Missions

For the IJL, the aim of this post-doc is to provide EDF with elements for understanding the formation of segregations in alloying elements, and in particular cobalt, which develop during the solidification of cast irons made from steels recycled by EDF. These steels contain radioactive elements for which the aim is to provide information on the risks of concentration of these elements either in a precipitate or at the level of the local average composition (macrosegregations). The link between this information and the induced radioactivity will have to be established by EDF. The project is structured around five work packages:
1. Literature review on the formation of macrosegregations during the solidification of cast iron;
2. Simulation with the software SOLID® developed at IJL of the formation of macrosegregations in the case of the solidification of an 11 kg ingot cast by EDF considering constant thermodynamic data;

The content of the work package 3 will be decided according to the results of the study of the work package 1:
3.
a) Taking into account the variability of the thermodynamic data in the software SOLID® by a neural network regression method based on ThermoCalc® calculations;
b) Taking into account the diffusion of chemical species in the liquid phase on a macroscopic scale;
4. Simulation of extreme scenarios that could lead to the formation of macrosegregations or phases concentrated in radioactive elements;
5. Writing of the final report.

Activités

Work package 1 : Literature review on the formation of macrosegregations during the solidification of cast iron.
A bibliographic study reporting the formation of macrosegregations during the solidification of cast iron will be carried out. This study will in particular make it possible to define the extreme cases of casting conditions likely to lead to the formation of macrosegregations. The ranges of alloy composition retained for this study will also be defined at this stage.

Work package 2 : Simulation with the SOLID® code of the formation of macrosegregations in the case of the solidification of an 11 kg ingot cast by EDF considering constant thermodynamic data.
The boundary conditions to be adopted to reproduce the solidification of the 11 kg ingot will be identified based on the temperature measurements made during the casting tests. The thermodynamic data required as input to the SOLID® code will be taken from the study carried out in the work package 1. The micrographs provided by EDF will be used to determine the microstructural characteristics required as input to SOLID®, such as the average secondary dendrites arms spacing. A reflection will be conducted on the best solution to adopt to describe as accurately as possible the end of solidification, i.e. the moment when a solid phase other than austenite is formed. This part will also allow the robustness of SOLID® in case of alloying elements whose partition coefficient is greater than unity, such as silicon. Finally, the phases and their composition will be calculated in post-processing of the SOLID® calculations with the ThermoCalc® software in order to characterise the risk of formation of phases rich in radioactive elements.
The content of work package 3 will be decided according to the results of the work package 1:
- If the partition coefficients and liquidus slopes vary significantly in the solidification interval and cannot be considered as constant, the work package 3 will consist in taking into account the variability of these thermodynamic data in SOLID® by a neural network regression method based on ThermoCalc® calculations (work package 3-a).
- If the variations of the partition coefficients and liquidus slopes prove to be negligible, work package 3 will consist of taking into account the diffusion of chemical species in the liquid phase on a macroscopic scale (work package 3-b).

Work package 3-a: Taking into account the variability of the thermodynamic data in SOLID® by a neural network regression method based on ThermoCalc® calculations. Thermophysical data will be generated using the ThermoCalc® software and will be used to feed a neural network that will generate regression matrices that will allow the variability of the thermophysical data to be taken into account with the SOLID® calculation code. Simulations with the SOLID® software will then be carried out to study the effect of the variability of these data on the macrosegregations and on the microsegregation in the case of the 11 kg ingot.

Work package 3-b: Taking into account the diffusion of chemical species in the liquid phase on a macroscopic scale.
Due to the high fraction of solid that forms over a small temperature range at the end of solidification, it is possible that macrosegregation can form at the bottom of the mushy zone. In this case, solute diffusion in the liquid phase at a macroscopic scale may play an important role. It is currently not described in the SOLID® software. The solute transport equation in the liquid phase will be completed to take this phenomenon into account and the additional terms will be implemented in SOLID®. The effect of diffusion on the formation of macrosegregations will then be studied.

Work package 4: Simulation of extreme scenarios that could lead to the formation of macrosegregations or phases concentrated in radioactive elements
The extreme cases for the formation of radioactive segregated zones defined in work package 1 will be simulated using the SOLID® software. A post-processing analysis with the ThermoCalc® software will complete this part to determine the risks of formation of phases enriched in radioactive elements.

Work package 5: Writing of the final report

Compétences

Heat, mass and momentum transport phenomena, solidification, numerical simulation (finite volume method), process simulation modeling
Technical skills: Fortran programming
Good command of scientific writing

Contexte de travail

The Institute Jean Lamour (IJL) is a joint research unit of CNRS and Université de Lorraine.
Focused on materials and processes science and engineering, it covers: materials, metallurgy, plasmas, surfaces, nanomaterials and electronics.
The IJL has 263 permanent staff (30 researchers, 134 teacher-researchers, 99 IT-BIATSS) and 394 non-permanent staff (182 doctoral students, 62 post-doctoral students / contractual researchers and more than 150 trainees), of 45 different nationalities.
Partnerships exist with 150 companies and our research groups collaborate with more than 30 countries throughout the world.
Its exceptional instrumental platforms are spread over 4 sites; the main one is located on Artem campus in Nancy.
The study will be carried out within the solidification team.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.