures. Part I: tensile properties, intergranular fracture strength and impact tests, International Journal of Pressure Vessels and Piping 68/1 (1996) 99-111.
• [6] M. Holzmann, L. Dlouhy, B. Vlach, J. Krumpos, Degradation of mechanical properties of Cr-Mo-V and Cr-Mo-V-W steam turbine rotors after long-term operation at elevated temperatures. Part II: fracture toughness, correlation of fracture toughness with Charpy V-notch results, International Journal of Pressure Vessels and Piping 68/1 (1996) 113-120.
• [7] Metals Handbook, Vol. 11 Failure Analysis and Prevention, ASM International, Metals Park Ohio, USA, 1995.
• [8] L.A. Dobrzański, Metal engineering materials, WNT, Warsaw, 2004 (in Polish).
• [9] L.A. Dobrzański, M. Kowalski, J. Madejski, Methodology of the mechanical properties prediction for the metallurgical products from the engineering steels using the Artificial Intelligence methods, Journal of Materials and Processing Technology 164 (2004) 1500-1509.
• [10] A. Hernas, Creep resistance of steel and alloys, Silesian University of Technology, Gliwice, 1999 (in Polish).
• [11] Atlases - changes in microstructure of creep resistant steels due to long term use, Institute of Power Energy, Warsaw, 1996.
• [12] S. Mrowec, T. Weber, The modern heat resistant materials, WNT, Warsaw, 1982 (in Polish).
• [13] A. Zieliński, J. Dobrzański, G. Golański, Estimation of the residual life of L17HMF cast steel elements after long-term service, Journal of Achievements in Materials and Manufacturing Engineering 34/2 (2009) 137-144.
• [14] J. Dobrzański, A. Zieliński, H. Krztoń, Mechanical properties and structure of the Cr-Mo-V low-alloyed steel after longterm service in creep condition, Journal of Achievements in Materials and Manufacturing Engineering 18 (2006) 39-42.
• [15] D. Renowicz, A. Hernas, M. Cieśla, K. Mutwil, Degradation of the cast steel parts working in power plant pipelines, Journal of Achievements in Materials and Manufacturing Engineering 23/1 (2007) 219-222.
• [16] PN-89/H-83157 Cast steels for elevated temperature applications, Grades.
• [17] PN-EN ISO 6507-1 Metallic materials - Vickers hardness test, Part 1: Test method.
• [18] PN-EN ISO 148-1 Metallic materials - Charpy pendulum impact test, Part 1: Test method.