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  • 1.
    Adegoke, Olutayo
    Dalarna University, School of Technology and Business Studies, Material Science.
    Homogenization of Precipitation Hardening Nickel Based Superalloys2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Allvac 718 Plus and Haynes 282 are relatively new precipitation hardening nickel based superalloys with good high temperature mechanical properties. In addition, the weldability of these superalloys enhances easy fabrication. The combination of high temperature capabilities and superior weldability is unmatched by other precipitation hardening superalloys and linked to the amount of the γ’ hardening precipitates in the materials. Hence, it is these properties that make Allvac 718 Plus and Haynes 282 desirable in the manufacture of hot sections of aero engine components.

    Studies show that cast products are less weldable than wrought products. Segregation of elements in the cast results in inhomogeneous composition which consequently diminishes weldability. Segregation during solidification of the cast products results in dendritic microstructure with the segregating elements occupying interdendritic regions. These segregating elements are trapped in secondary phases present alongside γ matrix. Studies show that in Allvac 718Plus, the segregating phase is Laves while in Haynes 282 the segregating phase is not yet fully determined.

    Thus, the present study investigated the effects of homogenization heat treatments in eliminating segregation in cast Allvac 718 Plus and Haynes 282. Paramount to the study was the effect of different homogenization temperatures and dwell time in the removal of the segregating phases. Experimental methods used to both qualify and quantify the segregating phases included SEM, EDX analysis, manual point count and macro Vickers hardness tests.

    Main results show that there is a reduction in the segregating phases in both materials as homogenization proceeds hence a disappearance of the dendritic structure. In Allvac 718 Plus, plate like structures is observed to be closely associated with the Laves phase at low temperatures and dwell times. In addition, Nb is found to be segregating in the interdendritic areas. The expected trend of increase in Laves as a result of the dissolution of the plate like structures at the initial stage of homogenization is only detectable for few cases. In Haynes 282, white and grey phases are clearly distinguished and Mo is observed to be segregating in interdendritic areas.

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  • 2. Arzpeyma, N.
    et al.
    Alam, Moudud
    Dalarna University, School of Information and Engineering, Statistics.
    Gyllenram, R.
    Jönsson, P. G.
    Model development to study uncertainties in electric arc furnace plants to improve their economic and environmental performance2021In: Metals, ISSN 2075-4701, Vol. 11, no 6, article id 892Article in journal (Refereed)
    Abstract [en]

    A statistical model is developed in order to simulate the melt composition in electric arc furnaces (EAFs) with respect to uncertainties in 1) scrap composition, 2) scrap weighing and 3) element distribution factors. The tramp element Cu and alloying element Cr are taken into account. The model enables simulations of a charge program as well as backwards estimations of the element concentrations and their variance in scrap. In the backwards calculation, the maximum likelihood method is solved by considering three cases corresponding to the involved uncertainties. It is shown that the model can estimate standard deviations for elements so that the real values lie within the estimated 95% confidence interval. Moreover, the results of the model application in each target product show that the estimated scrap composition results in a melt composition, which is in good agreement with the measured one. The model can be applied to increase our understanding of scrap chemical composition and lower the charged material cost and carbon footprint of the products. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

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  • 3. Cinca, N.
    et al.
    Gordon, S.
    Llanes, L. M.
    Olsson, Mikael
    Dalarna University, School of Information and Engineering, Materials Technology.
    Friction, Material Transfer And Wear Phenomena In Sliding Contacts Between Cemented Carbide And Aluminum During Metal Forming2022In: World PM 2022 Congress Proceedings, European Powder Metallurgy Association (EPMA) , 2022Conference paper (Refereed)
    Abstract [en]

    In many metal forming processes, certain texturization of the tool is necessary to have some friction that can make the operation possible. One example of that is the use of crosshatching methods on cemented carbide punches to manufacture beverage cans. The deep drawing and ironing process of the aluminum foil during can production involves different wear mechanisms being active on the punch surface, i.e. adhesive and abrasive wear. The present work examines the abrasive wear performance and friction characteristics of two cemented carbide grades by means of scratch and linear reciprocating sliding wear testing with Al2O3 counterparts under dry and lubricated conditions, with smooth polished and textured cemented carbide surfaces respectively. Additionally, well-controlled tests in which an aluminum pin is sliding over the cemented carbide surface were performed to evaluate the initial metal transfer. Post-test characterization shows the influence of carbide microstructure on the friction, material transfer and wear mechanisms. © European Powder Metallurgy Association (EPMA)

  • 4.
    Engkvist, Josefin
    et al.
    Dalarna University, School of Technology and Business Studies, Material Science.
    Canovic, Sead
    Hellström, Kristina
    Järdnäs, Anders
    Svensson, Jan-Erik
    Johansson, Lars-Gunnar
    Olsson, Mikael
    Dalarna University, School of Technology and Business Studies, Material Science.
    Halvarsson, Mats
    Alumina scale formation on a powder metallurgical FeCrAl alloy (Kanthal APMT) at 900-1100 °C in dry O2 and in O2 + H2O2010In: Oxidation of Metals, ISSN 0030-770X, E-ISSN 1573-4889, Vol. 73, no 1-2, p. 233-253Article in journal (Refereed)
    Abstract [en]

    A Rapidly Solidified Powder (RSP) metallurgical FeCrAl alloy, Kanthal APMT, was exposed in dry and humid O2 for 72 h at 900–1,100 °C. The formed oxide scales were characterized using gravimetry in combination with advanced analysis techniques (SEM, EDX, TEM, XRD, AES andSIMS). The oxide scales were at all exposures composed of two-layered α-Al2O3 scales exhibiting a top layer of equiaxed grains and a bottom layer containing elongated grains. A Cr-rich zone, originating in the native oxide present before exposure, separated these two layers. The top α-Al2O3layer is suggested to have formed by transformation of outwardly grown metastable alumina, while the inward-grown bottom α-Al2O3 layer had incorporated small Zr-, Hf- and Ti-rich oxide particles present in the alloy matrix. The scale also contained larger Y-rich oxide particles. Furthermore, in the temperature range studied, the presence of water vapour accelerated alloy oxidation somewhat and affected scale morphology.

  • 5.
    Ismail, Kasimagwa
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad processmetallurgi.
    Brabie, Voicu
    The use of thermodynamic computations to predict the phase transformation in MgO-C refractories during steel refining2008In: Refractories Manual, p. 42-47Article in journal (Refereed)
  • 6.
    Jacobson, S
    et al.
    Uppsala Universitet.
    Beste, U
    Uppsala Universitet.
    Heinrichs, J
    Uppsala Universitet.
    Olsson, Mikael
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Wiklund, U
    Uppsala Universitet.
    On the nature of cemented carbide wear in rock drilling2014In: Hard Rock Tribology Course and Seminar, Tampere, Finland, November 4-5, 2014, 2014Conference paper (Other academic)
  • 7.
    Jayamani, Jayaraj
    et al.
    National Institute for Materials Science, Tsukuba, Japan.
    Mendis, C. L.
    Ohkubo, T.
    Oh-ishi, K.
    Hono, K.
    Precipitation hardenable Mg-Ca-Al alloys2011In: Magnesium Technology 2011 / [ed] Sillekens, Wim H.; Agnew, Sean R.; Neelameggham, Neale R.; Mathaudhu, Suveen N, Cham: Springer International Publishing , 2011, p. 245-248Chapter in book (Refereed)
    Abstract [en]

    The age-hardening responses and the corresponding microstructures of Mg-0.5Ca-xAl (x = 0, 0.1, 0.3, 0.5, 1 wt. %) alloys were investigated by hardness tests and transmission electron microscopy. For the optimum Al addition of 0.3 wt. % an enhanced age-hardening response with the highest peak hardness of HV=72 was achieved. TEM analyses confirmed that the improvement in the peak hardness is associated with the dense precipitation of ordered monolayer G.P. zones. Whereas, lower content of Al resulted in the formation of G.P. zones and Mg2Ca and the excess addition of Al causes the formation of the G.P. zones and the grain boundary Al2Ca phase.

  • 8.
    Kamachi Mudali, U.
    et al.
    Corrosion Science and Technology Group Indira Gandhi Center for Atomic Research Kalpakkam, Tamilnadu.
    Jayamani, Jayaraj
    Corrosion Science and Technology Group Indira Gandhi Center for Atomic Research Kalpakkam, Tamilnadu.
    Bulk Metallic Glassy Alloys for Nitric Acid Applications2014Conference paper (Refereed)
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  • 9.
    Kasimagwa, Ismail
    Dalarna University, School of Technology and Business Studies, Material Science.
    A study of slag corrosion of oxides and oxide-carbon refractories during steel refining2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The use of ceramic material as refractories in the manufacturing industry is a common practice worldwide. During usage, for example in the production of steel, these materials do experience severe working conditions including high temperatures, low pressures and corrosive environments. This results in lowered service lives and high consumptions of these materials. This, in turn, affects the productivity of the whole steel plant and thereby the cost. In order to investigate how the service life can be improved, studies have been carried out for refractories used in the inner lining of the steel ladles. More specifically, from the slag zone, where the corrosion is most severe. By combining thermodynamic simulations, plant trails and post-mortem studies of the refractories after service, vital information about the behaviour of the slagline refractories during steel refining and the causes of the accelerated wear in this ladle area has been achieved. The results from these studies show that the wear of the slagline refractories of the ladle is initiated at the preheating station, through reduction-oxidation reactions. The degree of the decarburization process is mostly dependent on the preheating fuel or the environment. For refractories without antioxidants, refractory decarburization is slower when coal gas is used in ladle preheating than when a mixture of oil and air is used. In addition, ladle preheating of the refractories without antioxidants leads to direct wear of the slagline refractories. This is due to the total loss of the matrix strength, which results in a sand-like product. Thermal chemical changes that take place in the slagline refractories are due to the MgO-C reaction as well as the formation of liquid phases from impurity oxides. In addition, the decrease in the system pressure during steel refining makes the MgO-C reaction take place at the steel refining temperatures. This reduces the refractory’s resistance to corrosion. This is a serious problem for both the magnesia-carbon and dolomite-carbon refractories. The studies of the reactions between the slagline refractories and the different slag compositions showed that slags rich in iron oxide lead mostly to the oxidation of carbon/graphite in the carbon-containing refractories. This leads to an increased porosity and wettability and therefore an enhanced penetration of slag into the refractory structure. If the slag contains high contents of alumina and or silica (such as the steel refining slag), reactions between the slag components and the dolomite-carbon refractory are promoted. This leads to the formation of low-temperature melting phases such as calcium-aluminates and silicates. The state of these reaction products during steel refining leads to an accelerated wear of the dolomite-carbon refractory. The main products of the reactions between the magnesia-carbon refractory and the steel refining slag are MgAl2O4 spinels, and calcium-aluminates, and silicates. Due to the good refractory properties of MgAl2O4 spinels, the slag corrosion resistance of the magnesiacarbon refractory is promoted.

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  • 10.
    Kasimagwa, Ismail
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad processmetallurgi.
    Brabie, Voicu
    Thermo-calc and SEM analysis of the dolomite lining during steel refining2008In: Proceedings for the Third Nordic Symposium for Young Scientists in Metallurgy, 2008, p. 46-50Conference paper (Refereed)
  • 11.
    Kasimagwa, Ismail
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad processmetallurgi.
    Brabie, Voicu
    Dalarna University, School of Technology and Business Studies, Material Science.
    Jönsson, Pär G.
    Studies on decarburization of MgO-c refractories during ladle preheating2010In: Steel GRIPS Journal, ISSN 1611-4442, no 8, p. 357-363Article in journal (Refereed)
  • 12.
    Kasimagwa, Ismail
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad processmetallurgi.
    Eriksson, J.
    Brabie, Voicu
    Wahlberg, H.
    Slagline refractory2008In: Proceedings for the SCANMET III-3rd International Conference on Process Development in Iron and Steel making, 2008, p. 377-384Conference paper (Refereed)
  • 13.
    Kellner, Hans
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH, Tillämpad processmetallurgi.
    Study of Non-metallic Inclusion in Alloy 8252017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    It is well known that inclusions affect the properties of steels and alloys. Therefore, it is important to understand what type of inclusions that exist and how they behave and especially with a focus on large size inclusions. Thus, the large size non-metallic inclusions in ferroalloy FeTi70R were investigated in two dimensions (2D) by using scanning electron microscopy (SEM) in combination with an energy dispersive technique (EDS). It was found that the FeTi70R ferroalloy contain complex oxide inclusions consisting mostly of CaO, SiO2 and TiOx. Furthermore, experimental trials were performed to investigate how these inclusions behaved when entering a melt. More specifically, a comparison between pure Fe and an Alloy 825 grade were made. These results determined the parameters effect on the transformation of the inclusions in the melt.

    The large size non-metallic inclusions in Alloy 825 during the ladle treatment were investigated during industrial trials by using both two dimensional (2D) and three dimensional (3D) microscopic investigations. The results showed that inclusions consisted of spherical oxides and clusters made up of oxides and nitrides. Further investigations found that the spherical inclusions were transformed from existing NMI in the FeTi70R ferroalloy and slag particles. As for the clusters, they originate from deoxidation products. Furthermore, small inclusions precipitated in the local zones around the added FeTi70R ferroalloy and titanium nitrides. Investigations also found that only Al2O3-MgO and TiN clusters exist after casting.

    Industrial trials were performed during the last period of the ladle treatment and using a combined electromagnetic (EMS) and gas (GS) stirring. The purpose to investigate the effect of different EMS directions on the agglomeration and on the removal of Al2O3-MgO and TiN clusters. The investigations were then performed in 3D after an electrolytic extraction of the metal samples. The results show that electromagnetic stirring in the upwards direction is best for the agglomeration of the Al2O3-MgO and TiN clusters. However, electromagnetic stirring in the downwards direction is more effective to remove clusters from the melt. This is in agreement with the theoretical predictions based on Stokes’, Brownian and Turbulent collisions. Also, the calculations showed that for Al2O3-MgO clusters with sizes <20 μm the Turbulent collision is the defining factor for agglomeration. However, both Stokes’ and Turbulent collisions are dominant for larger inclusions. For the TiN clusters, turbulent collisions is the dominant factor.

    Further investigations with more heats and stirring modes were done by using 2D microscopic investigations. More specifically, the number, size, composition and morphology of different inclusions were determined by using SEM in combination with EDS and Inca Feature analyses. The results show that the EMS in downwards direction with a 0.04 m3 min-1 gas flow rate promotes a general removal of Al2O3-MgO and TiN inclusions. Furthermore, that the upwards EMS direction promotes a drastically increase of inclusions having an equivalent size smaller than 11.2 μm. Moreover, the stirring with a 0.02 m3 min-1 gas flow rate has a better removal rate for both downwards and upwards stirring directions compared to the stirring with a 0.04 m3 min-1 gas flow rate. However, no influence on the inclusion composition and morphology could be seen from the different stirring modes.

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  • 14. Lin, Z.
    et al.
    Surreddi, Kumar Babu
    Dalarna University, School of Information and Engineering, Materials Technology.
    Hulme, C.
    Dadbakhsh, S.
    Rashid, A.
    Influence of Electron Beam Powder Bed Fusion Process Parameters on Transformation Temperatures and Pseudoelasticity of Shape Memory Nickel Titanium2023In: Advanced Engineering Materials, ISSN 1438-1656, E-ISSN 1527-2648, Vol. 25, no 12, article id 2201818Article in journal (Refereed)
    Abstract [en]

    Electron beam powder bed fusion (PBF-EB) is used to manufacture dense nickel titanium parts using various parameter sets, including the beam current, scan speed, and postcooling condition. The density of manufactured NiTi parts is investigated in relation to the linear energy input. The results imply that the part density increases with increasing linear energy density to over 98% of the bulk density. With a constant energy input, a combination of low power and low scan speed leads to denser parts. This is attributed to lower electrostatic repulsive forces from lower number density of the impacting electrons. After manufacturing, the densest parts with distinct parameter sets are categorized into three groups: 1) high power with high scan speed and vacuum slow cooling, 2) low power with low scan speed and vacuum slow cooling, and 3) low power with low scan speed and medium cooling rate in helium gas. Among these, a faster cooling rate suppresses phase transformation temperatures, while vacuum cooling combinations do not affect the phase transformation temperatures significantly. Herein, all the printed parts exhibit almost 8% pseudoelasticity regardless of the process parameters, while the parts cooled in helium have a higher energy dissipation efficiency (1 − η), which implies faster damping of oscillations. © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.

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  • 15. Maurya, H. S.
    et al.
    Jayamani, Jayaraj
    Dalarna University, School of Information and Engineering, Materials Technology.
    Vikram, R. J.
    Juhani, K.
    Sergejev, F.
    Prashanth, K. G.
    Additive manufacturing of TiC-based cermets: A detailed comparison with spark plasma sintered samples2023In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 960, article id 170436Article in journal (Refereed)
    Abstract [en]

    The present work is a comparative study on the TiC-430 L ferritic stainless steel (FSS) cermets manufactured via two powder metallurgical processes, namely, conventional spark plasma sintering (SPS) and metal additive manufacturing (AM) process (laser powder-bed fusion process (LPBF)/selective laser melting (SLM)). The rescanning strategy has been used to preheat and melt the powder bed with different laser parameters during the SLM process to suppress the presence of residual thermal stress leading to the fabrication of cermets without cracks. The as-fabricated SPS samples (95 %) show a relatively lower density than the SLM-built parts (~98 %). A study of their mechanical properties such as hardness, compressive strength, and fracture toughness was conducted and discussed in detail. Further, the corrosion behavior of the fabricated cermets parts was evaluated in 3.5 wt% NaCl. The SLM-prepared specimens reveal finer microstructures and better mechanical properties (compressive strength and fracture toughness) due to the presence of fine microstructure. Furthermore, the corrosion current density of TiC-430 L fss-based cermets fabricated by SLM is approximately 270 times lower than that of cermets parts fabricated by SPS, indicating excellent corrosion resistance. On the other hand, the hardness shows an opposite trend, where the SPS samples show the maximum hardness as compared to the SLM counterparts due to the presence of hard and coarse TiC particles along with some metallic carbides formed during the SPS process. The results reveal that AM processes not only can fabricate cermets with intricate shapes but can also fabricate them with improved mechanical and corrosion properties.

  • 16.
    Memarpour, Arashk
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    An Experimental Study of Submerged Entry Nozzles (SEN) Focusing on Decarburization and Clogging2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The submerged entry nozzle (SEN) is used to transport the molten steel from a tundish to a mould. The main purpose of its usage is to prevent oxygen and nitrogen pick-up by molten steel from the gas. Furthermore, to achieve the desired flow conditions in the mould. Therefore, the SEN can be considered as a vital factor for a stable casting process and the steel quality. In addition, the steelmaking processes occur at high temperatures around 1873 K, so the interaction between the refractory materials of the SEN and molten steel is unavoidable. Therefore, the knowledge of the SEN behaviors during preheating and casting processes is necessary for the design of the steelmaking processes  The internal surfaces of modern SENs are coated with a glass/silicon powder layer to prevent the SEN graphite oxidation during preheating. The effects of the interaction between the coating layer and the SEN base refractory materials on clogging were studied. A large number of accretion samples formed inside alumina-graphite clogged SENs were examined using FEG-SEM-EDS and Feature analysis. The internal coated SENs were used for continuous casting of stainless steel grades alloyed with Rare Earth Metals (REM). The post-mortem study results clearly revealed the formation of a multi-layer accretion. A harmful effect of the SENs decarburization on the accretion thickness was also indicated. In addition, the results indicated a penetration of the formed alkaline-rich glaze into the alumina-graphite base refractory. More specifically, the alkaline-rich glaze reacts with graphite to form a carbon monoxide gas. Thereafter, dissociation of CO at the interface between SEN and molten metal takes place. This leads to reoxidation of dissolved alloying elements such as REM (Rare Earth Metal). This reoxidation forms the “In Situ” REM oxides at the interface between the SEN and the REM alloyed molten steel. Also, the interaction of the penetrated glaze with alumina in the SEN base refractory materials leads to the formation of a high-viscous alumina-rich glaze during the SEN preheating process. This, in turn, creates a very uneven surface at the SEN internal surface. Furthermore, these uneven areas react with dissolved REM in molten steel to form REM aluminates, REM silicates and REM alumina-silicates.

    The formation of the large “in-situ” REM oxides and the reaction of the REM alloying elements with the previously mentioned SEN´s uneven areas may provide a large REM-rich surface in contact with the primary inclusions in molten steel. This may facilitate the attraction and agglomeration of the primary REM oxide inclusions on the SEN internal surface and thereafter the clogging. The study revealed the disadvantages of the glass/silicon powder coating applications and the SEN decarburization.

    The decarburization behaviors of Al2O3-C, ZrO2-C and MgO-C refractory materials from a commercial Submerged Entry Nozzle (SEN), were also investigated for different gas atmospheres consisting of CO2, O2 and Ar. The gas ratio values were kept the same as it is in a propane combustion flue gas at different Air-Fuel-Ratio (AFR) values for both Air-Fuel and Oxygen-Fuel combustion systems. Laboratory experiments were carried out under nonisothermal conditions followed by isothermal heating. The decarburization ratio (α) values of all three refractory types were determined by measuring the real time weight losses of the samples. The results showed the higher decarburization ratio (α) values increasing for MgO-C refractory when changing the Air-Fuel combustion to Oxygen-Fuel combustion at the same AFR value. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times compared to heating at lower temperatures during longer holding times for Al2O3-C samples. Diffusion models were proposed for estimation of the decarburization rate of an Al2O3-C refractory in the SEN.

    Two different methods were studied to prevent the SEN decarburization during preheating: The effect of an ZrSi2 antioxidant and the coexistence of an antioxidant additive and a (4B2O3 ·BaO) glass powder on carbon oxidation for non-isothermal and isothermal heating conditions in a controlled atmosphere. The coexistence of 8 wt% ZrSi2 and 15 wt% (4B2O3 ·BaO) glass powder of the total alumina-graphite refractory base materials, presented the most effective resistance to carbon oxidation. The 121% volume expansion due to the Zircon formation during heating and filling up the open pores by a (4B2O3 ·BaO) glaze during the green body sintering led to an excellent carbon oxidation resistance.

    The effects of the plasma spray-PVD coating of the Yttria Stabilized Zirconia (YSZ) powder on the carbon oxidation of the Al2O3-C coated samples were investigated. Trials were performed at non-isothermal heating conditions in a controlled atmosphere. Also, the applied temperature profile for the laboratory trials were defined based on the industrial preheating trials. The controlled atmospheres consisted of CO2, O2 and Ar. The thicknesses of the decarburized layers were measured and examined using light optic microscopy, FEG-SEM and EDS. A 250-290 μm YSZ coating is suggested to be an appropriate coating, as it provides both an even surface as well as prevention of the decarburization even during heating in air. In addition, the interactions between the YSZ coated alumina-graphite refractory base materials in contact with a cerium alloyed molten stainless steel were surveyed. The YSZ coating provided a total prevention of the alumina reduction by cerium. Therefore, the prevention of the first clogging product formed on the surface of the SEN refractory base materials. Therefore, the YSZ plasma-PVD coating can be recommended for coating of the hot surface of the commercial SENs.

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  • 17.
    Memarpour, Arashk
    et al.
    Dalarna University, School of Technology and Business Studies, Material Science.
    Brabie, Voicu
    Jönsson, Pär
    Studies of Yttria Stabilized Zirconia (YSZ) plasma-PVD coated Al2O3-C refractory base materials of a commercial SEN with respect todecarburization and clogging2011In: Steel Grips - Journal of Steel and Related Materials, ISSN 1866-8453Article in journal (Refereed)
    Abstract [en]

    Carbon oxidation is a main industrial problem for alumina-graphite refractory base materials used in commercial Submerged Entry Nozzles (SEN) during preheating. Thus, the effects of the plasma spray-PVD coating of the Yttria Stabilized Zirconia (YSZ) powder on the carbon oxidation were investigated. Laboratory preheating trials were performed at non-isothermal heating conditions in a controlled atmosphere. Also, the applied temperature profile for the laboratory trials were defined based on industrial preheating trials. The controlled atmospheres consisted of CO2, O2 and Ar. The (CO2/O2) ratios were kept the same as for a propane combustion flue gas at an Air-Fuel-Ratio (AFR) value equal to 1.5 for heating in an air-fuel mixture and in air. The thicknesses of the decarburized layers were measured and examined using light optic microscopy, FEG-SEM and EDS. The YSZ plasma-PVD coated alumina-graphite refractory base materials, presented the effective resistance to carbon oxidation at different coating thicknesses from 160-480 μm in both combustion flue gas and air atmospheres. For the YSZ plasma coating that contained a thinner coating layer such as 160 μm, the uneven surface of the substrate may be reflected more than it could be reflected for a thicker coating. However, for the YSZ plasma coating with a coating thickness of 290 μm, the uneven surface of the substrate may be reflected much less than it could be reflected for thinner coatings. A 250μm and a 290μm YSZ coating may prevent the decarburization of an alumina-graphite refractory base materials during preheating in air at a maximum heating temperature of 1020°C. Moreover, in an oxidizing atmosphere with an AFR value equal to 1.5 at a maximum temperature of 1020°C and a holding time of 7200 seconds. A 250-290 μm YSZ coating is suggested to be an appropriate coating, as it provides both an even surface and prevention of the decarburization even during heating in air. In addition, the interactions between the YSZ coated alumina-graphite refractory base materials in contact with a cerium alloyed molten stainless steel were surveyed. The YSZ coating provided a total prevention of the alumina reduction by cerium. Therefore, the prevention of the first clogging product formed on the surface of the SEN refractory base materials. Therefore, the YSZ plasma-PVD coating can be recommended for coating of the hot surface of thecommercial SENs.

  • 18.
    Molnar, David
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH, Tillämpad materialfysik.
    Stacking fault energy and deformation behaviour of austenitic stainless steels: a joint theoretical-experimental study2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Austenitic stainless steels are primarily known for their exceptional corrosion resistance. They have the face centred cubic (FCC) structure which is stabilised by adding nickel, manganese or nitrogen to the Fe-Cr alloy. The Fe-Cr-Ni system can be further extended by adding other elements such as Mo, Cu, Ti, C, etc. to improve the properties. Since austenitic stainless steels are often used as structural materials, it is important to be able to predict their mechanical behaviour based on their composition, microstructure, magnetic state, etc.

    In this work, the plastic deformation behaviour of austenitic stainless steels is investigated by theoretical and experimental approaches. In FCC materials the stacking fault energy (SFE) plays an important role in the description and prediction of the deformation modes. Based on the magnitude of the SFE different deformation modes can be observed such as martensite formation, deformation twinning, or dislocation glide. All these deformation modes influence the material behaviour, therefore it is desired to predict and control their occurrence. Alloying elements and temperature have a strong effect on the SFE and thus on the mechanical properties of the alloys. Several models based on the SFE and more recently on the so-called generalised stacking fault energy (GSFE or γ-surface) are available to describe the alloy's affinity to twinning and the critical twinning stress representing the minimum resolved shear stress required to initiate the deformation twinning mechanism. One can employ well established experimental techniques to measure the SFE. On the other hand, one needs to resort to ab initio calculations based on density functional theory (DFT) to compute the GSFE of austenitic steels and derive parameters like the twinnability and the critical twinning stress.

    The correlation between the stacking fault energy and the deformation behaviour for four different austenitic stainless steels is discussed in this work. The SFE of the selected alloys is obtained by ab initio calculations and based on different models, their tendency for twinning and their critical twinning stress is predicted. The mechanical behaviour and the affinity for twinning and martensitic transformation is mapped across a broad range of temperature (-70°C to +500°C) for the four alloys. The theoretical predictions are contrasted with tensile tests and electron backscatter diffraction (EBSD) measurements. Several conventional and in situ tensile test are performed to verify the theoretical results. EBSD measurements on interrupted and fractured specimens, and during in situ tensile tests were carried out to closely follow the development of the microstructure. In the present thesis, a technique is proposed that can provide accurate unstable stacking fault energy values for any austenitic alloy exhibiting twinning at low stress values. The importance of temperature and interstitial alloying on mechanical behaviour is also investigated.

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  • 19.
    Molnar, David
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH.
    Engberg, Göran
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Li, Wei
    Lu, Song
    Hedström, Peter
    Kwon, Se Kyun
    Vitos, Levente
    Experimental study of the gamma-surface of austenitic stainless steels2019In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 173, p. 34-43Article in journal (Refereed)
    Abstract [en]

    We introduce a theory-guided experimental approach to study the γ-surface of austenitic stainless steels. The γ-surface includes a series of intrinsic energy barriers (IEBs), which are connected to the unstable stacking fault (USF), the intrinsic stacking fault (ISF), the unstable twinning fault (UTF) and the extrinsic stacking fault (ESF) energies. The approach uses the relationship between the Schmid factors and the effective energy barriers for twinning and slip. The deformation modes are identified as a function of grain orientation using in situ electron backscatter diffraction measurements. The observed critical grain orientation separating the twinning and slip regimes yields the USF energy, which combined with the universal scaling law provides access to all IEBs. The measured IEBs and the critical twinning stress are verified by direct first-principles calculations. The present advance opens new opportunities for modelling the plastic deformation mechanisms in multi-component alloys.

  • 20.
    Molnar, David
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH.
    Engberg, Göran
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Li, Wei
    Vitos, Levente
    Deformation properties of austenitic stainless steels with different stacking fault energies2018In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 941, p. 190-197Article in journal (Refereed)
    Abstract [en]

    In FCC metals a single parameter – stacking fault energy (SFE) – can help to predict the expectable way of deformation such as martensitic deformation, deformation twinning or pure dislocation glide. At low SFE one can expect the perfect dislocations to dissociate into partial dislocations, but at high SFE this separation is more restricted. The role of the magnitude of the stacking fault energy on the deformation microstructures and tensile behaviour of different austenitic steels have been investigated using uniaxial tensile testing and electron backscatter diffraction (EBSD). The SFE was determined by using quantum mechanical first-principles approach. By using plasticity models we make an attempt to explain and interpret the different strain hardening behaviour of stainless steels with different stacking fault energies.

  • 21.
    Molnar, David
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH.
    Lu, Song
    Hertzman, Staffan
    Engberg, Göran
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Vitos, Levente
    Study of the alternative mechanism behind the constant strain hardening rate in high‑nitrogen steels2020In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 170, article id 110726Article in journal (Refereed)
    Abstract [en]

    In this study, three austenitic stainless steels with different compositions are compared in terms of their deformation behaviour. Two of the investigated steels have considerable high nitrogen content which affects their deformation behaviour. The deformation properties and microstructure of the materials were studied by tensile testing and electron backscatter diffraction. We observe that the strain hardening rate curve of the alloy with low nitrogen content shows a continuously decreasing slope, whereas those of the high‑nitrogen steels exhibit a clear plateau. Since no twinning or ε-phase formation is observed at the corresponding strain levels, we suggest that the addition of a large amount of nitrogen suppresses cross-slip and promotes dislocation planarisation. The microstructural evolution of the materials during deformation supports the above scenario. Based on the results of the ab initio calculations, the deformation behaviour of high‑nitrogen alloys cannot be explained in terms of the stacking fault energy.

  • 22.
    Molnar, David
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH.
    Sun, Xun
    Lu, Song
    Li, Wei
    Engberg, Göran
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Vitos, Levente
    Effect of temperature on the stacking fault energy and deformation behaviour in 316L austenitic stainless steel2019In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 759, p. 490-497Article in journal (Refereed)
    Abstract [en]

    The stacking fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial dislocation ribbon, and shows strong correlation with the leading plastic deformation modes. Based on the SFE, one can estimate the critical twinning stress of the system as well. The SFE mainly depends on the composition of the system, but temperature can also play an important role. In this work, using first principles calculations, electron backscatter diffraction and tensile tests, we show a correlation between the temperature dependent critical twinning stress and the developing microstructure in a typical austenitic stainless steel (316L) during plastic deformation. We also show that the deformation twins contribute to the strain hardening rate and gradually disappear with increasing temperature. We conclude that, for a given grain size there is a critical temperature above which the critical twinning stress cannot be reached by normal tensile deformation, and the disappearance of the deformation twinning leads to lower strain hardening rate and decreased ductility.

  • 23.
    Molnar, David
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH.
    Vida, Ádám
    Huang, Shuo
    Chinh, Nguyen Q
    The effect of cooling rate on the microstructure and mechanical properties of NiCoFeCrGa high-entropy alloy2019In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 54, no 6, p. 5074-5082Article in journal (Refereed)
    Abstract [en]

    The effect of cooling rate on the microstructure and mechanical properties of equimolar NiCoFeCrGa high-entropy alloy (HEA) was studied by scanning electron microscopy, energy-dispersive X-ray spectroscopy and electron backscatter diffraction (EBSD), as well as by microhardness tests. Experimental results show that the cooling rate has a crucial impact on the developing microstructure which has a mixture of two—FCC and BCC—phases, leading to a self-similarity of the solidified structure formed in the sample. Furthermore, the cooling rate influences both the composition of the two phase-components and the ratio of their volume fractions, determining the mechanical properties of the sample. The present results confirm the grouping of Co, Fe and Cr in the FCC phase and that of Ni and Ga in BCC phase in the NiCoFeCrGa high-entropy alloy system. An empirical rule is suggested to predict how the phase-components can be expected in this complex high-entropy alloy.

  • 24.
    Nilsson, Maria
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Tribology in Metal Working2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis focuses on the tribological performance of tool surfaces in two steel working operations, namely wire drawing and hot rolling. In all forming operations dimensions and surface finish of the products are of utmost importance. Forming basically includes three parts – forming conditions excluded – that may be changed; work material, tool and (possibly) lubricant. In the interface between work material and tool, the conditions are very aggressive with – generally or locally – high temperatures and pressures. The surfaces will be worn in various ways and this will change the conditions in the process. Consequently, the surface finish as well as the dimensions of the formed product may change and in the end, the product will not fulfil the requirements of the customer. Therefore, research and development in regard to wear, and consequently tribology, of the forming tools is of great interest.

    The investigations of wire drawing dies focus on coating adhesion/cohesion, surface characteristics and material transfer onto the coated steel both in laboratory scale as well as in the wire drawing process. Results show that it in wire drawing is possible to enhance the tribological performance of drawing dies by using a lubricant together with a steel substrate coated by a polished, dual-layer coating containing both hard and friction-lowering layers.

    The investigations of hot rolling work rolls focus on microstructure and hardness as well as cracking- and surface characteristics in both laboratory scale and in the hot strip mill. Results show that an ideal hot work roll material should be made up of a matrix with high hardness and a large amount of complex, hard carbides evenly distributed in the microstructure. The surface failure mechanisms of work rolls are very complex involving plastic deformation, abrasive wear, adhesive wear, mechanical and thermal induced cracking, material transfer and oxidation.

    This knowledge may be used to develop new tools with higher wear resistance giving better performance, lower costs and lower environmental impact.

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  • 25.
    Nilsson, Maria
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Olsson, Mikael
    Dalarna University, School of Technology and Business Studies, Materials Technology. Tribomaterials group, The Ångström Laboratory, Uppsala University, Sweden.
    Microstructural, mechanical and tribological characterisation of roll materials for the finishing stands of the hot strip mill for steel rolling2013In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 307, no 1-2, p. 209-217Article in journal (Refereed)
    Abstract [en]

    The microstructure, mechanical and tribologicalproperties for three different materials, High Speed Steel, High Chromium ironand Indefinite Chill iron, used for hot strip mill work rolls have beenevaluated. Microstructural characterisation was performed using light opticalmicroscopy, scanning electron microscopy and energy dispersive X-rayspectroscopy. The mechanical and tribological properties were evaluated usingmicro Vickers indentation and scratch testing in combination with post-testmicroscopy. The microstructures of the investigated materials were found to berather complex with a number of secondary phases andalso materials with similar nominal composition display significant differenceswith respect to distribution, size and morphology of carbides. Scratch testing,including detection of friction coefficient, acoustic emission and penetrationdepth, gives valuable information concerning the mechanical and tribologicalresponse on a microscopic level of the investigated materials. Type,amount, distribution, size and morphology of the secondary phases in thematerials have a strong impact on the surface deformation and wear mechanismsduring scratching. Cracking and chipping are frequently observed in connectionto the ridges surrounding the scratches. However, cross-sectional analyses ofthe scratched microstructures reveal that cracking of the brittle carbidephases may extend to significant depths, >100 µm, reducing the mechanicalstrength of the material. Based on the results, it is believed that a moreisotropic microstructure, e.g. obtained via a powder metallurgy process, withfiner carbides would result in improved properties and performance in a hotrolling application.

  • 26.
    Olsson, Mikael
    Dalarna University, School of Information and Engineering, Materials Technology.
    Characterization of hot-rolled and annealed oxides on 2507 stainless steel – Microstructure, chemical composition, mechanical properties and pickling characteristics2021Report (Other academic)
    Abstract [en]

    The surface characteristics of samples of the 2507 super duplex stainless steel with three different surface conditions, i.e. after hot rolling, annealing and blasting plus pickling, respectively, have been investigated. High resolution scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and scratch testing have been used to characterize the oxide scales with respect microstructure, chemical composition, and mechanical properties.

    SEM and EDS-analysis of the oxide scales formed on hot rolled and hot rolled plus annealed samples, show an outer Fe-rich oxide layer and an inner Cr-rich oxide layer close to the interface. While the hot rolled oxide scale displays a relatively porous oxide composed of oxide particles, the hot rolled plus annealed oxide scale displays a more dense, “sintered”, microstructure, i.e. the high temperature during the annealing step promotes a densification of the oxide scale microstructure.

    The outer porous Fe-rich layer shows poor cohesive strength and poor adhesion to the underlying Cr-rich layer. In contrast, the inner more dense Cr-rich layer shows strong adhesion towards the 2507 duplex stainless steel. The dense microstructure and strong adhesion of the inner Cr-rich layer is believed to significantly influence the blasting and pickling characteristics.

  • 27.
    Olsson, Mikael
    Dalarna University, School of Information and Engineering, Materials Technology.
    Characterization of oxide layers formed on pilot plant Steckel rolled 304 stainless steel samples2021Report (Other academic)
    Abstract [en]

    The evolution of oxide scales on 304 austentic stainless steel during Steckel mill rolling has been investigated by performing pilot trials at Swerim Luleå. The influence of Steckel furnace conditions, i.e. temperature, time and excess oxygen, at reheating between subsequent roll passes have been evaluated and the resulting surface structures have been characterized.

    High resolution scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to characterize the oxide scales with respect microstructure and chemical composition. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyze the chemical composition at the stainless steel / oxide interface region.

    In common for all Steckell rolled samples investigated was a pronounced rough surface morphology and the surface roughness was found to increase with more iterations of rolling. Also, the amount of oxides in the rolled surface tends to increase with longer heating times and more iterations of rolling. 

    Surface areas, less distorted by the rolling contact, displayed an oxide scale with an outer rather well-defined oxide layer with relatively large grains and an inner, less well defined, oxide layer. EDS-analysis show that the outer oxide mainly consists of Fe2O3, whereas the inner oxide layer corresponds to a chromium rich (Cr,Fe)2O3 oxide combined with areas of partly oxidized stainless steel. ToF-SIMS analysis of the stainless steel / oxide interface region show the presence of B, Ca and Cu in connection to the oxides.

  • 28.
    Olsson, Mikael
    Dalarna University, School of Information and Engineering, Materials Technology.
    Influence of annealing on hot-rolled oxides on 2507 stainless steel2021Report (Other academic)
    Abstract [en]

    The influence of annealing on the oxide scale characteristics of hot rolled samples of 2507 super duplex stainless steel have been investigated using well controlled lab scale annealing experiments. High resolution scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to characterize the oxide scales with respect microstructure and chemical composition.

    SEM and EDS-analysis of the oxide scales after annealing show an outer Fe-rich oxide layer and an inner Cr-rich oxide layer close to the interface. While the hot rolled oxide scale displays a relatively porous oxide composed of oxide particles, the hot rolled plus annealed oxide scale displays a more dense, “sintered”, microstructure.

    The annealing peak temperature was found to have a strong impact on the oxide scale microstructure. The higher peak temperature, 1150 °C, resulted in a significantly denser microstructure with more pronounce, larger grains, as compared with the lower peak temperature, 1100 °C, which resulted in a less dense microstructure. In contrast, the influence of annealing atmosphere, was found to be relatively small.

  • 29.
    Pirouznia, Pouyan
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH Royal Institute of Technology; voestalpine Precision Strip AB, Munkfors.
    Andersson, N. ÅI.
    Tilliander, A.
    Jönsson, P. G.
    The impact of the gas inlet position, flow rate, and strip velocity on the temperature distribution of a stainless-steel strips during the hardening process2019In: Metals, E-ISSN 2075-4701, Vol. 9, no 9, article id 928Article in journal (Refereed)
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  • 30.
    Safara Nosar, Nima
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH Royal Institute of Technology.
    Engberg, Göran
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Ågren, John
    KTH Royal Institute of Technology.
    Modeling microstructure evolution in a martensitic stainless steel subjected to hot working using a physically based model2019In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50, no 3, p. 1480-1488Article in journal (Refereed)
    Abstract [en]

    The microstructure evolution of a martensitic Stainless steel subjected to hot compression is simulated with a physically based model. The model is based on coupled sets of evolution equations for dislocations, vacancies, recrystallization and grain growth. The advantage of this model is that with only a few experiments, the material dependent parameters of the model can be calibrated and used for a new alloy in any deformation condition. The experimental data of this work is obtained from a series of hot compression, and subsequent stress relaxation tests performed in a Gleeble thermo-mechanical simulator. These tests are carried out at various temperatures ranging from 900 to 1200⁰C, strains up to 0.7 and strain rates of 0.01, 1 and 10 s-1. The grain growth, flow stress, and stress relaxations are simulated by finding reasonable values for model parameters. The flow stress data obtained at the strain rate of 10 s-1 were used to calibrate the model parameters and the predictions of the model for the lower strain rates were quite satisfactory. An assumption in the model is that the structure of second phase particles does not change during the short time of deformation. The results show a satisfactory agreement between the experimental data and simulated flow stress, as well as less than 5% difference for grain growth simulations and predicting the dominant softening mechanisms during stress relaxation according to the strain rates and temperatures under deformation.

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  • 31.
    Safara Nosar, Nima
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH Royal Institute of Technology.
    Golpayegani, Ardeshir
    Sandvik Materials Technology.
    Engberg, Göran
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Ågren, John
    KTH Royal Institute of Technology.
    Study of the mean size and fraction of the second-phase particles in a 13% chromium steel at high temperature2019In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, p. 1-17Article in journal (Refereed)
    Abstract [en]

    The mean size and fraction of the second-phase particles in a 13% chromium steel are investigated, while no plastic deformation was applied. The results of the measurement are compared with the modelling results from a physicallybased model. The heating sequence is performed on samples using a Gleeble thermo-mechanical simulator over the temperature range of 850?1200°C. Using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), the size distribution and composition of the carbides were evaluated, respectively. For obtaining particle size distribution (PSD), an image-processing software was employed to analyse the SEM images. Additionally, the relation between the 2D shape factor and size of the particles is also studied at different temperatures and most of the particles turned out to have a shape factor close to two. In order to measure the carbide weight fraction, electrochemical phase isolation was employed. The Ms and fraction of the martensite phase after quenching of samples are calculated and the results were comparable with the measured hardness values at corresponding temperatures. The measured hardness of the samples is found to comply very well with the measured mean size of the precipitates. The calculated mean size of the particles from the model shows very good agreement with both hardness value and experimentally measured mean size, while the calculated volume fraction from simulation follows a slightly different trend.

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  • 32.
    Safara Nosar, Nima
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Sandberg, Fredrik
    Sandvik Materials Technology.
    Engberg, Göran
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Characterization of hot deformation behavior in a 13% chromium steel2018In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 941, p. 458-467Article in journal (Refereed)
    Abstract [en]

    The behavior of a 13% chromium steel subjected to hot deformation has been studied by performing hot compression tests in the temperature range of 850 to 1200 ⁰C and strain rates from 0.01 to 10 s-1. The uniaxial isothermal compression tests were performed on a Gleeble thermo-mechanical simulator. The best function that fits the peak stress for the material and its relation to the Zener-Hollomon parameter (Z) is illustrated. The average activation energy of this alloy for the entire test domain was reviled to be about 557 [kJ mol-1] from the calculations and the dynamic recrystallization (DRX) kinetic were studied to find the fraction DRX in the course of deformation.

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  • 33.
    Saketi, Sara
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. Uppsala University.
    Bexell, Ulf
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Olsson, Mikael
    Dalarna University, School of Technology and Business Studies, Materials Technology. Uppsala University.
    Wear mechanism of cemented carbide cutting tool in the turning of 316L stainless steel2018Conference paper (Refereed)
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  • 34.
    Sanni, Onimisi
    Dalarna University, School of Information and Engineering.
    Microstructural and Micro-Mechanical Characterization of As-built and Heat-treated samples of HASTELLOY X produced by Laser Powder Bed Fusion Process2022Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Microstructure and micro-mechanical characterization of as-built and heat-treated samples of Hastelloy X produced by laser powder bed fusion (LPBF) process has been carried out in this study. As-built LPBF blocks were solution heat-treated at 1177°C and 1220°C followed by fast cooling. The microstructure of as-built and heat-treated samples were studied by light optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Instrumented indentation micro Vickers testing was performed to obtain microhardness and elastic modulus of asbuilt and heat-treated samples. Microtensile samples from as-built and heat-treated blocks were prepared and polished for mechanical characterization. Microtensile testing inside the scanning electron microscope was performed to evaluate the mechanical properties and to get information about the microstructural changes during plastic deformation. Microstructure characterization revealed disrupted epitaxial grain growth for the as-built samples whereas the two heated-treated Hastelloy X samples exhibited equiaxed grains with varying twin fractions. As-built Hastelloy X samples exhibited higher mean hardness than heat-treated samples. The yield strength of as-built samples reveals higher values as compared to conventional wrought Hastelloy X samples, whereas lower yield strength and higher elongation were observed for heat-treated samples as compared to as-built samples. Higher elongation and lower yield strength values were observed for the samples solution heat-treated at 1220°C compared to the solution heat-treated at 1177°C. Microstructural evaluation at different plastic strains during in-situ microtensile testing reveals a clear difference in dislocation density for as-built and heat-treated samples.

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  • 35.
    Schütt, Viktor
    Dalarna University, School of Information and Engineering.
    Thermal Simulation Analysis of localized heating of martensitic steel in a continuous line2023Independent thesis Basic level (professional degree), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Advanced high strength low alloy steels are mainly used in the automotive industry where low weight and high strength are two of the main criteria, shortly followed by formability. These materials are under constant development to be stronger and perform better. These steels consist of a martensite structure and have high yield and tensile strength with limited ductility and formability. 

    To increase the formability of these martensitic steel types, it is necessary to apply tempering treatment, however, if done incorrectly, it might also deteriorate the properties of the material. 

    Earlier investigations have suggested that with the pre-known bending positions in roll forming operations of coiled steel, efforts can be made to preheat the pre-known areas, enabling better formability. 

    This report's main goal has been to investigate whether localized heating/tempering in pre-known specific locations is possible or not. The main criteria for the localized tempering task have been to apply the desired temperature which can be achieved at the rate at which the movement of the continuous annealing line at SSAB Borlänge. 

    This investigation will be made using FEA simulation software, to accurately depict localized heating scenarios considering different variables. The results of this investigation show that it is possible to apply the desired temperature with some local laser heating devices into the continuous annealing line, accommodating the rate of heating as well.

  • 36.
    Svensson, Jennie
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH, Tillämpad processmetallurgi.
    An Experimental Study to Improve the Casting Performance of Steel Grades Sensitive for Clogging2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this study, the goal is to optimize the process and to reduce the clogging tendency during the continuous casting process. The focus is on clogging when the refractory base material (RBM) in the SEN is in contact with the liquid steel. It is difficult or impossible to avoid non-metallic inclusions in the liquid steel, but by a selection of a good RBM in the SEN clogging can be reduced.

     

    Different process steps were evaluated during the casting process in order to reduce the clogging tendency. First, the preheating of the SEN was studied. The results showed that the SEN can be decarburized during the preheating process. In addition, decarburization of SEN causes a larger risk for clogging. Two types of plasma coatings were implemented to protect the RBM, to prevent reactions with the RBM, and to reduce the clogging tendency. Calcium titanate (CaTiO3) mixed with yttria stabilized zirconia (YSZ) plasma coatings were tested in laboratory and pilot plant trials, for casting of aluminium-killed low-carbon steels. For casting of cerium alloyed stainless steels, YSZ plasma coatings were tested in laboratory, pilot plant and industrial trials. The results showed that the clogging tendency was reduced when implementing both coating materials.

     

    It is also of importance to produce clean steel in order to reduce clogging. Therefore, the steel cleanliness in the tundish was studied experimentally. The result showed that inclusions originated from the slag, deoxidation products and tundish refractory and that they were present in the tundish as well as in the final steel product.

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  • 37.
    Svensson, Jennie
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH, Tillämpad processmetallurgi.
    The submerged entry nozzle: a study of how to reduce decarburization and clogging2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    During continuous casting the submerged entry nozzle (SEN) is one of the critical points. This since sequence casting is favourable, clogging of the SEN results in fever sequences or aborted castings. In this study the preheating of SEN has been evaluated at different steel plants. This since it earlier  has been found, preheating resulted in decarburisation of  the refractory base material (RBM)  which in turn resulted in clogging of the SEN. During the industrial preheating trials it was found that decarburisation of  the SEN is possible.The decarburisation results in a higher oxygen potential near the SEN wall. This in turn makes it possible for aluminum in the molten steel to form alumina particles. For low carbon aluminum killed steel grades, accretion of alumina clusters on the SEN internal wall result in clogging which hinder or reduces the steel flow through the SEN. For this reason, laboratory trials were performed to establish the formation of a liquid phase. Thus simulating the formation of liquid inclusions when alumina clusters attaches to the SEN wall, and which can be washed away by the steel flow. Formation of a liquid phase was indicated at the temperatures 1550-1600°C, and plasma coated nozzles were tried in pilot plant trials. The coating contained of 5-9 pct, 99.9 pct pure, calcium titanate (CaTiO3) mixed with yttria stabilized zirconia (YSZ).

  • 38.
    Svensson, Jennie
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Larsson, F.
    Memarpour, A.
    Ekerot, S.
    Brabie, Voicu
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Jönsson, P.G.
    Implementation of an YSZ coating material to prevent clogging of the submerged entry nozzle (SEN) during continuous casting of Ce-treated steels2018In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 45, no 2, p. 105-113Article in journal (Other academic)
    Abstract [en]

    The possibility to reduce clogging in the submerged entry nozzle (SEN) during continuous casting of Ce alloyed stainless steels has been studied. This was done by implementing a new plasma coating material, consisting of yttria stabilized zirconia (YSZ). The coating was first tested in pilot-plant trials; where the amount of steel teemed through the plasma coated nozzles was monitored. Thereafter, samples of the coatings from the nozzles were studied using a FEG-SEM equipped with EDS. In addition, the coating material was tested in industrial trials where the performance was judged with respect to the clogging tendency detected as the movement of the stopper rod. The results from both the pilot-plant trials and industrial trials showed that the use of an YSZ coating led to a decreased clogging tendency during the casting in comparison to when using an uncoated SEN. Specifically, the pilot-plant trials showed that the clogging factor was lowered when implementing the YSZ coatings. Furthermore, the industrial trials showed that the clogging tendency, measured by the stopper rod movements, were lower when implementing the YSZ coating.

  • 39.
    Svensson, Jennie
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Larsson, Fredrik
    Outokumpu, Avesta SE-774 22, Sweden.
    Memarpour, Arashk
    Dalarna University, School of Technology and Business Studies, Materials Technology. Sandvik Materials Technology, Sandviken SE-811 81, Sweden.
    Brabie, Voicu
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Jönsson, Pär
    Royal Institute of Technology, Brinellvägen 23, Stockholm SE-100 44, Sweden.
    Post-mortem Studies of Submerged Entry Nozzles (SEN) Coated with Yttria Stabilized Zirconia (YSZ)2015In: The 6th International Congress on the Science and Technology of Steelmaking (ICS2015), China, May 12-14, 2015 in Beijing / [ed] Organized by The Chinese Society for Metals (CSM), 2015Conference paper (Refereed)
    Abstract [en]

    During continuous casting of Ce-treated stainless steel grades an accumulation of inclusions at the submerged entry nozzle’s (SEN) internal wall can cause disturbances or even prevent the molten steel flow. Plasma sprayed yttria stabilized zirconia (YSZ) coating of the SEN’s inlet and stopper rod have been implemented in order to decrease the clogging effect. The coating thickness was 210 µm and the plant trials were conducted during continuous casting of slabs. The clogging tendency was measured by the stopper rod position during the casting process. The results of the measured positions were then compared to experimentally determined information from samples cut out from the SEN’s inlet after casting. Specifically, the samples were analysed using a field emission gun scanning electron microscope (FEG-SEM) equipped with an energy dispersive X-ray spectrometry (EDS). Sample with the dimensions of ̴15x10x5 mm were electrolytic extracted before analysis. The chemical composition at the interface between the solidified steel and refractory base materials were analysed, and traces of the YSZ coating were determined.

    The movement of the stopper position indicated that the clogging tendency was reduced for the coated SEN. However, the FEG-SEM analyses showed that clogging of the SEN had occurred during the casting.

  • 40.
    Svensson, Jennie
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH.
    Memarpour, Arashk
    Brabie, Voicu
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH.
    Jönsson, Pär
    KTH.
    Studies of the decarburisation phenomena during preheating of submerged entry nozzles (SEN) in continuous casting processes2017In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 44, no 2, p. 108-116Article in journal (Refereed)
    Abstract [en]

    Decarburisation of the submerged entry nozzles (SEN) during the preheating process wasinvestigated based on plant trials and thermodynamic modelling at three different steel plants.During the trials the preheating processes were mapped, the temperature profiles wereregistered and post-mortem studies of the SENs with scanning electron microscopy wereperformed. Typically, the glass/silicon powder will form a dense and protective layer inside theSEN when heated over 1100°C. However, this study found that the temperature distributioninside the SEN did not always reach this critical temperature. Thus, decarburisation of the SENwas found at all steel plants. The overall results illustrate that the control of the preheatingprocess needs to be improved at all steel plants. It is suggested that future research should befocused on the development of new coating materials to prevent decarburisation of therefractory base material, which would decrease the chances of clogging during casting.

  • 41.
    Svensson, Jennie
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology. KTH, Tillämpad processmetallurgi.
    Memarpour, Arashk
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Brabie, Voicu
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Jönsson, Pär
    KTH, Tillämpad processmetallurgi.
    Studies of the decarburization phenomena during heating of submerged entry nozzles (SEN) in continuous casting processesManuscript (preprint) (Other academic)
  • 42.
    Svensson, Jennie
    et al.
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Memarpour, Arashk
    Ekerot, Sven
    Brabie, Voicu
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Jönsson, Pär
    Tillämpad processmetallurgi, KTH.
    Studies of new coating materials to prevent clogging of the submerged entry nozzle (SEN) during continuous casting of Al killed low carbon steels2017In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 44, no 2, p. 117-127Article in journal (Refereed)
    Abstract [en]

    The possibility to reduce clogging in the submerged entry nozzle (SEN) when casting Al killed low carbon steel has been evaluated. The coating materials have been tested in laboratory trials and pilot plant trials. Totally, 2 mixes of the coating material has been tested; containing 4.8 and 9.1 % CaTiO3 mixed with yttria stabilised zirconia (YSZ) powder. The chemical composition was analysed by using an FEG-SEM equipped with EDS.The major result showed that alumina reacted with CaTiO3 in the temperature interval from 1550-1600°C. In addition, the clogging ratio showed a decreased clogging tendency for coated nozzles compared to standard nozzles. Also, the results showed that an approximately three times higher steel mass could be teemed through the coated nozzles before clogging occurred compared to trials with uncoated standard nozzles. Overall, it is concluded that plasma sprayed CaTiO3 coatings can reduce the clogging tendency during the continuous casting process.

  • 43.
    Vattur Sundaram, Maheswaran
    et al.
    Chalmers University of Technology.
    Surreddi, Kumar Babu
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Hryha, Eduard
    Chalmers University of Technology.
    Veiga, Angela
    CEIT, Pº Manuel Lardizábal 15, 20018 San Sebastián, Spain.
    Berg, Siguard
    Höganäs AB, SE-263 83 Höganäs, Sweden.
    Castro, Francisco
    CEIT, Pº Manuel Lardizábal 15, 20018 San Sebastián, Spain.
    Nyborg, Lars
    Chalmers University of Technology.
    Full Densification in PM Steels Through Liquid Phase Sintering and HIP Approach2020In: Euro PM 2018 Congress and Exhibition, 2020Conference paper (Refereed)
    Abstract [en]

    High-density powder metallurgy (PM) components are required for high-performance applications. Liquid phase sintering (LPS) is one such method to improve the densification, especially the master alloy route is preferred due to the flexibility in tailoring the alloying contents. In this study, gas atomised Ni-Mn-B master alloy powder of size fraction < 45 µm was admixed with water atomised iron and Mo-prealloyed powder. During sintering, there was a significant densification due to LPS where the liquid formation occurred in two stages, one from the master alloy melting and another from the eutectic liquid formation, enabling densities > 95%. The microstructural investigation revealed that the surface densification was achieved after sintering in H2 containing atmosphere. Capsule free hot isostatic pressing was performed on these samples to achieve full density. This approach of combining LPS and capsule free hot isostatic pressing demonstrates the potential in reaching full densification in high-performance PM steel components.

    Download full text (pdf)
    fulltext
  • 44. Vida, Ádám
    et al.
    Lábár, János
    Dankházi, Zoltán
    Maksa, Zsolt
    Molnar, David
    Dalarna University, School of Information and Engineering, Materials Technology. Royal Institute of Technology, Stockholm.
    Varga, Lajos K.
    Kalácska, Szilvia
    Windisch, Márk
    Huhn, Gabriella
    Chinh, Nguyen Q.
    A Sequence of Phase Transformations and Phases in NiCoFeCrGa High Entropy Alloy2021In: Materials, E-ISSN 1996-1944, Vol. 14, no 5, article id 1076Article in journal (Refereed)
    Abstract [en]

    The present investigation is directed to phase transitions in the equimolar NiCoFeCrGa high entropy alloy, which is a mixture of face-centered cubic (FCC) and body-centered cubic (BCC) crystalline phases. The microstructure of the samples was investigated by using scanning electron microscopy (SEM), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), transmission electron microscopy-based energy-dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), as well as X-ray diffraction (XRD) measurements. Based on the phases observed in different temperature ranges, a sequence of the phase transitions can be established, showing that in a realistic process, when freely cooling the sample with the furnace from high to room temperature, a microstructure having spinodal-like decomposition can also be expected. The elemental mapping and magnetic behaviors of this decomposed structure are also studied.

    Download full text (pdf)
    fulltext
  • 45. Wang, Z.
    et al.
    Prashanth, K. G.
    Surreddi, Kumar Babu
    Dalarna University, School of Technology and Business Studies, Materials Technology.
    Suryanarayana, C.
    Eckert, J.
    Scudino, S.
    Pressure-assisted sintering of Al–Gd–Ni–Co amorphous alloy powders2018In: Materialia, E-ISSN 2589-1529, Vol. 2, p. 157-166Article in journal (Refereed)
  • 46. Yaxuan, X.
    et al.
    Huixiang, W.
    Yuting, W.
    Jinhua, Z.
    Haimeng, L.
    Qian, X.
    Zhang, Xingxing
    Dalarna University, School of Information and Engineering, Energy Technology.
    Chuan, L.
    Yulong, D.
    Carbide slag based shape-stable phase change materials for waste recycling and thermal energy storage2022In: Journal of Energy Storage, ISSN 2352-152X, E-ISSN 2352-1538, Vol. 50, article id 104256Article in journal (Refereed)
    Abstract [en]

    Massive accumulation of industrial carbide slag tend to cause ecological environment pollution and greenhouse gas emission. This innovative work proposed to fabricate shape-stable phase change materials (SSPCMs) with carbide slag to utilise industrial solid waste and protect precious natural resources. Seven SSPCMs were fabricated with different mass ratios of industrial carbide slag to sodium nitrate by cold-compression hot-sintering method, and key performance was investigated on the SSPCMs. Results showed that the SSPCM (sample CC6) with the mass ratio of 5:5 of carbide slag to sodium nitrate presented the best performance: sample CC6 achieved a high thermal energy storage density of 447 J/g in the range of 100–400 °C and reached a mechanical strength of 73.6 MPa; sample CC6 demonstrated a good thermal stability and chemical compatibility between carbide slag and sodium nitrate during the heating/cooling cycles; the thermal conductivity of sample CC6 was 0.93 W/(m•K), and elements distributed uniformly in sample CC6. © 2022

1 - 46 of 46
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