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  • 1. Andersson, Henrik C. M.
    et al.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Creep crack growth in service-exposed weld metal of 2.25Cr1Mo2001In: International Journal of Pressure Vessels and Piping, ISSN 0308-0161, E-ISSN 1879-3541, Vol. 78, no 11-12, p. 749-755Article in journal (Refereed)
    Abstract [en]

    Creep crack growth (CCG) has been studied for ex-service weld metal of 2.25Cr1Mo (P22). The testing was conducted at a temperature of 550degreesC and prior to testing, the material had been exposed to high temperature service for 110 000 h at 530 C. The results show a marked effect of the service exposure on the CCG properties of the material when compared to similar testing performed on a new material. The CCG rate was higher by a factor 3.1 in the service-exposed material, which should be compared to the model value of 3.2 based on the relations between the elongation values. The consumed deformation capacity was also estimated with the omega model for tertiary creep. In this case, an enhanced growth rate of 2.4 was obtained. Within a distance of about 10 mm in front of the propagating cracks, the number of creep cavities was significantly higher than in the surrounding material. The variation of the density of cavities as a function of distance from the crack tip was successfully modelled. (C) 2002 Published by Elsevier Science Ltd.

  • 2. Andersson, Henrik C. M.
    et al.
    Seitisleam, Facredin
    Sandstrom, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Creep testing of thick-wall copper electron beam and friction stir welds2004In: Materials Research Society Symposium Proceedings, ISSN 0272-9172, E-ISSN 1946-4274, Vol. 824, p. 51-56Article in journal (Refereed)
    Abstract [en]

    Thick section copper canisters are planned to be used as a corrosion protection of nuclear waste disposal containers for long term underground deposal in Sweden. The copper canisters will have the top and possibly the bottom lid welded to the canister walls using electron beam or friction stir welding. Due to the high external hydrostatic pressure and the relatively high temperature of the waste during the first one hundred years the copper will creep. The creep process will close the manufacturing gap between the cast iron container and the copper canister. The creep ductility must be sufficient to avoid cracking of the weld. Specimens cut from the friction stir welds and the electron beam welds have been creep tested at temperatures ranging from 75 to 175 degreesC. Cross-weld specimens were used for both friction stir and electron beam welds. Weld metal, heat affected zone and base metal were also studied for friction stir welds. The results for the electron beam welds show that the main creep deformation is concentrated to the weld metal where the failure takes place. Weld metal and most cross-weld tests of friction stir weld material show similar creep lives and ductility as base metal tests. Ductility at rupture was found to exceed 30% for friction stir weld specimens, and the Norton power law exponent was determined to be between 30 and 50.

  • 3.
    Andersson, Henrik C.M.
    et al.
    Swedish Institute for Metals Research.
    Sandström, Rolf
    Swedish Institute for Metals Research.
    Segle, P.
    SAQ Kontroll.
    Andersson, Peter
    SAQ Kontroll.
    Creep crackgrowth in ex service weld metal of 0.5CrMoV1999In: Cape 99: Wilderness, Cape province, South Africa, 12-16 April (1999), 1999Conference paper (Refereed)
    Abstract [en]

    Accurate assessment of the integrity of high temperature components will be of ever increasing importance. The reason for this is that many power plants have reached and exceeded their design life and the number of detected defects increases. This is accentuated by the improvement of the methods for non-destructive testing which means that more and smaller defects will be detected. The possibility to assess the influence of defects on the integrity of high temperature components, will be of vital importance to maintain safe and cost effective power plants.

    The aim of the present work is to increase the understanding of the influence of service exposure on the remaining life of components in a high temperature plant. The investigation aims to creep test exserviceweld material, 14MoV 6 3, from a Swedish power plant. Thematerial has been in service for a period of about 80 000 hours at atemperature of 530-540 °C and with a nominal hoop stress of 52MPa.Both uniaxial and compact tension creep tests have been performedat a temperature of 550 °C. The stress range used was between 130MPa and 170 MPa for the uniaxial creep tests. For the creep crack growth tests the reference stress was ranging between 122 MPa and146 MPa.

    A remaining life assessment according to the R5 procedure is included, where material data from the present experimental study is used. The analysis suggests that a defect or a crack with a depth of 2 mm and a length of 5 mm can be left unattended for a season of service under the condition that the service parameters are not changed. A comparison with the assessment of cracks, found in the same plant as the material for the experimental studies came from, and their known extension during service, is included. A parametric study where load level and type of initial defect/crack are varied is also included.

  • 4. Andersson-Östling, Henrik C.M.
    et al.
    Seitisleam, Facredin
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Influence of phosphorus, sulphur and grain size on creep in pure copper2009In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847Article in journal (Other academic)
    Abstract [en]

    Uniaxial creep tests have been performed at 175 °C to study the influence of phosphorus, sulphur and grain size on the creep properties of oxygen free copper. Copper with no phosphorous content and copper with 2000 μm grain size showed lower creep strength and ductility than the reference material which contained 58 ppm phosphorous and had 350 μm average grain size. Phosphorous content of 29 and 106 ppm showed no difference in relation to the reference material, and neither did grain sizes of 100 and 800 μm average grain size. 6 or 12 ppm sulphur did not affect the creep properties at all. The main creep rupture mechanisms were found to be cavitation and microcracking at the grain boundaries. The observed influence of P on creep is consistent to previously published models both with respect to creep rate and creep ductility.

  • 5. Andersson-Östling, Henrik C.M.
    et al.
    Seitisleam, Facredin
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Testing and modelling of creep in copper friction stir welds2009In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847Article in journal (Other academic)
    Abstract [en]

    Specimens cut from friction stir welds in copper canisters for nuclear waste have been used for creep experiments at 75°C. The specimens were taken from a cross-weld position as well as heat affected zone and weld metal. The weld specimens exhibited shorter creep lives than the parent metal specimens by a factor of three in time. The cross weld and HAZ specimens were shorter by an order of magnitude when compared to the weld metal. The creep exponent was in the interval 50 to 69 implying that the material was well inside the power-law breakdown regime. The ductility properties expressed as reduction in area were not significantly different in the weld zones and all the rupture specimens demonstrated valu esexceeding 80%. The stationary creep rate for the parent metal was consistent with a previously developed model. The primary creep was successfully modelled. Weld reduction factors have been obtained by comparing the results from base metal tests and weld tests. Measured values at 75 °C for are about 6% for friction stir welds and 14% for electron beam welds.

  • 6. Bohnenkamp, U.
    et al.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Evaluation of the density of steels2000In: Steel research, ISSN 0177-4832, Vol. 71, no 3, p. 88-93Article in journal (Refereed)
    Abstract [en]

    Data on physical properties of steels have been collected from the open literature and put into a database. The influence of composition on the density of steels has been analyzed. An overview over former studies is given. The steels have been investigated by regression analysis in two groups, i) ferritic and low alloy steels, and ii) austenitic steels. For ferritic steels two models are provided. The first model is based on the assumption that all C is bound in cementite and other solutes are insoluble in cementite, The second model employs the result of a thermodynamic analysis where the amount of cementite and the solubilities in ferrite and cementite were determined with computational thermodynamics. The non-linear effect of Cr and Mn in cementite was computed and regression analysis of the effect of solutes on the density of ferrite was performed. For Ti-stabilized austenitic steels, the amount of TiC and the solubilities were assessed in a thermodynamic analysis. The effect of solutes on the density of austenite was studied by regression analysis. For estimations of the density of steels containing components that are not covered by the regression analysis, the regression coefficients can be supplemented with literature data or theoretically determined values. The results obtained by the present regression analysis are: Cu and Mo increase the density of ferritic steels, and C, Cr, Mn, S, Si, and V decrease it. TiC. C, Cr, Mn, N, Si, and Ti reduce the density of austenitic steels and Cu, Co. Mo, and Ni increase it.

  • 7. Bohnenkamp, U.
    et al.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Evaluation of the elastic modulus of steels2000In: Steel research, ISSN 0177-4832, Vol. 71, no 3, p. 94-99Article in journal (Refereed)
    Abstract [en]

    Literature data on the physical properties of steels have been collected and put into a database. The elastic modulus of steels has been analyzed as a function of composition. An overview over former studies is given. The steels have been investigated in three groups, martensitic and ferritic steels, ferritic steels separately, and austenitic steels. For the last two groups, a thermodynamic analysis with Thermo-Gale has been performed. Regression analysis on the influence of composition on the elastic modulus was then carried out. The results for ferritic steels reveal that cementite has no effect on the elastic modulus, whereas Cr, Mo, Si, Mn, and Cu increase it. The elastic modulus of austenitic steels is reduced by Ni and Mo and increased by N, NbC, TiC, and Cr. Cr23C6, while statistically significant in the analysis, has no effect on the elastic modulus of austenitic steels, The regression coefficients found can be used to predict the elastic modulus of steels with known composition.

  • 8. Bohnenkamp, U.
    et al.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Evaluation of the electrical resistivity of steels2000In: Steel research, ISSN 0177-4832, Vol. 71, no 10, p. 410-416Article in journal (Refereed)
    Abstract [en]

    Literature data on the physical properties of sleets have been collected and put into a database. The resistivity of steers has been analyzed as a function of composition and microstructure. An overview over former studies is given. The steels have been investigated in two groups, ferritic steels and austenitic steels. A thermodynamic analysis with ThermoCalc has been performed. Regression analysis on the influence of composition on the resistivity was then carried out. The results for ferritic steels are: Si and Al have the highest elemental resistivity, followed by Mn. Cu, Ni, Mo, and Cr. C precipitated in cementite shows a high coefficient in the analysis when the amount of Fe bound in cementite is not considered separately. C in solution with ferrite shows no significant effect. Cr bound in cementite shows a significant effect but Mn, though present in cementite in comparable amounts, has no significant effect on the resistivity. N and C have the highest elemental resistivity in austenite, followed by the substitutional solutes Nb, Si, Ti, Cu, Ni, Mo, and Cr. The carbides NbC and Tic appear with a higher coefficient in the regression model than can be explained by phase-mixture models providing upper and lower bounds for the resistivity of two-phase alloys. Cr23C6 shows no significant effect. The regression results can be used to predict the resistivity of steels with known composition. The model predicts the resistivity of ferritic steels with a maximum deviation between experimental and computed value of 12 n Omegam and a standard deviation of 5.6 n Omegam. For austenitic steels, the model prediction shows a maximum deviation of 52 mu Omegam and a standard deviation of 20 n Omegam.

  • 9. Bohnenkamp, U.
    et al.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Grimvall, G.
    Electrical resistivity of steels and face-centered-cubic iron2002In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 92, no 8, p. 4402-4407Article in journal (Refereed)
    Abstract [en]

    Literature data for the electrical resistivity of austenitic and ferritic steels are analyzed in a model that takes into account their composition and microstructure. The resistivity in these systems is so high that one must allow for its saturation. That effect is described with the shunt-resistor model, and saturation at 1.68 muOmegam. Our analysis yields the contributions to the resistivity of austenite per weight percent from the solute elements N, C, Nb, Si, Ti, Cu, Ni, Cr, and Mo, with the contribution decreasing in this order. For ferrite, the contribution of the solute elements Si, Mn, Cu, Ni, Mo, and Cr was determined, with the contribution decreasing in this order. The data allow us to obtain the previously unknown resistivity of metastable pure gamma-iron (fcc Fe) near room temperature, and find an interpolation formula for the resistivity up to 1183 K where the gamma phase becomes stable. The resistivity in metastable gamma-iron is significantly larger than the resistivity in stable alpha-Fe (bcc Fe) below 1000 K. This difference is attributed to a strong electron scattering due to spin disorder in gamma-iron at intermediate temperatures.

  • 10.
    Chen, Kaixuan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Chen, Xiaohua
    Univ Sci & Technol Beijing, State Key Lab Adv Met & Mat, Beijing 100083, Peoples R China..
    Wang, Zidong
    Univ Sci & Technol Beijing, Sch Mat Sci & Engn, Beijing 100083, Peoples R China..
    Mao, Huahai
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Optimization of deformation properties in as-cast copper by microstructural engineering. Part I. microstructure2018In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 763, p. 592-605Article in journal (Refereed)
    Abstract [en]

    The microstructural features required to optimize both the strength and ductility of copper are investigated by examining the as-cast pure Cu and Cu-(1.0e3.0)Fe-0.5Co and Cu-1.5Fe-0.1Sn (wt %) alloys. Uniaxial tensile tests show that (Fe, Co)- or (Fe, Sn)-doping improves both the strength and ductility of pure copper. The microstructure evolution with Fe, Co, or Sn doping is characterized by using optical and scanning and transmission electron microscopies. The effects of Fe, Co, and Sn doping on the microstructure clearly show that (i) iron-rich nanoparticles are dispersed inside the grains. The spherical nanoparticles grow in size with increasing Fe content, and when the Fe content exceeds 2.0 wt %, the particles transition into a petal-like morphology. (ii) The microstructure of the alloys (grain size and morphology) is notably influenced by the Fe and Co contents, and the grain size is reduced from an average of 603 mu m in pure Cu to an average of 26 mm in the Cu-3.0Fe-0.5Co alloy. (iii) The addition of 1.5wt % Fe and 0.1wt % Sn dramatically reduces the grain size to an average of 42 mu m, and this reduction is correlated with the appearance of smaller spherical iron-rich nanoparticles. The evolution mechanisms of the iron-rich nanoparticles and grain structure under the alloying effect are discussed.

  • 11.
    Delandar, Arash Hosseinzadeh
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Haghighat, Seyed Masood Hafez
    Korzhavyi, Pavel
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Dislocation dynamics modeling of plastic deformation in single-crystal copper at high strain rates2016In: International Journal of Materials Research - Zeitschrift für Metallkunde, ISSN 1862-5282, E-ISSN 2195-8556, Vol. 107, no 11, p. 988-995Article in journal (Refereed)
    Abstract [en]

    Tensile deformation of single-crystal copper along [001] orientation is modeled. Single crystal is deformed at three sets of high strain rates, ranging from 10(3) to 10(5) s(-1), using the three-dimensional dislocation dynamics technique to simulate dislocation microstructure evolution and the resultant macroscopic response. Two initial dislocation configurations consisting of straight dislocations and Frank-Read sources are randomly distributed over the simulation volume with an edge length of 1 mu m. For both initial setups, the mechanical response of the single crystal to the external loading demonstrates a considerable effect of strain rate. In addition, strain rate influences dislocation density evolution and consequently development of the dislocation microstructure. At all applied strain rates for both initial dislocation setups, dislocations evolve into a heterogeneous microstructure and this heterogeneity increases with plastic strain and strain rate.

  • 12.
    Delandar, Arash Hosseinzadeh
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    The role of glide during creep of copper at low temperaturesManuscript (preprint) (Other academic)
  • 13.
    Delandar, Arash Hosseinzadeh
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. KTH Royal Inst Technol, Mat Sci & Engn, SE-10044 Stockholm, Sweden..
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE. KTH Royal Inst Technol, Mat Sci & Engn, SE-10044 Stockholm, Sweden..
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics. KTH Royal Inst Technol, Mat Sci & Engn, SE-10044 Stockholm, Sweden..
    The Role of Glide during Creep of Copper at Low Temperatures2018In: METALS, ISSN 2075-4701, Vol. 8, no 10, article id 772Article in journal (Refereed)
    Abstract [en]

    Copper canister will be used in Scandinavia for final storage of spent nuclear fuel. The copper will be exposed to temperatures of up to 100 degrees C. The creep mechanism at near ambient temperatures has been assumed to be glide of dislocations, but this has never been verified for copper or other materials. In particular, no feasible mechanism for glide based static recovery has been proposed. To attack this classical problem, a glide mobility based on the assumption that it is controlled by the climb of the jogs on the dislocations is derived and shown that it is in agreement with observations. With dislocation dynamics (DD) simulations taking glide but not climb into account, it is demonstrated that creep based on glide alone can reach a quasi-stationary condition. This verifies that static recovery can occur just by glide. The DD simulations also show that the internal stress during creep in the loading direction is almost identical to the applied stress also directly after a load drop, which resolves further classical issues.

  • 14. Eliasson, J.
    et al.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Proof strength values for austenitic stainless steels at elevated temperatures2000In: Steel research, ISSN 0177-4832, Vol. 71, no 07-jun, p. 249-254Article in journal (Refereed)
    Abstract [en]

    The influence of the alloying elements C, N, Si, Mn, Cr, Ni, Mo, Cu, Ti on proof strength is studied in the temperature range 20-550 degrees C. High temperature data for the proof strength of austenitic stainless steels have been analysed. Using computational thermodynamics the amount of alloying elements in solid solution and the volume fractions of precipitates were assessed. These quantities were then applied in a regression analysis for the high temperature strength. Quantitative relationships for the proof strength as a result of the regression analysis are proposed as a function of temperature. They are to be used in materials design. The interstitial elements showed the largest effect. Si, Ni and Mo increased the strength at all temperatures. Cu and Mn reduced the strength and Cr gave an influence, which varied with temperature.

  • 15.
    Ericsson, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue of friction stir welded T-joints2005In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452Article in journal (Other academic)
  • 16.
    Ericsson, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue properties of friction stir overlap welds2007In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 29, no 1, p. 57-68Article in journal (Refereed)
    Abstract [en]

    Friction stir welding (FSW) is currently used for many applications involving lap or T-joints, e.g. hermetically closed boxes such as cooling elements and heat exchangers. The frequent pressure changes in these make them susceptible to fatigue. The fatigue characterization of lap joints involves a combination of shear and bending. Forces applied to the ends of lap joints result in non-axial stresses in the connection area. FSW lap joints of Al-Mg-Si alloy 6082 in the artificially aged condition T6 were studied. A pin (probe) based on the Triflute (TM) concept was used with two modifications to the pin, the pin end being either convex or concave. Tool shoulders of 15 and 18 turn respectively were utilized, producing four different weld series. Fracture was initiated in the highly stressed area where the weld cuts through the interface between the two sheets. The cracks typically propagated through the weld in the upper sheet (tool side). The broadest tool shoulder with a concave end of pin design gave the best fatigue performance. This was due to an improved flow path provided by the hollowed out end of the pin; allowing material flow around the pin which resulted in minimal hooking of the sheet interface adjacent to the weld nugget. Additionally heat energy was supplied by the increased contact area. The stress intensity factor Delta K was determined. It was found that a simplified approach, developed to estimate Delta K for overlap spot welds, could be applied to friction stir overlap joints. The corresponding crack propagation rates were in fair accordance with the experimental results.

  • 17.
    Ericsson, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue crack propagation in friction stir welded and parent AA60822006In: Steel Research International, ISSN 1611-3683, Vol. 77, no 6, p. 450-455Article in journal (Refereed)
    Abstract [en]

    The fatigue crack propagation characteristics of a friction stir welded AI-Mg-Si alloy, 6082, have been investigated. The electrical potential drop method was used for measurements. A low and a high load ratio (R) level were tested. At low load ratio (R=0.1) and a low stress intensity Delta K the propagation rate in the weld was higher than in the parent material by a factor of 3 to 5. However, the propagation rates were approaching each other close to fracture. At high load ratio (R=0.8) the propagation rate was similar in the parent material and weld. The weld crack growth rate was about the same at low and high R (except close to fracture), while the parent material growth rate increased at high R. Paris law was used to describe the measured crack propagation rates in the weld. In the case of the parent material, showing an R-dependence, Forman's law was used.

  • 18.
    Ericsson, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Fatigue performance of friction stir welded AlMgSi-alloy 60822001In: Aluminium, ISSN 0002-6689, Vol. 77, no 7-8, p. 572-575Article in journal (Other academic)
    Abstract [en]

    Friction stir welding is a relatively new solid-state welding method, ideal to join large, thin-walled aluminium profiles. In this paper results from tensile and fatigue testing of friction stir welds of the hardenable Al-Mg-Si alloy AA6082 is presented. For temper T4, post weld age treated (PWAT), the fatigue strength was found to be lower than for temper T6, and with a steeper slope of the stress-life curve. The lower fatigue strength for the statically stronger T4 + PWAT material was unexpected. Fracture in T6 initiated and propagated in the vicinity of the weak weld/HAZ border, directly outside the stirred up edge on the advancing (or shear-) side of the weld. For T4 + PWAT many fractures occurred in the weld, often initiated on the advancing side.

  • 19.
    Ericsson, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Influence of welding speed on the fatigue of friction stir welds, and comparison with MIG and TIG2003In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 25, no 12, p. 1379-1387Article in journal (Refereed)
    Abstract [en]

    The objective of this investigation was to determine whether the fatigue strength of friction stir (FS) welds is influenced by the welding speed, and also to compare the fatigue results with results for conventional arc-welding methods: MIG-pulse and TIG. The Al-Mg-Si alloy 6082 was FS welded in the T6 and T4 temper conditions, and MIG-pulse and TIG welded in T6. The T4-welded material was subjected to a post-weld ageing treatment. According to the results, welding speed in the tested range, representing low and high commercial welding speed, has no major influence on the mechanical and fatigue properties of the FS welds. At a significantly lower welding speed, however, the fatigue performance was improved possibly due to the increased amount of heat supplied to the weld per unit length. The MIG-pulse and TIG welds showed lower static and dynamic strength than the FS welds. This is in accordance with previous comparative examinations in the literature on the fatigue strength of fusion (MIG) and FS welds. The TIG welds had better fatigue performance than the MIGpulse welds. The softening, of the alloy around the weldline has been modelled. Using a model without adjustable parameters, a fair description of the hardness profiles across the weld as a function of welding speed was obtained. The softening in front of the Friction Stir Welding tool was also estimated. At the low and high welding speeds a full and partial softening is predicted, respectively.

  • 20. Eskner, M.
    et al.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Measurement of the ductile-to-brittle transition temperature in a nickel aluminide coating by a miniaturised disc bending test technique2003In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 165, no 1, p. 71-80Article in journal (Refereed)
    Abstract [en]

    Nickel aluminide coatings are often employed to enhance the corrosion and oxidation resistance of nickel base gas turbine blades and vanes, as the high near-surface content of Al increases the ability to form an Al2O3 protective scale. The ductility of the coating depends on the type of aluminisation process and Ni-base material. In order to prevent coating degradation during service it is important to assess the ductile-to-brittle transition temperature (DBTT) in ductility of the coating. To determine the DBTT a miniaturised disc bending test (MDBT) technique is used, where a biaxial tensile stress is applied to a disc specimen. The DBTT of a NiAl coating, applied by a high-activity aluminium pack cementation process to a polycrystalline Ni-base superalloy (IN738 LC), was evaluated using the MDBT technique between room temperature (RT) and 860 degreesC. Test results gave a DBTT in biaxial ductility of the coating of approximately 760 degreesC. Above 760 degreesC, a significant increase in ductility was noted. Fractographic examination showed that the coating fractures in a mainly transgranular mode at RT but in a predominately intergranular mode at elevated temperatures, even at temperatures above DBTT.

  • 21. Eskner, M.
    et al.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Mechanical properties and temperature dependence of an air plasma-sprayed NiCoCrAlY bondcoat2006In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 200, no 8, p. 2695-2703Article in journal (Refereed)
    Abstract [en]

    The elastic and inelastic properties were investigated for an air plasma-sprayed bondcoat (Ni-23Co-17Cr-12Al-0.5Y) at temperatures up to 800 degrees C. The yield strength was evaluated by using a miniaturised disc-bending test while the stress-strain behaviour and elastic modulus were determined through spherical indentation. The material was tested in both as-coated and heat-treated conditions (1500 h, 1000 degrees C). From the disc-bending tests it was observed that both conditions were macroscopically brittle both at low and high temperatures, with slow and stable crack propagation in the inter-splat region. Between room temperature and 500 degrees C the yield strength is constant and yield occurs by inter-splat shearing, but above this temperature the yield strength decreases due to yielding of the splats. This behaviour was confirmed by the fractographical examination that showed an inter-splat fracture surface at room temperature with small amount of plastic features, but at 800 degrees C a more dimpled and ruptured fracture appeared. Small difference in yield strength was found between the two conditions, and they had similar temperature dependence. However, their indentation stress-strain response was different, where the heat-treated condition had the largest resistance against inelastic deformation. The value of the elastic modulus was for the as-coated condition 137 GPa at 25 degrees C and 127 GPa at 800 degrees C, and for the heat-treated condition 226 GPa at 25 degrees C and 192 GPa at 800 degrees C, the difference between the two conditions being a result of internal oxidation.

  • 22.
    Eskner, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Measurement of the elastic modulus of a plasma-sprayed thermal barrier coating using spherical indentation2004In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 177, p. 165-171Article in journal (Refereed)
    Abstract [en]

    Failure in plasma-sprayed thermal barrier coatings systems mostly takes place in the ceramic topcoat or at the interface between the topcoat and the bondcoat. The failure normally occurs by spallation of the topcoat at shutdown operations from high temperatures where compressive thermal-mismatch stresses are induced in the topcoat. In order to analyse the residual stresses, knowledge about the elastic modulus for the different components in the coating system is required. In this work, a spherical indentation method has been used for room-temperature measurement of the elastic modulus of both the topcoat (300-mum-thick air-plasma sprayed ZrO2-8wt,%Y2O3) and the bondcoat (150-mum-thick air-plasma sprayed Ni-23Co-17Cr-12Al-0.5Y). This method gives the compressive modulus, which for the top-coat may be more adequate as it has different modulus in tensile and compressive stress states due to its microstructure. Measurements were made on specimens in as-coated condition and after heat treatment (1500 h at 1000 degreesC). A significant increase in the elastic modulus of the bondcoat was observed as a result from the heat treatment, from 137 to 226 GPa, which is explained by the internal oxidation at the initial weak inter-splat regions. A sintering effect and a change in the elastic modulus were also observed for the topcoat as a result of the beat-treatment, where the elastic modulus increased from 38 to 60 GPa.

  • 23.
    Eskner, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Mechanical property evaluation using the small punch test2004In: Journal of Testing and Evaluation, ISSN 0090-3973, E-ISSN 1945-7553, Vol. 32, no 4, p. 282-289Article in journal (Refereed)
    Abstract [en]

    The small punch (SP) test technique is an attractive method for evaluating mechanical behavior in materials where only small volumes of materials are available. An SP-test machine was developed where miniature disc specimens 5 and 3 mm in diameter and with thickness ranging between 50 to 400 mum can be investigated. For two materials, a 1Cr-0.5Mo low alloy steel and an 18Cr-9Ni austenitic stainless steel, the stress-strain flow curve was assessed and compared with that in tensile tests. The yield strength was determined from the SP-test by analyzing the initial elastic deformation with help of classical plate bending theory. The effective flow properties in the biaxial stress state were evaluated through analytical modelling of the stretching deformation. Good agreement between the uniaxial and biaxial flow properties was found for the 1wCr-0.5Mo steel and acceptable agreement for the 18Cr-9Ni steel.

  • 24.
    Farooq, Muhammad
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Influence of long time ageing on ductility and toughness in the stainless steel 310 in the presence of banded microstructure2011In: 7th European Stainless Steel Conference: Science and Market, Proceedings, Associazione Italiana di Metallurgia , 2011Conference paper (Refereed)
    Abstract [en]

    During service at elevated temperatures extensive formation of particles can take place that can have a dramatic influence on mechanical properties. Precipitation of σ-phase and M23C6-carbides have been studied both experimentally and with thermodynamic modelling for 25Cr20Ni austenitic stainless steels (AISI 310) at 800 °C for up to 5000 h. Previous work has demonstrated that the modelling could describe the nucleation and growth satisfactory. After long term ageing the particles form bands in the microstructure. In the present paper the influence of these bands on ductility and toughness at room temperature is analysed. For this purpose previously developed models for ductility and toughness are utilised. Model values for banded and non-banded microstructures have been generated for casts of 310 in fine and coarse grained conditions with 0.04 and 0.12%N. The model values show that in the coarse grained condition, no reduction in ductility and toughness can be expected in the banded microstructure. In the fine grained condition a modest reduction is predicted.

  • 25.
    Farooq, Muhammad
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Influence of long time ageing on ductility and toughness in the stainless steel 310 in the presence of banded microstructure2012In: La Metallurgia Italiana, ISSN 0026-0843, Vol. 104, no 11-12, p. 33-38Article in journal (Refereed)
    Abstract [en]

    During service at elevated temperatures extensive formation of particles can take place that can have a dramatic influence on mechanical properties. Precipitation of sigma-phase and M23C6-carbides have been studied both experimentally and with thermodynamic modelling for 25Cr20Ni austenitic stainless steels (AISI 310) at 800 degrees C for up to 5000 h. Previous work has demonstrated that the modelling could describe the nucleation and growth satisfactory. After long term ageing the particles form bands in the microstructure. In the present paper the influence of these bands on ductility and toughness at room temperature is analysed. For this purpose previously developed models for ductility and toughness are utilised. Model values for banded and non-banded microstructures have been generated for casts of 310 in fine and coarse grained conditions with 0.04 and 0.12%N The model values show that in the coarse grained condition, no reduction in ductility and toughness can be expected in the banded microstructure. In the fine grained condition a modest reduction is predicted.

  • 26.
    Farooq, Muhammad
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Lundberg, Mats
    Precipitation during long time ageing in the austenitic stainless steel 3102012In: Materials at High Temperature, ISSN 0960-3409, E-ISSN 1878-6413, Vol. 29, no 1, p. 8-16Article in journal (Refereed)
    Abstract [en]

    Extensive precipitation can occur in high temperature materials during service. This is known to significantly affect the mechanical properties. To be able to study this phenomenon, particle formation in the austenitic stainless steel 310 has been quantified. In particular, sigma-phase and M23C6-carbides have been analysed. Specimens have been aged up to 5000 h at 800 degrees C. The precipitation of the two phases has been modelled. The basis of the model is diffusion-controlled growth. sigma-phase shows a spherical growth and an M23C6 planar growth. The soft impingement of the diffusion zones with respect to both chromium and carbon is taken into account. The influence on mechanical properties is analysed in a parallel paper.

  • 27.
    Farooq, Muhammad
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Tassa, Oriana
    Precipitation hardening and other contributions to the creep strength of an 23Cr25NiWCuCo austenitic stainless steelManuscript (preprint) (Other academic)
  • 28.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Basic modelling of creep rupture in austenitic stainless steels2017In: Theoretical and applied fracture mechanics (Print), ISSN 0167-8442, E-ISSN 1872-7638, Vol. 89, p. 139-146Article in journal (Refereed)
    Abstract [en]

    Creep rupture can happen in two ways, brittle and ductile creep rupture. Brittle creep rupture of austenitic stainless steels proceeds with the nucleation, growth and coalescence of grain boundary cavities. A creep cavity nucleation model has been developed previously, which considers cavity nucleation at particles and sub-boundary corners due to grain boundary sliding. A modified constrained cavity growth model has been used to describe the cavity growth behavior with combination of the cavity nucleation models. In this paper, the brittle creep rupture has been analyzed by combining the cavity nucleation and growth models. The physically based models where no adjustable parameters were involved were used to predict the brittle creep rupture strength. On the other hand, previously developed basic models for ductile creep rupture based on exhaustion of the creep ductility have been used. The creep rupture strength of common austenitic stainless steels has been predicted quantitatively by taking both ductile and brittle rupture into account. The predicted rupture times for ductile rupture are longer than those for brittle rupture at high stresses and low temperatures with a reversed situation at low stresses and high temperatures. This reproduces the characteristic change in slope in the creep rupture curves.

  • 29.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Brittle rupture of austenitic stainless steels due to creep cavitation2016In: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), Elsevier, 2016, p. 863-870Conference paper (Refereed)
    Abstract [en]

    Basic creep cavitation models have been used to predict brittle rupture of austenitic stainless steels. It involves the grain boundary sliding models, which is the basis of the creep cavitation models, the recently developed cavity formation models and the constrained cavity growth models. The individual creep cavitation models are verified with experimental observations. Brittle rupture due to creep cavitation that appears as intergranular failure is found to be dominant at high temperatures and long creep exposure times.

  • 30.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Creep cavity growth models for austenitic stainless steels2016In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 674, p. 328-334Article in journal (Refereed)
    Abstract [en]

    Diffusion controlled cavity growth models tend to exaggerate the growth rate. For this reason it is essential to take into account the restrictions caused by creep rate of the surrounding material, so called constrained growth. This has the consequence that the stress that the cavities are exposed to is reduced in comparison to the applied creep stress. Previous constrained growth models have been based on linear viscoplasticity. To avoid this limitation a new model for constrained growth has been formulated. Part of the work is based on a FEM study of expanding cavities in a creeping material. Compared with the previous constrained cavity growth models, the modified one gives lower reduced stresses and thereby lower cavity growth rates. By using recently developed cavity nucleation models, the modified creep cavity growth model can predict the cavity growth behaviour quantitatively for different types of austenitic stainless steels, such as 18Cr10Ni, 17Cr12NiNb and 17Cr12NiTi.

  • 31.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Formation of creep cavities in austenitic stainless steels2016In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 51, no 14, p. 6674-6685Article in journal (Refereed)
    Abstract [en]

    The possibility of creep cavity formation at subboundaries in austenitic stainless steels is analysed. It is demonstrated that such nucleation is thermodynamically feasible. A minimum stress must be exceeded in order to create cavities. The nucleation is assumed to take place where subboundaries on one side of a sliding grain boundary meet subgrain corners on the other side (double ledge models). Alternative cavitation positions can be found where particles meet subboundaries. The nucleation model can quantitatively predict the observed nucleation rate. The model gives a nucleation rate that is proportional to the creep rate in agreement with many experiments

  • 32.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Modelling grain boundary sliding during creep of austenitic stainless steels2016In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 51, no 6, p. 2926-2934Article in journal (Refereed)
    Abstract [en]

    Two models are presented for grain boundary sliding (GBS) displacement during creep. GBS is considered as crucial for the formation of creep cavities. In the first model, the shear sliding model, GBS is accommodated by grains freely sliding along the boundaries in a power-law creeping material. The GBS rate is proportional to the grain size. In the second model, the shear crack model, the sliding boundaries are represented by shear cracks. The GBS rate is controlled by particles in the boundaries. In both models, the GBS displacement rate is proportional to the creep strain rate. Both models are consistent with existing experimental observations for GBS during creep of austenitic stainless steels. For cavity nucleation at particles, Harris’ model (1965) for the relationship between GBS and a critical particle size has been analysed and found to be in agreement with observations.

  • 33.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Notargiacomo, Sandro
    Low-Cycle Fatigue Properties of a Nickel-Based Superalloy Haynes 282 for Heavy Components2017In: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 26, no 5, p. 2257-2263Article in journal (Refereed)
    Abstract [en]

    Low-cycle fatigue (LCF) tests of the nickel-based superalloy Haynes 282 from a large forged ingot were conducted at 25 and 750 degrees C with total strain ranges from 0.7 to 1.7%. Compared with other tests on this alloy, it was found that the LCF properties showed similar results at room temperature, but improved number of cycles to failure at high temperatures. The number of cycles at a given total strain range showed no large differences between the core and rim positions. By comparing with two other types of low gamma' volume fraction nickel-based superalloys, Haynes 282 gave the best LCF properties at high temperatures. The reason may be due to the dominating transgranular fracture in the current work. A mixture of intergranular and transgranular fractures had been observed in the other alloys. The results demonstrate that heavy components of Haynes 282 can be produced with good LCF properties.

  • 34.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Vujic, Stojan
    Creep, low cycle fatigue and creep-fatigue properties of a modified HR3C2016In: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), Elsevier, 2016, p. 871-878Conference paper (Refereed)
    Abstract [en]

    Creep, low cycle fatigue (LCF) and creep fatigue tests have been conducted for modified HR3C (25Cr20NiNbN) at high temperatures ranging of 650-750 degrees C. Both LCF and creep fatigue test results could be described with the Coffin-Manson relationship. The number of cycles to failure in the creep fatigue tests was more than one order of magnitude lower compared with LCF. The effect of the total hold time in tension (the total creep time) was compared to creep rupture data. The creep fatigue results were in reasonable agreement with the creep tests. The short creep fatigue lives may be due to the low creep ductility which was found in the creep tests. Fractography showed that the rupture mode was intergranular. Cavities were observed at grain boundaries due to the fracture of the primary Z phase particles in both LCF and creep fatigue tests. In comparison to Sanicro 25, the modified HR3C showed better LCF properties.

  • 35.
    Hosseinzadeh Delandar, Arash
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Hafez Haghigha, S. M.
    Korzhavyi, Pavel
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Three-dimensional dislocation dynamics simulation of plastic deformation in copper single crystalManuscript (preprint) (Other academic)
    Abstract [en]

    Dynamic deformation of single crystal copper at high strain rates ranging from 103 to 105 s-1 is modeled using three dimensional discrete dislocation dynamics method. Numerical uniaxial tensile test is performed on a model crystal along [0 0 1] orientation to examine the resulting macroscopic behavior along with microstructure evolution at high strain rates. Twenty-four straight dislocations of mixed character are randomly distributed inside a model crystal with an edge length of 1  subjected to period boundary conditions. In the simulated single crystal with the considered initial dislocation microstructure, plastic flow demonstrates a significant strain rate dependency at imposed strain rates. Rate sensitivity of flow stress observed at strain rates >> 103 s-1 agrees well with the reported experimental studies on copper single crystal.  Furthermore, strain rate considerably affects the microstructure evolution of the sample crystal as a result of influence of strain rate on dislocations generation and interactions. Formation of heterogeneous microstructure is observed at all imposed strain rates. We find that heterogeneity of microstructure escalates as strain rate increases.

  • 36.
    Hosseinzadeh Delandar, Arash
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Korzhavyi, Pavel
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Investigation of loading orientation effect on dynamic deformation of single crystal copper at high strain rates: Discrete dislocation dynamics studyManuscript (preprint) (Other academic)
    Abstract [en]

    Uniaxial tensile loading of copper single crystal along [001] and [111] orientations is modeled at two high strain rates of 105 and 106 s-1. Discrete dislocation dynamics method is used to study the anisotropic characteristic of plastic deformation in the model crystal. Furthermore, strain rate sensitivity of the flow stress in copper crystal is examined. Investigation of mechanical response of single crystal to the external loading demonstrates a substantial effect of loading orientation on the plastic flow. We find that at both imposed strain rates flow stress increases significantly when tensile load is applied along [111] crystallographic axis. Similarly, plastic anisotropy is observed in dislocation density evolution such that more dislocations are generated as straining direction of single crystal is changed from [001] to [111] axis. Moreover, plastic flow behavior exhibits a profound strain rate sensitivity at both loading orientations which agrees well with experimental observations regarding strain rate dependency of flow stress in copper single crystal as strain rate exceeds 103 s-1. At both applied strain rates dislocations evolve into a heterogeneous microstructure and highest heterogeneity is observed as model crystal is loaded along [111] direction at strain rate of    106 s-1. Formation of slip bands and consequently localization of plastic deformation are detected for all considered cases. However, at the higher strain rate of 106 s-1, slip band formation is more pronounced for both loading orientations.

  • 37. Högberg, J.
    et al.
    Chai, G.
    Kjellström, Patrik
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Sandvik Materials Technology.
    Boström, M.
    Forsberg, U.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Creep behavior of the newly developed advanced heat resistant austenitic stainless steel grade UNS S310352010In: Proceedings of the ASME Pressure Vessels And Piping Conference 2010, ASME Press, 2010, p. 421-428Conference paper (Refereed)
    Abstract [en]

    The UNS S31035 austenitic stainless steel grade is a newly developed advanced heat resistant material for use in coal fired boilers at material temperatures up to about 700°C. This new grade has good resistance to oxidation and hot corrosion, and shows higher creep rupture strength than other austenitic stainless steels available today. This paper will mainly focus on the study of the creep mechanisms in this grade from 550?C up to 800°C by using TEM, SEM and LOM. the creep mechanisms at different temperatures and loading conditions have been identified. the interaction between dislocations and precipitates and their contribution on the creep rupture strength and fracture mechanisms have been discussed. In this paper, different models have been used to evaluate the long term creep behavior of the grade. A creep rupture strength near 100MPa at 700°C for 100 000h has been predicted. This makes it an interesting alternative for super-heaters and reheaters in future high-efficient coal fired boilers.

  • 38. Izadi, Hossein
    et al.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Gerlich, Adrian P.
    Grain Growth Behavior and Hall-Petch Strengthening in Friction Stir Processed Al 50592014In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45A, no 12, p. 5635-5644Article in journal (Refereed)
    Abstract [en]

    Friction stir processing (FSP) of Al 5059 is studied in which subsequent heat treatment is conducted to investigate its effects on grain size and hardness. It was found that mainly elongated and rhomboidal morphologies of Al-6(Mn,Fe) particles are present in the alloy both before and after FSP, where the rhomboidal particles are more effective in pinning grain boundaries during heat treatment. The stir zone reached a temperature of 705 K (432 A degrees C), and ThermoCalc modeling confirmed that the Al-6(Mn,Fe) particles will remain stable at this temperature. Negligible grain growth was observed during FSP of the Al 5059 due to low grain boundary mobility resulting from slow diffusion associated with a high Mg content in the alloy. During heat treatment at 448 K (175 A degrees C) grain growth could be correlated with time using a particle-controlled grain growth model. Microhardness values indicate that Hall-Petch behavior occurs in the processed alloy, while dislocation density and particle dispersion play a minor role in strengthening.

  • 39.
    Jin, Lai -Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Non-stationary creep simulation with a modified Armstrong-Frederick relation applied to copper2009In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 46, no 2, p. 339-346Article in journal (Refereed)
    Abstract [en]

    A previously formulated model using back stress to handle non-stationary creep during power-law breakdown is further developed. In particular, the way to integrate the back stress is modified. Usually the Armstrong-Frederick relation has been applied, but it can give unphysical results in the sense that the back stress exceeds the tensile strength of the material. Such a problem can be solved by replacing the back stress term in this relation with the back stress deviator. The creep model is applied to copper canister in waste packages intended for encapsulating spent nuclear fuel. These waste packages will be placed in the bedrock at a depth of about 500 m as a final stage of disposal. During storage, radioactivity-induced thermal evolution raises temperature in repositories and water-saturation generates pressure directly on the copper canister. The thermally activated creep in copper canister occurs readily. To estimate the amount of creep deformation, a finite element model is set up to compute the evolution of creep deformation in copper canister. The creep model takes both stationary and non-stationary creep into account The computed maximum creep strain is shown to be 7.8% over 10 years, which should not cause failure since measured creep elongations are in the range of 15-40%.

  • 40.
    Jin, Lai-Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Creep of copper canisters in power-law breakdown2008In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 43, no 3, p. 403-416Article in journal (Refereed)
    Abstract [en]

    According to the Swedish KBS-3 concept the spent nuclear fuel will be placed in copper canisters 500 m down in the bedrock. In thestorage, the canister will creep under conditions that are well inside the power-law breakdown regime. To prevent creep rupture fromoccurring that could cause leakage of nuclides, finite element models are set up to study the evolution of creep deformation in the coppercanisters. In this paper, two finite element models for the secondary creep are formulated. The first one is based on a fundamental climb–glide creep law valid over a wide range of temperatures. The second one is on the basis of a generalised Norton equation fitted to secondarycreep data of phosphorus doped pure copper. The creep deformation is shown to be much larger in the lid and the bottom of the canistersthan in the cylindrical wall. In the latter a stationary creep state is reached only after very long time (30000 years). Since the deformation inthe copper canister is restricted by a cast iron insert and stress concentrations are reduced with time, the total creep strain is limited.

  • 41.
    Jin, Lai-Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Numerical simulation of residual stresses for friction stir welds in copper canisters2012In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 14, no 1, p. 71-81Article in journal (Refereed)
    Abstract [en]

    In an attempt to map the residual stress distributions after friction stir welding of copper canisters, a three-dimensional thermo-mechanical model has been formulated by coupling heat transfer and elastoplasticity analyses. The transient temperature field around the tool is simulated by a moving heat source. The simulation shows that the residual stress distribution in a thick-wall copper canister is sensitive to the circumferential angle and asymmetrical to the weld line. Both tensile and compressive stresses emerge along the weld line and its vicinity. The maximum tensile stress appears in the circumferential direction on the outer surface. The maximum tensile stress, whether it is predicted by the finite element method or measured by the hole-drilling technique and the X-ray diffraction method, does not exceed 50 MPa in general.

  • 42.
    Jin, Lai-Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Steady non-Newtonian flows in copper and iron aluminide at elevated temperatures2007In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 189, no 1-3, p. 428-434Article in journal (Refereed)
    Abstract [en]

    On the basis of dislocation climb and mobility, a steady-state non-Newtonian flow law is derived for two metallic and one intermetallic material,namely electrolytic tough pitch copper, phosphorus alloyed pure copper, and Fe24AlMo iron aluminide. The purpose is to develop a flow lawapplicable to the finite element simulation for hydrodynamic flow of incompressible metals. The model can accurately represent the experimentalflow stress values and the viscosity of the materials. The mathematical form of the model is similar to that of the free-volume approach, which isused for liquids and amorphous metals. The study indicates that in the temperature and strain-rate regimes that are appropriate to the hot-workingprocesses, the Cohen–Grest model, which is essentially related to the total thermal expansion of fluid, can phenomenologically be extended to thecrystalline solid-state materials for the depiction of viscosity data.

  • 43.
    Korzhavyi, Pavel A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Monovacancy in copper: Trapping efficiency for hydrogen and oxygen impurities2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 84, p. 122-128Article in journal (Refereed)
    Abstract [en]

    The structure and binding energy of vacancy-impurity complexes in copper are studied using first-principles calculations based on density functional theory. A single vacancy is found to be able to trap up to six hydrogen atoms which tend to be situated inside the vacancy at off-center positions (related to the octahedral interstitial positions of the ideal fcc lattice). The binding energy of an H atom dissolved in the Cu lattice (octahedral interstitial position) to a vacancy is calculated to be about 0.24 eV, practically independent of the number of H atoms already trapped by the vacancy, up to the saturation with 6 hydrogens. For an oxygen impurity in Cu, a monovacancy is shown to be a deep trap (with a binding energy of 0.95 eV). The position of a trapped O atom inside a vacancy is off-center, almost a half-way from the nearest octahedral interstitial to the vacancy center. Such a vacancy-O cluster is shown to be a deep trap for dissolved hydrogen (the calculated binding energy is 1.23 eV). The trapping results in the formation of an OH-group, where the H atom is situated near the vacancy center, and the O atom is displaced from the center along a 〈100〉 direction towards a nearby octahedral interstitial position. Further hydrogenation of the monovacancy-OH cluster is calculated to be energetically unfavourable. McNabb-Forster's equations are generalised to describe the competition between a deep hydrogen trap and a shallow one. It is demonstrated that the deep trap is almost fully filled, which explains why some of hydrogen is strongly bound and cannot be removed without vacuum treatment at elevated temperatures.

  • 44.
    Korzhavyi, Pavel A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    First-principles evaluation of the effect of alloying elements on the lattice parameter of a 23Cr25NiWCuCo austenitic stainless steel to model solid solution hardening contribution to the creep strength2015In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 626, p. 213-219Article in journal (Refereed)
    Abstract [en]

    By the use of first-principles calculations based on density functional theory, lattice misfit parameters for alloying elements in the austenitic stainless steel 23Cr25NiWCuCo have been derived. These lattice misfit parameters have been applied to determine the solid solution hardening of the elements W, Nb, and Cu in the steel. The model for solid solution hardening is based on work by Hirth and Lothe, where solutes are creating Cottrell clouds around the dislocations and slow down their motion. The model is also verified by comparison to creep tests for Ni-20%Cr and Ni-20%Cr-6W, where W is almost completely in solid solution and no other strengthening mechanism than solid solution hardening should be active. The contribution from the interstitial elements C and N to the solid solution hardening is found to be negligibly small for the studied steel.

  • 45.
    Källgren, Therese
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Finite Element Modelling of Friction Stir Welding on Copper Canister2004In: 5th International Friction Stir Welding symposium, 2004Conference paper (Refereed)
    Abstract [en]

    In an effort to enhance safety for long time deposit of waste nuclear fuel, friction stir welding has been tentatively used to seal copper canisters. To avoid the formation of voids and cracks during the welding process, and to understand the heat and material flow as well as the evolution of the microstructure, are of great importance. Finite element modelling has been used to simulate the friction stir welding process.

    A model involving heat transfer, material flow, and continuum mechanics has been developed. The steady state solutions have been compared with experimental temperature observations as well as analytical solutions, showing good agreement. Temperature distribution is affected by the welding speed. For a given reference point perpendicular to the welding direction, a lower welding speed corresponds to a higher peak temperature. The plunging position of welding tool influences the temperature distribution and therefore also the thermal distortion of the weldment.

  • 46.
    Källgren, Therese
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Finite element modelling of temperature distribution in friction stir welding process and its influence on distortion of copper canisters2004In: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXVIII, 2004, Vol. 824, p. 57-62Conference paper (Refereed)
    Abstract [en]

    In an effort to enhance safety for long time disposal of waste nuclear fuel, friction stir welding has been developed as one alternative to seal copper canisters. To avoid the formation of voids and cracks during the welding process, an understanding of the heat and material flow and thereby the evolution of the microstructure, is of great importance. Finite element modelling has been used to simulate the heat and material flow as well as thermal expansion during the friction stir welding process. A model involving heat transfer, material flow, and continuum mechanics has been developed. The steady state solutions have been compared with experimental temperature observations as well as analytical solutions, showing good agreement. Temperature distribution is affected by the welding speed. For a given reference point perpendicular to the welding direction, a lower welding speed corresponds to a higher peak temperature. The plunging position of welding tool influences the temperature distribution and therefore the displacement distribution of the weldment.

  • 47.
    Källgren, Therese
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Flow modelling of friction stir welding using brass markers in copperIn: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936Article in journal (Other academic)
    Abstract [en]

    This paper describes the behaviour of the material flow in 50 mm thick friction stir welded phosphorous-alloyed copper with both experiments and modelling work. In Sweden the friction stir welding is going to be used for sealing copper canisters for storage of nuclear waste from fuel elements. An understanding of the material flow during the process is essential in order to predict the weld quality and thereby the maintenance of the weld. A marker technique is used which involves inserting dissimilar material into the weld zone before joining. The material flow can then be examined. In these experiments Ø 9 brass rods are used. After welding, the copper was sliced, etched and examined by optical microscopy. The results show that the weld tool pushes the brass as far as 37 mm backwards from the inserted location, which is little more than one pin diameter. This brass is first seen on the advancing side. Most brass is found behind its inserted location, on both sides of the centre line. The material movement underneath the pin in the weld root is insignificant. The pin area has a nugget with a fine grain structure, but also a part near the shoulder with coarser grains. A finite element model is developed in order to describe the temperature and material flow. The model and the examined welds were compared showing a satisfactory agreement for most features.

  • 48.
    Källgren, Therese
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Influence of tool geometry on the material flow during friction stir welding of copperIn: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774Article in journal (Other academic)
    Abstract [en]

    The understanding of the material flow during friction stir welding of thick sections of copper is of great interest. Sweden plans to use this method to seal 50 mm thick copper canisters which will contain spent nuclear fuel. In this study two different weld tools are investigated. The first experiment uses a pin length of 52 mm and the second a pin length of 30 mm. The shoulder diameter, process parameters and materials are the same for both experiments. Experiments as well as FEM modelling work are reported. The experimental part includes a marker inserting technique using brass rod, which allows the material flow to be examined. After welding, the weld line is sliced, etched and examined by optical microscope. The experiments show more brass in the weld with shorter pin length and the nugget zone has different appearances in the two cases. A finite element model is applied to describe the temperature, material flow, dynamic viscosity, strain rate and shear stress in the welds. The outcome of the model and the feature of the experimental welds are compared showing a satisfactory result. The size of the thermomechanically affected zone is predicted by this model.

  • 49.
    Källgren, Therese
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Material Flow during Friction Stir Welding of Copper2008In: 7th international Friction Stir Welding symposium: Awaji Island, Japan, 20-22 May 2008, 2008Conference paper (Refereed)
    Abstract [en]

    SKB (Swedish Nuclear Fuel and Waste Management Co) intends to use the FSW method to seal copper canisters for nuclear fuel waste. The understanding of the material flow in this process is essential in order to obtain good weld quality. Material and temperature flow in 50 mm thick friction stir welded (FSW) copper has been investigated in this study. The main experiments have been performed at SKB’s canister laboratory in Oskarshamn. By inserting dissimilar material into the weld zone before welding and then join the material, the flow can be analyzed. Ø4 mm brass rods were used. After welding, the weld line was sliced, etched and examined by optical microscope. A three-dimensional finite element model was used to describe mass and momentum transport as well as heat transfer. The shoulder moved the first brass to its new position after welding. Independent of where the rod was inserted, the brass was first observed in the area where the tool has passed at the advancing side close to the uppe rsurface. Most of the brass was seen in front of the position of the inserted rod, i.e. the brass had moved backwards. After the inserted rod, no brass was observed at the retreating side, but some was found at the advancing side. The pin moved the brass from the root of the weld upward towards the shoulder both on the advancing and retreating sides. The flow around the pin never appeared more than a few mm below the pin, or more than 6 and 8 mm outside the pin on the advancing and retreating sides, respectively. The model and the examined welds were compared showing a satisfactory agreement except for some of the vertical flow.

    In the presence of brass, a distinct nugget, often with onion rings, could be observed. The material velocity was highest near the weld tool, but at the advancing side there was a clear drop in velocity a few mm from the pin. This drop could cause void formation if the welding parameters are not chosen properly. The retreating side has a higher temperature and lowerhardness compared to the advancing side in this weld. One of the reasons for the higher temperature is that the component width is less at the retreating side than at the advancing side. Consequently generated heat is concentrated to a smaller area at the retreating side.

  • 50.
    Källgren, Therese
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Microstructure and temperature development in copper welded by the FSW-process2003In: 4th International Symposium on FSW, Park City, USA, April, 2003, 2003Conference paper (Refereed)
    Abstract [en]

    The use of Copper canisters with cast iron inserts is one of the proposed methods for long time deposition of nuclear fuel waste. The joining of the lid and possibly the base of the canister can be performed with Friction Stir Welding (FSW). A thorough understanding of the microstructure development in these welds is of major importance, since defects must be avoided. The microstructure and hardness profiles were investigated for two FSW conditions, at the beginning of the weld when there are essentially cold conditions, and when the steady state is reached. In cold welds the features of the nugget are very similar to that observed when FSW-joining aluminium. However, when the welding conditions reach the steady state, the nugget gets wider and the hardness is lower than in the cold weld.

    Electron Back Scatter Diffraction (EBSD) visualisation has been used to determine the grain size distribution of, twins and misorientation within grains. The orientation maps show a fine uniform equiaxed grain structure. There is no major misorientation within the grains in the nugget or at the root, but 25 mm from the weld centre the misorientation is large. This confirms that this area lies within the Thermal Mechanical Affected Zone (TMAZ) and is partially recrystallised.  The root has many more annealing twins than the nugget. This indicates that the nugget has experienced deformation after recrystallisation. A model has been used to correlate heat flow to the grain size and the hardness distributions.

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