Influência do processo de ferroxidação nas propriedades de ferro sinterizado

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 1996A ferroxidação ou tratamento a vapor é um processo termoquímico largamente utilizado na indústria de componentes ferrosos sinterizados, com o objetivo de melhorar suas propriedades, tais como: resistência ao desgaste, resistência à corrosão, resistência à compressão e promover o selamento da rede comunicante de poros. Neste trabalho realizou-se um estudo com o objetivo de avaliar a influência do processo de ferroxidação sobre o limite de resistência à tração, o alongamento, a dureza aparente e a adesividade da camada superficial, de componentes sinterizados em ferro puro. Tomando-se como material dois tipos de pó de ferro, um atomizado (ANCOR 1000C) e outro reduzido (ANCOR MH-100), produziu-se corpos de prova do tipo cotonete (MPA), em diferentes níveis de densidade. As amostras foram sinterizadas e ferroxidadas em condições industriais. Através dos resultados obtidos constatou-se que os níveis de oxidação atingido pelas amostras é função de sua densidade inicial e do tipo de pó de ferro utilizado. O limite de resistência a tração e a dureza aparente aumentam em todas as densidades ensaiadas, independente do tipo de pó de ferro. O alongamento tem seus valores severamente reduzidos após a ferroxidação. As amostras de média densidade produzidas com o pó de ferro atomizado (ANCOR 1000C), apresentaram os melhores resultados quanto a desividade da camada superficial.The steam treatment is broadly a process thermochemical used in the industry of components ferrous sinterizados, with the objective of improving its properties, such as: resistance to the wear, resistance to the corrosion, resistance to the compression and to promote the sealing of the communicating net of pores. In this work it was made a study with the objective of evaluating the influence of the steam treatment process on the resistance limit to the traction, the prolongation, the apparent hardness and the adhesiveness of the superficial layer, of component sinterizados in pure iron. Using as material, two types of powder of iron, an atomized (ANCOR 1000C) and another reduced (ANCOR MH-100), it was produced bodies of test of the type "swab " (MPA), in different density levels. The samples were sinterizadas and steam treatment in industrial conditions. Through the obtained results it was verified that, the level of oxidation of the samples is function of your initial density and of the type of powder of used iron. The resistance limit to the traction and the apparent hardness increase in all the rehearsed densities, independent of the type of powder of iron. The prolongation has your values severely reduced after the steam oxidation The samples of medium density produced with the powder of atomized iron (ANCOR 1000C), presented the best results as the adhesiveness of the superficial layer

Point defect dynamics in bcc metals

We present an analysis of the time evolution of self-interstitial atom and vacancy (point defect) populations in pure bcc metals under constant irradiation flux conditions. Mean-field rate equations are developed in parallel to a kinetic Monte Carlo (kMC) model. When only considering the elementary processes of defect production, defect migration, recombination and absorption at sinks, the kMC model and rate equations are shown to be equivalent and the time evolution of the point defect populations is analyzed using simple scaling arguments. We show that the typically large mismatch of the rates of interstitial and vacancy migration in bcc metals can lead to a vacancy population that grows as the square root of time. The vacancy cluster size distribution under both irreversible and reversible attachment can be described by a simple exponential function. We also consider the effect of highly mobile interstitial clusters and apply the model with parameters appropriate for vanadium and $\alpha-$iron.Comment: to appear in Phys. Rev.

Topology and phase transitions: a paradigmatic evidence

We report upon the numerical computation of the Euler characteristic \chi (a topologic invariant) of the equipotential hypersurfaces \Sigma_v of the configuration space of the two-dimensional lattice $\phi^4$ model. The pattern \chi(\Sigma_v) vs. v (potential energy) reveals that a major topology change in the family {\Sigma_v}_{v\in R} is at the origin of the phase transition in the model considered. The direct evidence given here - of the relevance of topology for phase transitions - is obtained through a general method that can be applied to any other model.Comment: 4 pages, 4 figure

Stretched--exponential relaxation in arrays of coupled rotators

We consider the non--equilibrium dynamics of a chain of classical rotators coupled at its edges to an external reservoir at zero temperature. We find that the energy is released in a strongly discontinuous fashion, with sudden jumps alternated with long stretches during which dissipation is extremely weak. The jumps mark the disappearance of strongly localized structures, akin to the rotobreather solutions of the Hamiltonian model, which act as insulating boundaries of a hot central core. As a result of this complex kinetics, the ensemble--averaged energy follows a stretched exponential law until a residual pseudo--stationary state is attained, where the hot core has reduced to a single localized object. We give a statistical description of the relaxation pathway and connect it to the properties of return periods of rare events in correlated time series. This approach sheds some light into the microscopic mechanism underlying the slow dynamics of the system. Finally, we show that the stretched exponential law remains unaltered in the presence of isotopic disorder.Comment: 13 Figure

Topological aspects of geometrical signatures of phase transitions

Certain geometric properties of submanifolds of configuration space are numerically investigated for classical lattice phi^4 models in one and two dimensions. Peculiar behaviors of the computed geometric quantities are found only in the two-dimensional case, when a phase transition is present. The observed phenomenology strongly supports, though in an indirect way, a recently proposed topological conjecture about a topology change of the configuration space submanifolds as counterpart of a phase transition.Comment: REVTEX file, 4 pages, 5 figure

Mesoscopic lattice Boltzmann modeling of flowing soft systems

A mesoscopic multi-component lattice Boltzmann model with short-range repulsion between different species and short/mid-ranged attractive/repulsive interactions between like-molecules is introduced. The interplay between these composite interactions gives rise to a rich configurational dynamics of the density field, exhibiting many features of disordered liquid dispersions (micro-emulsions) and soft-glassy materials, such as long-time relaxation due to caging effects, anomalous enhanced viscosity, ageing effects under moderate shear and flow above a critical shear rate.Comment: 4 pages, 4 figure

Liguria: aperture e chiusure di un’isola fra due pianure

This paper discusses Neolithic and Copper Age Liguria, Braudel’s ‘island’, a mountain chain situated between the Mediterranean and the Po plain. The Neolithic colonisation of western Liguria and the Mediterranean coasts of France happened a short time (as little as one century) after that of southern Italy and before either central Italy, Corsica or Sardinia; excavations at the Arene Candide cave confirm that the initial Neolithic is characterised by Impressa ware, followed by a Cardial phase, and that it is a full Neolithic, complete with cereals and domestic animals. The colonists appear to have chosen an area where later Mesolithic hunter-gatherers are not attested, although they traded for lithic materials with the neighbouring hunter-gatherer populations of eastern Liguria. The importance of the ophiolites of Liguria is indicated by the trade in polished axes as far as Puglia, by inclusions in early Neolithic pottery and by the finding of Hordeum pollen dated 6222-5990 cal BC in a core from the prehistoric lagoon at Sestri Levante. In the Cardial phase, western Liguria became part of a larger cultural area including southern France. In the following centuries, in the context of the middle Neolithic Square-Mouthed Pottery culture, the region took on its historical role as the gateway between the Mediterranean and the Po plain. The later Neolithic Chassey culture, shared with southern France, comprised the development of mountain pastoralism and cheesemaking, which facilitated the exploitation of mountain resources such as copper that began in this period

Storm event impact on organic matter flux, composition and reactivity in Taskinas Creek, VA

Carbon export from the land to the ocean are an important part of the global carbon cycle, linking terrestrial watersheds and the global carbon cycle. Burial of terrestrial organic carbon represents a long term sink for atmospheric CO2. Approximately 0.4 Pg Cy-1 is delivered to the global ocean from rivers, equally divided between POC and DOC. However, the amount of carbon entering the ocean is a small portion of the total amount entering rivers from the terrestrial environment, suggesting a large amount of processing in inland waters and estuaries. Most monitoring efforts have focused the processing of organic matter on baseflow conditions. However, recent studies have shown that POC and DOC exported during storm events, a small time period during a hydrologic year, can account for the majority of the annual carbon exported from small watersheds. This dissertation identifies the impact different magnitudes of storm events have on the source, composition and reactivity of organic carbon released to downstream waters from the terrestrial environment at Taskinas Creek, Virginia. The proximity of the Creek to the York River estuary, the changes in water table at the site, along with the small size of the watershed allowing opportunity to examine the connectivity between the watershed processes and delivery of organic matter made the site ideal for identifying how hydrology and environment alter POM and DOM export and reactivity. The sources, composition and flux of DOM and POM were measured during four storm events of different magnitudes to determine how events impacted the sources and fluxes of organic matter and the % reactive DOC exported. Events of different magnitudes with varying sources of DOC and POC had similar % reactive DOC that was not predicted using excitation emission spectroscopy. The events resulted in DOC fluxes 1.5-490 fold higher than baseflow. POC fluxes for storm were 6.7-55 times higher than DOC fluxes. Although the % reactive DOC did not increase during storm event conditions, coupled with the overall flux, storm events represent a considerable pulse of % reactive DOC to downstream waters, well above baseflow levels. When considered with increases in storm intensity due to climate change, storm event fluxes of reactive OM may have broad impacts on estuaries and the global carbon cycle through changes in carbon storage