Crack evaluation of the fourth stage blade in a low-pressure steam turbine
Crack evaluation of the fourth stage blade in a low-pressure steam turbine Hyojin Kim ?
Dept.of Aeronautical Engineering,Korea Aviation Polytechnic College,438Igeum-dong,Sacheon,Gyeongnam 664-708,Republic of Korea
a r t i c l e i n f o Article history:Received 10June 2010Received in revised form 10October 2010Accepted 28November 2010Available online 15December 2010Keywords:Blade Low-pressure steam turbine Replication Corrosion pit Corrosion fatigue
a b s t r a c t
A crack of the fourth stage blade in a low-pressure turbine of a 500MW 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.
ó2010Elsevier Ltd.All rights reserved.1.Introduction
Blades of a steam turbine are critical components in power plants that convert the available energy in steam into mechan-ical energy [1].If the blades of turbine fail,power plants will shut down.This case can cause long time current failure and economic loss.So,it is necessary to settle the failure analysis of the blades in order to increase the reliability of turbine sys-tem.The low-pressure turbine blades,designed to extract the ?nal remnants of energy from the passing steam ?ow,are rel-atively large scale rotating airfoils due to the signi?cant centrifugal forces experienced during normal operation [2].Previous researches have shown that the low-pressure blades of a steam turbine are generally found to be more susceptible to failure than intermediate-pressure and high-pressure blades [3].The most common failure mechanisms,which occur within the low-pressure blade,are normally those associated with either sympathetic or forced vibrations,those caused by transient operating conditions,and those that occur as the result of the transported and accumulated corrosive ions in working ?uid
[4–7].Failures occur from time to time in power plants,as they do in other engineering structures.However,they are not always examined closely to identify the causes.Similarly,when the failures are observed from time to time,repairs are often made without any careful analysis being undertaken.But to correct a blade problem requires more than positive identi?ca-tion of the mechanisms involved [5].
The turbine section of low-pressure turbine of a 500MW steam turbine consists of six stages of turbine stator and rotor.The 4–6stages of the low-pressure turbine are operated in the wet steam region.On the routine inspection after 10years service,a crack was observed at the vane of the fourth stage rotor blade in the low-pressure turbine.The cover above the cracked blade was previously replaced since cracks had occurred in that.In this paper,it has been studied to see the cause and process of the crack at the blade vane.Material composition analysis,hardness measurement and microstructure 1350-6307/$-see front matter ó2010Elsevier Ltd.All rights reserved.
doi:10.1016/j.engfailanal.2010.11.004
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908H.Kim/Engineering Failure Analysis18(2011)907–913
analysis were performed for this study.The ultimate goal of this study is to improve the safety of the turbine and the main-tenance practice.
2.Experimental procedure
The cracked blade is examined.Experimental procedure consists of macroscopic inspection,material veri?cation,hard-ness measurement,microscopic examination,and metallographic analysis.To assess a crack,the cracked area of the blade vane was examined in detail.The area was prepared by techniques of a?nal polish with0.5l m,and was etched in a solution of Kalling’s acid to reveal the microstructure,and replicated with acetate?lm[8].The replicas were taken from the cracked area.These replicas were used to assess the crack morphology and its relationship to the microstructure.The replicas,which are the microstructure of the area,were examined by an optical microscope and a scanning electron microscopy(SEM).
The fracture surface of the blade was examined to analyze the causes and process of the cracked blade.For the fracture surface,the cracked part was cut precisely from the blade.Then the top and bottom parts of the crack,which were cooled with liquid nitrogen,were broken.The fracture surface of the blade was examined by a visual stereoscopy and a SEM after the surface was cleaned ultrasonically.
3.Results and discussion
3.1.Visual examination
The cracked blade is presented in Fig.1.The blade was cracked at the leading edge of approximately1/3length of the blade vane.The crack size reaches about26mm.The crack seems to penetrate the leading edge of the blade vane.The crack path is observed to be perpendicular to the direction of maximum tensile stress by large centrifugal force acting on the blade.
3.2.Material veri?cation
Using inductively coupled plasma spectrometer(ICP),the chemical compositions of the cracked blade were examined,as shown in Table1.The chemical analysis indicates that the blade material is12Cr alloy steel.It is con?rmed that the design material was used in the cracked blade.The hardness of the cracked blade was measured in order to verify the mechanical
Fig.1.View of the cracked blade in a low-pressure steam turbine.
H.Kim/Engineering Failure Analysis18(2011)907–913909
Table1
Chemical compositions of the cracked blade(wt.%).
C Cr Ni Mo Cu Mn V
Speci?cations0.10–0.1711.25–13.000.60max.0.30max.0.5max.0.25–0.80Info.only Measured0.1311.4390.310.1080.4820.4790.067
Table2
Results of hardness measurement for the cracked blade.
cracked region and is no evidence of any microstructural abnormality.The typical morphology of the crack found at the fourth blade is shown in Fig.4.The crack is very straight with little evidence of crack branching.It can be judged primarily that the crack has no feature of the stress corrosion cracking.On the concave surface of the blade vane,the crack of about 2mm away from the leading edge is divided into two cracks.A corrosion pit at the point of the crack separation is recog-nized.One of the cracks stopped growing at some point,and the other crack grew into a main crack.The other corrosion pit is identi?ed above approximately1.5mm from the crack.A crack in the second corrosion pit is found as shown in Fig.5b.
Fig.4.Crack micrograph on the polished surface of the cracked blade.
of the crack area on the concave surface of the blade vane:(a)view of the cracked blade vane,(b)a corrosion at the point of crack separation,(d)magni?ed crack.
H.Kim/Engineering Failure Analysis18(2011)907–913911
X-ray electron dispersive spectroscopy(EDS)analysis is performed to detect the components of nonmetallic inclusions and corrosion media inside the corrosion pits.The internal components of the corrosion pits are identi?ed as the oxide-scale as shown in Fig.6.The deposit samples collected from the power plant is detected with the same oxide-scale as the internal components of the corrosion pits.The source of the corrosive media was not found because the surface of the blade was cleaned in order to perform non-destructive inspection for periodic overhaul.Generally,if seawater is used as condensing water in power plants,Na+and Clàare known as the corrosive media of turbine component.So 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 treat-ment.Corrosion can affect blade structural integrity since fatigue cracks can nucleate from the corrosion pits and grow accel-erated rate[9].
3.4.Analysis of the fracture surface
The fracture surface of the blade was examined to analyze the causes and process of the cracked blade.Fig.7shows the fracture surface of the cracked blade.Beach marks are observed,which proves that the failure mechanism of the blade is fatigue.The beach marks often bow out in the direction of crack propagation and generally tend to align in the perpendicular to the principal crack-propagation direction[10].Following the direction of the beach marks,I could?nd the origin of the crack.It is con?rmed that the crack appeared to have initiated at the leading edge of the blade vane.The crack has propa-gated over a distance of about26mm by fatigue due to the vibration of the blade.Figs.8and9show the SEM fractographs of the fracture surface.Two corrosion pits are found at the leading edge.The corrosion pits,once formed,act as stress concen-tration sites and facilitate crack initiation under cyclic and sustained loading[11].It is con?rmed that the crack was started at the corrosion pits1and2.The?rst crack is believed to begin at the pit1because it had the thicker oxide-scale than that of the pit2.Based on the fact,cracks are assumed to have started at the pits1and2respectively.They were combined at a certain location after they had been grown respectively.The corrosion pit at the point of the crack separation shown in Fig.5is consistent with the pit1in Fig.8.The SEM fractographs of the crack tip is presented in Fig.9.The right fracture sur-face has a smooth appearance indicative of the progressive fatigue crack.The left surface is rough,which indicates the instant fracture region by forced break.
From the results of the analysis above,the direct cause of the crack initiation in the fourth stage blade of low-pressure
7.Fracture surface of the cracked blade:(a)upper side,(b)lower side.
the corrosion pit alone cannot cause the crack in the same operating conditions.It is assumed that the local stress concen-tration and residual stress had been acting on the corrosion pits by external forces of disassembling and assembling process of the cracked blade for turbine repair such as cover replacement.The crack can be occurred by these factors.In addition to
these factors,it is considered that the crack occurred because the level of the material property of the cracked blade,statis-tically,is low though the material speci?cations of the blade were satis?ed.So,the blade was relatively vulnerable to corrosion fatigue as long-term use of the blade.
4.Conclusions
The study was conducted to see the cause and process of the crack at the vane of the fourth stage blade in the low-pressure turbine.Corrosion pits at the leading edge of the blade are found by the microstructure and EDS analysis of the blade.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.The corrosion pits acted as stress concentration site and facilitated crack initiation under cyclic loading experienced during normal operation of the blade.It is concluded that the crack is initiated from the corrosion pits at the leading edge of the blade,and propagated inside by fatigue due to the vibration of the blade.On the basis of the results,corrosion fatigue crack is likely to occur in blades that corrosion pits exist.So,it is recommended that periodic inspection is needed for blades with corrosion pits.
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