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  • Grolig, Jan Gustav, 1986- (författare)
  • Coated Ferritic Stainless Steels as Interconnects in Solid Oxide Fuel Cells - Material Development and Electrical Properties
  • 2015
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Solid oxide fuel cells (SOFCs) are attracting increasing interest as devices with potentialuses in decentralized and clean electricity and heat production. Several challengeswith respect to materials have to be overcome to achieve efficiencies and life-spansthat are sufficient for long-term applications.An important element of an SOFC stack is the interconnect component, which connectstwo adjacent fuel cell elements. Interconnects, which are commonly composedof ferritic stainless steels, have to be corrosion-resistant, mechanically stable and costoptimized.This work aimed to investigate economic solutions for interconnect materials and tounderstand the underlying mechanisms of degradation and electrical conduction ofthese materials. Mainly two substrates, a commercially available steel (AISI 441) anda ferritic stainless steel that was optimized for an SOFC application (Sandvik SanergyHT) were combined with different barrier coatings and exposed to a cathode-sideatmosphere. A method was developed that allows for the electrical characterizationof promising material systems and model alloys, thereby facilitating a fundamentalunderstanding of the dominant electrical conduction processes linked to the oxidescales that grow on interconnects. The AISI 441 steel coated with reactive elementsand cobalt showed good corrosion and chromium evaporation profiles, while AISI 441coated with cerium and cobalt also had promising electrical properties. The SanergyHT steel was examined with coatings of copper and iron and copper and manganese,respectively. The corrosion and chromium evaporation profiles of Sanergy HT wereimproved by coating with copper and iron. The copper and iron-coated Sanergy HTshowed lower area specific resistance values than cobalt-coated Sanergy HT. Chromia,which is the main constituent of oxide scales, was synthesized using differentmethods. The electrical properties of chromia were found to be sensitive to not onlyimpurities, but also heat treatment. Finally the electrical properties of cobalt- andcobalt cerium-coated Sanergy HT steels were investigated. It was revealed that theaddition of cerium improved the conductivity of the interconnect by both slowingdown chromia growth and preventing the outward diffusion of iron into the spinel.
  • Goel, Sneha, et al. (författare)
  • The Effect of Location and Post-treatment on the Microstructure of EBM-Built Alloy 718
  • 2018
  • Ingår i: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications. - 2367-1181 .- 2367-1696. - 978-3-319-89479-9 ; s. 115-129
  • Konferensbidrag (refereegranskat)abstract
    • Alloy 718, usually referred to as Inconel 718, is a Ni-Fe based superalloy widely used in the aerospace industry by virtue of the excellent balance of mechanical properties and corrosion resistance that it affords at high temperature [1]. While Alloy 718 parts have been traditionally fabricated using conventional cast and wrought (C&W) techniques, there has been a growing interest in production of the complex high temperature service components through additive manufacturing (AM) processes due to the significant design freedom offered by the technology. Alloy 718 has been processed by laser (laser metal deposition, LMD; selective laser melting, SLM) and electron (Electron Beam Melting, EBM) beam based technologies [2]–[4]. AM, in general, involves high solidification rates and high thermal gradients, thus resulting in a finer microstructure compared to C&W products [5]. In EBM, a relatively high temperature is maintained throughout the process compared to SLM, thus resulting in lower residual stresses in as-manufactured parts [6]. However, as a relatively newer technology, the degree to which the EBM processing conditions (scanning strategies, preheat and melting parameters) affect the properties of EBM-built Alloy 718 material is less understood compared to the laser based techniques. Also, it has been challenging to date to build Alloy 718 parts that are free from issues such as defects, inhomogeneity, anisotropy, etc., thereby demanding post-treatment to achieve the required microstructure and intended part performance. Although post treatment of C&W Alloy 718 has been widely investigated over several decades [7]–[9], the inadequacy of conventional post-treatment protocols for EBM Alloy 718 [10]–[12], justifies further investigation of microstructural changes that occur in EBM-built Alloy 718 during post-treatment. A comprehensive study may eventually allow optimization of the combined build and post-treatment processes to obtain components with properties deployable in severe environments.
  • Jaladurgam, Nitesh Raj, 1993- (författare)
  • Heterogeneous deformation of multi-phase engineering materials - an in-situ neutron diffraction study
  • 2019
  • Licentiatavhandling (övrigt vetenskapligt)abstract
    • Gas turbines are complex power generation systems used in aerospace or land-based-power stations. Materials such as Ni-base superalloys are involved in the combustion zone of these machines, which continuously experiences harsh environments with loading at high temperatures. Moreover, the continuous demand for increasing operating temperature to achieve higher efficiencies and reduced emission levels opens the scene to new heat resistant materials like the state-of-the-art high entropy alloys (HEAs), which require a thorough understanding of the structure-process-property relationships. The microstructures of these advanced multi-phase, multi-component alloys are complex, and the deformation is generally heterogeneous both with respect to the different phases and to the crystallographic orientation within each phase. Hence, it is important to understand their behavior and performance during processing and service. In-situ neutron diffraction is a unique technique to probe the deformation behaviour during service/processing-like conditions, including plastic deformation at various temperatures, in order to provide insights into the structure-property relations. In the first part of this work the deformation mechanisms of a newly developed Ni-base superalloy was investigated using in-situ neutron diffraction and electron microscopy at room temperature. In addition, elasto-plastic self-consistent (EPSC) crystal plasticity simulations are used to obtain insights into the operating deformation mechanisms. In the second part, the as-cast eutectic high entropy alloy AlCoCrFeNi2.1 was studied using in-situ neutron diffraction at temperatures from 77 to 673 K. These investigations provide unique insights into the complex heterogeneous deformation behavior of these high-performance multi-phase engineering materials.
  • Ringsberg, Jonas, 1971-, et al. (författare)
  • Sheet metal fatigue near nuts welded to thin sheet structures
  • 2008
  • Ingår i: International Journal of Fatigue. - 0142-1123. ; 30:5, s. 877-887
  • Tidskriftsartikel (refereegranskat)abstract
    • One of the fatigue design processes of a car body is stress-based sheet metal fatigue near nuts welded to thin sheets (weld nuts). In this investigation, the influence from nut geometry and dimension, as well as sheet material and thickness on sheet metal fatigue life was studied by fatigue experiments and numerical analyses. The fatigue experiments were force-controlled and carried out on a weld nut specimen which, during the reversed loading that was applied, was designed to result in bending fatigue loading conditions of the sheet metal near the weld nut. The results from the experiments were Fa–N curves for various combinations of nut geometry and dimension, sheetmaterial and thickness. Numerical analyses of the experiments were carried out by linear elastic finite element (FE) analyses, in order to transform the Fa–N curves to Wöhler (sig_a–N) curves. A simplified FE model of the weld nut, which is suitable for structural analysis of car body structures, was used. The structural stresses calculated using the FE model showed good agreement with the structural stresses calculated using an analytical model that is based on plate theory. The conclusion was that the simplified representation of the weld nut, and the mesh density used in the FE model, was appropriate for computation of the stress response near a weld nut subjected to bending fatigue loading conditions. Finally, ra–N curves were computed for each weld nut configuration using its experimental data set. A sig_a–N curve fitted to all data sets resulted in the relation sig_a = 9888N^0.312. The sig_a–N curves for the 21 weld nut configurations tested were gathered within a factor of three in fatigue life from this curve.
  • Ryttberg, Kristina, 1977-, et al. (författare)
  • Electron microscopy of white-etching band generated by high-velocity parting-off of 100CrMn6 steel
  • 2008
  • Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - 0921-5093. ; 480:1-2, s. 489-495
  • Tidskriftsartikel (refereegranskat)abstract
    • High-velocity parting-off has been applied to 80 mm bars of pearlitic 100CrMn6, resulting in shear localisation and white-etching bands in a severely deformed region below the fracture surface. Electron microscopy showed that going from the bulk material towards the fracture surface the grains become elongated and refined. The region below the fracture surface can be divided into three subzones: 50–100 μm below the surface grains are elongated, cementite lamellae are distorted, break up and the lamellar spacing decreases.
  • Ryttberg, Kristina, 1977-, et al. (författare)
  • High strain rate deformation induced by high-velocity forming of 100Cr6 steel
  • 2008
  • Ingår i: proceedings of The 9th International Conference on Technology of Plasticity (ICTP), Gyeongju, Korea, September 7-11, 2008. - 978-89-5708-152-5 ; s. On CD or www.ictp2008.org
  • Konferensbidrag (refereegranskat)abstract
    • This paper concerns the development of the near-net-shape manufacturing technique of high-velocity forming. The microstructural response to the high strain-rate deformation was evaluated for 100Cr6 steel in two differently heat treated conditions: spheroidise annealed (hardness 250 HV) and quenched and tempered (330 HV). A high-velocity pressing machine was used to conduct the forming tests on cylindrical specimens. By applying different impact velocities on samples with varying sizes deformation mechanisms could be related to strain and strain rate. Extreme localization of deformation resulting in white etching bands was seen in quenched and tempered samples but not in spheroidise annealed samples. Furthermore, the strain and strain rates were shown to affect the tendency for extreme localization. No white etching bands were found in samples strained to less than 1. Appearance of white etching bands could be seen in the macroscopic shape of the samples where the more homogeneously deformed samples showed a typical barrel shape while the strongly localized samples were slightly more bell shaped. It can be concluded that for high-velocity forming to be applicable to 100Cr6 steel a careful choice of the combination strain/strain rate needs to be made.
  • Ryttberg, Kristina, 1977-, et al. (författare)
  • Microstructural evolution during fracture induced by high strain rate deformation of 100Cr6 steel
  • 2009
  • Ingår i: Journal of Materials Processing Technology. - 0924-0136. ; 209:7, s. 3325-3334
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper investigates the influence on parting-off mechanisms of a significantly higher impact velocity compared to commercial methods. A specially developed method, allowing for parting-off velocities ranging from 38 to 285 m/s has been applied to 100Cr6 steel bars. Two different heat treatments; spheroidise annealing (SA) and quench and tempering (QT) were employed to produce two different microstructures of a hardness of 255 and 310HV, respectively. It appears that the failure mechanisms active in the current process are virtually identical to failure occurring during high-velocity parting-off using commercially available machines, which displays shear fracture and adiabatic shear banding. It was also concluded that the estimated energy consumed during parting-off is not dependent on microstructure. Prior to parting-off, impact results in shear localisation, which is somewhat increased with increased impact speed. Furthermore, shear strains at fracture also increaseswith impact speed, from a value of about 2.2 at the lowest impact velocity to almost 3.5 at an impact velocity of 285 m/s. The heavy deformation causes a grain refinement. Right below the fracture surface three subzones can be found in the microstructure; a white etching band (WEB) (only present on some locations), equiaxed grains and then elongated subgrains. The grain size within these zones varies between 50 and 150 nm. The findings of elongated subgrains of a mutual orientation, adjacent subgrains having {110} type of planes in parallel, support the theory of formation of white etching bands being a mechanically rate controlled process.
  • Ryttberg, Kristina, 1977-, et al. (författare)
  • Microstructure and texture development during ring rolling of 100Cr6
  • 2008
  • Ingår i: Ceramic Transactions. - 1042-1122. - 978-1-57498-296-1 ; 200, s. 301-309
  • Konferensbidrag (refereegranskat)abstract
    • Pre-turned 100Cr6 steel rings with spheroidised annealed microstructure have been cold ring rolled to two levels: expanding the outer diameter by 10 and 20%. The final microstructure was shown to be varying throughout the cross-section of the ring, where the largest deformation was found close to the surface of the inner diameter. This was reflected in the hardness profile showing the highest hardness values close to the inner surface. Scanning electron microscopy showed that the microstructure below the surface of the outer diameter consisted of a greatly refined ferrite. Close to the surface of the inner diameter the structure was even more refined. Electron back scatter diffraction together with X-ray diffractometry showed that cold ring rolling changed the existing texture of the ring blanks, where {110}-planes were parallel to the axial direction of the rings, towards {110}-planes becoming parallel to the rolling direction. This resulted in a weak {110}-texture. Close to the surface of the outer diameter the greatly refined ferrite had decomposed into a more random orientation distribution.
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