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[en] Full Text: Nitriding is a superficial treatment of great importance for the industry in general as it provides improvements in hardness, resistance to wear, fatigue and corrosion, so it is widely used in various industries due to its great advantages. This thermo-chemical treatment can be done by bath salts, gas or plasma, the latter has many advantages; among them we can mention the low environmental impact that allows a high control over the process variables, managing to control the thicknesses of the composite layer and the exterior which guarantees the repeatability and accuracy of the properties obtained. In Costa Rica, this technique has not been applied yet and no studies have been carried out, so this project intends to take a first step to create a procedure to carry out this surface treatment in steels commonly used in Costa Rica. The research and development of the experiments will focus on American Iron and Steel Institute (AISI) 4140 steel, this is because it is one of the most used in various applications in the industry of our country, due to its high mechanical properties that a common carbon steel is not able to meet and the best options that are counted are the tool-type steels or micro-alloy steels; tool types are steels of very high mechanical properties and are characterized by high performance under extreme conditions and commonly with a heat treatment these properties are achieved, but have the disadvantage that their cost is very high; so they appear micro-alloyed steels, specifically AISI 4140 steel, which, by itself has good qualities, but adding a surface treatment such as nitriding make it very useful in various industries, as the properties they obtain they can be compared with those of a steel tool but for a much lower cost. It is intended to perform 5 experiments at different times to obtain a curve of thickness of layer vs. time that allows to predict the time required for a certain thickness, at each time 6 specimens will be nitrided, 3 for the tensile test and 3 for the impact test, with which we will obtain enough data to support the results of this project
[es]Full text: La nitruracion es un tratamiento superficial de suma importancia para la industria en general ya que brinda mejoras en la dureza, resistencia al desgaste, a la fatiga y a la corrosion, por lo que es muy utilizado en diversas industrias debido a sus grandes ventajas. Este tratamiento termo-quimico puede hacerse por bano de sales, gaseosa o plasma, este ultimo presenta bastantes ventajas; entre ellas se puede mencionar el bajo impacto ambiental que permite un alto control sobre las variables de proceso, logrando controlar los espesores de la capa compuesta y la exterior lo que nos garantiza la repetitividad y la exactitud de las propiedades obtenidas. En Costa Rica esta tecnica no se ha aplicado aun y tampoco se han realizado estudios, por lo que este proyecto pretende dar un primer paso que permita crear un procedimiento para llevar a cabo este tratamiento superficial en los aceros de uso comun en Costa Rica. La investigacion y el desarrollo de los experimentos se enfocaran en el acero American Iron and Steel Institute (AISI) 4140, esto debido a que es uno de los mas utilizados en diversas aplicaciones en la industria de nuestro pais, debido a sus altas propiedades mecanicas que un acero al carbono comun no es capaz de cumplir y las mejores opciones con las que se cuentan son los aceros tipo herramientas o aceros micro-aleados; los tipo herramienta son aceros de muy elevadas propiedades mecanicas y se caracterizan por un alto rendimiento ante condiciones extremas y comunmente con un tratamiento termico se logran dichas propiedades, pero tienen la desventaja de que su costo es muy elevado; por lo que aparecen los aceros micro-aleados, especificamente el acero AISI 4140, el cual, por si solo presenta buenas cualidades, pero al sumarsele un tratamiento superficial como la nitruracion lo hacen de mucha utilidad en diversas industrias, ya que las propiedades que obtienen logran ser comparables con las de un acero herramienta pero por un costo mucho menor. Se pretende realizar 5 experimentos a distintos tiempos para lograr obtener una curva de espesor de capa vs tiempo que permita predecir el tiempo requerido para cierto espesor, a cada tiempo se nitruraran 6 probetas, 3 para el ensayo de traccion y 3 para el de impacto, con lo cual obtendremos suficientes datos que respalden los resultados de este proyecto
[en] It has been demonstrated that toughness of structural steels depends mainly on the ratio of the balance established between precipitation hardening and grain refinement as well as the morphological and dispersion changes of microstructural components. With the aid of the controlled evolution of the structure of metals in the thermomechanical and controlled cooling processes the required increase in the yield strength and improvement of fracture toughness of microalloyed 18G2Va and 16G2Vn steels and ultra low carbon bainitic steels (ULCB) with higher manganeses content has been achieved. (author). 31 refs, 6 figs
[en] In this study, the fatigue properties of SAE 8620 case hardened steel was investigated. Carburizing, shot peening and their combinations were applied to SAE 8620 steel fatigue specimens. Different fabrication parameters of carburizing were applied. Shot peening was also applied to carburized and non carburized specimens. Hardness and residual distribution of mechanical and heat-treated fatigue specimens were determined. Rotating bending fatigue tests were performed using treated test specimens. Plotted S-N curves, for treated at different conditions and different ways, were compared to curves for drawn specimens. The maximum increase in fatigue limit is determined for double quenched and shot peened specimens as 72%. (author)
[en] The paper presents results from studies on the thermal processes taking place during electron-beam treatment of tool steels. A thermal model is presented describing these processes and results are given of calculations performed concerning the thermal field and the dimensions of the zone of structural changes due to electron-beam modification of specimens of tool steels.
[en] Graphite flakes were divided into two equal parts Part A and Part B. Part A was thermally shocked at 800 degree C under inert atmosphere of argon for two min before exfoliation under sonic tip while Part B was exfoliated as such using acetonitrile (ACN) as exfoliating media in both cases at 15-20 degree C. The concentration of the graphene as a result of exfoliation and centrifugation at 500 rpm for 45 min in both cases were determined. After ten hours of sonication the concentration of graphene in Part A is >12mg/ml with few layered thickness while that of Part B was <5mg/ml consisting of more layers than Part A. UV spectroscopy, Raman, Transmission Electron Microscopy and Scanning Electron Microscopy (SEM) were used for the study of these nanoflakes. (author)
[en] The understanding and modelling of PCI failure mechanism is a key aspect to demonstrate fuel design suitability for more flexible/demanding modes of operation, like load following. In order to assess PCI properly, it is a must to accurately estimate the driving force of the clad mechanical load (PCMI). Even though a lot of work has been carried out on this issue, less work has been conducted on the gaseous swelling contribution when high burnups are reached. The main goal of this work is the assessment of the gaseous swelling effect on PCMI under ramping conditions, taking into account a typical PWR fuel rod irradiated up to 75 GWd/tU. Particularly, this study is focused on reactor startups and anticipated operational occurrences (AOOs) supposed to occur at different burnups. To perform this work, the thermo-mechanical code FRAPCON-3 is used along with some key hypotheses concerning the irradiation history and the ramps studied. The study carried out has pointed out a noticeable contribution of the gaseous swelling effect on PCMI under the ramps considered. It can be inferred implications on safety margins to be considered, especially at high burnups (> 55 GWd/tU): increase of the margin related to cladding stress under reactor startups and decrease of the acceptable power increment in AOOs. Given the significant impact of gaseous swelling found in the study, a critical review of the model used in FRAPCON-3 is to be conducted.
[en] Agar-Agar is an algal polysaccharide ingredient used recently, in indigenously developed polyvinyl alcohol based radiation processed hydrogel wound dressing( HWD). As it is obtained from sea weeds, a natural source, variation in molecular weight and possible presence of other polymeric structures, the required properties, specially the gelling property, may vary. These variations could change HWD characteristics. Thermomechanical technique in static and dynamic load can be used to study the agar gel properties. The studies reveal that irradiation degrades agar in dry state and the gel formed by such agar shows significant deviations in softening point, peak melting temperature and compression characteristics under cyclic load. (author)
[en] In this work, the effects of different amounts (5, 10, 20, 40%) of compressive deformation of austenite on the isothermal transformation of bainite in a Fe-Mn-Si-C alloy steel have been investigated. It is found that the prior deformation of austenite retards significantly the bainitic transformation. At the same isothermal transformation temperature, as the amount of prior deformation is higher, the quantity of bainite (which can be obtained after the isothermal transformation is completed) becomes less and it is also more difficult to develop the classical sheaf-like structure. The evidence implies that severe deformation causes mechanical stabilisation of austenite for bainitic transformation. (orig.)
[en] The prediction of welding distortions of large welded assemblies is a complex issue which is even more difficult when an accurate and fast analysis is demanded. Distortion simulation is always associated with huge computational costs in terms of computer memory and CPU time. The vertical tool RED-WeldS “Rapid Evaluation of Distortions in Welding Structures” developed in commercial software NSYS®, allows obtaining a fast and enough accurate solution for the prediction of structure distortions due to the welding process. Thus, the manufacturing of large welded assemblies can be simulated considering different welding sequences in order to choose the one which distortion is minimal. In-plane and out-ofplane distortions are taken into account through thermal shrinkage model, which combined with a static linear-elastic analysis leads to fast solutions. A very important reduction of computational time is achieved when using this simplified mechanical model instead of more traditional transient thermo-mechanical-metallurgical or thermo-mechanical non-linear analyses. As application / validation, the distortions due to the welding operations for the connection of two carbon steel nozzles to the shell of a pressure vessel were simulated and the results were compared to the true measurements performed in the workshop after welding. Pre-processing and computational times are summarised to show the power of this numerical approach which will help manufacturers to determine optimum welding sequences to fulfill dimensional criteria.