Root Cause of Degradation in the Creep Strength of Martensitic Steel

Creep curves of Grade 91 and 92 steels were analyzed by applying an exponential law to the temperature, stress, and time parameters to investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel. The activation energy (Q ), activation volume (V ), and Larson–Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs because of dislocations that are swept out of the sub-grains, and this is followed by strengthening owing to the rearrangement of M23C6 and the precipitation of the Laves phase. Heterogeneous recovery and subsequent heterogeneous deformation start at an early stage of transient creep near several of the weakest boundaries because of the coarsening of the precipitates; this results in the simultaneous decreases in Q , V , and C  even in transient creep. Further, this activity triggers an unexpected degradation in strength because of the accelerated formation of the Z-phase even in transient creep. The stabilization of M23C6 and the Laves phase is important to mitigate the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for Cr-Mo systems because Fe2Mo is easily coarsened at approximately 600 °C compared to Fe2W in Grade 92 steel.

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Analysis on Degradation in Creep Strength of 9Cr-W Martensitic Steel

Journal of Materials Science Research ◽

10.5539/jmsr.v10n1p1 ◽

2021 ◽

Vol 10 (1) ◽

pp. 1

Author(s):

Manabu TAMURA

Keyword(s):

Activation Volume ◽

Martensitic Steel ◽

Rupture Strength ◽

Early Stage ◽

Laves Phase ◽

Creep Mechanism ◽

Steel Grades ◽

Time Parameters ◽

Heterogeneous Deformation

In order to clarify the creep mechanism of high Cr martensitic steel, creep curves of 9Cr-1W and 9Cr-4W steels were analyzed applying an exponential law to the temperature, stress, and time parameters. The activation energy, Q, the activation volume, V, and the Larson-Miller constant, C, are obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening by swept dislocations out of sub-grains occurs followed by strengthening due to the rearrangement of M23C6 and the precipitation of Laves phase. After Q reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of some weakest boundaries due to coarsening of the precipitates, which triggers the unexpected degradation in strength due to the accelerating coarsening of precipitates. Stabilizing not only M23C6 but also Laves phase is important to mitigate the degradation of rupture strength of martensitic steel. The above creep mechanism for martensitic steel can be applicable to the explanation for the degradation in long term rupture strength of high Cr martensitic steel, Grades 91 and 92.

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Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

Journal of Materials Science Research ◽

10.5539/jmsr.v10n2p1 ◽

2021 ◽

Vol 10 (2) ◽

pp. 1

Author(s):

Manabu TAMURA ◽

Fujio ABE

Keyword(s):

Martensitic Steel ◽

Rupture Strength ◽

Early Stage ◽

High Strength ◽

Laves Phase ◽

Martensitic Steels ◽

Short Period ◽

Creep Curves ◽

Si Content

To investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel, the creep curves of Grades T91, T92, and P92 were analyzed along with the experimental steels of 9Cr-1W and 9Cr-4W by applying an exponential law to the temperature, stress, and time parameters. The activation energy (Q ), activation volume (V ), and Larson-Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs due to dislocations that are swept out of the sub-grains, which is followed by strengthening due to the rearrangement of M23C6 and the precipitation of the Laves phase. After Q  reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of several of the weakest boundaries due to coarsening of the precipitates. This activity triggers an unexpected degradation in strength due to the accelerated formation of the Z-phase. Stabilization of M23C6 and the Laves phase is important for mitigating the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for the Cr-Mo systems because Fe2Mo is easily coarsened at ~600 °C as compared to Fe2W. Lowering the hardness and Si content also prevents excess hardening due to the Laves phase, which also mitigates the degradation. The online monitoring of creep curves and the QVC  analysis render it possible to detect signs of long-term degradation under targeted conditions within a relatively short period.

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Influence of Stress on Degradation and Life Prediction of High Strength Ferritic Steels

Experience With Creep-Strength Enhanced Ferritic Steels and New and Emerging Computational Methods ◽

10.1115/pvp2004-2566 ◽

2004 ◽

Cited By ~ 10

Author(s):

Kazuhiro Kimura ◽

Kota Sawada ◽

Kiyoshi Kubo ◽

Hideaki Kushima

Keyword(s):

Yield Stress ◽

Elastic Limit ◽

High Strength ◽

Time To Rupture ◽

Limit Stress ◽

Creep Resistant Steels ◽

Grade 91 ◽

Term Creep ◽

Grade 91 Steel

Influence of stress on creep deformation and degradation behavior has been investigated. Corresponding to inflection of stress vs. time to rupture curve, difference in recovery phenomena, that was homogeneous in short-term and inhomogeneous in long-term, was observed. Inflection of stress vs. time to rupture curve took place at the stress condition corresponding to half of 0.2% offset yield stress at the temperature. Elastic limit stress of Grade 91 steel was evaluated to be 150MPa at 600°C and 100MPa at 650°C, by means of stress abrupt change test. These stresses were found to be almost the same as half of 0.2% offset yield stress at the temperatures. Inflection of stress vs. time to rupture curve is caused by transient of applied stress from higher level than elastic limit to within elastic range. It has been concluded that long-term creep strength of ferritic creep resistant steels should be predicted from the selected creep rupture data under the stresses lower than elastic limit by considering half of 0.2% offset yield stress at the temperature, by means of Larson-Miller parameter with a constant of 20.

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

Journal of Pressure Vessel Technology ◽

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 ◽

Grade 91 Steel

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.

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Evaluation of Stress Rupture Factors for Grade 91 Weldments

Volume 6B: Materials and Fabrication ◽

10.1115/pvp2018-84572 ◽

2018 ◽

Author(s):

Kazuhiro Kimura

Keyword(s):

Base Metal ◽

Rupture Strength ◽

Stress Rupture ◽

Creep Rupture ◽

Weld Strength ◽

Creep Rupture Strength ◽

Premature Failure ◽

Type Iv ◽

Grade 91

Stress rupture factors and weld strength reduction factors for Grade 91 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 the 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 weldments in the long-term, however, is caused by Type IV failure which takes place in the fine grained 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. Time and temperature dependent stress rupture factors for Grade 91 have been estimated based on the average creep rupture strength of cross weld test specimen to the average creep rupture strength of base metal.

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Long Term Creep Rupture Strength of 9Cr1MoNbV Narrow Gap Welded Joints Improved by Normalizing and Tempering After Welding

Volume 9: Eighth International Conference on Creep and Fatigue at Elevated Temperatures ◽

10.1115/creep2007-26570 ◽

2007 ◽

Cited By ~ 1

Author(s):

Takashi Sato ◽

Kohji Tamura ◽

Koichi Mitsuhata ◽

Ryuichi Ikura

Keyword(s):

Welded Joints ◽

Power Plants ◽

Rupture Strength ◽

Thermal Power ◽

High Strength ◽

Creep Rupture ◽

Narrow Gap ◽

Parent Metal ◽

Welding Procedure

With the increase of steam parameters of coal-fired thermal power plants, high strength 9%Cr steel containing niobium and vanadium became major material in high temperature boiler components. As the microstructure of these steels is tempered martensite, it is known that the softening occurs in HAZ of the weldment. In the creep rupture test of these welded joints the rupture strength is lower than that of the parent metal, and sometimes this reduction of strength is caused by Type IV cracking. To develop an effective method to improve the rupture strength of welded joints, a normalizing-tempering heat treatment after weld was proposed. 9Cr1MoNbV plates with a thickness of 40–50 mm were welded by 10 mm width automatic narrow gap MAG welding procedure using specially modified welding material. After normalizing at 1050C and tempering at 780C, material properties of the welded joints were examined. Microstructure of the HAZ was improved as before weld, and rupture strength of the welded joints was equal to that of the parent metal. The long term rupture strength of the welded joints as confirmed in the test exceeded 30,000 hours. This welding procedure has been applied to the seam weld of boiler hot reheat piping in USC plants successfully.

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

ASME 2018 Symposium on Elevated Temperature Application of Materials for Fossil, Nuclear, and Petrochemical Industries ◽

10.1115/etam2018-6706 ◽

2018 ◽

Author(s):

Kouichi Maruyama ◽

Nobuaki Sekido ◽

Kyosuke Yoshimi

Keyword(s):

Rupture Strength ◽

Creep Rupture ◽

Rupture Data ◽

Data Points ◽

Grade 91 ◽

Term Creep ◽

Off Time ◽

Term Data ◽

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.

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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 ◽

Grade 91 Steel

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.

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A Grain Size-Dependent Equation for Creep Rupture Life of Grade 91 Steel Verified Up To 233,000 Hours

Journal of Pressure Vessel Technology ◽

10.1115/1.4047442 ◽

2020 ◽

Vol 142 (6) ◽

Author(s):

K. Maruyama ◽

N. Sekido ◽

K. Yoshimi ◽

Y. Yamamoto

Keyword(s):

Grain Size ◽

Rupture Life ◽

Creep Rupture ◽

Life Assessment ◽

Size Dependent ◽

Grade 92 Steel ◽

Grade 91 ◽

Steam Pipes ◽

Grade 91 Steel

Abstract Grade 91 steel is widely used as steam pipes in ultrasupercritical (USC) steam boilers. In residual creep life assessment of the pipes by calculation, one needs creep rupture life of the steel as a function of stress and temperature in a time range longer than 105 h. Four regions with different creep rupture characteristics appear in a stress versus creep rupture life diagram of the steel. Main steam pipes made of the steel are used in a long-term region with low values of stress exponent and activation energy for creep rupture life (referred to as region G in this paper). Creep rupture lives of the steel in this region vary from heat to heat depending on their prior austenite grain size. This paper proposes a grain size-dependent equation representing creep rupture life of the steel in region G. The equation is verified with creep rupture data up to 232,833 h at 600 °C. Region G is absent in some heats with a large grain size. The equation can rationalize the absence in the heats. In a stress versus creep rupture life diagram of grade 92 steel, there is the same long-term region G. In the region, a creep rupture life of each heat is dependent on its grain size as is the case in grade 91 steel. The proposed equation accords well with the creep rupture lives of the grade 92 steel in region G.

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