Diffusion Of Carbon In Ferrite Research Articles

Kinetics of Solute Partitioning During Intercritical Annealing of a Medium-Mn Steel

The evolution of austenite fraction and solute partitioning (Mn, Al, and C) during intercritical annealing was calculated for a medium-Mn steel containing 11 pct Mn. Austenite growth takes place in three stages. The first stage is growth under non-partitioning local equilibrium (NPLE) controlled by carbon diffusion in ferrite. The second stage is growth under partitioning local equilibrium (PLE) controlled by diffusion of Mn in ferrite. The third stage is shrinkage of austenite under PLE controlled by diffusion of Mn in austenite. During PLE growth, the austenite is progressively enriched in Mn. Compositional spikes evolve early during NPLE growth and broaden with annealing temperature and time.

Carbon diffusion in supersaturated ferrite: a comparison of mean-field and atomistic predictions

Hillert's mean-field elastic prediction of the diffusivity of carbon in ferrite is regularly used to explain the experimental observation of slow diffusion of carbon in supersaturated ferrite. With increasing carbon supersaturation, the appropriateness of assuming that many-body carbon interactions can be ignored needs to be re-examined. In this work, we have sought to evaluate the limits of such mean-field predictions for activation barrier prediction by comparing such models with molecular dynamics simulations. The results of this analysis show that even at extremely high levels of supersaturation (up to 8 at% C), mean-field elasticity models can be used with confidence when the effects of carbon concentration on the energy of carbon at octahedral and tetrahedral sites are considered. The reasons for this finding and its consequences are discussed.

Modeling decarburization kinetics of grain-oriented silicon steel

A mathematic model has been presented to predict the decarburization kinetics of grain-oriented silicon steel sheet in the gas mixture of N2-H2-H2O during annealing. This model is based on the carbon flux balance between the oxidation reaction at the surface and the carbon diffusion inside the steel sheet. It can be numerically solved to quantify the influences of annealing temperature and atmosphere on decarburization kinetics when the boundary conditions are properly determined. In case that a humid gas mixture is employed during annealing, the most influential process parameter is temperature rather than compositions of the gas mixture, because the diffusion of carbon in ferrite is the rate-limiting step. Therefore, a higher temperature is required for the efficient decarburization of the thicker silicon steel sheet using the industrial continuous annealing production line.

The role of carbon diffusion in ferrite on the kinetics of cooperative growth of pearlite: A multi-phase field study

Cooperative growth of pearlite is simulated for eutectoid steel using the multi-phase field method. The model considers diffusion of carbon not only in γ phase, but also in α phase. The lamellar spacing and growth velocity are estimated for different undercoolings and compared with experimental results from the literature and theoretical results from analytical models. The important finding of this work is that carbon diffusion in ferrite and growth of cementite from the ferrite increase the kinetics of the pearlitic transformation by a factor of four as compared to growth from austenite only, which is assumed by the classic Zener–Hillert model. This growth mode therefore must be considered to be the dominating growth mode and it explains at least some of the differences between experiment and theory, where diffusion in ferrite is excluded.

The influence of diffusion of carbon in ferrite as well as in austenite on a model of reaustenitization from ferrite/cementite mixtures in Fe-C steels

The kinetics of formation of austenite from ferrite and cementite mixtures has been modelled by assuming the local equilibrium at the planar phase interfaces. The exact solutions to the diffusion equations governing the volume diffusion of carbon in austenite and ferrite are presented. The concurrent motions of the two interfaces are calculated via solving a set of transcendental equations derived from the flux balance conditions. At low isothermal transformation temperatures, it is found that the time required for reaustenitization is slightly greater than the time previously calculated with no diffusion of carbon in ferrite.

Kinetics of thermal aging and its effect on the proneness of hull steels to brittle failure

1. The hull steels 10GN2MFA and 15Kh2NMFA-A have little proneness to aging) this is in keeping with the requirements that modern shell materials have to fulfill. 2. The activation energy of the process of thermal aging (Q=70–80 kJ/mole) is close to the activation energy of diffusion of carbon in ferrite. The magnitude of the activation energy, and also the nature of the change of strength and fracture toughness make it reasonable to assume that in the steels 15Kh2NMFA-A and 10GN2MFA the aging process is determined by the state of the carbide phase. 3. The use of the constant in the Khollomon equation (α=3.5–4.5), which was experimentally established for the temperature range 250–400°C, and 10GN2MFA upon aging. 4. Aging for up to 6500 h causes a shift of the temperature dependence of the fracture toughness of steel 15Kh2NMFA-A into the range of higher temperatures, and it also increases fracture toughness at low temperatures (below −100°C).

Resistivity and kinetic measurements of carbon dissolution in alpha and gamma iron

The kinetics of carbon dissolution in polycrystalline iron foils have been determined from electrical resistance measurements. Reaction with methane at 10–100 Torr pressure has been carried out at temperatures in the range 766–1163 K. Incremental resistance changes for unit increase in carbon concentration have been determined for temperatures between 838 and 1163 K. Breaks in the specific resistivity increment curves occur at temperatures associated with (i) significant structural rearrangements in the metal, (ii) nucleation of precipitates and (iii) phase transitions.Three stages of reaction with different rate-determining steps have been identified. At temperatures up to 1023 K, the first-stage reaction occurs with zero activation energy and its rate-determining step is attributed either to the chemisorption of methane molecules or formation of adsorbed CH3 radicals. Between 913 and 1008 K it is followed by a second-stage reaction in which carbon diffusion in ferrite is rate-determining, with an activation energy of 176 ± 15 kJ mol–1. Above the Fe–C eutectoid temperature (996–1011 K) the first stage is followed by a third-stage reaction, for which it is proposed that dehydrogenation of methyl radicals is rate-determining with an activation energy of 190 ± 5 kJ mol–1. The first and third stages follow the Langmuir isotherm and an isosteric heat of adsorption of 170 ± 11 kJ mol–1 has been determined for the third-stage reaction.Carbide formation occurs in both the second- and third-stage reactions, and in the latter the total carbon uptake in γ-iron corresponds closely with the iron–iron carbide phase boundary. The significance of these results for the mechanism of carbon deposition in transition-metal–hydrocarbon systems is discussed.