[PDF] Microstructure Development And Precipitation Behavior Of Laves Phase During Thermomechanical Process In High Chromium Ferritic Stainless Steel eBook

Author : Ali Aghajani Bazazi
Publisher : Cuvillier Verlag
Page : 118 pages
File Size : 21,77 MB
Release : 2009-12-08
Category : Technology & Engineering
ISBN : 3736931824

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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 : James C M Li
Publisher : World Scientific Publishing Company
Page : 454 pages
File Size : 10,23 MB
Release : 2000-10-09
Category : Technology & Engineering
ISBN : 9813105658

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This is the second volume of an advanced textbook on microstructure and properties of materials. (The first volume is on aluminum alloys, nickel-based superalloys, metal matrix composites, polymer matrix composites, ceramics matrix composites, inorganic glasses, superconducting materials and magnetic materials). It covers titanium alloys, titanium aluminides, iron aluminides, iron and steels, iron-based bulk amorphous alloys and nanocrystalline materials.There are many elementary materials science textbooks, but one can find very few advanced texts suitable for graduate school courses. The contributors to this volume are experts in the subject, and hence, together with the first volume, it is a good text for graduate microstructure courses. It is a rich source of design ideas and applications, and will provide a good understanding of how microstructure affects the properties of materials.Chapter 1, on titanium alloys, covers production, thermomechanical processing, microstructure, mechanical properties and applications. Chapter 2, on titanium aluminides, discusses phase stability, bulk and defect properties, deformation mechanisms of single phase materials and polysynthetically twinned crystals, and interfacial structures and energies between phases of different compositions. Chapter 3, on iron aluminides, reviews the physical and mechanical metallurgy of Fe3Al and FeAl, the two important structural intermetallics. Chapter 4, on iron and steels, presents methodology, microstructure at various levels, strength, ductility and strengthening, toughness and toughening, environmental cracking and design against fracture for many different kinds of steels. Chapter 5, on bulk amorphous alloys, covers the critical cooling rate and the effect of composition on glass formation and the accompanying mechanical and magnetic properties of the glasses. Chapter 6, on nanocrystalline materials, describes the preparation from vapor, liquid and solid states, microstructure including grain boundaries and their junctions, stability with respect to grain growth, particulate consolidation while maintaining the nanoscale microstructure, physical, chemical, mechanical, electric, magnetic and optical properties and applications in cutting tools, superplasticity, coatings, transformers, magnetic recordings, catalysis and hydrogen storage.

Author : Benjamin Allen Shassere
Publisher :
Page : 202 pages
File Size : 40,32 MB
Release : 2016
Category : Alloys
ISBN :

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Type IV cracking in weldments of steel pipes after creep deformation is a concern in modern fossil-fueled power plants. Two possible methods for minimizing or eliminating Type IV cracking will be discussed. The first method alters the initial microstructure of typical Grade 91 steel base metal before welding, while the second provides baseline microstructure characteristics and creep performance of a new alloy that is strengthened by the intermetallic Fe2Nb Laves phase. The initial microstructure of the Grade 91 steel can be controlled by Thermo-Mechanical Treatments, which aids in precipitation of fine (5-10 nm) MX particles in austenite before transformation to martensite on cooling. Results also reveal that MX is shown to have a reduced volume percent and size of M23C6 carbide formation upon tempering. A strain rate model has been created based on particle sizes and interparticle spacing to predict creep deformation strain rates from the microstructure. The model reveals that the minimum creep rate can be altered with a change in precipitate distribution, and may have competing creep mechanisms in the heterogeneous microstructure of weldments. The model indicates that Type IV cracking may not have a solution based only on precipitation control. Based on this idea a new alloy is being developed which precipitates a thermally stable Fe2Nb Laves phase with the average size between 80-150 nm. The alloy has a ferritic matrix and general composition of Fe-30Cr-3Al-.2Si-xNb. The initial base metal creep results indicate the new alloy has similar creep properties to the Grade 92 steel at 700 °C. The microstructure characterization has revealed that a precipitate free zone (PFZ) forms parallel to all the grain boundaries with larger (200-300 nm) Laves phase decorating the grain boundary. Utilizing the strain rate model, a Grain Boundary Sliding type mechanism was identified as the deformation mechanism. In these alloys, it will be discussed how the areal fraction of grain boundary Laves phase and the width of the PFZ controls the cavitation nucleation and eventual grain boundary ductile failure. This reveals the competition between the intragranular creep resistance of the Laves phase and the intergranular deformation along the grain boundaries.