846 research outputs found
Characterization by Electron Diffraction of Two Thermodynamical Phases of Precipitation in Nb-Microalloyed Steels
Excellent mechanical properties (high strength and toughness) of microalloyed steels are mainly caused by induced precipitation during thermomechanical treatment (TMT) and grain refinement. It has been recently found that TMT of Nb-microalloyed steels can give rise to two different kinds of precipitates, manifested by the double plateau in the statically recrystallised fraction (Xa) against time curves. This work presents an electron diffraction study performed in a transmission electron microscope, equipped with an EDS analytical system. Lattice parameters of a great deal of particles, smaller than 200 nm and with face cubic centred structure, have been measured. Frequency distribution of the values of lattice parameters shows that these are grouped in two sets whose mean values are close. Comparison of these values with those found in the literature for carbides, nitrides and carbonitrides usually present in microalloyed steels demonstrates that they are Nb carbonitrides with slight stoichiometric differences (NbCxNy).Peer reviewe
MICROSTRUCTURAL STUDIES FOR OPTIMIZATION OF HEAT TREATMENT IN COMPONENTS OF STEEL X38CrMoV5-1 SUBJECTED TO HIGH STRESSES
This material X38CrMoV5-1 is an alloyed steel used for hot working, with good toughness and high resistance to thermal shock. The presence of Cr, Mo and V gives this steel a high resistance to wear, keeping its hardness properties at high temperature. Cr and Mo delay softening annealing and inhibit the grain growth. The great resistance to high temperatures of this type of steels is related with an easy martensitic transformation. This transformation happens even at low cooling speeds. The properties of these types of martensitic steels result as a consequence of their complex microstructure that is obtained by an extremely controlled thermal treatment. Dilatometric testing was performed on continuous cooling from austenization temperature (1050ºC). This testing shows the high hardenability of this type of steels. ATD studies have been done to complement the dilatometric testing. After the previous results, it has been considered that the optimal treatment to get tough and tenacious structure, consists in submitting material to an annealing processing at 780ºC/1hour, followed by a quenching treatment at 1020ºC/1hour and finally cooling in oil with a double tempering at 580ºC/2 hours. This treatment provides the best properties that guarantee service with safety parts.Peer Reviewe
Paper-based chromatic toxicity bioassay by analysis of bacterial ferricyanide reduction
Water quality assessment requires a continuous and strict analysis of samples to guarantee compliance with established standards. Nowadays, the increasing number of pollutants and their synergistic effects lead to the development general toxicity bioassays capable to analyse water pollution as a whole. Current general toxicity methods, e.g. Microtox®, rely on long operation protocols, the use of complex and expensive instrumentation and sample pre-treatment, which should be transported to the laboratory for analysis. These requirements delay sample analysis and hence, the response to avoid an environmental catastrophe. In an attempt to solve it, a fast (15 min) and low-cost toxicity bioassay based on the chromatic changes associated to bacterial ferricyanide reduction is here presented. E. coli cells (used as model bacteria) were stably trapped on low-cost paper matrices (cellulose-based paper discs, PDs) and remained viable for long times (1 month at -20 °C). Apart from bacterial carrier, paper matrices also acted as a fluidic element, allowing fluid management without the need of external pumps. Bioassay evaluation was performed using copper as model toxic agent. Chromatic changes associated to bacterial ferricyanide reduction were determined by three different transduction methods, i.e. (i) optical reflectometry (as reference method), (ii) image analysis and (iii) visual inspection. In all cases, bioassay results (in terms of half maximal effective concentrations, EC50) were in agreement with already reported data, confirming the good performance of the bioassay. The validation of the bioassay was performed by analysis of real samples from natural sources, which were analysed and compared with a reference method (i.e. Microtox). Obtained results showed agreement for about 70% of toxic samples and 80% of non-toxic samples, which may validate the use of this simple and quick protocol in the determination of general toxicity. The minimum instrumentation requirements and the simplicity of the bioassay open the possibility of in-situ water toxicity assessment with a fast and low-cost protocolPostprint (author's final draft
Etude méthodologique des transferts d'eau et des sels dans un monolithe de sol argileux : 2ème partie. Caractérisation hydrodynamique et modélisation des transferts d'eau
TPM Maintenance Management Model Focused on Reliability that Enables the Increase of the Availability of Heavy Equipment in the Construction Sector
The purpose of this paper is to present a maintenance study focused on total productive maintenance (TPM) and reliability-centered maintenance (RCM). Its approach is based on the first pillars of TPM, preventive and autonomous maintenance, as well as the FMEA analysis of RCM for maintenance analysis, which was conducted in this study. The implementation of TPM was successful in that various preventive maintenance (PM) policies assigned to the assets were implemented and it was demonstrated that TPM application in the construction industry could reduce the excessive accumulation of maintenance with the same effective optimization, and with support from RCM analysis and its heavy equipment systems analysis. Excessive corrective maintenance accounts for high investment and delay rates in work times of the assigned project. Traditional methods of availability guarantee, such as reactive or routine maintenance, are insufficient to satisfy a heavy equipment maintenance plan; therefore, what is called for is the systematic application of RCM and TMP because they allow the selection and application of effective PM tasks. An approach that develops and thoroughly analyzes the strategies of continuous corrective and PM is used with an atmosphere of uncertainty and with operational data limited by criticism. Results show a 90% improvement in availability.Revisión por pare
Influence of the hydrocarbons diesel, gasoline, and benzene on the growth and mineral and antioxidant concentrations of tomato plants
The produced water is obtained during the extraction process of hydrocarbons, whose characteristics, composition and concentration depend on the reservoir that contains them. The waters produced contain hydrocarbons and heavy metals, and may contain essential elements for plant nutrition. Some studies indicate that for plants the most toxic components of the produced water are the hydrocarbons. This research aimed to evaluate the response in the pH and the electrical conductivity (EC) of irrigation leachate, morphological variables, mineral concentration and the generation of antioxidants in the tomato plants treated with diesel, gasoline and benzene in concentrations of 15 and 30 mg L-1, simulating the use of water produced for irrigation. An analysis of variance and tests of means of least significant difference was performed. The hydrocarbon treated plants reached the fifth cut of ripe fruits, except the treatment of diesel at 30 mg L-1, in which only 45% of the plants survived, and only the first harvest of ripe fruits was obtained. According to their type and concentration, the hydrocarbons produced both favourable and unfavourable changes in the pH, EC, stem diameter, plant height and dry fruit weight. Also, the hydrocarbons produced both beneficial and detrimental changes in the mineral concentration of the plants; however, the hydrocarbons inhibited the mineral concentration in the fruits. The level of ascorbate in the fruits was decreased, and the diesel treatments limited the accumulation of lycopene
Evolution of the Density of States Gap in a Disordered Superconductor
It has only recently been possible to study the superconducting state in the
attractive Hubbard Hamiltonian via a direct observation of the formation of a
gap in the density of states N(w). Here we determine the effect of random
chemical potentials on N(w) and show that at weak coupling, disorder closes the
gap concurrently with the destruction of superconductivity. At larger, but
still intermediate coupling, a pseudo-gap in N(w) remains even well beyond the
point at which off-diagonal long range order vanishes. This change in the
elementary excitations of the insulating phase corresponds to a crossover
between Fermi- and Bose-Insulators. These calculations represent the first
computation of the density of states in a finite dimensional disordered fermion
model via the Quantum Monte Carlo and maximum entropy methods.Comment: 4 pages, 4 figure
Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities
A model for the onset of the reduction in SRF cavity quality factor, the
so-called Q-drop, at high accelerating electric fields is presented. Breakdown
of the surface barrier against magnetic flux penetration at the cavity equator
is considered to be the critical event that determines the onset of Q-drop. The
worst case of triangular grooves with low field of first flux penetration Hp,
as analyzed previously by Buzdin and Daumens, [1998 Physica C 294: 257], was
adapted. This approach incorporates both the geometry of the groove and local
contamination via the Ginzburg-Landau parameter kappa, so the proposed model
allows new comparisons of one effect in relation to the other. The model
predicts equivalent reduction of Hp when either roughness or contamination were
varied alone, so smooth but dirty surfaces limit cavity performance about as
much as rough but clean surfaces do. When in combination, contamination
exacerbates the negative effects of roughness and vice-versa. To test the model
with actual data, coupons were prepared by buffered chemical polishing and
electropolishing, and stylus profilometry was used to obtain distributions of
angles. From these data, curves for surface resistance generated by simple flux
flow as a function of magnetic field were generated by integrating over the
distribution of angles for reasonable values of kappa. This showed that
combined effects of roughness and contamination indeed reduce the Q-drop onset
field by ~30%, and that that contamination contributes to Q-drop as much as
roughness. The latter point may be overlooked by SRF cavity research, since
access to the cavity interior by spectroscopy tools is very difficult, whereas
optical images have become commonplace. The model was extended to fit cavity
test data, which indicated that reduction of the superconducting gap by
contaminants may also play a role in Q-drop.Comment: 15 pages with 7 figure
Quantum superconductor-metal transition
We consider a system of superconducting grains embedded in a normal metal. At
zero temperature this system exhibits a quantum superconductor-normal metal
phase transition. This transition can take place at arbitrarily large
conductance of the normal metal.Comment: 13 pages, 1 figure include
The Superconductor-Insulator Transition in a Tunable Dissipative Environment
We study the influence of a tunable dissipative environment on the dynamics
of Josephson junction arrays near the superconductor-insulator transition. The
experimental realization of the environment is a two dimensional electron gas
coupled capacitively to the array. This setup allows for the well-controlled
tuning of the dissipation by changing the resistance of the two dimensional
electron gas. The capacitive coupling cuts off the dissipation at low
frequencies. We determine the phase diagram and calculate the temperature and
dissipation dependence of the array conductivity. We find good agreement with
recent experimental results.Comment: 4 pages, 4 .eps figures, revte
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