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Materials and Fabrication

Creep Rupture Ductility of Creep Strength Enhanced Ferritic Steels

[+] Author and Article Information
Kazuhiro Kimura

Materials Reliability Unit,  National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japankimura.kazuhiro@nims.go.jp

Kota Sawada

Materials Reliability Unit,  National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japansawada.kota@nims.go.jp

Hideaki Kushima

Materials Information Station,  National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japankushima.hideaki@nims.go.jp

J. Pressure Vessel Technol 134(3), 031403 (May 18, 2012) (7 pages) doi:10.1115/1.4005876 History: Received February 17, 2011; Revised November 09, 2011; Published May 17, 2012; Online May 18, 2012

Creep rupture strength and ductility of creep strength enhanced ferritic steels of Grades 23, 91, 92, and 122 was investigated with particular emphasis on remarkable drop in the long-term. Large difference in creep rupture strength and ductility was observed on three heats of Grade 23 steels. Remarkable drop of creep rupture strength in the long-term of T91 was comparable to those of Grades 92 and 122. Remarkable drop in creep rupture ductility in a stress regime below 50% of 0.2% offset yield stress was observed on Grade T23 steel, however, that of Grade P23 steel did not indicate any degradation of creep rupture ductility. Higher creep rupture ductility of Grade P23 steel was considered to be caused by its lower creep strength than that of T23 steels. Creep rupture ductility of Grades 92 and 122 steels indicated rapid and drastic decrease with decrease in stress at 50% of 0.2% offset yield stress. Stress dependence of creep rupture ductility of Grades 92 and 122 steels was well described by a ratio of stress to 0.2% offset yield stress, regardless of temperature. On the other hand, large drop in creep rupture ductility of Grade 91 steel was observed only in the very low-stress regime at 650 °C. Alloying elements including impurities and changes in precipitates may influence on creep rupture ductility, however, remarkable drop in ductility of the steels cannot be explained by chemical composition and precipitates. High ductility in the high-stress regime above 50% of 0.2% offset yield stress should be provided by easy plastic deformation, and it has been concluded that a remarkable drop in ductility in the low-stress regime is derived from a concentration of creep deformation into a tiny recovered region formed at the vicinity of grain boundary.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 2

Stress versus time to rupture curves of (a) Grade 91, (b) Grade T/P92, and (c) Grade T/P122 steels

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Figure 3

Comparison of creep rupture strength of ASME SA-213 Grades T91, T92, and T122 steels

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Figure 4

Reduction of area of (a) T23 (MLA), (b) T23 (MLB), (c) P23, (d) Grade 91, (e) Grade 92, and (f) Grade 122 steels

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Figure 5

Relation between stress ratio to 0.2% offset yield stress and reduction of area of the steels

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Figure 6

Changes in Vickers hardness in the gauge portion of the creep ruptured specimen of Grade T/P23 steels

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Figure 7

Bright field TEM images of the MLA heat of T23 steels (a) in the as tempered condition (HV207), and (b) creep ruptured after 1835 h at 650 °C (HV148)

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Figure 8

Reduction of area of (a) T23 (MLA), (b) T23 (MLB), and (c) P23 steels plotted against a Larson–Miller parameter with a parameter constant of 20

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Figure 9

Changes in Vickers hardness with increase in time to rupture of (a) Grade 91, (b) Grade 92, and (c) Grade 122 steels

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Figure 10

Relation between stress ratio to 0.2% offset yield stress and Vickers hardness in the creep ruptured specimen of (a) Grade 91, (b) Grade 92, and (c) Grade 122 steels

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Figure 11

Bright field TEM images of MGC heat of T91 steel (a) in the as tempered condition, creep ruptured at 600 °C after (b) 971 h at 160 MPa, (c) 12,859 h at 120 MPa, and (d) 34,141 h at 100 MPa

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Figure 12

Comparison of changes in Vickers hardness of T91 and T92 steels in the head portion of the specimen creep ruptured at 600 °C

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Figure 1

Stress versus time to rupture curves of Grade T/P23 steels at 500, 550, and 600 °C

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