Experimental study and modeling of complex carburizing thermochemical treatments: application to case-hardening steels M/F

Job Description

Missions

Carburizing thermochemical treatments allow to reinforce steel components (gears, transmission…) at the surface. Resistance to wear and fatigue are increased thanks to hardening of the surface, while the ductility is kept in the core, and thanks to the formation of compressive residual stresses near the surface. Research and development activity is ongoing to apply carburizing to steel grades richer in alloying elements (Mo, V, Cr…), for applications at higher temperature dedicated to
aeronautical industry. These elements have a significant impact on microstructural evolutions, which need to be studied because these evolutions influence the profiles of microstructures, the mechanical behavior and the formation of residual stresses.

The project will focus on the carburizing step, which is carried out at high temperature (e.g. 950°C), in the domain of stability of the austenite. Two phenomena occur and interact: the long-distance carbon diffusion (up to several mm from the surface) and the carbides precipitation. Precipitation kinetics depend on the local carbon concentration and in return, the precipitation of carbides disrupts the macroscopic flow of carbon. Additional complexities come from the process, which includes pulses of enrichment separated by diffusion stages, and from the coexistence of interacting carbides
populations, which undergo precipitation and dissolution. There is a lack of research on these phenomena and most observations in literature have been done after enrichment, cooling and tempering [1]. The enrichment step remains to be studied specifically

Activity

Objectives of the postdoc will be to study, by modeling and advanced characterization techniques, carburizing of a high-alloyed tool steel containing carbide-forming alloying elements such as Mo, Cr and V.
Main activities will consist of:
• Studying the influence of enrichment treatment parameters by varying the temperature, the number of pulses and the duration of the diffusion stages
• Performing interrupted enrichment treatments followed by quenching
• Examining enriched case of laboratory samples by EPMA to measure the carbon concentration profiles, and by electron microscopies (SEM, TEM) to observe the precipitation and the matrix (martensite, retained austenite) microstructures
• Establishing prior austenite grain sizes by crystallographic reconstructions (EBSD).
• Performing in situ high-energy XRD (HEXRD) experiments to track the carbides fractions and the carbon concentration in the austenite [2]. In these experiments, the samples will have beenpreviously subjected, in laboratory, to different sequences of pulses and diffusion and quenching.
In situ HEXRD experiments at synchrotron beamlines will be done on a last diffusion stage.
Additionally, a model introduced previously for carburizing will be further developed in this study. In this model, carbon diffusion is predicted using Fick's law and finite difference scheme. A source term accounts for the consumption of carbon by carbides precipitation. Precipitation / dissolution model predicts nucleation (classical theory), diffusive growth / dissolution and coarsening of coexisting populations of carbides [3].
By combining simulations and experiments, comprehensive analysis of the process will be achieved and guidelines for optimization will be proposed. The analysis could then be extended to broader range of steel compositions. The future potential of the model / numerical tool to replace costly trial- error method will also be evaluated.

Your Profil

Skills

PhD in Materials Science. Metallurgy, phase transformations, microscopy, X-Ray diffraction, modeling.

Your Work Environment

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.
By 2026, IJL has 258 permanent staff (33 researchers, 133 teacher-researchers, 92 IT-BIATSS) and 389 non-permanent staff (146 doctoral students, 43 post-doctoral students / contractual researchers and more than 200 trainees), from some seventy different nationalities.
Partnerships exist with 150 companies and our research groups collaborate with more than XX countries throughout the world.
Its exceptional instrumental platforms are spread over 4 sites ; the main one is located on Artem campus in Nancy.
The work will be carried out within the “Microstructures and Stresses” team.
The postdoc will take place inside a research collaboration between IJL and Safran

Compensation and benefits

Compensation

3071.50€ gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

The research professions