A Proposal for Post-Assessment Test of Long-Term Creep Rupture Strength of Grade 91 Steel

Predictions as to 105 hrs creep rupture strength of grade 91 steel have been made recently. The predictions should be verified by some means, since they are based on certain assumptions. A formula for predicting long-term creep rupture lives should correctly describe long-term data points used in its formulation. Otherwise the formula cannot properly predict further longer-term creep rupture lives. On the basis of this consideration, the predictions are examined with long-term creep rupture data of the steel. In the predictions three creep rupture databases were used: data of tube products of grade 91 steel reported in NIMS Creep Data Sheet (NIMS T91 database), data of T91 steel collected in Japan, and data of grade 91 steel collected by an ASME code committee. Short-term creep rupture data points were discarded by the following criteria for minimizing overestimation of the strength: selecting long-term data points with low activation energy (multi-region analysis), selecting data points crept at stresses lower than a half of proof stress (σ0.2/2 criterion), and selecting data points longer than 1000 hrs (cut-off time of 1000 hrs). In the case of NIMS T91 database, a time-temperature parameter (TTP) analysis of a dataset selected by the multi-region analysis can properly describe the long-term data points. However, the TTP analyses of datasets selected by the σ0.2/2 criterion and by the cut-off time of 1000 hrs from the same database overestimate the long-term data points. The different criteria for data selection have more substantial effects on predicted values of the strength of the steel than difference of the databases.

A Postassessment Test of 100,000 h Creep Rupture Strength of Grade 91 Steel at 600 °C

10.1115/1.4037446 ◽

2017 ◽

Vol 139 (5) ◽

Cited By ~ 4

Author(s):

K. Maruyama ◽

N. Sekido ◽

K. Yoshimi

Keyword(s):

Rupture Strength ◽

Creep Rupture ◽

Creep Rupture Strength ◽

Data Points ◽

Predicted Values ◽

Grade 91 ◽

Term Creep ◽

Term Data ◽

Predictions as to 105 h creep rupture strength of grade 91 steel have been made recently. The predicted values are examined with long-term creep rupture data of the steel. Three creep rupture databases were used in the predictions: data of tube products of grade 91 steel reported in National Institute for Materials Science (NIMS) Creep Data Sheet (NIMS T91 database), data of T91 steel collected in Japan, and data of grade 91 steel collected by an American Society of Mechanical Engineers (ASME) code committee. Short-term creep rupture data points were discarded by the following criteria for minimizing overestimation of the strength: selecting long-term data points with low activation energy (multiregion analysis), selecting data points crept at stresses lower than a half of proof stress (σ0.2/2 criterion), and selecting data points longer than 1000 h (cutoff time of 1000 h). In the case of NIMS T91 database, a time–temperature parameter (TTP) analysis of a dataset selected by multiregion analysis can properly describe the long-term data points and gives the creep rupture strength of 68 MPa at 600 °C. However, TTP analyses of datasets selected by σ0.2/2 criterion and cutoff time of 1000 h from the same database overestimate the data points and predict the strength over 80 MPa. Datasets selected by the same criterion from the three databases provide similar values of the strength. The different criteria for data selection have more substantial effects on predicted values of the strength of the steel than difference of the databases.

Re-Evaluation of Long-Term Creep Strength of Welded Joint of ASME Grade 91 Type Steel

Volume 6B: Materials and Fabrication ◽

10.1115/pvp2016-63316 ◽

2016 ◽

Cited By ~ 3

Author(s):

Masatsugu Yaguchi ◽

Kaoru Nakamura ◽

Sosuke Nakahashi

Keyword(s):

Creep Strength ◽

Welded Joints ◽

Rupture Strength ◽

Creep Rupture ◽

Base Metals ◽

Rupture Data ◽

Grade 91 ◽

Term Creep ◽

Creep rupture data of welded joints of ASME Grade 91 type steel have been collected from Japanese plants, milling companies and institutes, and the long-term creep rupture strength of the material has been evaluated. This evaluation of welded joints of Grade 91 steel is the third one in Japan as similar studies were conducted in 2004 and 2010. The re-evaluation of the creep rupture strength was conducted with emphasis on the long-term creep rupture data obtained since the previous study, with durations of the new data of up to about 60000h. The new long-term data exhibited lower creep strength than that obtained from the master creep life equation for welded joints of Grade 91 steel determined in 2010, then the master creep life equation was again reviewed on the basis of the new data using the same regression method as that used in 2010. Furthermore, the weld strength reduction factors obtained from 100000h creep strength of welded joints and the base metals are given as a function of temperature, where the master creep equations of the base metals are also redetermined in this study.

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

10.1115/1.4032647 ◽

2016 ◽

Vol 138 (3) ◽

Cited By ~ 7

Author(s):

K. Maruyama ◽

J. Nakamura ◽

K. Yoshimi

Keyword(s):

Rupture Strength ◽

Creep Rupture ◽

Data Selection ◽

Creep Rupture Strength ◽

Rupture Data ◽

Data Points ◽

Service Conditions ◽

Term Creep ◽

Term Data

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.

Re-Evaluation of Long-Term Creep Strength of Base Metal of ASME Grade 91 Type Steel

Volume 6B: Materials and Fabrication ◽

10.1115/pvp2016-63355 ◽

2016 ◽

Cited By ~ 6

Author(s):

Kazuhiro Kimura ◽

Masatsugu Yaguchi

Keyword(s):

Rupture Strength ◽

Creep Rupture ◽

Creep Rupture Strength ◽

Creep Data ◽

High Chromium ◽

Rupture Data ◽

Assessment Committee ◽

Grade 91 ◽

Creep rupture strength of ASME Grades 91, 92, 122 and 23 type steels were evaluated by the SHC committee in 2004 and 2005, and the Assessment Committee on Creep Data of High Chromium Steels in 2010. According to the evaluation of creep rupture strength, allowable stress of the steels was revised and weld strength reduction factor (WSRF) was established. In 2015, the creep rupture data of those steels was collected from materials producers, power plant manufacturers and institutes in Japan and a review of long-term creep rupture strength of the steels was conducted by the Assessment Committee on Creep Data of High Chromium Steels in reference to the previous evaluation. It has been confirmed with the latest dataset that re-evaluation of long-term creep rupture strength is not required for Grades 92, 122 and 23 type steels. On the other hand, lower creep rupture strength compared with the previous evaluation was recognized on the new creep rupture data of Grade 91 steels, therefore, re-evaluation of creep rupture strength was conducted on Grade 91 steels. Creep rupture strength was assessed by means of region splitting analysis method in consideration of 50% of 0.2% offset yield strength, in the same way as the previous study. According to the evaluation of long-term creep strength of the steels, allowable tensile stress was reviewed and proposed revision was concluded.

Evaluation of Stress Rupture Factors for Grade 91 Weldments

10.1115/1.4046431 ◽

2020 ◽

Vol 142 (4) ◽

Author(s):

Kazuhiro Kimura ◽

Masatsugu Yaguchi

Keyword(s):

Base Metal ◽

Rupture Strength ◽

Stress Rupture ◽

Creep Rupture ◽

Creep Rupture Strength ◽

Premature Failure ◽

Rupture Data ◽

Grade 91 ◽

Abstract Stress rupture factors and weld strength reduction factors for Grade 91 steel weldments in the codes and literatures have been reviewed. Stress rupture factors for weld metals proposed for code case N-47 in the mid 1980's was defined as a ratio of average rupture strength of the deposited filler metal to the average rupture strength of the base metal. Remarkable drop in creep rupture strength of weldments is significant issue of Grade 91, especially in the low-stress and long-term regime. A premature failure of Grade 91 steel weldments in the long-term, however, is caused by type IV failure which takes place in the fine grain heat affected zone (FG-HAZ), rather than fracture in the deposited weld metal. The stress rupture factor of the Grade 91 steel, therefore, was based on the creep rupture strength of cross weld test specimens. Creep rupture data of Grade 91 steel weldments reported in the publication of ASME STP-PT-077 were integrated with the creep rupture data collected in Japan and used for this study. Time- and temperature-dependent stress rupture factors for Grade 91 steel have been evaluated based on the consolidated database as a ratio of average creep rupture strength of cross weld test specimen to the average creep rupture strength of base metal.

Evaluation of Long-Term Creep Strength of Welded Joints of ASME Grades 91, 92 and 122 Type Steels

Volume 6: Materials and Fabrication, Parts A and B ◽

10.1115/pvp2012-78393 ◽

2012 ◽

Cited By ~ 10

Author(s):

Masatsugu Yaguchi ◽

Takuaki Matsumura ◽

Katsuaki Hoshino

Keyword(s):

Creep Strength ◽

Welded Joints ◽

Rupture Strength ◽

Reduction Factor ◽

Creep Rupture ◽

Creep Rupture Strength ◽

Strength Reduction Factor ◽

Rupture Data ◽

Creep rupture data of welded joints of ASME Grades 91, 92 and 122 type steels have been collected and long-term creep rupture strength of the materials has been evaluated. Similar study was conducted by the SHC Committee in 2004 and 2005, therefore, the evaluation of the creep rupture strength was conducted with emphasis on the long-term creep rupture data obtained after the previous study, in addition to discussion of the effects of product form, welding procedure and test temperature etc. on the creep strength. Almost the same results were obtained on the welded joint of Grade 92 as the previous study, however, the master creep life equations for the welded joints of Grades 91 and 122 were lower than the previous results, especially in the case of Grade 122. Furthermore, the creep strength reduction factor obtained from 100,000 hours creep strength of welded joints and base metal was given as a function of temperature.

Evaluation of Short- and Long-Term Creep Rupture Properties for Grade 91 Materials

Volume 6: Materials and Fabrication ◽

10.1115/pvp2005-71674 ◽

2005 ◽

Author(s):

Marvin J. Cohn

Keyword(s):

Good Method ◽

Creep Rupture ◽

Material Degradation ◽

Rupture Data ◽

Short And Long Term ◽

Grade 91 ◽

Metal Properties ◽

Term Creep ◽

Rupture Properties

Recent literature indicates that there is a concern regarding the short-term vs. long-term creep rupture base metal properties for Grade 91 material. Evaluations of recent creep rupture data suggest that the material properties degrade more severely than expected and extrapolated creep rupture properties may be very optimistic. One of the approaches to evaluate creep rupture data is with a parameterized master curve such as the Larson-Miller parameter. Evaluations of creep rupture data indicate that the effects of material degradation can be considered with appropriate stress, time and temperature relationships. Using the Larson-Miller parameter methodology, the selected heats of Grade 91 creep rupture data indicate a reasonable relationship that does not appear to degrade rapidly for the longer term data. If even longer term creep rupture data suggest severe aging degradation as compared to current extrapolations, a transition of the Larson-Miller parameter constant from 31 to 20 does not appear to be a good method to calculate the degraded life estimates. As longer term creep rupture data become available, resulting oxide thicknesses should be measured and reported. The adverse effect of oxidation at longer times, resulting in loss of material and effectively higher stress, should be evaluated.

Evaluation of Long-Term Creep Strength of ASME Grades 91, 92, and 122 Type Steels

Volume 6: Materials and Fabrication, Parts A and B ◽

10.1115/pvp2012-78323 ◽

2012 ◽

Cited By ~ 16

Author(s):

Kazuhiro Kimura ◽

Yukio Takahashi

Keyword(s):

Base Metal ◽

Creep Strength ◽

Rupture Strength ◽

Nickel Content ◽

Creep Rupture ◽

Tube Steel ◽

Creep Rupture Strength ◽

Rupture Data ◽

Creep rupture data of ASME Grades 91, 92 and 122 type steels have been collected and long-term creep rupture strength of the steels has been evaluated. Similar study was conducted by the SHC committee in 2004 and 2005, therefore, the evaluation of long-term creep rupture strength was conducted with emphasis on the long-term creep rupture data obtained after the previous study. Creep rupture strength was analyzed by means of region splitting analysis method in consideration of 50% of 0.2% offset yield strength, in the same way as the previous study. Almost the same results were obtained on base metal of Grade 92 as the previous study, however, evaluated 100,000 hours creep rupture strength of base metal of Grades 91 and 122 were lower than the previous results. For Grades 91 and 122 type steels, moreover, creep rupture strength of the plate steel were lower than those of pipe and forging steels. Tendency to decrease with increase in nickel content was observed on long-term creep rupture strength of tube steel of Grade 91 at 600°C. According to the evaluation of long-term creep strength of the steels, allowable tensile stress was reviewed and proposed revision was concluded.

Influence of Oxidation on Estimation of Long-Term Creep Rupture Strength of 2.25Cr–1Mo Steel by Larson–Miller Method

10.1115/1.4044264 ◽

2019 ◽

Vol 141 (6) ◽

Author(s):

Fujio Abe

Keyword(s):

Materials Science ◽

Rupture Strength ◽

Creep Rupture ◽

Creep Rupture Strength ◽

Creep Life ◽

Creep Tests ◽

Initial Strength ◽

Rupture Data ◽

Abstract The influence of oxidation on the estimation of long-term creep rupture strength is investigated for 2.25% chromium (Cr)–1% molybdenum (Mo) steel specified as JIS STBA 24, JIS SCMV 4 NT, and ASTM A542/A542M by the Larson–Miller method using creep rupture data in the National Institute for Materials Science (NIMS) Creep Data Sheets at 450–650 °C for up to 313,000 h. The creep rupture data exhibit a change in slope of the stress versus time to rupture curves due to oxidation in air during 600 °C creep tests at 15,000–40,000 h and 650 °C tests at 2000–3500 h for different size specimens, which indicates degradation in creep life by the oxidation. The estimated 100,000 h creep rupture strength using regression analysis is increased by the elimination of long-term data degraded by the oxidation. Several metallurgical factors, such as the initial strength represented by the 0.2% proof stress at the creep test temperature and the concentration of aluminum (Al) impurity, also affect the creep life of the tested steel.

Re-Evaluation of Stress Rupture Factors for Grade 91 Weldments Based on the Extended Database With the Data Collected in Japan

Volume 6B: Materials and Fabrication ◽

10.1115/pvp2019-93331 ◽

2019 ◽

Author(s):

Kazuhiro Kimura ◽

Masatsugu Yaguchi

Keyword(s):

Base Metal ◽

Rupture Strength ◽

Stress Rupture ◽

Creep Rupture ◽

Weld Strength ◽

Creep Rupture Strength ◽

Premature Failure ◽

Grade 91 ◽

Abstract Stress rupture factors and weld strength reduction factors for Grade 91 steel weldments in the codes and literatures have been reviewed. Stress rupture factors for weld metals proposed for Code Case N-47 in the mid 1980’s was defined as a ratio of average rupture strength of the deposited filler metal to the average rupture strength of the base metal. Remarkable drop in creep rupture strength of weldments is significant issue of Grade 91, especially in the low-stress and long-term regime. A premature failure of Grade 91 steel weldments in the long-term, however, is caused by Type IV failure which takes place in the fine grain heat affected zone (FG-HAZ), rather than fracture in the deposited weld metal. The stress rupture factor of the Grade 91 steel, therefore, was based on the creep rupture strength of cross weld test specimens. Creep rupture data of Grade 91 steel weldments reported in the publication of ASME STP-PT-077 was incorporated in the creep database collected in Japan which was used for the previous study. Time and temperature dependent stress rupture factors for Grade 91 steel have been re-evaluated based on the extended database as a ratio of average creep rupture strength of cross weld test specimen to the average creep rupture strength of base metal.