Research Papers: Materials and Fabrication

Assessment of Long-Term Creep Rupture Strength of T91 Steel by Multiregion Rupture Data Analysis

[+] Author and Article Information
K. Maruyama

Department of Materials Science,
Graduate School of Engineering,
Tohoku University,
6-6-02 Aramaki-Aoba, Aoba-ku,
Sendai 980-8579, Japan
e-mail: maruyama@material.tohoku.ac.jp

J. Nakamura, K. Yoshimi

Department of Materials Science,
Graduate School of Engineering,
Tohoku University,
Sendai 980-8579, Japan

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 7, 2015; final manuscript received January 14, 2016; published online February 23, 2016. Assoc. Editor: Haofeng Chen.

J. Pressure Vessel Technol 138(3), 031407 (Feb 23, 2016) (9 pages) Paper No: PVT-15-1152; doi: 10.1115/1.4032647 History: Received July 07, 2015; Revised January 14, 2016

Creep rupture strength of creep strength enhanced ferritic steels is often overestimated, and its evaluated value has been reduced repeatedly. In this paper, the cause of the overestimation is discussed, and the creep rupture strength of T91 steel is assessed with its updated creep rupture data. Effects of residual Ni concentration on the creep rupture strength and necessity of F factor in T91 steel are also discussed. Decrease in activation energy Q for rupture life in long-term creep is the cause of the overestimation, since conventional time–temperature parameter (TTP) methods cannot deal with the change in Q. Due to the decrease in Q, long-term creep rupture strength evaluated decreases as longer-term data points are added or shorter-term data points are discarded in the conventional TTP analysis. The long-term region with small values of activation energy and stress exponent is named region L2 in this paper. Region L2 appears in all the heats of T91 steel and plate products of Gr.91 steel. Since service conditions of the T91 steel are usually in region L2, the creep rupture strength under the service conditions should be evaluated from the rupture data in region L2 only. The 5 × 105 hrs rupture strength at 550 °C decreases from 129 MPa (evaluated from the whole data of T91 steel) to 79 MPa (evaluated from the data in region L2 only) with increasing cut-off time for data selection. The 105 hrs rupture strength at 600 °C also decreases from 87 MPa (whole data) to 70 MPa (region L2 only) despite sufficient number of long-term data points at 600 °C. Careful consideration on the data selection is necessary in evaluation of creep rupture strength of the T91 steel. A multiregion rupture data analysis (MRA) is helpful to select data points belonging to region L2.

Copyright © 2016 by ASME
Topics: Creep , Steel , Rupture
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Fig. 1

Stress dependence of rupture life together with regression curves (solid line) and lower boundary of 90% confidence bands (dotted line) obtained by conventional LM analysis of all the data. Data points of heat MGC are represented with the solid symbols.

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Fig. 2

(a) SEE, (b) LM constant C, and rupture strength of (c) 5 × 105 hrs at 550 °C and (d) 105 hrs at 600 °C as a function of cut-off time taken for rupture data selection. σ0.2/2 and MRA stand for data selection based on σ0.2/2 criterion and multiregion analysis, respectively.

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Fig. 4

(a) Stress and (b) temperature dependence of rupture life of heat MGC. The solid lines are regression curves determined by the multiregion analysis. Activation energies Q in the four regions are given in (b).

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Fig. 3

Correlation between data band (gray line) and its regression line (thin solid line) determined by SD analysis of the creep rupture data. The dotted lines are the upper and lower boundaries of a 90% confidence band.

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Fig. 5

Creep rupture data (a) selected by σ0.2/2 criterion and (b) longer than 30,000 hrs. Solid lines are regression curves, and dotted lines are the lower boundary of 90% confidence bands. The solid symbols represent data points of heat MGC.

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Fig. 6

(a) Rupture life and (b) minimum creep rate of T91 steel in region L2. The solid regression lines were determined by LM analyses of the data in region L2 only.

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Fig. 7

Ni concentration dependence of rupture strength at (a) 5 × 105 hrs at 550 °C and (b) 105 hrs at 600 °C, and (c) deformation resistance at 10 −7 hr−1 at 600 °C

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Fig. 8

Rupture lives of three heats of plate products of Gr.91 steel together with the regression lines obtained in Fig. 6(a). Residual Ni concentrations of the three heats are given in the figure.

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Fig. 9

Stress exponent n* at design lives (5 × 105 hrs at 550 °C and 105 hrs at 600 °C) as a function of cut-off time. MRA stands for the results obtained from analyses of the data in region L2 only.




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