Modelling of solidification is of industrial and theoretical relevance. An accurate estimation of the actual liquidus and solidus temperatures leads to significant improvements in quality and efficiency of steel production as well as substantially reduces the energy consumption and the ecological footprint.An optimisation of the Scheil-Gulliver model for solidification with the aim to predict the solidus temperature of steels (Scheil-Gulliver for Steel, SGS) is presented. The SGS model allows an easy and accurate simulation of the solidification interval using software based on the CALPHAD approach. Based only on the steel composition, the model consistently chases between full equilibrium for ferrite and partial redistribution of alloying elements (Scheil-Gulliver approach) for austenite. The predictions of the model were compared to differential thermal analysis (DTA) measurements of industrial heats, which represent a wide range of compositions. The agreement of the data calculated with the SGS model with the values measured by DTA represents an improvement compared to existing models. (C) 2015 Elsevier Ltd. All rights reserved.

The austenite grain growth of a microalloyed steel was investigated via annealing experiments and phase-field simulations using the phase-field code Micress. The technique described in a previous work was enhanced and applied to an Nb, Ti microalloyed linepipe steel for the case of isothermal heat treatment between 1050 and 1200 degrees C. The input parameters for the phase-field simulations were deduced from physical models based on the results of isothermal holding experiments. A further improvement was the use of the software package MatCalc to simulate at a lower scale the coarsening of the pinning particles. The results of these simulations showed good agreement with the experimental results.

In this paper, the effect of particle pinning on grain boundary motion is investigated by phase-field modeling. In general, the kinetics of grain growth in multicrystalline materials is determined by the interplay of curvature driven grain boundary motion and the balance of interfacial tension at the vertices of a grain boundary network. A comprehensive way to treat both effects in one model is given by the phase-field approach. The specific feature of the multiphase-field model used for this investigation is its ability to treat each grain or phase boundary with its individual characteristics, together with a thermodynamic coupling which allows a sound treatment of phase transformation, e.g. the formation of precipitates of a second phase.The pinning effect itself is simulated on the nanometer scale resolving the interaction of individual inert or reactive precipitates with a curved grain boundary. From these simulations an effective pinning force is deduced, and a model for a driving force dependent grain boundary mobility is formulated accounting for the pinning effect in the grain growth simulation on the mesoscopic scale. These simulations demonstrate how particle pinning leads to much slower growth kinetics and a different grain morphology with higher boundary curvatures in the stationary state. Finally, an increase of the pinning force due to a changing particle density, e.g. during heat treatment, is shown to result in a transition between normal and abnormal grain growth before grain coarsening is inhibited completely.

The challenge of simulating microstructure formation in technical castings implies not only the necessity to handle multicomponent and multiphase problems, but also to cope with the complexity of the thermal coupling between the scales: Local microstructure formation depends on the macroscopic heat fluxes but, in turn, the macroscopic temperature solution depends on the latent heat release, and thus on the microstructure formation, in all parts of the casting. This general micro-macro problem cannot be solved without proper approximations if time-consuming micro-models like phase-field are involved.In this paper different approximations for coupling of the macroscopic heat flux to complex microstructure models are discussed and compared. Simulations are per-formed for the case of the binary alloy Al3at%Cu and a multiphase-field solidification model. A macroscopic 1D-temperature field is coupled to the microscopic simulation domain in order to evaluate the interaction of the long-range thermal diffusion with the local release of latent heat caused by the growth of the primary dendrites and secondary Al2Cu particles.For the case of equiaxed solidification a homoenthalpic approximation is proposed and applied to a hypothetical plate casting. Three inoculation case scenarios are assumed and the resulting local cooling curves and grain sizes are compared. The validity of the model assumptions is discussed.

Calculating microstructures for technical materials is an ambitious task which not only implies different length scales but also the complex thermodynamic properties of multicomponent and multiphase alloys. We report some of the recent progress in simulating microstructure evolution in multicomponent steels using the multiphase-field software MICRESS®. Several applications are reviewed in order to demonstrate the current status of applied phase-field techniques.

This article deals with the austenite (γ) decomposition to ferrite (α) during cooling of a 0.10 wt pct C-0.49 wt pct Mn steel. A phase-field model is used to simulate this transformation. The model provides qualitative information on the microstructure that develops on cooling and quantitative data on both the ferrite fraction formed and the carbon concentration profile in the remaining austenite. The initial austenitic microstructure and the ferrite nucleation data, derived by metallographic examination and dilatometry, are set as input data of the model. The interface mobility is used as a fitting parameter to optimize the agreement between the simulated and experimental ferrite-fraction curve derived by dilatometry. A good agreement between the simulated α-γ microstructure and the actual α-pearlite microstructure observed after cooling is obtained. The derived carbon distribution in austenite during transformation provides comprehension of the nature of the transformation with respect to the interface-controlled or diffusion-controlled mode. It is found that, at the initial stage, the transformation is predominantly interface-controlled, but, gradually, a shift toward diffusion control takes place to a degree that depends on cooling rate.

This paper deals with the kinetics of the austenite to ferrite (gamma --> alpha) transformation during controlled cooling of C-Mn steel. A phase field model is used to simulate the gamma --> alpha transformation. The coupled differential equations governing the phase field development, characterising the phase of the system at each point in space and time, and the Solute diffusion are presented. The numerical Solution of the. equations yields the time evolution of the microstructure. The gamma --> alpha transformation kinetics is also investigated experimentally, using dilatometry and laser scanning confocal microscopy. A remarkably good agreement between the experimental and simulated development of the phase fractions, as a function of temperature and time, is obtained using realistic values for the physical material parameters featured in the model. Moreover, the simulated microstructure is shown to have the same characteristics as the experimental one. The interface velocity is investigated in more detail.

Phase transformations play a significant role during the technical heat treatment of steel. For instance a controlled transformation in an annealing process allows for the production of multiphase steels. In this investigation the procutectoid ferrite precipitation will be looked into. The transformation kinetics can be described as diffusion controlled,yet another type of kinetics is also thinkable Which Would be, controlled by interface mobility. Isothermal transformation dilatometry has been performed oil high carbon steel grade samples. In a detailed metallographic analysis the samples were examined according to probable nucleation sites of proeutectoid ferrite. The simulations in this work have been performed using the software, MICRESS. MICRESS is based oil the multicomponent multiphase-field model. Besides diffusion it also takes finite interface nobilities into account. It allows for multi-dimensional calculations, which illustrates the ferrite precipitation in real microstructures. For comparison reasons, simulations in a one-dimensional domain have been performed using the commercial program DICTRA.

Phase transformation is a powerful tool to change the properties of steels. Of the known transformations especially the gamma-alpha -transformation is utilised. It occurs in a temperature range relevant for heat treating and hot deformation processes. In this paper an approach is presented in which the gamma-alpha -transformation is simulated with Micress.: This software applies the multicomponent multiphase-field model, which is based on the reduction of total free enthalpy. Two different steels have been selected for the simulations, an ULC and an IF steel. Dilatometric tests serve as a basis for the simulations. These tests have shown that the transformation behaviours of the two steel grades are governed by two different kinetics. The transformation kinetics of the IF grade is influenced by the microalloying concept applied, resulting in a very slow start of the transformation. This has also been incorporated in the simulations by choosing two different grain boundary mobilities, one main parameter of the simulation. The simulation results of the ULC grade show the huge influence of nucleation undercooling as another one of the main parameters. Both simulation results are satisfying. They show that the phase-field method offers a strong simulation tool in the area of phase transformation.

The multi-phase field method is a modem and flexible numerical tool to calculate the structural development during phase transitions. Primarily being developed for solidification problems, it can be effectively applied to solid state transformations driven by diffusion and/or reduction of interfacial area. Curvature effects and topological rearrangement of grain boundary networks are incorporated in a comprehensive way. The paper presents simulations of grain growth scenarios in 2D and 3D isotropic growth and the effect of impurity drag. Impurity drag is described by a simple model, where the "loaded" and "free" part of the grain boundary is distinguished depending on the local velocity of the grain boundary. This treatment allows for the consideration of hysteresis effects. The model is calibrated by comparison with bicrystal experiments taken from the literature.