Research Papers: Materials and Fabrication

Modeling and Verification of Creep Strain and Exhaustion in a Welded Steam Mixer

[+] Author and Article Information
Stefan Holmström

 VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finlandstefan.holmstrom@vtt.fi

Juhani Rantala, Anssi Laukkanen, Kari Kolari, Heikki Keinänen

 VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland

Olli Lehtinen

 Fortum Power and Heat, 00048 Fortum, Finland

J. Pressure Vessel Technol 131(6), 061405 (Oct 13, 2009) (5 pages) doi:10.1115/1.4000201 History: Received August 13, 2008; Revised June 10, 2009; Published October 13, 2009

Structures operating in the creep regime will consume their creep life at a greater rate in locations where the stress state is aggravated by triaxiality constraints. Many structures, such as the welded steam mixer studied here, also have multiple material zones differing in microstructure and material properties. The three-dimensional structure as such, in addition to interacting material zones, is a great challenge for finite element analysis (FEA), even to accurately pinpoint the critical locations where damage will be found. The studied steam mixer, made of 10CrMo 9-10 steel (P22), has after 100,000 h of service developed severe creep damage in several saddle point positions adjacent to nozzle welds. FE-simulation of long term behavior of this structure has been performed taking developing triaxiality constraints, material zones, and primary to tertiary creep regimes into account. The creep strain rate formulation is based on the logistic creep strain prediction model implemented to ABAQUS , including primary, secondary, and tertiary creep. The results are presented using a filtering technique utilizing the formulation of rigid plastic deformation for describing and quantifying the developing “creep exhaustion.”

Copyright © 2009 by American Society of Mechanical Engineers
Topics: Creep , Steam , Modeling , Stress
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Figure 1

(a) Upper nozzle of high pressure mixer and (b) Detail of 200 mm crack at nozzle weld (HAZ)

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Figure 2

Creep cavitation and crack formation in one of the upper nozzles

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Figure 3

ECCC short term 10CrMo 9–10 strain data (0.1 to fracture strain) presented in time-temperature parameter form with modeled LCSP predictions of longest and shortest test, and for the standard value stress 500°C/103 MPa, giving 1% at 100,000 h (marked as a star (24))

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Figure 4

Simulated creep curves at service pressure and temperature (first 200,000 h) for BM, HAZ (WSF=0.8), and WM2 as in Table 1. The dashed line represents the accumulated strain if using minimum creep rate only (BM).

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Figure 5

Creep exhaustion (Λ-values) after 100,000 h/525°C

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Figure 6

Creep strain (maximum principal) after 100,000 h/525°C, note location (inner surface BM)

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Figure 7

Stress (von Mises) after 100,000 h/525°C, note location (inner surface WM)

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Figure 8

Creep exhaustion after 1000 h at 600°C with overmatched weld metal (WM2). Compare with 525°C simulation in Fig. 5.

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Figure 9

Creep exhaustion after 422 h at 600°C with weak weld metal (WM3). Note the failing weld metal.




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