Materials and Fabrication

Evaluation of Creep Strength Reduction Factors for Welded Joints of Modified 9Cr-1Mo Steel

[+] Author and Article Information
Masaaki Tabuchi

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japantabuchi.masaaki@nims.go.jp

Yukio Takahashi

Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japanyukio@criepi.denken.or.jp

J. Pressure Vessel Technol 134(3), 031401 (May 18, 2012) (6 pages) doi:10.1115/1.4006131 History: Received February 08, 2009; Revised February 25, 2011; Published May 17, 2012; Online May 18, 2012

Creep strength of welded joint for high Cr ferritic heat resisting steels decreases due to Type-IV failure in heat-affected zone (HAZ) during long-term use at high temperatures. In order to review the allowable creep strength of these steels, creep rupture data of base metals and welded joints have been collected, and long-term creep strength has been evaluated in the SHC (strength of high-chromium steel) committee in Japan. In the present paper, the creep rupture data of 370 points for welded joint specimens of modified 9Cr-1Mo steel (ASME Grade 91 steel) offered from seven Japanese companies and institutes were analyzed. These data clearly indicated that the creep strength of welded joints was lower than that of base metal due to Type-IV failure in HAZ at high temperatures. From the activities of this committee, it was concluded that the weld strength reduction factor (WSRF) should be taken into consideration for the design and residual life assessment of boiler components in fossil power plants. The committee recommended the WSRF for 100,000 h creep of Gr.91 steel as 0.85 at 575 °C, 0.75 at 600 °C, 0.74 at 625 °C, and 0.70 at 650 °C. The master curve for residual life assessment of Gr.91 steel welds using Larson-Miller parameter was also proposed.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

Comparison of creep rupture data between welded joints and base metals for Gr.91 steel at 550, 600, 650, and 700 °C

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Figure 2

Fracture locations of the welded joints for Gr.91 steel

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Figure 3

Results of regression for all data of Gr.91 steel welded joints by LMP parameter (C = 31.2, a0  = 33,689, a1  = 3697, a2  = −2594, SEE = 0.286)

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Figure 4

Results of regression for all data of Gr.91 steel welded joints by ORNL equation (Ch  = − 21.83, C0  = − 0.0157, C1  = − 3.57, C2  = 30,069, SEE = 0.283)

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Figure 5

Relationship between stress versus LMP parameter for Gr.91 steel welds (C = 31.2)

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Figure 6

Results of regression for the data that fractured in HAZ (C = 28.8, a0  = 36,549, a1  = − 2255, a2  = − 880, SEE = 0.216)

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Figure 7

Results of regression for the selected data, of which conditions are close to structural components (C = 31.4, a0  = 34,154, a1  = 3494 and a2  = − 2574, SEE = 0.267)

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Figure 8

Results of regression for the data that fractured in HAZ within the selected data of Fig. 7 (C = 29.4, a0  = 34,446, a1  = 736, a2  = − 1712, SEE = 0.200)

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Figure 9

Creep rupture strength of the Gr.91 steel welded joints with the postweld normalizing-tempering (NT) heat treatment

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Figure 10

Creep rupture times of the longitudinal tube and pipe welds under internal pressure for the Gr.91 steel




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