A Quantitative Assessment of Microstructural Evolution for Grade 91

2018 ◽  
Author(s):  
Chang Che ◽  
Gong Qian ◽  
Xisheng Yang
Keyword(s):  
High Temperature ◽  
Microstructural Evolution ◽  
Quantitative Assessment ◽  
Creep Damage ◽  
Precipitated Phase ◽  
High Temperature Components ◽  
Grade 91 ◽  
Term Creep ◽  
Grade 91 Steel

China has the most supercritical boilers in the world. Grade 91 steels are widely used for high temperature components of supercritical boiler. During high temperature service, microstructural evolution of Grade 91 steel may affect the mechanical properties, including creep strength. However, there are very few studies on quantitative assessment of microstructural evolution for Grade 91 steel, especially on precipitates content. In this article, microstructural evolution of Grade 91 was studied. A quantitative assessment of microstructure evolution was given during long-term creep, focusing on the precipitated phase content in Grade 91 steel. The results show, the precipitates content of Grade 91 steel has a corresponding relationship with creep damage.

Prediction of long-term creep behaviour of Grade 91 steel at 873 K in the framework of microstructure-based creep damage mechanics approach

2018 ◽  
Vol 28 (6) ◽  
pp. 877-895 ◽  
Author(s):  
J Christopher ◽  
BK Choudhary
Keyword(s):  
Damage Mechanics ◽  
Creep Behaviour ◽  
Creep Damage ◽  
Time Data ◽  
Grade 91 ◽  
Term Creep ◽  
Kinetic Creep ◽  
Grade 91 Steel

A detailed analysis has been performed for the prediction of long-term creep behaviour of tempered martensitic Grade 91 steel at 873 K using the microstructure-based creep damage mechanics approach. Necessary modifications have been made into the original kinetic creep law proposed by Dyson and McLean in order to account for the influence of microstructural damages arising from the coarsening of M23C6 and conversion of useful MX precipitates into deleterious Z-phase on creep behaviour of the steel. An exponential rate relationship has been introduced for the evolution of number density of MX precipitates with time. It has been shown that the developed model adequately predicts the experimental long-term creep strain–time as well as creep rate-time data. The role of Z-phase on long-term creep behaviour of Grade 91 steel has also been discussed.

Unified Viscoplastic Model Coupled With Damage for Evaluation of Creep-Fatigue of Grade 91 Steel

2017 ◽  
Author(s):  
Nazrul Islam ◽  
David J. Dewees ◽  
Tasnim Hassan
Keyword(s):  
Damage Mechanics ◽  
Creep Damage ◽  
Continuum Damage Mechanics ◽  
Creep Model ◽  
Damage Evaluation ◽  
Creep Fatigue ◽  
Grade 91 ◽  
Term Creep ◽  
Grade 91 Steel

A continuum damage mechanics (CDM) coupled unified viscoplasticity model has been developed to predict the creep-fatigue life of modified Grade 91 steel. A tertiary creep model termed MPC-Omega codified in Part 10 of API (and also implemented in the ASME BP&V Code for Grade 22V and more recently Grade 91 Steel) is also employed for creep damage evaluation. As MPC-Omega has a direct relationship with Larson-Miller parameter (LMP) coefficients, creep damage coefficients in the unified constitutive model (UCM) are tied with MPC-Omega coefficients in order to utilize WRC and API 579-1 Grade 91 creep rupture database. The model is validated against long-term creep, LCF, creep-fatigue and TMF experimental responses at T = 20–600°C.

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

2004 ◽  
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.

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

2017 ◽  
Vol 139 (5) ◽  
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.

Characterisation of the Microstructure Evolution of Aged Grade 91 Steel

2020 ◽  
Author(s):  
Chang Che ◽  
Xiang Liu ◽  
Youqiao Huang ◽  
Qingchuan Pan ◽  
Gong Qian
Keyword(s):  
Microstructural Evolution ◽  
Microstructure Evolution ◽  
Power Plants ◽  
Optical Microscope ◽  
Laves Phases ◽  
Transmission Electron ◽  
Materials Used ◽  
Grade 91 ◽  
Grade 91 Steel

Abstract Grade 91 steel has high creep strength and has been used as the material of piping in 600°C USC power plants in China. The Grade 91 materials used in actual power plants are useful in estimating the changes of material properties caused by long-term aging and damage at low stress conditions. An understanding of the long-term microstructural evolution under actually used conditions is a key for the improvement of these heat resistant steels. In this article, microstructural evolution of Grade 91 steel under different service conditions in Chinese power plants was studied using optical microscope (OM), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) techniques. The results show, M23C6, MX (V-rich particles; Nb-rich particles), and Laves phases were found to precipitate. A quantitative characterisation of microstructure evolution was evaluated during long-term exposure, focusing on the size of precipitates (M23C6 carbides, Laves phase, MX phase) for the Grade 91 steel after long-term service.

Creep damage in long-term grade 91 steel component tests

2020 ◽  
Vol 37 (6) ◽  
pp. 425-433
Author(s):  
Jonathan Parker ◽  
John Siefert
Keyword(s):  
Creep Damage ◽  
Steel Component ◽  
Grade 91 ◽  
Grade 91 Steel

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

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.

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

2016 ◽  
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.

The Performance of Creep-Strengthened Ferritic Steels in Power Generating Plant

2007 ◽  
Author(s):  
Jonathan Parker ◽  
Jeff Henry
Keyword(s):  
Heat Treatment ◽  
High Temperature ◽  
Ferritic Steels ◽  
Stress Concentrations ◽  
Actual Case ◽  
Type Iv ◽  
Temperature Performance ◽  
Grade 91 ◽  
Term Creep

Creep-strengthened ferritic steels, such as Grade 91, offer the potential for excellent high-temperature performance. To realize the benefits for these alloys requires careful control of original composition and manufacturing processes, such as welding and bending, as well as the associated heat treatments. Laboratory tests indicate that long-term lives may be below the original estimates made based on Larson Miller extrapolation. Furthermore, accelerated rates of damage accumulation in in-service Grade 91 components can occur due to a number of factors including: • Problems associated with design, for example with reinforcement at nozzles and with stress concentrations in piping systems. • Incorrect heat treatment, in addition to proper instrumentation appropriate heat treatment schedules should consider specific compositions. • Bending, problems may be introduced following both hot or cold bending. • High-temperature operation in tubing leading to excessive scale formation and overheating. • Type IV cracking in welds which results from the local reduction in the heat affected zone strength resulting from welding thermal cycles. Review of key information regarding the high-temperature performance of creep strengthened ferritic steels shows that the long-term creep strength may not achieve the levels expected from simple extrapolation of short term data. The problems experienced are highlighted with reference to actual case histories. The additional challenges associated with the development of creep-fatigue damage in high-temperature plant operated in a cyclic mode are also discussed.

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