Papers by Author: Carlos Capdevila

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Authors: David San Martín, Francisca García Caballero, Carlos Capdevila, Carlos García de Andrés
Abstract: Austenite grain growth in microalloyed steels is governed by the coarsening of fine precipitates present at grain boundaries below the grain coarsening temperature. Zener model is widely used in metals to describe the pinning effect of second phase particles precipitated in the matrix. In this work it has been discussed whether grain boundary or volume diffusion is the rate controlling process for the coarsening of the niobium carbonitrides. Calculations on austenite grain growth kinetics, obtained coupling Zener theory and both rate controlling processes of precipitate coarsening, have been compared against experimental austenite grain size results under nonisothermal heating conditions. In this sense, it has been concluded that the coarsening of niobium carbonitrides is mainly controlled by volume diffusion of Nb in austenite.
Authors: Carlos García de Andrés, Carlos Capdevila, Francisca García Caballero
Authors: Carlos Capdevila, Tommy De Cock, Francisca García Caballero, Carlos García-Mateo, Carlos García de Andrés
Abstract: The influence of the deformation grade on the recrystallised grain size has been studied in the AISI 304 stainless steel. Therefore, cold rolled samples of this material with reductions varying between 30% and 80% were annealed at different temperatures and subsequently quenched. The mean austenitic grain sizes were measured and compared. Moreover, the correlation between the variation of the thermoelectric power and the grain growth was investigated for each degree of prior deformation.
Authors: Carlos Capdevila, I. Toda, J. Chao, Carlos García de Andrés
Abstract: The ferritic oxide dispersion strengthened alloys (ODS) are manufactured using the mechanical alloying process. The development of a coarse grained microstructure during the recrystallization has been noted and discussed by a number of authors but, the mechanism of grain control remains uncertain. Recent work has emphasized the large influence of non-uniformities on the development of the recrystallized microstructure. The purpose of the present work was to study the effect of non-uniform plastic strain on recrystallization of Fe-base ODS alloy named MA 957.
Authors: Carlos Capdevila, Tommy De Cock, Carlos García-Mateo, Francisca García Caballero, Carlos García de Andrés
Abstract: The recrystallisation behaviour of two cold-rolled low carbon microalloyed steels with different Ti content was investigated by thermoelectric power (TEP) measurements and metallography. It was shown that recrystallisation was more sluggish in the higher Ti grade steel as compare with traditional ELC and LC steels. The existence of Ti in solid solution together with the pinning effect of Nb-, V- and Ti-rich particles are likely to be responsible for such behaviour. Metallographic etching with picral revealed that cementite act as preferential nucleation site for recrystallisation.
Authors: David San Martín, Francisca García Caballero, Carlos Capdevila, C. Carcía de Andrés
Abstract: Grain growth is a thermally activated process in which the average grain size increases as temperature and time increases. The driving force for grain growth results from the decrease in the free energy associated with the reduction in total grain boundary energy. There are several known factors that influence the migration of grain boundaries such as second phase particles precipitated in the matrix and the solute elements segregated at grain boundaries. The austenite grain boundaries are revealed using the thermal etching method. Carbon extraction replicas were prepared to determine the composition and size of precipitates present in the matrix. In this work, the evolution of the average prior austenite grain size (PAGS) of a low carbon steel microalloyed with niobium is studied as a function of temperature and heating rate. Austenite grains show a two-stage growth. It has been found that as heating rate increases, the grain coarsening temperature (TGC) increases and the grain size at that temperature decreases. TGC temperature lies around 40-60°C below the temperature for complete dissolution of carbonitrides (TDISS).
Authors: Carlos Capdevila, Tommy De Cock, Francisca García Caballero, Carlos García de Andrés
Abstract: In this work the prominent influence of Particle Stimulated Nucleation (PSN) on the overall process of nucleation and subsequent grain growth is discussed and a global recrystallisation kinetics model is proposed. This model accounts for the effect of the most relevant industrial parameters, such as cold reduction and annealing temperature. Moreover, not only the role of the cementite content, which is function of the nominal carbon content, is included in the model, but also the morphology of the second phase particles. Experimental evidence is given to ascertain the accuracy of the theoretical predictions.
Authors: K. Tanaka, M. Hara, Yasu Yogo, Kou Nakanishi, Carlos Capdevila
Abstract: The kinetics of phase transformations in medium-carbon forging steels (MCFS) have been modeled based on CALPHAD multicomponent thermodynamics and the classical nucleation-growth theory. New treatments include the time dependency of parabolic growth rate of proeutectoid ferrite (α) , which account for the soft impingement effect by carbon enrichment in austenite (γ). And a potential transition of γ/α interface equilibrium has also been considered depending on temperatures and velocity of the moving interface. To make a realistic prediction of the onset of pearlite (P) transformation, a normal distribution of γ grain size has been assumed and successive α→P transformation kinetics in each grain size have been summated. The developed program coupled with thermodynamic solver, 'ThermoCalc', calculated the isothermal kinetics of MCFS and has been found to predict well the effect of minor difference of chemical composition / holding temperatures.
Authors: Carlos Capdevila, Francisca García Caballero, Carlos García de Andrés
Authors: Hamed Hoseiny, Francisca García Caballero, David San Martín, Carlos Capdevila
Abstract: The many practical difficulties and longer lead time with heat treatment of mould steels after machining have led to increased demand for steels in prehardened condition, typically ~ 40HRC. At this hardness the steel possesses an optimal combination of high strength and machinability. The steels used for moulds require a wide range of demanding properties, among which high enough strength and toughness are the primary necessities in order to resist any deformation and dimensional change in mould during use. Uddeholm Impax HH which resembles the modified AISI P20 has been widely used for moulding of plastics and die-casting of low melting temperature metals. The common hardening process for Impax HH is conventional quenching and tempering. Hence, investigating the effect of hardening parameters on the required properties upon this steel grade is beneficial in improving it for better performance as a prehardened mould steel. In the present work, the effect of changes in austenitization temperature and consequently the prior austenite grain and martensite packet sizes on the tensile properties and impact toughness of Uddeholm Impax HH at the hardness of ~40HRC is studied. The results have shown reduction in impact toughness but no considerable change in yield and ultimate tensile strength upon increasing the austenitization temperature.
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