Defect and Diffusion Forum Vols. 283-286

Abstract: For simulating flows in a porous medium, a numerical tool based on the Lattice Boltzmann Method (LBM) is developed with regards to the classical D2Q9 model. A short description of this model is presented. This technique, applied to two-dimensional configurations, indicates its ability to simulate phenomena of heat and mass transfer. The numerical study is extended to estimate physical parameters that characterize porous materials, like the so-called Effective Thermal Conductivity (ETC) which is of our interest in this paper. Obtained results are compared with those which could be found analytically and by theoretical models. Finally, a porous medium is considered to find its ETC.

Abstract: Microstructure influence on hydrogen trapping in a Cr-Mo type steels −2.25Cr 1Mo and 2.25Cr 1Mo 0.25V− was studied by means of electrochemical permeation test, thermal desorption spectrometry, scanning and transmission electron microscopy analysis. Both steels, used in hydrogenation reactors, in as received and artificial aged conditions exhibit a bainitic microstructure with CrxMoy and CrxMoyVz carbides finely dispersed. The hydrogen diffusivity for the 2.25Cr-1Mo-0.25V is lower than 2.25Cr-1Mo due to its higher carbide precipitation. At aged conditions TDS on samples cathodically charged with hydrogen showed an increase on the hydrogen trapping capacity for 2.25Cr-1Mo and a reduction for the vanadium modified steel, compared with the as-received state.

Abstract: In foundry technology the modeling of heat transfer in materials containing exothermic components must take into consideration the presence of heat sources in the Fourier–Kirchhoff equation. The aim of this investigation was the identification of real and effective thermophysical parameters of the insulating and insulating –exothermic materials used as riser sleeves containing these exothermic heat sources. The experiments of liquid steel or cast iron pouring into the mould, containing different insulating and exothermic sleeves were carried out, using thermocouples meas-urement systems (thermal analysis of casting–mould system). Then the thermo-physical coefficients of these materials were calculated using inverse problem solution. The numerical system Calcosoft and its Inverse Solution procedure were applied.

Abstract: The problems of thermomechanical phenomena, which occur in low stability – porous ceramic material (LS-PCM) submitted to thermal shock, are discussed. The authors tried to answer the question concerning the usefulness of Hot Distortion tests (HD) for estimation of parameters necessary in data base of simulation codes, which permit to simulate the phenomena in casting-mould system. These parameters should be accessible as temperature function, especially in high temperature range of LS-PCM used as mold heating using poured casting. The new methodology and equipment, based on British Cast Iron Association (BCIRA) method, allowed to study LS-PCM specimen feature called Hot Distortion method. In this paper the modified HD methodology, are shown. The modified apparatus DMA includes two thermal sources (electric and gaseous) used to 114 x 25 x 6 mm and 120 x 22 x 22 mm sample heating were tested and analyzed. The measurement and recording of chosen parameters, i.e. deformation (distortion) during heating and variability of surface temperature fields in heated sample, by use of pyrometer and thermal camera are presented. The virtual volume temperature field in heating sample was simulated. The comparative analysis of both, experimental and virtual results are realized. This permitted also to estimate the substitute thermo-physical coefficients using inverse solution by try & error method for predefined boundary conditions on bottom and upper specimen surfaces.

Abstract: This article presents and discusses a network model to describe and predict the behaviour and performance of catalyst particles. The differences and advantages of this approach when compared to the continuous models currently used in practice are highlighted and critically assessed. The local structure of the catalyst particle is modelled using a three dimensional network model made up of cylindrical pores and nodes of negligible volume. In the pores a homogenous first-order reaction takes place, coupled with the diffusion. For steady state conditions the concentration field can be obtained solving a sparse linear system of equations, obtained by solving the mass balance equations written for the network nodes and using the concentration profile in the network pores. The influence of the boundary conditions and the network sizes was investigated, showing the results in particular that the nature of the boundary conditions can have a profound impact in the predictions of the model.

Abstract: The movement of multiple markers (M-M) embedded in a multiple phases diffusion couple (M-couple) has been numerically analyzed for binary two phases models taking the molar volume change effect to the diffusion direction into account. From the results obtained by this analysis the places where vacancies are annihilated or generated can be visualized. It has been clarified that a part of M-M is necessarily shown by a linear line due to parabolic movement of the inter-phase interface. Some other interesting results obtained in this study will be reported.

Abstract: In order to improve the poor surface hardness and the wear resistance, titanium has been nitrided with plasma (ion) nitriding which is one of the methods to treat surface properties of titanium alloys. The increment at surface hardness and so the wear resistance of nitrided titanium alloys has been provided by means of compound layer (ε-Ti2N+δ-TiN) and diffusion zone (α-Ti) occurred by plasma ion nitriding. The goal of the present paper is to investigate effects of nitriding temperature and nitriding time on the microstructure and hardness value of nitrided surface layers. A systematic study was undertaken with specimens of commercial pure Ti and Ti-6Al-4V alloy. As treatment parameters, we have used; nitriding time (from 2 to 9 hour), nitriding atmosphere (H2-80%N2), total pressure (1 kPa) and cathode temperature (from 600 to 800 oC). The Vickers indenter was used for analysis of the micro hardness measurements. The thin hardened layer at the nitrided surface was characterized by glancing-angle X-ray difractometer. X-ray diffraction analysis has confirmed the formation of ε-Ti2N and δ-TiN phases on the nitrided specimens. Experimental details and characterization of plasma (ion) nitrided titanium have reported and discussed.

Abstract: Magnetic properties of nano-crystalline yttrium iron garnet, Y3Fe5O12 and bismuth iron garnet, Bi3Fe5O12 were studied. The samples were synthesized by using sol gel route. The gel were prepared using nitrates of yttrium and iron for the yttium iron garnet (YIG) and nitrates of bismuth and iron for the bismuth iron garnet (BIG). The raw materials were mixed and dissolved in citric acid and were stirred for 3 months in room temperature until the gel was observed. The X-ray diffraction pattern reveals the cubic structure of YIG and BIG samples at 7000C and 5000C respectively. From the M-H diagrams it was found that the YIG and BIG samples have saturation magnetization, Ms of 27.2 Gs with rectangular hysteresis loop and 24.4 Gs with S hysteresis loop, respectively. It is evident from the FESEM micrographs that the rectangular loop shape (YIG) arises from the homogeneous and rounded microstructure as compared to the inhomogenous and random orientation microstructure (BIG) of the samples. From the Electron diffraction done on Transmission Electron Microscope, both of the samples were shown to be single crystals.

Abstract: Steel containing a high Si-content is mainly used as electrical steel in flux carrying electrical machines. These materials are divided in the categories: grain oriented and non oriented electrical steels, mainly used in transformers and electrical motors, respectively. Their industrial production is normally limited to silicon contents lower than 3.5 m.-%, due to the generation of brittle ordered structures if the Si content is increased beyond this value. The paper reports on microstructure and texture evolution during processing by rolling of electrical steel in the high Si-range. The materials studied are two industrial electrical steels with a silicon content of 2.4 and 3.2 m.-%, their situation was as-received after hot rolling and industrial annealing. The different processing parameters, as rolling temperatures and cooling conditions have a strong influence on the final microstructures and textures. The importance of hot rolling and intermediate annealing processes is enhanced since above 2 m.-% Si these steels do not experience the usual α-γ-α phase transformation, because they present a bcc crystal structure over the entire solidus domain. Consequently, their microstructures and textures are strongly inherited from the earlier processing steps into the final product. The as-received materials were cold rolled with a nominal reduction of 75%. Their microstructures and textures were analysed by EBSD. The results obtained were compared with those of the industrial hot band. The textures were studied by the interpretation of the most important crystallographic fibre textures, extracted from the ODF’s of φ2 = 45o section of the Euler space. Special attention was given to the evolution of the most important magnetic textural components. Although in terms of grain shape, IQ, texture and normalised thickness position or ‘s’-parameter the microstructures obtained before and after cold rolling are totally different, the overall crystallographic textures seem not to differ very much.

Abstract: Oxide scales growing during hot rolling of steel represent an industrial and environmental problem. Tertiary oxide, which starts to form before entering the finishing stands, remains on the steel surface until the end of the process, affecting the final surface quality and the response to downstream processing. Characterizing scale layers and the scale/steel interface in terms of phase morphology, texture, grain structure and chemical composition is fundamental for a better understanding of their behaviour and the effect of thermomechanical cycles on the material response to further processing. Thin tertiary scale layers have been grown on ULC steel under controlled conditions in a laboratory device adequately positioned in a compression-testing machine, immediately before plane strain deformation. After heating under a protective atmosphere (nitrogen), the samples have been oxidized in air at 1050°C for a short oxidation time. Immediately after this controlled oxidation, some of the samples were subjected to plane strain compression (PSC) inside the experimental device, in order to simulate the finishing hot rolling process. Direct observations of oxide scale layers are impossible. The EBSD technique has been identified as a powerful tool that can be used to reveal the microstructure within the oxide scale and to distinguish between its constitutive phases, based on their distinct crystal lattices. The texture of the deformed oxide scales, originally grown on ULC steel, was determined in a SEM using the EBSD technique. This will help to achieve a better understanding of their complex deformation behaviour. Because the substrate deformation affects the oxide layer, orientation relationships between scale layer and substrate were measured and the crystallographic orientation between undeformed and deformed areas was determined. Strongly textured wustite grains with a clearly pronounced columnar structure were observed after oxidation at 1050°C. The detailed EBSD study reveals that the oxide layer is able to accommodate a significant amount of deformation.