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creep analysis
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Proceedings Papers
AM-EPRI2016, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Eighth International Conference, 466-477, October 11–14, 2016,
... Abstract A methodology is developed for evaluating its creep rupture life from analysis of an on-going creep curve with the aid of an Ω creep curve equation. The method is applied to on-going creep curves of grade 91 steel for evaluating their rupture lives. Quick decrease in creep rupture...
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A methodology is developed for evaluating its creep rupture life from analysis of an on-going creep curve with the aid of an Ω creep curve equation. The method is applied to on-going creep curves of grade 91 steel for evaluating their rupture lives. Quick decrease in creep rupture strength has been reported recently in long-term creep of grade 91 steel. The quick decrease of the steel is discussed by using the rupture lives evaluated. The quick decrease is confirmed in the present study in the time range longer than 3 x 10 4 h at 600°C.
Proceedings Papers
AM-EPRI2024, Advances in Materials, Manufacturing, and Repair for Power Plants: Proceedings from the Tenth International Conference, 1300-1312, October 15–18, 2024,
... more than 160 heats and 2,400 creep testing data, covers a wide spectrum of elemental compositions and product forms. To perform a prudent analysis of the creep property dataset, a statistical overview was first implemented to understand the data distribution relevant to data sources, chemistries...
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This study investigates the influences of product chemistry and grain size on the high-temperature creep properties of 316 stainless steels by analyzing an extensive range of historical and modern literature data. The investigated 316 stainless steel creep property dataset, including more than 160 heats and 2,400 creep testing data, covers a wide spectrum of elemental compositions and product forms. To perform a prudent analysis of the creep property dataset, a statistical overview was first implemented to understand the data distribution relevant to data sources, chemistries, product forms, testing temperatures, and grain sizes. The creep data of 550°C, 600°C, 650°C, 700°C, and 750°C with ±10°C were grouped together, and the analytical study was performed on each sub dataset to investigate the temperature-specific creep performance. The creep strength was evaluated using the average stress ratio (ASR) between the experimental and predicted creep data of tested 316SS heats. The influence of composition and grain size on the creep strength ratio were evaluated using linear correlation analysis. Effects of specified and non-specified elements including C, N, and B were specifically investigated to understand their impacts on the creep strength with regards to the variation of creep temperature. In addition to the literature data, the most recent EPRI creep data of three commercial heats were used to validate the correlations from the historical creep property dataset.
Proceedings Papers
AM-EPRI2016, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Eighth International Conference, 347-355, October 11–14, 2016,
... steel with the extension of service time, sigma precipitates form at grain boundaries by continuous chain. Sigma precipitates are hard and brittle, weaken grain boundaries and cause microscopic damage, eventually lead to boiler tubes failure. austenitic stainless steel creep failure analysis...
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Up to now, the amount of supercritical boilers in China has ranked number one in the world. Many supercritical boilers have run for more than 100,000 hours. Creep becomes one of the main reasons for supercritical boiler tubes failure. In this article, the failure of superheater tubes in a supercritical boiler was analyzed, the microstructural evolution of austenitic stainless steel tubes were studied, a full investigation into the failure cause was carried out involving in visual examination, optical microscope, SEM, TEM and XRD. The results show, sigma phase precipitates in this austenitic steel with the extension of service time, sigma precipitates form at grain boundaries by continuous chain. Sigma precipitates are hard and brittle, weaken grain boundaries and cause microscopic damage, eventually lead to boiler tubes failure.
Proceedings Papers
AM-EPRI2024, Advances in Materials, Manufacturing, and Repair for Power Plants: Proceedings from the Tenth International Conference, 219-234, October 15–18, 2024,
..., and Repair for Power Plants: Proceedings from the Tenth International Conference October 15 18, 2024, Bonita Springs Florida, USA httpsdoi.org/10.31399/asm.cp.epri2024p0219 Copyright © 2024 ASM International® All rights reserved. www.asminternational.org CREEP CAVITATION IMAGING AND ANALYSIS IN 9%CR 1%MO P91...
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The current research adopts a novel approach by integrating correlative microscopy and machine learning in order to study creep cavitation in an ex-service 9%Cr 1%Mo Grade 91 ferritic steel. This method allows for a detailed investigation of the early stages of the creep life, enabling identification of features most prone to damage such as precipitates and the ferritic crystal structure. The microscopy techniques encompass Scanning Electron Microscopy (SEM) imaging and Electron Back-scattered Diffraction (EBSD) imaging, providing insights into the two-dimensional distribution of cavitation. A methodology for acquiring and analysing serial sectioning data employing a Plasma Focused Ion Beam (PFIB) microscope is outlined, complemented by 3D reconstruction of backscattered electron (BSE) images. Subsequently, cavity and precipitate segmentation was performed with the use of the image recognition software, DragonFly and the results were combined with the 3D reconstruction of the material microstructure, elucidating the decoration of grain boundaries with precipitation, as well as the high correlation of precipitates and grain boundaries with the initiation of creep cavitation. Comparison between the 2D and 3D results is discussed.
Proceedings Papers
AM-EPRI2013, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Seventh International Conference, 702-713, October 22–25, 2013,
... portion of the test pipe and voids density was measured along the thickness direction in the HAZ region. To clarify the stress/strain distribution in the welded portion, creep analysis was conducted on the test pipe, where the materials are assumed to consist of base metal, weld metal and HAZ. After...
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An internal pressure creep test has been carried out on a Gr. 91 steel longitudinal welded pipe at 650°C to examine the type IV failure behavior of actual pipes, using a large-scale experiment facility “BIPress”, which can load internal pressure and bending force on large diameter pipes at high temperatures. The creep test was also interrupted three times to measure hardness and voids density in the HAZ region of the outer surface of the test pipe. Results of the measurement of the hardness and voids density at the interruption did not indicate creep damage accumulation. The welded pipe suddenly ruptured with large deformation, which caused crushing damage to the surrounding facility. Type IV cracking occurred in the longitudinal welded portion of the test pipe, and the length of the crack reached 5000mm. SEM observation was carried out at the cross section of the welded portion of the test pipe and voids density was measured along the thickness direction in the HAZ region. To clarify the stress/strain distribution in the welded portion, creep analysis was conducted on the test pipe, where the materials are assumed to consist of base metal, weld metal and HAZ. After stress redistribution due to creep deformation, stress and strain concentrations were observed inside the HAZ region. Then, the authors' creep life prediction model was applied to the creep test result to examine its validity to actual size pipes. It was demonstrated that the life prediction model can evaluate damage of the Gr. 91 steel longitudinal welded pipe with sound accuracy.
Proceedings Papers
AM-EPRI2019, 2019 Joint EPRI – 123HiMAT International Conference on Advances in High-Temperature Materials, 1282-1293, October 21–24, 2019,
... considered that the fractal dimension of the grain boundary in FGHAZ could be an indication of creep damage and studied its change as creep proceeded. First, creep tests were conducted to produce damaged materials, and their fractal dimensions were measured. Next, FEM analysis was conducted to obtain...
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Type IV creep damage of high chromium steel is a problem in thermal power plants and a method of evaluating remaining life is required. Type IV creep damage is characterized by many voids that initiate in the weldment fine grain heat affected zone (FGHAZ), where the stress multiaxiality (expressed by the Triaxiality Factor, TF) is high. As the creep continues, the shape of the grain boundary becomes simple; that is, close to a straight line. It is known that the grain boundary is fractal. The complexity of the fractal is represented by the fractal dimension. Therefore, we considered that the fractal dimension of the grain boundary in FGHAZ could be an indication of creep damage and studied its change as creep proceeded. First, creep tests were conducted to produce damaged materials, and their fractal dimensions were measured. Next, FEM analysis was conducted to obtain the distribution of the principal stress, TF, and creep strain of the observed surface. The distribution of creep damage was obtained by the time fraction rule. The results of this evaluation confirmed that the fractal dimension of the grain boundary decreases with creep time and that the principal stress and TF affect it.
Proceedings Papers
AM-EPRI2010, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Sixth International Conference, 530-553, August 31–September 3, 2010,
.... Two forms of simplified method will be described. Use of a creep analysis reflects the change from elastic to steady state. A limit load analysis gives stress distributions varying from elastic to a distribution representing rupture, depending on inelastic strain. Thus use can be made of the whole...
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The paper describes methods for practical high temperature weldment life assessment, and their application to the analysis of notable high energy piping weldment failures and interpretation of cross-weld data. The methods described in the paper are simplified versions of full continuum damage mechanics (CDM) analysis techniques which have been developed over the last 20 years. The complexity of the CDM methods and their data requirements has been a barrier to their more widespread use. The need for simplified methods has been driven by the need for risk assessment of in-service high temperature welded piping and headers around the world, the need to connect cross-weld data to weld joint design and assessment, and in general, the need to develop suitable guidelines for evaluating the strength of weldments relative to that of base metal.
Proceedings Papers
AM-EPRI2016, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Eighth International Conference, 169-180, October 11–14, 2016,
... a simulated structure made of 23Cr-45Ni-6W alloy (HR6W) [12], which is one of the materials being considered for making large-diameter piping, creep analysis was carried out through the FEM analysis. EXPERIMENTAL The material of the test pieces was HR6W, which has the nominal chemical composition shown...
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23Cr-45Ni-7W alloy (HR6W) is a material being considered for use in the high temperature parts of A-USC boilers in Japan. In order to establish an assessment method of creep damage for welded components made using HR6W, two types of internal pressure creep tests were conducted. One is for straight tubes including the circumferential weld and the other is for welded branch connections. The test results for the circumferential welds ensured that the creep rupture location within the area of the base metal, as well as the time of rupture, can be assessed by mean diameter hoop stress. On the other hand, the creep rupture area was observed in the weld metal of the branch connections, although the creep strength of Inconel filler metal 617 was higher than that of HR6W. FE analyses were conducted using individual creep strain rates of the base metal, the heat affected zone and the weld metal to clarify this difference in the failures of these two specimens. Significant stress was only produced in the weld metal as opposed to the base metal, due to the difference in creep strain rates between the welded branch connections and creep crack were initiated in the weld metal. The differences between the two failure types were assessed using the ductility exhaustion method.
Proceedings Papers
AM-EPRI2019, 2019 Joint EPRI – 123HiMAT International Conference on Advances in High-Temperature Materials, 1330-1339, October 21–24, 2019,
... of the observed parts *2) fracture TF*1) 2.5~3.0 test time (hr) 1117*2) 1054 934 454 2049*2) 1321 1031 Stress analysis of test pieces The observation cross section should be satisfy three conditions; a) TF is larger than 2. b) Maximum principal stress and TF uniformly distribute. c) Creep damage is maximum...
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Type IV creep damage is a problem in high-temperature steam piping made of high chromium steel at thermal power plants, and a method for evaluating the remaining life is required. In this study, we considered that void’s initiation and growth can be expressed by initiation rate f, growth rate h, and initiation start time t 1 , and that stress and TF affect f, h and t 1 . We also proposed the method to estimate f, h and t 1 by measuring the change of the distribution of radius of voids during creep test. The creep test conditions are (1) test temperature of 650 C, maximum principal stress σ 1 of 79.5MPa, and TF of 2.5 ~ 3.0, and (2) test temperature of 650C, maximum principal stress of 71.5MPa, and TF of 2.5 ~ 3.0. The influence of σ 1 to f, h and t 1 was quantified by comparing the result of test (1) and that of test (2).
Proceedings Papers
AM-EPRI2019, 2019 Joint EPRI – 123HiMAT International Conference on Advances in High-Temperature Materials, 1322-1329, October 21–24, 2019,
... of taking miniature sample scoops on the creep life of a pipe. This paper, reports on the results of internal pressure creep tests and FE analysis on ASME Grade 91 steel pipes with defects on their outer surface, which simulate sampling scoops. EXPERIMENTAL PROCEDURE Materials The two types of material...
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The effect of taking miniature sample scoops on the creep life of ASME Grade 91 steel pipes was experimentally and analytically assessed in this work. Internal pressure tests were conducted on tubular specimens having defects on their outer surface, which simulate sampling scoops. The creep life did not decrease until the depth ratio of the defect to the wall thickness of the specimens was about 5%, and the creep life decreased with increasing defect depth when the depth ratio exceeded about 5%. When the depth ratio was about 11%, the creep life decreased to four-fifths of that of a specimen with no defects. In addition, as a result of investigating the stress concentration around a defect with a depth ratio of about 5% by the finite element method, stress concentration was clearly observed around the defect. These results suggest that taking a miniature sample up to a depth of 5% of the thickness of a Grade 91 steel pipe in service has a negligible effect on the creep life of the pipe.
Proceedings Papers
AM-EPRI2013, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Seventh International Conference, 1244-1255, October 22–25, 2013,
... analysis to determine a set of calibration curves for inferring strain at any given vertical displacement. Some creep strain data are also presented. creep strain displacement finite element analysis small punch creep test test specimens Advances in Materials Technology for Fossil Power Plants...
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A prototype small punch test rig has been developed to extend the range of data output. Through the introduction of a probe, vertical displacements can be measured across a region of the specimen underside. This information provides much greater understanding of the specimen deformation. Having displacement data at a series of measurement points also facilitates the calculation of strains across the sample. The probe can also be used during a test to provide time dependent data from small punch creep tests. The measured displacement data have been used in conjunction with FE analysis to determine a set of calibration curves for inferring strain at any given vertical displacement. Some creep strain data are also presented.
Proceedings Papers
AM-EPRI2016, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Eighth International Conference, 1182-1193, October 11–14, 2016,
... methodology to identify welds suspected to have received non-standard PWHT cycles on Grade 91 pipework systems. KEY WORDS Grade 91 creep resistant steel / post weld heat treatment / TIG weld process / MMA weld process / mechanical tests / dilatometry / SEM / TEM analysis INTRODUCTION Grade 91 is a creep...
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There is a constant need for improved knowledge of the influence of non-standard processing on the expected performance of creep strength enhanced ferritic (CSEF) materials as the total installed tonnage of these materials is rapidly increasing across the power generation industry. Cr-Mo-V steel grades micro-alloyed with niobium and titanium designed for pressurized equipment operating in the supercritical steam range proved to be very sensitive to relative minor variations in the principal heat treatment parameters time and temperature, when compared to the traditional Cr-Mo-V grades. A key component for successful welds is optimised post weld heat treatment (PWHT). Under certain conditions premature failures of welds can occur when incorrect weld and heat treatment performance result in a reduction of specified mechanical properties and high temperature creep performance, it is therefore of significant importance to have a good understanding of actual material properties for effective operation and plant life studies. This study investigated the effect and impact variations of post weld heat treatment time and temperature on mechanical properties of tungsten inert gas (TIG) and manual metal arc (MMA) welds on Grade 91 pipes from a set of reference samples. This is in preparation of establishing a benchmark set of tests to determine the integrity and expected long-term performance of butt-welds from limited site sample volumes, providing a non-intrusive methodology to identify welds suspected to have received non-standard PWHT cycles on Grade 91 pipework systems.
Proceedings Papers
AM-EPRI2016, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Eighth International Conference, 125-130, October 11–14, 2016,
... for the analysis on Alloy 617 and Alloy 740, respectively. The data were obtained by Special Metals. In these temperature ranges, Ni3Al can precipitate in Alloy 617 and Alloy 740 during creep. The maximum time to rupture was 40,126.7 and 24,066 h for Alloy 617 and Alloy 740, respectively. The rupture data...
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The reasonable procedures for estimation of 100,000 h creep rupture strength have been investigated for Alloy 617 and Alloy 740 for A-USC power plants by Larson Miller method. The creep rupture data of longer duration than 500 h in the temperature range between 593 and 816 °C and between 600 and 850 °C were used for the analysis on Alloy 617 and Alloy 740, respectively. The data were obtained by Special Metals. In these temperature ranges, Ni3Al-γ’ can precipitate in Alloy 617 and Alloy 740 during creep. The maximum time to rupture was 40,126.7 and 24,066 h for Alloy 617 and Alloy 740, respectively. The rupture data for Alloy 617 exhibit large scattering, especially at 760 °C, showing a split into two groups. After eliminating the shorter time to rupture data at 760 °C, the regression analysis using the second order equation of Larson-Miller parameter gives us the Larson-Miller constant C of 12.70 and the 100,000 h creep rupture strength of 100 MPa at 700 °C. The regression analysis underestimates the constant C and corresponding 100,000 h creep rupture strength of Alloy 617, as shown by the regression curves locating below the rupture data at long times, while those locating above the rupture data at short times. The underestimation of constant C is caused by large data scattering. The linear extrapolation of log tr versus reciprocal temperature 1/T plot to 1/T = 0 at constant stresses gives us the constant C of 18.5, which is much larger than that by the regression analysis. Using an appropriate constant C of 18.45, the 100,000 h creep rupture strength of Alloy 617 is estimated to be 123 MPa at 700 °C. On the other hand, the rupture data for Alloy 740 exhibit only a little bit scattering. The regression analysis gives us C = 18.45, which agrees very well with that by the linear extrapolation of log tr versus 1/T plot to 1/T = 0. The 100,000 h creep rupture strength of Alloy 740 is estimated to be 214 and 109 MPa at 700 and 760 °C, respectively.
Proceedings Papers
AM-EPRI2019, 2019 Joint EPRI – 123HiMAT International Conference on Advances in High-Temperature Materials, 1048-1059, October 21–24, 2019,
... good creep ductility due to the absence of γ’ phase precipitates. A method to evaluate stress relaxation cracking susceptibility was developed by applying a three-point bending test using a specimen with a V-notch and finite element analysis (FEA), and it was shown that stress relaxation cracking...
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The susceptibilities of hot cracking and reheat cracking of A-USC candidate Ni-based alloys were evaluated relatively by Trans-Varestraint testing and Slow Strain Rate Tensile (SSRT) testing. In addition, semi-quantitative evaluation of the stress relaxation cracking susceptibility of Alloy 617 was conducted, because stress relaxation cracking in the heat affected zone (HAZ) has actually been reported for repair welds in Alloy 617 steam piping in European A-USC field-testing. Solidification cracking susceptibilities of Alloy 617 were the highest; followed by HR35, Alloy 740 and Alloy 141, which were all high; and then by HR6W and Alloy 263, which were relatively low. In addition, liquation cracking was observed in the HAZ of Alloy 617. The reheat cracking susceptibilities of Alloy 617, Alloy 263, Alloy 740 and Alloy 141 were somewhat higher than those of HR6W and HR35 which have good creep ductility due to the absence of γ’ phase precipitates. A method to evaluate stress relaxation cracking susceptibility was developed by applying a three-point bending test using a specimen with a V-notch and finite element analysis (FEA), and it was shown that stress relaxation cracking of aged Alloy 617 can be experimentally replicated. It was proposed that a larger magnitude of creep strain occurs via stress relaxation during the three-point bending test due to a higher yield strength caused by γ’ phase strengthening, and that low ductility due to grain boundary carbides promoted stress relaxation cracking. The critical creep strain curve of cracking can be created by means of the relationship between the initial strain and the creep strain during the three-point bending tests, which were calculated by FEA. Therefore, the critical conditions to cause cracking could be estimated from the stress relaxation cracking boundary from of the relationship between the initial strain and the creep strain during the three-point bending test.
Proceedings Papers
AM-EPRI2024, Advances in Materials, Manufacturing, and Repair for Power Plants: Proceedings from the Tenth International Conference, 1269-1278, October 15–18, 2024,
... the analysis results. And then LCF and creep considering the actual operating conditions were evaluated. The calculated life of fatigue and creep life is compared to the hot gas path inspection interval. For the rejuvenated blades, the creep life and the LCF interval were reviewed based on the temperature...
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Gas turbine blades are operated in a high temperature and a high pressure. In order to cope with that harsh condition, the blades are made of Nickel based superalloys which show excellent performance in such environment. Manufacturers of the blades usually provide the standards for the blade inspection and replacement. According to their guide, the blades are replaced after 3 times of operations and 2 times of refurbishments. Howsoever, purchase the new blades is always costly and burdensome to the power plant owners hence, the assessment of the blade lifespan and the rejuvenation of the degraded blades are indeed crucial to them. In this study, the optimal rejuvenation conditions for gas turbine blades were derived and verified. In addition to that, the creep durability was evaluated based on the actual blade inspection interval. LCF tests have been carried out on the rejuvenated blade and the result was compared with the fatigue life of the new blades. In order to secure the safety of the rejuvenated blade during operation, a heat flow analysis was performed to simulate the operating conditions of the gas turbine during operation, and the main stress and strain areas were investigated through the analysis results. And then LCF and creep considering the actual operating conditions were evaluated. The calculated life of fatigue and creep life is compared to the hot gas path inspection interval. For the rejuvenated blades, the creep life and the LCF interval were reviewed based on the temperature, stress, and strain acquired by computational analysis. The creep life was calculated as 59,363 hours by LMP curve, and the LCF was calculated as 2,560 cycles by the Manson Coffin graph.
Proceedings Papers
AM-EPRI2013, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Seventh International Conference, 690-701, October 22–25, 2013,
... fraction rule as almost 110,000 h. Finite-element analysis was also conducted to assess the damage and remaining life, and the results were compared with the experimental results. creep damage creep rupture test creep-strength enhanced ferritic steel fine-grain heat-affected zone finite-element...
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Type IV damage was found at several ultra-supercritical (USC) plants that used creep-strength-enhanced ferritic (CSEF) steels in Japan, and the assessment of the remaining life of the CSEF steels is important for electric power companies. However, there has been little research on the remaining life of material that has actually served at a plant. In this study, the damage and remaining life of a Gr.91 welded elbow pipe that served for 54,000 h at a USC plant were investigated. First, microscopic observation and hardness testing were conducted on specimen cut from the welded joint; the results indicated that the damage to the elbow was more severe in the fine-grain heat-affected zone near the inner surface. Furthermore, creep rupture tests were performed using specimens cut from the welded joint of the elbow, and from these results, the remaining life was evaluated using the time fraction rule as almost 110,000 h. Finite-element analysis was also conducted to assess the damage and remaining life, and the results were compared with the experimental results.
Proceedings Papers
AM-EPRI2010, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Sixth International Conference, 705-714, August 31–September 3, 2010,
... Abstract A new methodology challenges the conventional use of a constant C-value in the Larson-Miller Parameter (LMP) for 9-12% Cr ferritic steels, proposing instead a multi-C region analysis to address creep strength breakdown issues. Using NIMS data and other publications, the study...
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A new methodology challenges the conventional use of a constant C-value in the Larson-Miller Parameter (LMP) for 9-12% Cr ferritic steels, proposing instead a multi-C region analysis to address creep strength breakdown issues. Using NIMS data and other publications, the study demonstrates that C-values vary both between steel types and across stress regions. The new approach enables prediction of long-term (10 5 hours) creep rupture properties using only short-term (5×10 3 hours) test data, while d[g(σ)]/d[P(t r ,T)] versus P(t r ,T) analysis provides insight into property stability. This methodology offers a more cost-effective and accurate approach to acquiring and assessing long-term creep rupture data for these heat-resistant steels.
Proceedings Papers
AM-EPRI2024, Advances in Materials, Manufacturing, and Repair for Power Plants: Proceedings from the Tenth International Conference, 1195-1206, October 15–18, 2024,
... samples. creep cavitation creep damage creep strength enhanced ferritic steel heat affected zone inclusions microstructural analysis post weld heat treatment Advances in Materials, Manufacturing, and Repair for Power Plants: Proceedings from the Tenth International Conference October 15 18...
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Creep strength enhanced ferritic (CSEF) steels have shown the potential for creep failure in the weld metal, heat affected zone (HAZ) or fusion line. Details for this behavior have been frequently linked to metallurgical risk factors present in each of these locations which may drive the evolution of damage and subsequent failure. This work is focused on three weld samples fabricated from a commercially sourced Grade 92 steel pipe section. These weld samples were extracted from the same welded section but were reported to exhibit failure in different time frames and failure locations (i.e., HAZ of parent, fusion-line, and weld metal). The only variables that contribute to this observed behavior are the post weld heat treatment (PWHT) cycle and the applied stress (all tests performed at 650 °C). In this work detailed microstructural analysis was undertaken to precisely define the locations of creep damage accumulation and relate them to microstructural features. As part of this an automated inclusion mapping process was developed to quantify the characteristics of the BN particles and other inclusions in the parent material of the samples. It was found that BN particles were only found in the sample that had been subjected to the subcritical PWHT, not those that had received a re-normalizing heat treatment. Such micron sized inclusions are a known potential nucleation site for creep cavities, and this is consistent with the observed failure location in the HAZ of the parent in the sample where these were present. In the absence of BN inclusions, the next most susceptible region to creep cavitation is the weld metal. This has an intrinsically high density of sub-micron sized spherical weld inclusions and this is where most of the creep damage was located, in all the renormalized samples.
Proceedings Papers
AM-EPRI2013, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Seventh International Conference, 732-743, October 22–25, 2013,
... Abstract Conventional time-temperature-parameter (TTP) methods often overestimate long-term creep rupture life of creep strength enhanced high Cr ferritic steels. The cause of the overestimation is studied on the basis of creep rupture data analysis on Gr.91, 92 and 122 steels. There are four...
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Conventional time-temperature-parameter (TTP) methods often overestimate long-term creep rupture life of creep strength enhanced high Cr ferritic steels. The cause of the overestimation is studied on the basis of creep rupture data analysis on Gr.91, 92 and 122 steels. There are four regions with different values of stress exponent n for creep rupture life commonly in stress-rupture data of the three ferritic steels. Activation energies Q for rupture life in the regions take at least three different values. The values of n and Q decrease in a longer-term region. The decrease in Q value is the cause of the overestimation of long-term rupture life predicted by the conventional TTP methods neglecting the change in Q value. Therefore, before applying a TTP method creep rupture data should be divided into several data sets so that Q value is unique in each divided data set. When this multi-region analysis is adopted, all the data points of the steels can be described accurately, and their long-term creep life can be evaluated correctly. Substantial heat-to-heat and grade-to-grade variation in their creep strength is suggested under recent service conditions of USC power boilers.
Proceedings Papers
AM-EPRI2013, Advances in Materials Technology for Fossil Power Plants: Proceedings from the Seventh International Conference, 586-595, October 22–25, 2013,
...-term creep test results and the precise precipitation analysis will be disclosed by the presentation. creep strain curves creep strength creep test electron backscatter diffraction ferritic stainless steel microstructure degradation misorientation precipitation strengthening transmission...
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In order to study the effect of precipitation strengthening by MX precipitates on the restriction of microstructure degradation in 9 mass% Cr ferritic heat-resistant steels, V, Nb additioned model steels were evaluated by microstructure analysis through TEM and EBSD with reference to the creep test and creep interrupting test. VN precipitation increased the creep strength if the content was higher than 0.02%. Simultaneous addition of Nb and V in the specimen resulted in the complex NbC-VN precipitates even in the as-heat-treated specimens. The coherent and fine-needle-type VN was also detected in the steel. These precipitates are expected to increase the creep strength according to the creep strain curves. V variation up to 0.02% did not affect the crystallographic character of the grain boundary in the as-heat-treated specimens. Nb variation affected the crystallographic character of the grain boundary significantly because of the grain refinement effect of NbC. VN precipitation during the creep test restricted the crystallographic misorientation-angle-profile degradation. Integrating all intragranular precipitates, VN, restricts the crystallographic degradation significantly. The long-term creep test results and the precise precipitation analysis will be disclosed by the presentation.
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