Author: Shirin Fahimi
Publisher:
ISBN: 9783844048681
Category :
Languages : en
Pages :

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Author: Ali Aghajani Bazazi
Publisher: Cuvillier Verlag
ISBN: 3736931824
Category : Technology & Engineering
Languages : en
Pages : 118

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Tempered martensite ferritic steels are used for critical components in fossil fired power plants that operate in the creep range. The materials contain a high density of dislocations and precipitates form on all types of internal interfaces, the majority of which represent subgrain boundaries. Most previous studies suffer from either only relating to short term creep experiments or from being incomplete in not considering all relevant elements of the microstructure. No systematic effort was made to investigate the evolution of microstructures under conditions of long term creep. In the present study the evolution of the microstructure of a 12% Cr tempered martensite ferritic steel was investigated under conditions of long term aging and creep. Transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD) techniques were used to characterize materials from interrupted creep tests (0.5%, 1%, 1.6% and rupture at 11.9%; creep conditions: 550°C, 120 MPa, rupture time: 139 971 h). It is shown that subgrains coarsen, that the close correlation between carbides and subgrain boundaries loosens during long term creep, and that the frequency of small angle boundaries increases. In addition, the evolution of dislocation densities during long term aging and creep was studied using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). During aging the dislocation density remains constant, while during long term creep the dislocation density continuously decreases. All these elementary deformation processes have already been discussed in short term creep studies. The present study shows that they also govern long term creep, however, during long term creep, precipitation and coarsening reactions occur which are not observed during short term creep. Cr rich M23C6, VX carbides and Laves phase were identified as the major precipitates in the microstructure of the 12% Chromium tempered martensite ferritic steel. Their chemical compositions, sizes, volume fractions and number densities were evaluated in all interrupted specimens. M23C6 particles coarsen and establish their equilibrium concentration after 51072 hours. VX particles are stable. The Laves phase particles do not reach thermodynamic equilibrium as they form and grow during long term creep. This is due to Silicon which is found in the Laves phase particles and which diffuses slowly in the steel matrix.

Author: Ali Aghajani Bazazi
Publisher:
ISBN: 9783869551821
Category :
Languages : en
Pages : 0

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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 19

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The ferritic-martensitic stainless steel HT-9 exhibits an anomalously high creep strength in comparison to its high-temperature flow strength from tensile tests performed at moderate rates. A constitutive relation describing its high-temperature tensile behavior over a wide range of conditions has been developed. When applied to creep conditions the model predicts deformation rates orders of magnitude higher than observed. To account for the observed creep strength, a fine distribution of precipitates is postulated to evolve over time during creep. The precipitate density is calculated at each temperature and stress to give the observed creep rate. The apparent precipitation kinetics thereby extracted from this analysis is used in a model for the rupture-time kinetics that compares favorably with observation. Properly austenitized and tempered material was aged over times comparable to creep conditions, and in a way consistent with the precipitation kinetics from the model. Microstructural observations support the postulates and results of the model system. 16 refs., 10 figs.

Author: Lun Wang
Publisher:
ISBN:
Category :
Languages : en
Pages : 111

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Abstract: The creep strength enhanced ferritic (CSEF) steels P91 and P92 are extensively used in fossil supercritical power plants due to their improved creep strength at elevated temperatures. Loss of creep strength and/or toughness may occur in CSEF steel welds due to formation of fresh martensite, ferrite, or retained austenite during welding and post weld heat treatment (PWHT). Predicting the critical phase transformation temperatures is of practical importance for the development of appropriate welding and PWHT procedures. The available empirical formula proposed by Andrew for the determination of the A1 temperature does not cover the composition range of CSEF steels and the effect of carbon and nitrogen has not been taken into account. Thermodynamic simulation software such as Thermo-Calc and JMat-Pro could predict equilibrium transformation temperatures (i.e. equilibrium formation of austenite, ferrite in steels). However, the microstructure in CSEF steels which is of practical importance for determination of the A1 temperature is tempered martensite (in as delivered condition). Santella developed two formulae for P91 and P92 steels based on thermodynamic simulation data recently. Predictions by Santella's formulae tend to be conservative estimates of the A1 temperature in P91 and P92 steels. Thermodynamic simulations and experimental measurement based Design of Experiment (DOE) approaches and fractional data analysis were applied to develop formulae for predicting the A1 temperature in P91 and P92 steels. The alloying elements with a significant effect on the A1 temperature in P91 steels screened out by the thermodynamic simulation based DOE approach are: Ni, Mn, Si, N, Cr, Mo, C, V. No interactions were found between these significant alloying elements. A measurement based DOE was developed based on the elements with significant effect on the A1 temperature determined by the thermodynamic simulation based DOE. The test samples were melted using a button melting system in argon environment. These samples were subjected to homogenizing, rolling, normalizing, and tempering in order to reproduce to the manufacturing process of commercial CSEF steels. The A1 temperatures in these samples were measured by Single Sensor Differential Thermal Analysis (SS-DTA). The results of the measurement based DOE were processed using fractional data analysis to develop predictive formulae for the A1 temperature in P91 and P92 steels. It was found that C, N, Si and Cr could influence the A1 temperature largely; Ni and Mn were not the only determining factor of the A1 temperature. The relations between the concentrations of most alloying elements and the A1 temperature of P91 and P92 steels were quadratic function. The predictions of these formulae were validated by comparison to measured values of the A1 temperature in P91 and P92 steels by dilatometry.

Author: Fujio Abe
Publisher: Elsevier
ISBN: 1845694015
Category : Technology & Engineering
Languages : en
Pages : 701

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Creep-resistant steels are widely used in the petroleum, chemical and power generation industries. Creep-resistant steels must be reliable over very long periods of time at high temperatures and in severe environments. Understanding and improving long-term creep strength is essential for safe operation of plant and equipment. This book provides an authoritative summary of key research in this important area.The first part of the book describes the specifications and manufacture of creep-resistant steels. Part two covers the behaviour of creep-resistant steels and methods for strengthening them. The final group of chapters analyses applications in such areas as turbines and nuclear reactors.With its distinguished editors and international team of contributors, Creep-resistant steels is a valuable reference for the power generation, petrochemical and other industries which use high strength steels at elevated temperatures. - Describes the specifications and manufacture of creep-resistant steels - Strengthening methods are discussed in detail - Different applications are analysed including turbines and nuclear reactors

Author:
Publisher:
ISBN:
Category : Metallurgy
Languages : en
Pages : 1216

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Author: Cem Topbasi
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Ferritic-martensitic steels are primary candidate materials for fuel cladding and internal applications in the Sodium Fast Reactor, as well as first-wall and blanket materials in future fusion concepts because of their favorable mechanical properties and resistance to radiation damage. Since microstructure evolution under irradiation is amongst the key issues for these materials in these applications, developing a fundamental understanding of the irradiation-induced microstructure in these alloys is crucial in modeling and designing new alloys with improved properties.The goal of this project was to investigate the evolution of microstructure of two commercial ferritic-martensitic steels, NF616 and HCM12A, under heavy ion irradiation at a broad temperature range. An in situ heavy ion irradiation technique was used to create irradiation damage in the alloy; while it was being examined in a transmission electron microscope. Electron-transparent samples of NF616 and HCM12A were irradiated in situ at the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory with 1 MeV Kr ions to ~10 dpa at temperatures ranging from 20 to 773 K. The microstructure evolution of NF616 and HCM12A was followed in situ by systematically recording micrographs and diffraction patterns as well as capturing videos during irradiation.In these irradiations, there was a period during which no changes are visible in the microstructure. After a threshold dose (~0.1 dpa between 20 and 573 K, and ~2.5 dpa at 673 K) black dots started to become visible under the ion beam. These black dots appeared suddenly (from one frame to the next) and are thought to be small defect clusters (2-5 nm in diameter), possibly small dislocation loops with Burgers vectors of either 1/2111 or 100.The overall density of these defect clusters increased with dose and saturated around 6 dpa. At saturation, a steady-state is reached in which defects are eliminated and created at the same rates so that the defect density is constant. After saturation, defects constantly appeared and disappeared in a time that is shorter than the time in between frames (normally 34 ms). The average diameter and size distribution of the irradiation-induced defect clusters did not vary with dose during a single irradiation in the temperature range of 50 to 573 K in NF616, and 20 to 673 K in HCM12A. At 673 K, the defects in NF616 grew and coalesced under irradiation which led to larger average defect sizes and low defect density. At high doses extended defect structures in NF616 formed as short segments aligned along 100 directions. At 773 K, the frequency of defect formation per unit area was the lowest amongst all irradiations and all the visible defect clusters that formed eventually faded out gradually (in ~28 seconds) leading to no net defect accumulation in NF616 even at the highest irradiation dose of 10 dpa.Under irradiation, a significant fraction of these defect clusters exhibited sudden one-dimensional jumps (over ~5nm) between 20 and 573 K, that is, some defect clusters move "or jump" along 211 directions which is consistent with the expected Burgers vector direction of (111). Interestingly, at 673 and 773 K, defects in NF616 and HCM12A did not exhibit the sudden jumps and jerks that were frequently observed during lower temperature irradiations. No resolvable loops, voids or precipitates were formed in NF616 and HCM12A. Furthermore, no significant interaction of the irradiation induced defects with the foil surface, pre-existing dislocation network or grain boundaries was observed between 20 and 773 K.A simplified rate theory model was developed to describe the initial defect formation processes. The model is based on the reactions between intra-cascade clusters driven by the one-dimensional movement of sub-visible interstitial clusters in their glide cylinder under irradiation after detrapping from interstitial and substitutional solute atoms by cascade impact. Multiple cascade impacts on previously existing clusters allow them to gather clusters during their glide, leading to the formation of TEM-visible (~2 nm) defects. The low dose defect density approximated by model is in good agreement with the experimental results. In addition, the model rationalizes the threshold dose before which no visible defect clusters were formed.

Author: Yong-Yi Wang
Publisher:
ISBN:
Category : Technology & Engineering
Languages : en
Pages : 290

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Author: Carlos Garcia-Mateo
Publisher: MDPI
ISBN: 3039288571
Category : Technology & Engineering
Languages : en
Pages : 166

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The microstructures of both martensite and bainite, although sharing some common features, depict a plethora of subtle differences that made them unique when studied in further detail. Tailoring the final properties of a microstructure based on one or the other as well as in combination with others and exploring more sophisticated concepts, such as Q&P and nanostructured bainite, are the topics which are the focus of research around the world. In understanding the key microstructural parameters controlling the final properties as well as definition of adequate process parameters to attain the desired microstructures requires that a proper understanding of the mechanism ruling their transformation and a detailed characterization first be acheived. The development of new and powerful scientific techniques and equipment (EBSD, APT, HRTEM, etc.) allow us to gain fundamental insights that help to establish some of the principles by which those microstructures are known. The developments accompanying such findings lead to further developments and intensive research providing the required metallurgical support.