Papers by Keyword: Carbon Diffusion

Abstract: Forging of ductile cast iron with pure iron by the Damascus technique, results in a new composite material. The combination of cast iron and pure iron is unusual because of its rather different properties. After forging these two materials a small diffusion zone of about 150 µm was observed. Various heat treatments at 900 °C for 2, 4 or 20 hours and 950 °C for 4 h were performed to increase the diffusion zone up to 2.4 mm. At 900 °C carbon solubility in austenite is about 1.2 wt. % and at 950 °C 1.4 wt. %. During the heat treatment carbon diffuses from cast iron into the pure iron and the diffusion gradient grows with time and temperature. Furthermore, the samples were air cooled or water quenched. In the ductile cast iron, graphite nodules are surrounded by ferrite. During the heat treatment graphite is dissolved and pores are observed. In the diffusion gradient layer, a broad range of microstructures observed in hyper- and hypoeutectoid steels could be found. The microstructures were revealed by different etchants and moreover, hardness measurements were performed.

Abstract: The studies on cementation focus exclusively on the carbon’s movement. It is described by diffusion equations, often with constant coefficients and without regard to the liaising with temperature. It does not allow to have regard to the further carbon diffusion into the workpiece with the lower temperature range. The most accurate prediction of carbon concentration profiles depending on the parameters of the carburization regime and the chemical composition of steel is possible with the mathematical models using. However, most models show good results for Fe-C austenite without affecting the effect of alloying substitution elements. Taking into account the influence of alloying elements leads to complex empirical dependencies with difficult selected coefficients. It makes their use difficult. The study describes the simulation using the finite element method for the process of austenite’s diffusion saturation Fe-C-Cr system with carbon during cementation. Here is an example of a steel gear 15Cr2 with the temperature influence. The COMSOL Multiphysics program is used to solve the problem numerically. It is found that the model of carbon diffusion in unalloyed austenite for the single-stage cementation regime is in good contact with the experimental data for the Fe-C-Cr austenite of 15Cr2 steel. For a two-stage process, the calculation of the carbon concentration in the surface layer has a slightly greater deviation from the experimental data than it is at a greater depth.

Abstract: Critical component of boiler that frequently experiencing failure is on the junction of boiler tube consist of two different materials. This failure mechanism would lead to tube rupture and would be followed by power plant shutdown. Failure analysis has been conducted on dissimilar metal weld (DMW) of Ferritic SA-213 T22 welded with Austenitic SA-213 TP 304H. This tube is used in Suralaya Steam Power Plant. Operating temperature and pressure of the steam inside the tube in normal condition are respectively 196.8kg/cm² and 540 °C.In order to understand the real cause of failure several tests are ran, which are chemical composition test, micro vickers, and metallographic test on based metal, heat effective zone, and filler area. The tests are conducted in a layered manner to clearly understand the cause of the failure. Causes of the failure are due to the contribution of carbon diffusion and disparity of expansion coefficient of two materials.

Abstract: In this paper, a formula for the calculation of the carbon content during the austenitizing of cast iron was deduced, considering the effect of silicon content upon the heat-treatment parameter. According to this formula, the carbon content of the austenite at a certain austenization temperature for a cast iron with given components can be easily calculated, and the austenization temperature required to give the expected carbon content in the austenite can also be determined. Moreover, according to the relationship between the austenization temperature Tx and the associated carbon content Cax,, and considering the effect of the silicon content, a diagram showing Cax, Tx and the silicon content during the austenitizing of cast iron was prepared.

Abstract: Dusting of iron results from partial disintegration of a cementite scale which grows on the metal surface during reaction with carbon-supersaturated gas. Scaling kinetics are shown to be consistent with diffusion of carbon through the cementite. Further diffusion of carbon into the iron supersaturates it to a very high degree. Dusting of nickel and austenitic alloys leads to no carbide formation. Instead graphite grows into the metal, supported by diffusion of dissolved carbon to growth sites. Variations in rate with alloy iron content reflect the known effects of iron on carbon solubility and diffusivity. Alloying with copper also changes coking and dusting rates, although copper does not affect carbon permeability. The effect is shown to be due to interaction of copper with graphite nucleation sites.

Abstract: . In this paper, carbon diffusion in cementite is studied by molecular dynamics simulation. An assumption that carbon-carbon interaction occurs only indirectly via neighbouring iron atoms is used. An interstitial mechanism of carbon diffusion in cementite is revealed. The principal tracer diffusion coefficients and activation parameters of carbon diffusion in cementite are calculated for the temperature range 1223-1373 K and compared with the available published experimental data.

Abstract: Molecular dynamics is employed to investigate carbon diffusion in cementite. An approximation that carbon atoms can interact with each other only indirectly (via neighbouring iron atoms) is used. The interstitial mechanism of carbon diffusion in cementite is elucidated. The formation energy of defects (a carbon atom on an interstitial position and a vacant site on a regular carbon position) as well as the migration energy of carbon atoms are estimated in the temperature range 1273–1373 K.

Abstract: Fractography was used in order to investigate the effects of Ta addition on the diffusion behavior of carbon in molybdenum. Mo-1.5mass%Ta alloy was carburized by the solid-state carburization process at a temperature between 1073 and 1473 K. Fracture surface of the specimen was observed before and after the carburization process and carbon diffusion distance was estimated from the change in fracture mode as a function of distance from the specimen surface. Results are as follows. (1) Ta addition slightly decreased the carbon diffusion distance. (2) Ta addition had almost no effect on the activation energy for diffusion. (3) These results suggest preferential combination between carbon atoms and Ta atoms is only minor.

Abstract: Magnesium alloys show promise in meeting the demand for materials of lighter weight and higher rigidity. Mg alloys are hard to process and normally require grain refining for improved formability and mechanical properties. To process these fine-grained Mg alloys effectively, it is important to relate their load stress and mechanical properties to changes in their microstructures. Using a biaxial tensile machine and cruciform specimens, to evaluate the mechanical properties, microstructure, and plasticity, in a high temperature biaxial stress state, used of AZ31 Mg alloy sheet. With biaxial deformation, grain boundary slide occurred more frequently than with uniaxial deformation, causing grain boundary separation and formation of micro-voids between the grains. In the vicinity of the cracks and at the locations of grain boundary separation, although deformation temperature at higher than the recrystallization temperature, fine grains (about 2 μm) showing in duplex grain structures were formed locally. The formation of duplex grain structures as a result of local formation of fine grains during the deformation process is a major issue to be solved from the viewpoint of plasticity processing.

Abstract: In this study, it will be investigated the diffusion of critical elements, namely, carbon (C) and iron (Fe), into a steel substrate (Impax Supreme) during the diamond chemical vapour deposition (CVD) process. The substrate temperature was varied from 700 to 850°C by plasma power manipulations to enable the correlation of substrate temperature with diffusion length and depth of the above mentioned critical elements into steel during film growth conditions. Methane concentration is also a parameter which has been considered during the parametric analysis. The crystalline compounds formed during the diamond growth process are studied using XRD analysis. In addition, SIMS technique is used with depth profiling to monitor the diffusion of elements during the process. The results obtained enabled to improve traditional understanding about the mechanisms relating to diamond deposition on steel substrates using CVD processes.