Cast steel is commonly used to produce structural and safety parts. Foundry processes allow producing parts from scrap steel directly to the required dimensions without any forming operation. Cast components may, however, exhibit macro- and micro-shrinkage porosities. The combined effect of macro- and micro-shrinkages on the fatigue behavior of cast steel has been characterized in the literature. Macro-shrinkages may nowadays be eliminated by adequate positioning of risers. However, micro-shrinkages will always be present in cast steel components. Present work addresses the influence of micro-shrinkage porosity on a G20Mn5 cast steel. G20Mn5 (normalized) ingots have been cast under industrial conditions, but ensuring the absence of macro-porosities. Solidification leads to two very different microstructures prior to the normalization treatment: columnar dendrites beneath the surface (Skin) and equiaxed microstructures close to the center (Core). First, metallographic observations of the whole ingot revealed the same grain size in both areas. Fatigue samples were extracted, by differentiating two sampling volumes corresponding to columnar (S) and equiaxed solidification (C), respectively. The distribution of micro-porosities was determined on all samples by Micro-CT-scans. Core samples exhibit micro-porosities with volumes 1.7 larger than Skin samples. Low cycle fatigue tests (3 levels of fixed plastic strain) were run on both sample series (C, S). Results follow a Manson–Coffin law. Core specimens exhibit lower fatigue life than Skin specimens. The differences in fatigue life have been related successfully to the differences in micro-porosities sizes.

Deoxidation is an unavoidable step in the elaboration of steel. The study of its influence could improve the quality of low-carbon steel (0.20-0.25 wt.% of carbon). There are many deoxidation methods, and the most-common one consists of adding aluminum. Although it is a classic method, determining the optimal process parameters (quantity, yield, etc.) could be very sensitive. Deoxidation plays a determining role on inclusion cleanliness, especially on sulfide morphology. In order to control the efficiency of deoxidation, different techniques can be used. In this paper, an automated counting procedure on a scanning electron microscope with a field emission gun (FEG-SEM) is presented. This method was applied on samples cast in our laboratory under different deoxidation conditions. According to this, the resulting inclusion population is correlated with the aluminum content to find the optimal process parameters.

L’étude porte sur les aciers bas carbone (0,2 %) faiblement alliés bruts de fonderie. Les différentes opérations effectuées dans le bain liquide de l’acier influent sur la population inclusionnaire ; ces inclusions sont elles-mêmes responsables d’une partie des propriétés mécaniques de l’acier. Leur contrôle et leur étude revêtent donc une grande importance. L’objet de ce travail est de mettre au point une méthode de comptage, d’en évaluer la fiabilité et la reproductibilité ainsi que d’en montrer la limite. D’ordinaire, cette opération est réalisée par microscopie optique, méthode longue à mettre en œuvre et ne couvrant qu’une surface d’échantillon très limitée. La méthode présentée ici requiert un microscope électronique à balayage (MEB) doté d’une forte stabilité du faisceau d’où l’utilisation d’un canon à émission de champs (FEG). Cette méthode est plus rapide et couvre une surface plus large. Elle est déjà utilisée en sidérurgie pour des aciers laminés mais est beaucoup moins courante pour des contrôles métallurgiques sur des aciers moulés. Pour transposer cette méthode à ce type d’acier il est donc nécessaire de confirmer sa fiabilité et sa répétabilité. Dans un premier temps, un opérateur définit des critères morphologiques et chimiques sur les inclusions rencontrées dans une zone représentative très réduite. Dans un second temps, un logiciel scanne automatiquement une zone beaucoup plus large et classe les inclusions détectées selon les critères pré-définis (taille, forme, nature). Afin de statuer sur la répétabilité et la reproductibilité de cette méthode, une étude statistique est réalisée sur une surface test ; cette zone est analysée plusieurs fois dans le but de définir des intervalles de tolérances. Si les résultats obtenus en termes de population inclusionnaire sont significatifs, il sera alors possible de statuer sur la pertinence des différentes opérations effectuées dans les bains d’acier liquide.

In many foundries, numerical simulation is used to determine the origins of different defects as this tool allows the acceleration of the design process. However, the databases provided by different software do not seem to tally with the actual properties of the material. In fact, every foundry uses a different grade of steel and varying mixtures of sand. An evaluation of the impact of different material properties showed the importance of measuring every physical property to improve the database of the software. Following this, an experiment was conducted to evaluate the gap between numerical simulations and the results obtained through experimentation. This experiment, called thermal analysis, consists in measuring the solidification and cooling of a cylinder filled with liquid steel. After the calculation of the steel properties and a simulation with real experimental parameters, a comparison between each cooling curve was realized. This comparison shows that the calculated properties provide a simulated cooling curve which is closer to the experimental curve than the properties in the original database. We did not explore all of the metal properties in this study, but the modification of the sand properties was explored, together with the thermal conductivity of the steel and sand. These other measurements will be obtained in a future study.

Ceroxides are surface defects caused by a mold-metal reaction during the casting process of steels. This type of defect may affect a large area, but it is located only on the skin of the parts. It does not affect the core of the parts nor its mechanical properties. Nevertheless, ceroxides induce a lack of material on the surface, forming a kind of crater and needing complementary surfacing. The defect is also composed by several non-metallic inclusions containing Al, Mg, Si and O. An EDX analysis by mapping show these elements involved in the mold-metal reaction. The presence of these oxides could confirm the hypothesis of the deoxidizer reoxidation found in the bibliography. To better understand the specific conditions of ceroxide formation, the first step was to find of way to generate systematically this defect at each casting. Two patterns with different filling rate were designed, simulated on Quikcast and tested. The pattern with turbulent filling rate allowed the formation of ceroxide at each casting and so was used during this study. This result shows that the filling rate of the mold could be considered as a first order parameter in ceroxide formation. Then, a specific experimental set up was designed to characterize this defect. The analysis of the defect was done for sizing it: surface and depth. Finally, some key parameters on defect formation were determined like the nature of deoxidizer or the amount of oxygen in the mould. Some laboratory tests were lead to show the influence of these parameters by characterization of the casted parts in comparison with a reference sample. This study allowed us to find process parameters responsive of ceroxide formation and to propose some way of improvement to reduce the size and the occurrence of ceroxides.