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Research Papers: Materials and Fabrication

Evaluation of Microstructures and Creep Damages in the HAZ of P91 Steel Weldment

[+] Author and Article Information
Masaaki Tabuchi

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

Hiromichi Hongo, Yongkui Li, Takashi Watanabe

 National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Yukio Takahashi

 Central Research Institute of Electric Power Industry, 2-11-1 Iwado Kita, Komae, Tokyo 201-8511, Japan

J. Pressure Vessel Technol 131(2), 021406 (Jan 06, 2009) (6 pages) doi:10.1115/1.3028021 History: Received December 28, 2007; Revised October 09, 2008; Published January 06, 2009

The creep strength of welded joints in high Cr steels decreases due to the formation of Type IV creep damage in heat-affected zones (HAZs) during long-term use at high temperatures. This paper aims to elucidate the processes and mechanisms of Type IV creep damage using Mod.9Cr–1Mo (ASME Grade 91) steel weldments. Long-term creep tests for base metal, simulated fine-grained HAZ, and welded joints were conducted at 550°C, 600°C, and 650°C. Furthermore, creep tests of thick welded joint specimens were interrupted at 0.1, 0.2, 0.5, 0.7, 0.8, and 0.9 of rupture life and damage distributions were measured quantitatively. It was found that creep voids were initiated at an early stage of life inside the specimen thickness and coalesced to form cracks at a later stage of life. Creep damage was observed mostly at 25% below the surface of the plate. Experimental creep damage distributions were compared with computed versions using finite element method and damage mechanics analysis. Both multi-axial stress state and strain concentration in fine-grained HAZ appear to influence the formation and distribution of creep voids.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 2

Creep test specimens of welded joints

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Figure 3

Creep test results for base metal, simulated fine-grained HAZ, and welded joints of Mod.9Cr–1Mo steel

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Figure 4

Procedures for measurement of creep voids in the HAZ of welded joints

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Figure 5

Distributions of creep voids in the HAZ of the S-welded joint fractured in Type IV mode at 650°C and 50MPa(tr=3568h). (a) Cross-sectional view of damage, (b) area fraction of creep voids, and (c) number density of creep voids along the thickness direction.

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Figure 7

Binary images of creep voids and cracks observed in the HAZ of a central cross section of crept L-welded joint specimens

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Figure 8

Changes in number of creep voids per mm2 (number density of voids) in the HAZ during creep

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Figure 10

FE model of L-welded joint specimen

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Figure 12

Parameter fitting for creep curves of the simulated fine-grained HAZ of Mod.9Cr–1Mo steel at 600°C

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Figure 13

Distributions of creep damage ω in the HAZ along the thickness direction of the S-welded joint specimen

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Figure 14

Changes in the maximum value of creep damage ω in the HAZ during creep in an S-welded joint specimen

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Figure 15

Distributions of creep damage based on the ductility exhaustion approach in the HAZ along the thickness direction of an S-welded joint specimen

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Figure 9

Changes in deformation caused by local necking in the HAZ on the specimen’s surface during creep

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Figure 11

Distributions of equivalent creep strain and stress triaxial factor in the central cross section of L-welded joint specimen (600°C, 90MPa, and t=7000h)

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Figure 6

Conditions for creep interruption tests using L-welded joint specimens

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Figure 1

Cross-sectional view of the present welded joint

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