Application of thermal analysis techniques to study the oxidation/reduction phenomena during sintering of steels containing oxygen-sensitive alloying elements

The final publication is available at Springer via https://doi.org/10.1007/s10973-016-5508-5.For the consolidation of steel parts manufactured by powder metallurgy (PM) techniques, removal of the surface oxides covering metallic powder particles is a necessary prerequisite. In PM steels with conventional compositions, reduction of the oxides is easily achieved in traditional sintering furnaces. However, processing steels containing alloying elements with a high oxygen affinity represents a big challenge that requires a deeper understanding of the chemical processes occurring during sintering. In the present work, thermogravimetry analysis coupled with mass spectrometry is used to describe the oxidation/reduction phenomena that take place when sintering steel powders and how these processes are modified by the addition of admixed particles containing oxygen-sensitive elements. Carbothermal reduction processes are studied using pure oxides (Fe2O3, MnO2, Cr2O3 and SiO2) as well as water-atomized Fe powders mixed with small amounts—4 mass/%—of Cr, Mn and Si powders or Fe–Mn–Si–(Cr) master alloy powders. The results show that there is an oxygen transfer from the base iron particles to the oxidation-sensitive elements—“internal getter effect”—taking place mostly through the gas phase. Different alloying elements (Cr, Mn, Si) show different temperature ranges of susceptibility to oxidation. Combination of these oxygen-sensitive alloying elements in the form of a master alloy powder reduces their sensitivity to oxidation. Also, the use of master alloys promotes the concentration of the oxides on the surface of the alloying particles and not in the grain boundaries of the surrounding iron particles—as occurs when using Mn carriers—which should have a beneficial impact on the final mechanical performance.European Union Seventh Framework, People Programme (Marie Curie Actions) FP7/2007-2013911051

The role of oxygen transfer in sintering of low alloy steel powder compacts: a review of the "internal getter" effect

The chemical aspects of sintering have to be considered, in particular the role of oxygen. For sintered alloy steels used for highly stressed components, traditional alloy elements have been Cu, Ni and Mo, which in their oxygen affinity are very similar to the base constituent iron. Advanced alloying systems however contain Cr, Mn and/or Si. In the present study it is shown that one of the principal aspects of sintering to be considered is oxygen transfer from the base iron oxides to the alloy elements, which then form oxides that are more difficult to reduce. This process, defined as “internal gettering”, occurs both in mixed powder compacts and in prealloyed materials, although through different mechanisms. The effect can at least be alleviated by presintering in H2 in the 400°C range, part of the oxygen being removed as H2O before internal gettering becomes kinetically effective. However, in industrial practice, this collides with delubricaton. Furthermore for both alloy variants high temperature sintering is advantageous because it enhances reduction of the more stable oxides, thus eliminating the effects of internal gettering.European Union Marie Sklodowska-Curie scholarshi

Dynamic analysis of physiological properties of Torulaspora delbrueckii in wine fermentations and its incidence on wine quality

This work examines the physiology of a new commercial strain of Torulaspora delbrueckii in the production of red wine following different combined fermentation strategies. For a detailed comparison, several yeast metabolites and the strains implantation were measured over the entire fermentation period. In all fermentations in which T. delbrueckii was involved, the ethanol concentration was reduced; some malic acid was consumed; more pyruvic acid was released, and fewer amounts of higher alcohols were produced. The sensorial properties of final wines varied widely, emphasising the structure of wine in sequential fermentations with T. delbrueckii. These wines presented the maximum overall impression and were preferred by tasters. Semi-industrial assays were carried out confirming these differences at a higher scale. No important differences were observed in volatile aroma composition between fermentations. However, differences in mouthfeel properties were observed in semi-industrial fermentations, which were correlated with an increase in the mannoprotein content of red wines fermented sequentially with T. delbrueckii

Dynamic analysis of physiological properties of Torulaspora delbrueckii in wine fermentations and its incidence on wine quality.

This work examines the physiology of a new commercial strain of Torulaspora delbrueckii in the production of red wine following different combined fermentation strategies. For a detailed comparison, several yeast metabolites and the strains implantation were measured over the entire fermentation period. In all fermentations in which T. delbrueckii was involved, the ethanol concentration was reduced; some malic acid was consumed; more pyruvic acid was released, and fewer amounts of higher alcohols were produced. The sensorial properties of final wines varied widely, emphasising the structure of wine in sequential fermentations with T. delbrueckii. These wines presented the maximum overall impression and were preferred by tasters. Semiindustrial assays were carried out confirming these differences at a higher scale. No important differences were observed in volatile aroma composition between fermentations. However, differences in mouthfeel properties were observed in semiindustrial fermentations, which were correlated with an increase in the mannoprotein content of red wines fermented sequentially with T. delbrueckii