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Research Papers: Materials and Fabrication

High Temperature, Low Cycle Fatigue Characterization of P91 Weld and Heat Affected Zone Material

[+] Author and Article Information
T. P. Farragher

Mechanical Engineering,
College of Engineering and Informatics,
NUI Galway,
Galway, Ireland

S. Scully

ESB Energy International,
ESB Head Office,
27 FitzWilliam Street,
Dublin 2,
Dublin, Ireland

N. P. O'Dowd

Department of Mechanical, Aeronautical,
and Biomedical Engineering,
Materials and Surface Science Institute,
University of Limerick,
Limerick, Ireland

C. J. Hyde

Department of Mechanical, Materials
and Manufacturing Engineering,
University of Nottingham,
Nottingham NG7 2RD, UK.

S. B. Leen

Mechanical Engineering,
College of Engineering and Informatics,
NUI Galway,
Galway, Ireland

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received May 25, 2013; final manuscript received October 16, 2013; published online January 8, 2014. Assoc. Editor: Marina Ruggles-Wrenn.

J. Pressure Vessel Technol 136(2), 021403 (Jan 08, 2014) (10 pages) Paper No: PVT-13-1088; doi: 10.1115/1.4025943 History: Received May 25, 2013; Revised October 16, 2013

The high temperature low cycle fatigue behavior of P91 weld metal (WM) and weld joints (cross-weld) is presented. Strain-controlled tests have been carried out at 400 °C and 500 °C. The cyclic behavior of the weld material (WM) and cross-weld (CW) specimens are compared with previously published base material (BM) tests. The weld material is shown to give a significantly harder and stiffer stress–strain response than both the base material and the cross-weld material. The cross-weld tests exhibited a cyclic stress–strain response, which was similar to that of the base material. All specimen types exhibited cyclic softening but the degree of softening exhibited by the cross-weld specimens was lower than that of the base material and all-weld tests. Finite element models of the base metal, weld metal and cross-weld test specimens are developed and employed for identification of the cyclic viscoplasticity material parameters. Heat affected zone (HAZ) cracking was observed for the cross-weld tests.

Copyright © 2014 by ASME
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References

Figures

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Fig. 1

Photograph of weld, positioned within a section of the P91 header material

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Fig. 2

Test specimen geometry (all dimensions in mm)

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Fig. 3

Representation of specimen position within a welded section of the header showing (a) the all-weld specimen and (b) the cross-weld specimen positions (all dimensions in mm)

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Fig. 4

Measured first cycle (N = 1) WM stress–strain response at 400 °C

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Fig. 5

Measured first cycle (N = 1) CW stress–strain response at 400 °C

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Fig. 6

WM cyclic softening behavior at 400 °C

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Fig. 7

CW stress–strain response at N = 1 at a temperature of 500 °C for various strain ranges

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Fig. 8

CW cyclic softening behavior at 500 °C

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Fig. 9

Comparison between BM, WM, and CW stress–strain response (1st cycle) for various strain ranges at a constant strain rate of 0.033%/s for 400 °C (Figs. 9(a) and 9(b)) and 500 °C (Figs. 9(c) and 9(d))

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Fig. 10

Cyclic stress-plastic strain curves for BM, WM and CW tests at 400 °C at half-life

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Fig. 11

Cyclic stress-plastic strain curves for BM, WM and CW tests at 500 °C at half-life

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Fig. 12

Stress relaxation data for different specimen types, held at a maximum tensile strain of 0.5% for a period of 120 s

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Fig. 13

Optical micrograph of (a) primary crack (occurring in the HAZ) and (b) magnified view of primary crack along with secondary (surface) cracking for cross-weld specimen tested at 1% strain range at 500 °C

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Fig. 14

Hardness traverse across (post test) CW specimen

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Fig. 15

Micrographs taken at different locations across the CW specimen of Fig. 14, showing (a) WM microstructure, (b) BM microstructure, (c) WM—fine grain HAZ fusion boundary, and (d) coarse grain HAZ, intercritical HAZ and BM boundaries.

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Fig. 16

Rheological diagram of two-layer viscoplasticity model, adapted [24]

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Fig. 17

Representation of FE models showing (a) BM single-material model, (b) WM single-material model, and (c) multimaterial CW model

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Fig. 18

Identification of C and γ for WM at 400 °C and 500 °C

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Fig. 19

Comparison of half-life experimental and NLKH WM responses at 500 °C

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Fig. 20

Comparison of half-life experimental and NLKH CW responses at 500 °C

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Fig. 21

Comparison of predicted isotropic and measured softening behavior for WM and BM at 500 °C

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Fig. 22

Comparison of predicted (combined isotropic-NLKH) and measured CW response at 500 °C for N = 1

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Fig. 23

Comparison of softening behavior predicted by the two-layer viscoplasticity CW model and measured softening behavior for the CW test at 500 °C at Δε = 0.8%

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Fig. 24

Stress relaxation behavior of CW test, compared against the stress relaxation response of the CW FE model at 500 °C

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Fig. 25

Validation cyclic two-layer viscoplasticity model for WM at N = 1 and at the half-life, at a temperature of 500 °C and for Δε = 0.8%

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Fig. 26

Comparison of FE-predicted (cyclic viscoplasticity) and measured CW response at N = 1 at 500 °C for two strain ranges

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Fig. 27

Comparison of FE-predicted (cyclic viscoplasticity) and measured CW response at half-life (softened) 500 °C for Δε = 0.8%

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Fig. 28

Comparison between predicted HAZ stress–strain response and measured BM, WM, and CW responses at N = 1

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