Research Papers: Operations, Applications & Components

Long-Term Stability of Residual Stress Improvement by Water Jet Peening Considering Working Processes

[+] Author and Article Information
Tadafumi Hashimoto

e-mail: hashimoto@hashimoto-tekko.com

Yusuke Osawa

e-mail: yusuke_osawa@mapse.eng.osaka-u.ac.jp

Shinsuke Itoh

e-mail: itoh@mapse.eng.osaka-u.ac.jp

Masahito Mochizuki

e-mail: nishimoto@mapse.eng.osaka-u.ac.jp

Kazutoshi Nishimoto

e-mail: mmochi@mapse.eng.osaka-u.ac.jp
Graduate School of Engineering,
Osaka University,
Japan, 2-1 Yamadaoka, Suita,
Osaka 565-0871, Japan

1Present address: Hashimoto Iron Works CO., LTD., 7-15 Chikkouhamaderanishi, Nishi, Sakai, Osaka 592-8352, Japan.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the Journal of Pressure Vessel Technology. Manuscript received October 18, 2011; final manuscript received March 13, 2012; published online May 21, 2013. Assoc. Editor: Allen C. Smith.

J. Pressure Vessel Technol 135(3), 031601 (May 21, 2013) (8 pages) Paper No: PVT-11-1188; doi: 10.1115/1.4023417 History: Received October 18, 2011; Revised March 13, 2012

To prevent primary water stress corrosion cracking (PWSCC), water jet peening (WJP) has been used on the welds of Ni-based alloys in pressurized water reactors (PWRs). Before WJP, the welds are machined and buffed in order to conduct a penetrant test (PT) to verify the weld qualities to access, and microstructure evolution takes place in the target area due to the severe plastic deformation. The compressive residual stresses induced by WJP might be unstable under elevated temperatures because of the high dislocation density in the compressive stress layer. Therefore, the stability of the compressive residual stresses caused by WJP was investigated during long-term operation by considering the microstructure evolution due to the working processes. The following conclusions were made: The compressive residual stresses were slightly relaxed in the surface layers of the thermally aged specimens. There were no differences in the magnitude of the relaxation based on temperature or time. The compressive residual stresses induced by WJP were confirmed to remain stable under elevated temperatures. The stress relaxation at the surface followed the Johnson–Mehl equation, which states that stress relaxation can occur due to the recovery of severe plastic strain, since the estimated activation energy agrees very well with the self-diffusion energy for Ni. By utilizing the additivity rule, it was indicated that stress relaxation due to recovery is completed during the startup process. It was proposed that the long-term stability of WJP under elevated temperatures must be assessed based on compressive stresses with respect to the yield stress. Thermal elastic–plastic creep analysis was performed to predict the effect of creep strain. After 100 yr of simulated continuous operation at 80% capacity, there was little change in the WJP compressive stresses under an actual operating temperature of 623 K. Therefore, the long-term stability of WJP during actual operation was analytically predicted.

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Fig. 1

Schematics of C-shaped ring specimen proposed to simulate weld residual stress

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Fig. 2

Mechanism of residual stress improvement by water jet peening

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Fig. 3

Geometry of diffraction system for stress measurement

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Fig. 4

FE mesh and boundary conditions

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Fig. 5

Material properties used in thermal elastic–plastic creep FE analysis

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Fig. 6

2θ-sin2Ψ diagram under applied stress in Ni{220}

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Fig. 7

Changes in slope M with applied stress in Ni{220}

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Fig. 8

Change of intercept 2θΨ = 0 with applied stress in Ni{220}

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Fig. 9

X-ray elastic constants calculated using the Kröner model with single-crystal constants and from experiment

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Fig. 10

Residual stress improvement by WJP

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Fig. 11

Comparison of residual stress distributions with correction

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Fig. 12

Thermal stability of compressive residual stress with surface depth

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Fig. 13

Relationship between stress relaxation and microstructure evolution during thermal aging

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Fig. 14

Residual stress changes caused by thermal aging in liquid tin bath

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Fig. 15

Johnson–Mehl plots for stress relaxation under each aging condition

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Fig. 16

Accession of activation energy for stress relaxation

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Fig. 17

Prediction of stress relaxation during actual plant startup

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Fig. 18

Creep strain versus time in accelerated conditions for Alloy 600. (a) at 773 K and (b) at 723 K.

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Fig. 19

Temperature dependence of steady creep rate for Alloy 600

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Fig. 20

Stress dependence of steady creep rate for Alloy 600

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Fig. 21

Change of residual stress distribution by working processes

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Fig. 22

Evaluation of long-term stability over 100 yr at a temperature of 623 K




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