Failure Analysis on PWSCC of Alloy 600 Plugs for SG Tube in Korea

M. H. Song, H. S. Shin, S. C. KangKorea Institute of Nuclear Safety

15151515----18 October 2007 18 October 2007 18 October 2007 18 October 2007

Shanghai, ChinaShanghai, ChinaShanghai, ChinaShanghai, China

2222ndndndnd International Symposium on International Symposium on International Symposium on International Symposium on PLiMPLiMPLiMPLiM

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Contents

1.1. IntroductionIntroduction

2.2. Material and Experimental ProceduresMaterial and Experimental Procedures

3.3. Results and DiscussionResults and Discussion

4.4. SummarySummary

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1. Introduction�Boron deposits were detected on the bottom surface of the tube-sheet of steam generator (S/G). �To investigate the cause of cracks, micro-structural and fracto-graphic examination were performed� by optical microscope (OM) and scanning electron

microscope (SEM)�An elasto-plastic analysis using finite element techniques was also conducted.� to estimate the residual stress applied to failed plugs

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1. Introduction�Boron deposits were detected on the bottom surface of the tube-sheet of steam generator (S/G). � boron deposit on one of the plugs installed in the hot leg of

steam generator to prevent the flow of media through the defective tube

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2. Material� Material and geometry of tube plug

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� Nondestructive Examination was conducted on the plug using a penetrating test (PT)

� Micro-structural and fracto-graphic examination was also performed� The residual stress analyses were performed using ANSYS 10.0.

2. Experimental Procedures

(a) Composed geometry (b) Finite Element Model

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3.1 Penetrating Test

3. Results and Discussion

Cracks of S/G plugs identified by penetrating test

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3.2 Micro-structural examination

3. Results and Discussion

� In the failed plugs, the distribution of Cr-rich carbides is not continuous but isolated, and their size is smaller than that of a sound plug

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3.3 Fractography

3. Results and Discussion

(a) Profile of a crack (b) Fracture surface(IG)

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3.4 Residual Stress Analysis(1)

3. Results and Discussion

Thermo-physical Properties of plug material

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3.4 Residual Stress Analysis(2)

3. Results and Discussion

Thermo-mechanical Properties of plug material

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3.4 Residual Stress Analysis(3)

3. Results and Discussion

� Temperature distribution at 1021.5 seconds for pass 2 welding process in thermal finite element analysis

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3.4 Residual Stress Analysis(4)

3. Results and Discussion

� Temperature variation of Node C during entire analysis process for thermal finite element analysis of plug weld

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3.4 Residual Stress Analysis(5)

3. Results and Discussion

� Hoop stress distribution (a) after weld pass 1, (b) after welding pass 2, (c) at normal operation in S/G plug

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3.4 Residual Stress Analysis(6)

3. Results and Discussion

� Axial and hoop stresses along the (a) inner path, (b) middle path, and (c) outer path in the plug

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4. Summary�� Cracks were typical interCracks were typical inter--granular cracking granular cracking �� Residual stress beyond the yield stress is estimated to Residual stress beyond the yield stress is estimated to be applied in the failed plugs. be applied in the failed plugs.

�� The size of CrThe size of Cr--rich carbides of failed plugs is smaller rich carbides of failed plugs is smaller than that of sound plugs than that of sound plugs

�� Grain boundary coverage (GBC) by the CrGrain boundary coverage (GBC) by the Cr--rich rich carbides in the failed plugs is less than that of the carbides in the failed plugs is less than that of the sound plugs. sound plugs. � It was concluded that poor carbide precipitation and high

residual stress around the plugs caused the failure.