Crack evaluation of the fourth stage blade in a low-pressure steam turbine.pdf

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Crack evaluation of the fourth stage blade in a low-pressure steam turbine.pdf

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Crack evaluation of the fourth stage blade in a low-pressure steam turbine

Engineering Failure Analysis 18 (2011) 907–913Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier .com/locate /engfai lanalCrack evaluation of the fourth stage blade in a low-pressure steam turbine Hyojin Kim ? Dept. of Aeronautical Engineering, Korea Aviation Polytechnic College, 438 Igeum-dong, Sacheon, Gyeongnam 664-708, Republic of Korea a r t i c l e i n f oArticle history: Received 10 June 2010 Received in revised form 10 October 2010 Accepted 28 November 2010 Available online 15 December 2010 Keywords: Blade Low-pressure steam turbine Replication Corrosion pit Corrosion fatigue1350-6307/$ - see front matter 2010 Elsevier Ltd doi:10.1016/j.engfailanal.2010.11.004 ? Tel.: +82 55 830 3442; fax: +82 55 830 3440. E-mail address: hjkimprof@a b s t r a c t A crack of the fourth stage blade in a low-pressure turbine of a 500 MW steam turbine is studied. From non-destructive inspection, the crack was found at the leading edge vane of the fourth stage blade. Material composition analysis, hardness measurement and microstructure analysis were performed to study the cause and process of the cracked blade. On further examination using the replication of the cracked surface for the blade vane, the crack was induced by corrosion pits. It is assumed that the causes of the corrosion pits are the induction of seawater from the condenser tube that has small leakage, and improper water treatment. It is shown that the corrosion pits acted as stress concentration site and facilitated crack initiation under cyclic loading experienced during normal opera- tion of the blade. From the fracture surface of the blade, it is concluded that the crack is initiated from the corrosion pits at the leading edge of the blade vane, and propagated inside by fatigue due to the vibration of the blade. 2010 Elsevier Ltd. All rights reserved.1. Introduction Blades of a steam turbine are critical components in power plants that convert the available