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  • 1.
    Siafakas, Dimitrios
    Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    On deoxidation practice and grain size of austenitic manganese steel2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The exceptional wear resistance and work hardenability, place Hadfield steel as one of themost important materials for manufacturing cast components used in the mining, crashing,drilling, and excavation industries. In all metallic alloys used for component casting, themechanical properties are highly influenced by the microstructure of the material. Castcomponents with finer microstructural characteristics are known to present bettermechanical properties and reduced risk of defects when compared with components witha coarser microstructure. A reduced grain size in Hadfield steel can increase the strengthof the material up to 30% and reduce the risk of porosity formation during solidification.The practice of adding selected compounds or alloying elements in a metal melt to modifyand refine the microstructure is called inoculation. It is currently one of the trendingmethods utilized in light-metal alloys and cast-iron components production but has not,yet, gained adequate acceptance in the steel casting industry because researchers have notbeen able to find proper inoculants.The main objective of this work is to investigate the qualitative and quantitativecharacteristics of the by-products of deoxidation of Hadfield steel that remain in thematerial after solidification and their positive or negative effect on the coarseness of thefinal as-cast microstructure. This type of research can help to identify the type of particlesor alloying elements that are most effective for refining the microstructure of austeniticsteels and pave the way for developing new or improving conventional deoxidation andinoculation processes that will, in turn, result in the improvement of the properties of thecomponent.The precipitation of particles and the as-cast grain size are studied in aluminum andtitanium deoxidized Hadfield steel samples acquired under pilot scale experimentalconditions. In the first part of this work, the qualitative and quantitative characteristics ofparticles such as type, morphology, composition amount and size are identified. Thesequence of precipitation is established. A model for predicting particle size and growth isdeveloped. The experimental results are compared against thermodynamic equilibriumcalculations and the precipitation mechanisms for each type of particles are described. Inthe second part, the as-cast grain size of samples with varying deoxidation treatments ismeasured. Then, the grain-size is correlated with certain particle characteristic and theparticles are ranked according to their ability to refine the microstructure. The particledisregistry with austenite is calculated and compared to the experimentally acquiredranking.

  • 2.
    Siafakas, Dimitrios
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    On particles and slags in steel casting2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Hadfield steel is widely accepted as one of the most important steel alloys utilized in industrial applications where high impact strength and wear resistance is required. Like in most metallic alloys used for component casting, the mechanical properties of Hadfield steel are directly connected with the microstructure of the material. It has been reported that Hadfield steel components with fine microstructure can present up to 30% increased strength and reduced risk of porosity formation during solidification when compared with their coarser microstructure counterparts.

    In the light-metal alloy and cast-iron industry, one of the most widely used methods for achieving refinement of the microstructure of the material is known as inoculation. As the name implies, inoculation is the practice ofadding selected compounds or alloying elements in a metal melt that have the ability to promote rapid grain nucleation during solidification. Even though it has been proved that inoculation is one of the most efficient methods for the refinement of a wide variety of metallic alloys, it has not yet gained adequate acceptance in the steel casting industry because researchers have not yet been able to identify proper inoculants for steel.

    The efficiency of the microstructural refinement when inoculating is influenced by several factors like the type of inoculant used and the processing conditions during melting, deoxidation, casting and heat treatment. Following proper deoxidation methods and application of tailored oxidic slags during melting could significantly promote the precipitation of desired inclusions that can act as potent nucleation sites for grains or as grain growth inhibitors.

    In any case, efficient inoculation is influenced by the complex interaction between the inoculant, the oxide slag, and the melt. The way this interaction happens is in many ways dictated by the chemical and thermophysical properties of the substances involved. Therefore, obtaining accurate values of basic thermophysical properties like viscosity and interfacial tension by improving current and utilizing novel measurement methods could significantly help in the effort of identifying and efficiently utilizing potent inoculants for austenitic steels.

    Considering the above, this work has a dual objective. The primary aim is to investigate if any of the by-products of deoxidation of Hadfield steel that remain in the material after solidification can act as potent inoculants by examining their qualitative and quantitative characteristics and their influence on the as-cast microstructure of the steel. The secondary aim is to acquire accurate values for oxide slag viscosity and slag-iron interfacial tension at high temperatures using different measurement methods and investigate how thermophysical properties are influenced by thermal and compositional conditions. This type of research is important because not only it can help to identify which substances are potent inoculants for austenitic steels but also pave the way for developing new or improving conventional deoxidation and inoculation processes with the ultimate goal of improving the cast component’s mechanical properties.

    The work is divided into 3 different stages. The first stage is dedicated to high-temperature oxide slag viscosity measurements. The viscosity of oxide slags with varying composition is measured in a wide temperature range utilizing the rotational bob and aerodynamic levitation methods. The systematic error is defined, and the compositional and thermodynamic dependence of viscosity is explained. In the second stage, the precipitation of particles in aluminum and titanium deoxidized Hadfield steel is investigated. The characteristics of particles, including type, size, morphology, composition, population, and sequence of precipitation are identified. The results are then compared against thermodynamic equilibrium calculations, a particle growth mathematical model is developed and the precipitation mechanism of each type of particle is described. Finally, in the third stage, the as-cast grain size of samples produced with varying deoxidation procedures is measured and the relationship between particle characteristics and grain size is determined. The particles are ranked according to their refining potency and compared to a ranking based on their disregistry with austenite.

  • 3.
    Siafakas, Dimitrios
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Matsushita, Taishi
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Hakamada, Shinya
    Gakushuin Univ, Dept Phys, Tokyo, Japan.
    Onodera, Kenta
    Gakushuin Univ, Dept Phys, Tokyo, Japan.
    Kargl, Florian
    German Aerosp Ctr DLR, Inst Mat Phys Space, Cologne, Germany.
    Jarfors, Anders E.W.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Watanabe, Masahito
    Gakushuin Univ, Dept Phys, Tokyo, Japan.
    Measurement of Viscosity of SiO2-CaO-Al2O3 Slag in Wide Temperature Range by Aerodynamic Levitation and Rotating Bob Methods and Sources of Systematic Error2018In: International journal of microgravity science and application, ISSN 0915-3616, Vol. 35, no 2, article id 350204Article in journal (Refereed)
    Abstract [en]

    Viscosity measurements for SiO2-CaO-Al2O3 based ternary slags with low SiO2 content were performed for a wide temperature range utilizing the aerodynamic levitation and rotating bob methods. Aerodynamic levitation was used for temperatures >= 2229 K and the viscosity was calculated by the sample oscillation decay time. The rotating bob method was used for temperatures <= 1898 K and the viscosity was determined by the variation of the torque at different rotation speeds. Fitting curves were created using Mauro’s viscosity equation. The main sources of systematic error were identified to be the sample weight measurement, the resolution of the high-speed camera, the fitting of the linear trend line in the torque against rpm diagrams and the vertical position of the bob. The combined standard uncertainty from all error sources was calculated for both measurement methods.

  • 4.
    Siafakas, Dimitrios
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Matsushita, Taishi
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Jarfors, Anders E.W.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Hakamada, Shinya
    Department of Physics, Gakushuin University, Tokyo, Japan.
    Watanabe, Masahito
    Department of Physics, Gakushuin University, Tokyo, Japan.
    Viscosity of SiO2–CaO–Al2O3 slag with low silica – Influence of CaO/Al2O3, SiO2/Al2O3 ratio2018In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 58, no 12, p. 2180-2185Article in journal (Refereed)
    Abstract [en]

    The viscosity of low SiO2 (10–20 mass%)-CaO-Al2O3 slag system was measured in a wide temperature range (1 623–2 800 K) using the rotational bob method and the aerodynamic levitation method. The influence of SiO2/Al2O3 ratio and CaO/Al2O3 ratio on the viscosity was examined. It was concluded that the SiO2/Al2O3 ratio did not affect the degree of polymerization of the aluminosilicate network in the composition range of the present study. An abnormal behaviour of the viscosity was observed at a CaO/Al2O3 ratio of 1.57 which was attributed to the formation of 12CaO·7Al2O3-like clusters. It was concluded that the overall influence on the viscosity could be expressed as the summation of the influence from the aluminosilicate network and the influence from the cluster formation of the primary precipitating solid phase. The temperature dependence of the cluster formation was coupled to the driving force of precipitation of the 12CaO·7Al2O3 phase.

  • 5.
    Siafakas, Dimitrios
    et al.
    Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Matsushita, Taishi
    Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Jarfors, Anders
    Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Lauenstein, Åsa
    Swerea SWECAST AB.
    Ekerot, Sven
    Comdicast AB.
    Particles precipitation in Ti and Al deoxidized Hadfield steels2016In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 87, no 10, p. 1344-1355Article in journal (Refereed)
    Abstract [en]

    The characteristics and precipitation mechanism of particles in titanium and aluminum treated Hadfield steel casted during pilot scale experiments have been studied. Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were utilized for the particle analysis and characterization. Additionally, thermodynamic equilibrium calculations were performed using Thermo-Calc software. Aluminum oxides, titanium carbon nitrides, titanium carbides and manganese sulfides were the main types of particles found. The order of precipitation during solidification and chemical composition range of each type of particle was determined. Aluminum Oxides were found to act as nucleation sites fortitanium carbon nitrides. Thermodynamic equilibrium calculation for particles characteristics were in good agreement with the experimental findings. Titanium carbides were found to form during initial stages of the Ferro-titanium additions dissolution.

  • 6.
    Siafakas, Dimitrios
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Matsushita, Taishi
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Lauenstein, Åsa
    Swerea SWECAST AB, Sweden.
    Ekerot, Sven
    Comdicast AB, Sweden.
    Jarfors, Anders
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    A particle population analysis in Ti- and Al-deoxidized Hadfield steels2018In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 31, no 3, p. 125-134Article in journal (Refereed)
    Abstract [en]

    A quantitative analysis of the amount, size and number of particles that precipitate in situ in titanium- and aluminium-treated Hadfield steel cast during pilot-scale experiments has been performed. SEM with EDS and automated particle analysis abilities was utilized for the analysis. Additionally, Thermo-Calc was used for thermodynamic calculations and Magma 5 for solidification and cooling simulations. Predicted particles sizes calculated with a model based on the Ostwald ripening mechanism were compared with the experimental data. The effect of solute availability, cooling rate and deoxidation practice on the particle population characteristics was determined. It was concluded that the amount, size and number of precipitating particles in Hadfield steel castings is possible to be controlled according to certain requirements by a careful selection of proper additives in proper amounts and also by the optimization of the casting process in aspects of deoxidation timing and control of the cooling rate of the castings.

  • 7.
    Siafakas, Dimitrios
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Matsushita, Taishi
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Lauenstein, Åsa
    Swerea SWECAST AB, Sweden.
    Jarfors, Anders E.W.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Ekengård, Johan
    Sandvik SRP, Sweden.
    The Influence of Deoxidation Practice on the As-Cast Grain Size of Austenitic Manganese Steels2017In: Metals, ISSN 2075-4701, Vol. 7, no 6, article id 186Article in journal (Refereed)
    Abstract [en]

    The effect of in-situ precipitating particles on the grain size of Al-Ti-treated and untreated Hadfield steel cast in a pilot scale environment was studied. Hadfield steel was melted in an induction furnace and cast in Y-Block samples. Light Optical Microscopy (LOM) and the intercept method were utilized for the grain size measurements. Additionally, Thermo-Calc Software TCFE7 Steels/Fe-alloys database version 7 was used for thermodynamic equilibrium calculations of the mole fraction of particles. The planar disregistry values between the austenite and the precipitating particles were calculated. It was observed that increasing oxide content in samples with low Ti(CN) content resulted in a finer microstructure, while increasing the Ti(CN) content under similar oxide content levels led to a coarser microstructure. The potency of each type of particle to nucleate austenitic grains was determined. Spinel (MnAl2O4, MgAl2O4) particles were characterized as the most potent, followed by olivine (Mn2SiO4), corundum (Al2O3, TiO2), and finally Ti(CN), the least potent particle.

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