Modelling of delta ferrite to austenite phase transformation kinetics in martensitic steels: application to rapid cooling in additive manufacturing

International audienceIn this paper, the high temperature transformation kinetics of delta ferrite to austenite (δ → γ) phase transformation is modeled by thermodynamic and diffusion calculations. It appears that, in martensitic steels, the δ → γ transformation is very fast (a few nanoseconds) as soon as the first austenite nuclei appears. Classically the austenitic phase will thus systematically be observed in the material during conventional elaboration processes. However, in powder metallurgy and additive manufacturing, it is possible to obtain sufficiently high quenching rates (up to 106 °C/s) so that the γ phase does not have time to appear. The calculations presented here allow to rationalize the understanding of the microstructures of powders and different additive manufacturing materials. They enable to understand why ferrite or martensite is sometimes obtained in the final microstructure. From the calculations made, an original CCT (Continuous Cooling Transformations) diagram starting from the δ phase is proposed. It allows to set up a strategy of grade design or process definition according to the final microstructure targeted

Laser Beam Direct Energy Deposition of graded austenitic-to-martensitic steel junctions compared to dissimilar Electron Beam welding

International audienceThis article presents the Direct Metal Deposition (DMD) process as a method to build a graded austenitic-to-martensitic steel. Builds are obtained by varying the ratio of the two powders upon DMD processing. Samples with gradual transitions were successfully obtained thanks to the use of a high dilution rate from a layer to another. Long austenitic grains are observed on 316L side when martensitic grains are observed on Fe-9Cr-1Mo side. In the transition zone, the microstructure is mainly martensitic.Characterizations were performed after building and after a tempering heat treatment at 630°C during 8h and compared to dissimilar Electron Beam welds. Before heat treatment, DMD graded area has high hardness values (around 430 HV) due to fresh martensite formed during building. Tempering heat treatment allows reducing hardness in this area to 300 HV. EDS measurements indicate that the chemical gradient between 316L and Fe-9Cr-1Mo obtained by DMD is smoother than the chemical change obtained in Electron Beam (EB) welds. Microstructures in DMD are quite different from those obtained by EB welding. Hardness measurements in DMD samples and in welds exhibit similar behaviours: the weld metal and the Fe-9Cr-1Mo heat affected zone are relatively hard after welding because of fresh martensite, such as the DMD transition zone. These areas are all softened by the tempering heat treatment

On dislocation hardening in a new manufacturing route of fer-ritic ODS Fe-14Cr cladding tube

International audienceThe microstructure evolution associated to the manufacturing of a Fe-14Cr-1W-0.3Ti-0.3Y$_2$O$_3$ grade ferritic stainless steel strengthened by dispersion of nano oxides (ODS) was investigated. The material was initially hot extruded, then shaped into cladding tube geometry via cold pil-gering and stress release heat treatments. Each step of the process was analyzed to better under-stand the high microstructure stability of the material. Slight grain refinement and alpha fiber texture reinforcement was obtained after cold pilgering. Despite high levels of stored en-ergy the following heat treatment did not recrystallize the material. Only the Vickers hardness showed significant variations in the manufacturing steps, which was attributed to the recovery of statistically stored dislocations (SSD), as concluded from a combination of EBSD and X-ray diffraction measurements

Insights from Microstructure and Mechanical Property Comparisons of Three Pilgered Ferritic ODS Tubes

International audienceThree oxide dispersion strengthened alloys were fabricated into thin-walled (~500 µm wall thickness) tubes and characterized using x-ray, electron microscopy, and atom probe tomography methods. The three iron-based alloys included the 14%Cr alloy 14WYT, the 12%Cr alloy OFRAC, and a 10%Cr-6%Al alloy CrAZY. Each tube was subjected to a different thermal history during the pilgering process, which allowed for a detailed comparison between varying grain structures and alloy compositions. Atom probe tomography and energy-filtered transmission electron microscopy (TEM) comparisons showed good agreement in precipitate distributions, which matched predicted values using state-of-the-art nanoprecipitate coarsening models. The grain size, precipitate dispersion characteristics, and dislocation densities were then used to estimate yield strengths that were compared against room temperature axial and ring-pull tensile test data. For all three alloys, axial tensile specimens exhibited high tensile strength (>1 GPa) and reasonable plastic strains (10-17%). Ring tensile specimens, conversely, showed limited ductility (~1%) with similar strengths to those measured in the axial orientation. The strengthening models showed mixed agreement with experimentally measured values due to the highly anisotropic microstructures of all three ODS tubes. These results illustrate the need for future model optimization to accommodate non-isotropic microstructures associated with components processed using rolling/pilgering approaches