Toward a heuristic optimum design of rolling schedules for tandem cold rolling mills

Scheduling for tandem cold mills refers to the determination of inter-stand gauges, tensions and speeds of a specified product. Optimal schedules should result in maximized throughput and minimized operating cost. This paper presents a genetic algorithm based optimization procedure for the scheduling of tandem cold rolling mills. The optimization procedure initiates searching from a logical staring point — an empirical rolling schedule — and ends with an optimum cost. Cost functions are constructed to heuristically direct the genetic algorithm’s searching, based on the consideration of power distribution, tension, strip flatness and rolling constraints. Numerical experiments have shown that the proposed method is more promising than those based on semi-empirical formulae. The results generated from a case study show that the proposed approach could significantly improve empirically derived settings for the tandem cold rolling mills.     

Research status and issues of tungsten plasma facing materials for ITER and beyond

This review summarizes surface morphology changes of tungsten caused by heat and particle loadings from edge plasmas, and their effects on enhanced erosion and material lifetime in ITER and beyond. Pulsed heat loadings by transients (disruption and ELM) are the largest concerns due to surface melting, cracking, and dust formation. Hydrogen induced blistering is unlikely to be an issue of ITER. Helium bombardment would cause surface morphology changes such as W fuzz, He holes, and nanometric bubble layers, which could lead to enhanced erosion (e.g. unipolar arcing of W fuzz). Particle loadings could enhance pulsed heat effects (cracking and erosion) due to surface layer embrittlement by nanometric bubbles and solute atoms. But pulsed heat loadings alleviate surfaces morphology changes in some cases (He holes by ELM-like heat pulses). Effects of extremely high fluence (∼10³⁰ m⁻²), mixed materials, and neutron irradiation are important issues to be pursued for ITER and beyond. In addition, surface refurbishment to prolong material lifetime is also an important issue.     

Influence of uniaxial stress on the morphology of coherent precipitates during coarsening—elastic energy considerations

The theory of inclusions and inhomogeneities by Eshelby is applied to predict the influence of uniaxial applied stress on the morphology of coherent precipitates during coarsening. The case where the misfit between the matrix and the precipitates is a pure dilatation is considered. The elastic energy is calculated as a function of the particle shape, the applied stress and the ratio between the Young's modulus of the precipitates and that of the matrix. Three shapes are considered: spheres, plates perpendicular to the stress axis and needles aligned with the stress axis. The calculations are used to construct a map which gives the conditions leading to the lowest elastic energy. The results are compared with observations in Ni base alloys which have shown that a stress applied during coarsening can alter the shape of γ particles.     

Mechanical and ablative properties improvement of HfC-SiC coatings upon introduction of TiC

A novel structural Hf-Ti-Si-C multiphase solid solution coating was designed and manufactured by chemical and solid solution reactions to improve the mechanical and ablation behavior of HfC-SiC coatings. Results show that, with TiC addition, the formed Hf_xTi_1?xC and (Ti_1?xHf_x)₃SiC₂ solid solutions can significantly enhance the micro-mechanical and ablation properties of the coating. The improved hardness and modulus, as well as the reduced ablation rates are mainly attributed to the optimized structure and solid solution reinforcing effect of coating. Moreover, the Ti-additives are conducive to restrain the active oxidation of SiC. Furthermore, HfTiO₂ can reduce the oxides cracking due to the inhibited crystal transformation of HfO₂ and its good self-healing ability, forming a dense and stable oxide scale with superior thermal protection.     

On the origin of the high coarsening resistance of Ω plates in Al–Cu–Mg–Ag Alloys

The thickening kinetics of Ω plates in an Al–4Cu–0.3Mg–0.2Ag (wt. %) alloy have been measured at 200, 250 and 300°C using conventional transmission electron microscopy techniques. At all temperatures examined the thickening showed a linear dependence on time. At 200°C the plates remained less than 6 nm in thickness after 1000 h exposure. At temperatures above 200°C the thickening kinetics are greatly increased. Atomic resolution Z-contrast microscopy has been used to examine the structure and chemistry of the (001)_Ω6(111)_α interphase boundary in samples treated at each temperature. In all cases, two atomic layers of Ag and Mg segregation were found at the broad face of the plate. The risers of the thickening ledges and the ends of the plates were free of Ag segregation. The necessary redistribution of Ag and Mg accompanying a migrating thickening ledge occurs at all temperatures and is not considered to play a decisive role in the excellent coarsening resistance exhibited by the Ω plates at temperatures up to 200°C. Plates transformed at 200°C rarely contained ledges and usually exhibited a strong vacancy misfit normal to the plate. A large increase in ledge density was observed on plates transformed at 300°C, concomitant with accelerated plate thickening kinetics. The high resistance to plate coarsening exhibited by Ω plates at temperatures up to 200°C, is due to a prohibitively high barrier to ledge nucleation in the strong vacancy field normal to the broad face of the plate. Results also suggest that accommodation of the large misfit that exists normal to the broad face of the plate is unlikely to provide the driving force for Ag and Mg segregation.     

A role of fractions of mobile grain boundaries in secondary recrystallization of Fe–Si steels

Grain growth during annealing of polycrystalline materials is influenced by the type of grain boundaries a grain encounters as it grows. Simple computer experiments have been performed to ascertain which type of boundaries are responsible for the abnormal grain growth (AGG) in Fe–3% Si steels. In modelling abnormal grain growth, two different assumptions are used, the first one is that the coincidence site lattice (CSL) boundaries have high mobility and the second is that high energy boundaries are more mobile than other boundaries. The results of the computer experiments support the latter model for abnormal grain growth in Fe–3% Si steels. Finally, the importance of fractions of mobile grain boundaries on the development of Goss texture is discussed.