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

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.