Abstract: Ferritic heat resistant steels are strongly desired to expand their maximum use temperature up to 650°C for application to the next highest temperature components of highly efficient, low emission ultra-supercritical (USC) power plant with maximum steam temperature of 700 °C. This minimizes the requirement of expensive nickel base superalloys. Critical issues for the development of ferritic steels for 650 °C USC boilers are the improvement of oxidation resistance as well as long-term creep rupture strength, including welded joints. The optimized combination of boron addition and dispersion of nanometer-sized vanadium nitrides significantly improves long-term creep strength of 9% Cr steel and also exhibits no degradation in welded joints at 650 °C. The protective Cr2O3-rich scale forms on the surface of 9% Cr steel by pre-oxidation treatment in Ar gas, which significantly improves the oxidation resistance in steam at 650°C.
Abstract: Conventional material developments have emphasized ultrafine grain refinement and homogenization. However, “nano- and homo-” materials do not usually satisfy the need to be both strong but ductile, which are of course rather contradictory characteristics. To solve such a problem, we have succeeded in designing a “Harmonic Structure Material” that is both a “nano- and harmonic” material which has overcome that antinomy through use of one of the non-equilibrium powder metallurgy (PM) processes called the severe plastic deformation (SPD) PM process. In the present study, pure Ti, Ti-6Al-4V alloy and SUS316L stainless steel powders are subjected to mechanical milling (MM) for various periods of time. The MM powders have two kinds of microstructure, which can be controlled by the MM conditions. They include ultra fine and coarse grain structures known as “shell” and “core”, respectively. Subsequently, these MM powders are sintered using conventional sintering processes. The sintered materials with the shell and the core have a network structure of continuously connected shells, which we refer to as a harmonic structure. The sintered materials with the harmonic structure simultaneously demonstrate both high strength and elongation. These outstanding mechanical properties are influenced by harmonic structure characteristics such as shell and core grain sizes, shell area fraction and shell network size. Thus, the harmonic structure can be considered as a remarkable design for improving the mechanical properties of materials
Abstract: The very high strength now achievable in low carbon HSLA steel plates is caused by the formation of bainite or martensite during the post-hot rolling cooling in interrupted direct quenching. Modern electron optical examination, especially FEG-SEM, has allowed the microstructural features such as packet, block and lath dimensions and crystallography to be quantitatively determined. Several recent studies have attempted to relate the strength and toughness to these features, with limited success. However, one observation is clear, these microstructural features scale with the prior-austenite grain size and state of recrystallization. The role of microalloying, beyond grain refinement, remains inconclusive. This paper will discuss these microstructures and suggest possible ways of further refining them.
Abstract: In the field of advanced ceramics, Spark Plasma Sintering (SPS) is known to be very efficient for superfast and full densification of ceramic nanopowders. This property is attributed to the simultaneous application of high density dc pulsed current and load, even though the sintering mechanisms involved remain unclear. In the first part of the paper, the mechanisms involved during SPS of two insulating oxide nanopowders (Al2O3 and Y2O3) are discussed while in the second part illustrations of the potential of SPS will be given for (i) Consolidation of mesoporous or unstable nanomaterials like SBA-15 or biomimetic apatite, respectively; (ii) Densification of core (BT or BST)/shell (SiO2 or Al2O3) nanoparticles with limited or controlled reaction at the interface. (iii) In-situ preparation of surface-tailored Fe–FeAl2O4–Al2O3 nanocomposites, and finally (iv) One-step preparation of multilayer materials like a complete thermal barrier system on single crystal Ni-based superalloy.
Abstract: In this paper the recent development of NiMnGa-particles-embedded polymer-matrix magnetodriven composites achieved by our group is described. The NiMnGa single-crystal particles can be easily fabricated by mechanically crushing the polycrystalline ingots due to intrinsic intergranular brittleness. The elastic back stress from the matrix polymer induces the reverse reorientation of martensite variants after removing the magnetic field. However, the actuation strain observed was very small around 10ppm which was 1/1000 times lower than the calculated value. Some possible reasons for the disagreement are that the crystallographic orientation of NiMnGa particles is random distribution, lattice defects introduced during crushing suppress reorientation of martensite variants, and that the elastic restriction from the matrix polymer is higher than expected. Therefore, the martensite variant reorientation behavior of the NiMnGa/silicone composites has been investigated from the viewpoint of (1) volume fraction of matrix polymer, (2) elastic modulus of polymer and (3) direction of magnetic field applied. And also, the internal structures of the composites were directly evaluated by microfocused X-ray computed tomography (µ-CT).
Abstract: There currently is a strong interest in using ‘as-hot rolled’ steels instead of ‘heat treated’ steels within the field of microalloyed steels with superior strength and improved ductility. However, mechanical and microstructural characterization of these steels is less elaborated. The present work is an effort in that direction, focusing on the evaluation of the impact of a variable Nb and C content on the microstructure and mechanical properties for seven hot rolled microalloyed steels and deriving an empirical relation between the mechanical properties and the Nb/C ratio.
Abstract: Thermomechanical simulation serves to simulate industrial conditions under closer control and much more cheaply than through works trials, and also under ideal conditions (e.g. constant temperature and strain rate) more suitable for input to metallurgical models which can then be applied to real cases. It is remarkable how much effort in Industry and Academia, and how many conferences, are devoted to this issue. Moreover, many of the questions being addressed do not appear to change very much over the years . The continuing stream of new rolled grades requires new experimental quantification and adjustment to models, because we do not as yet have a physically-based model powerful enough to extrapolate safely to substantially different compositions and process conditions. But that is only part of the story. Quantitatively sufficient simulation of the existing grade portfolio is a surprisingly complex, multi-facetted problem. An example is presented in Fig.1  of the model in routine use for Tata Steel’s plate mill at Scunthorpe, UK. Many variables are involved in the prediction of the optimum rolling schedule in terms of productivity, plate geometry, and final properties, each of which will be associated with its own errors. Its configuration around the microstructural development during rolling brings in a range of variables which are very difficult to measure on plant, and are usually implied from laboratory studies. Extensive plant data exist but are seldom transferable to other mills owing to the individual set-up of each mill and its associated data measurement facilities. Similarly, great care has to be taken to avoid systematic differences between laboratory simulation, pilot rolling, and results on plant, owing to subtleties of the set conditions and accuracy of measurement. An overview of such experience in Tata Steel’s European plate and strip mills is given here.
Abstract: Recent observations regarding the transformation of deformed austenite are reviewed. It is shown that superequilibrium ferrite and pearlite can be formed at temperatures well above the Ae3 and Ae1, respectively. The role of the stored energy associated with the introduction of the dislocations introduced by the deformation is discussed. It is shown that the forward dynamic transformation into ferrite and pearlite is several orders of magnitude faster than the reverse static transformation back into austenite. The retarding effect of alloying additions such as niobium is also outlined. The results are interpreted in terms of the effect of deformation on the modified phase diagrams pertaining to the transformation of deformed austenite.
Abstract: It is shown that implementation of high strains through equal-channel angular pressing (ECAP) and/or rolling into alloys belonging to Al-Mg-Sc-Zr system allows achieving high strength and satisfactory ductility. It was shown that strain hardening gives a main contribution to overall strength increment attributed to intense plastic straining; the role of grain size hardening is minor. However, extensive grain refinement is a necessary condition for retaining sufficient ductility in full-hardened condition for these materials.