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  • 1.
    Hosseini, Vahid
    University West, Department of Engineering Science, Division of Welding Technology.
    Influence of multiple welding cycles on microstructure and corrosion resistance of a super duplex stainless steel2016Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Super duplex stainless steel (SDSS) has found a wide use in demanding applications such as offshore, chemical and petrochemical industries thanks to its excellent combination of mechanical properties and corrosion resistance. Welding of SDSS, however, is associated with the risk of precipitation of secondary phases and formation of excessive amounts of ferrite in the weld metal and heat affected zone. The present study was therefore aimed at gaining knowledge about the effect of multiple welding thermal cycles on the microstructure and possible sensitization to corrosion of welds in SDSS.Controlled and repeatable thermal cycles were produced by robotic welding. Oneto four autogenous TIG-remelting passes were applied on 2507 type SDSS plates using low or high heat inputs with pure argon as shielding gas. Thermal cycles were recorded using several thermocouples attached to the plates. Thermodynamic calculations and temperature field modelling were performed in order to understand the microstructural development and to predict the pitting corrosion resistance. Etching revealed the formation of different zones with characteristic microstructures: the fused weld zone (WZ) and the heat affected zone composed of the fusion boundary zone (FBZ), next to the fusion boundary, and further out Zone 1 (Z1) and Zone 2 (Z2). The WZ had a high content of ferrite and often nitrides which increased with increasing number of passes and decreasing heati nput. Nitrogen content of the WZ decreased from 0.28 wt.% to 0.17 wt.% after four passes of low heat input and to 0.10 wt.% after four passes of high heatinput. The FBZ was reheated to high peak temperatures (near melting point) and contained equiaxed ferrite grains with austenite and nitrides. Zone 1 was free from precipitates and the ferrite content was similar to that of the unaffected base material. Sigma phase precipitated only in zone 2, which was heated to peak temperatures in the range of approximately 828°C to 1028°C. The content of sigma phase increased with the number of passes and increasing heat input. 

    All locations, except Z1, were susceptible to local corrosion after multiplere heating. Thermodynamic calculations predicted that a post weld heat treatment could restore the corrosion resistance of the FBZ and Z2. However, the pitting resistance of the WZ cannot be improved significantly due to the nitrogen loss. Steady state and linear fitting approaches were therefore employed to predict nitrogen loss in autogenous TIG welding with argon as shielding gas. Two practical formulas were derived giving nitrogen loss as functions of initial nitrogen content and arc energy both predicting a larger loss for higher heat input and higher base material nitrogen content. A practical recommendation based on the present study is that it is beneficial to perform welding with a minimum number of passes even if this results in a higherheat input as multiple reheating strongly promotes formation of deleterious phases.

  • 2.
    Hosseini, Vahid
    University West, Department of Engineering Science, Division of Welding Technology.
    Super duplex stainless steels: Microstructure and propertiesof physically simulated base and weld metal2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    High-temperature processing and application of super duplex stainless steel(SDSS) are associated with the risk of changes in the ferrite/austenite balance and precipitation of secondary phases. This study was therefore aimed at improving knowledge about effects of thermal cycles on the microstructure and properties of SDSS base and weld metal. Controlled and repeatable thermal cycles were physically simulated using the innovative multiple TIG reheating/remelting and the arc heat treatment techniques. In the first technique, one to four autogenous TIG-remelting passes were applied. During arc heat treatment, a stationary arc was applied on a disc mounted on a water-cooled chamber thereby subjecting the material to a steady state temperature gradient from 0.5 minute to 600 minutes. Microstructures and properties were assessed and linked to thermal history through thermal cycle analysis, thermodynamic calculations and temperature field modelling, Remelting studies showed that nitrogen loss from the melt pool was a function of arc energy and initial nitrogen content and could cause highly ferritic microstructures. Heat affected zones were sensitized by nitride formation next to the fusion boundary and sigma phase precipitation in regions subjected to peak temperatures of 828-1028°C. Accumulated time in the critical temperature range, peak temperature and the number of thermal cycles are the most relevant criteria when evaluating the risk of sigma phase precipitation. Arc heat treatment produced graded microstructures in SDSS base and weld metal with the formation of a ferritic region at high temperature due to solid-state nitrogen loss, precipitation of sigma, chi, nitrides, and R-phase with different morphologies at 550-1010°C and spinodal decomposition below 500°C. This caused sensitization and/or increased hardness and embrittlement. Results were summarized as time-temperature-precipitation and properties diagrams for base and weld metal together with guidelines for processing and welding of SDSS.

  • 3.
    Hosseini, Vahid
    University West, Department of Engineering Science, Division of Welding Technology.
    Super duplex stainless steels Microstructure and propertiesof physically simulated base and weld metal2019In: Stainless Steel World, ISSN 1383-7184, no December, p. 8-9Article in journal (Other academic)
    Abstract [en]

    High-temperature processing and application of super duplex stainless steel(SDSS) are associated with the risk of changes in the ferrite/austenite balance and precipitation of secondary phases. This study was therefore aimed at improving knowledge about effects of thermal cycles on the microstructure and properties of SDSS base and weld metal. Controlled and repeatable thermal cycles were physically simulated using the innovative multiple TIG reheating/remelting and the arc heat treatment techniques. In the first technique, one to four autogenous TIG-remelting passes were applied. During arc heat treatment, a stationary arc was applied on a disc mounted on a water-cooled chamber thereby subjecting the material to a steady state temperature gradient from 0.5 minute to 600 minutes. Microstructures and properties were assessed and linked to thermal history through thermal cycle analysis, thermodynamic calculations and temperature field modelling, Remelting studies showed that nitrogen loss from the melt pool was a function of arc energy and initial nitrogen content and could cause highly ferritic microstructures. Heat affected zones were sensitized by nitride formation next to the fusion boundary and sigma phase precipitation in regions subjected to peak temperatures of 828-1028°C. Accumulated time in the critical temperature range, peak temperature and the number of thermal cycles are the most relevant criteria when evaluating the risk of sigma phase precipitation. Arc heat treatment produced graded microstructures in SDSS base and weld metal with the formation of a ferritic region at high temperature due to solid-state nitrogen loss, precipitation of sigma, chi, nitrides, and R-phase with different morphologies at 550-1010°C and spinodal decomposition below 500°C. This caused sensitization and/or increased hardness and embrittlement. Results were summarized as time-temperature-precipitation and properties diagrams for base and weld metal together with guidelines for processing and welding of SDSS.

  • 4.
    Hosseini, Vahid A.
    et al.
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Aashuri, H.
    Materials Science and Engineering Dep., Sharif University of Technology, Tehran, Iran.
    Kokabi, A. H.
    Materials Science and Engineering Dep., Sharif University of Technology, Tehran, Iran.
    Study of the effect of tool geometry on semisolid stir welding of a AZ91 magnesium alloy2015In: Proceedings of the 18th International Conference on Joining Materials, JOM-Institute , 2015, p. 1-10Conference paper (Refereed)
    Abstract [en]

    Semisolid stir welding is a newly developed method suitable for joining of the magnesium alloy AZ91. In this study, the effect of tool geometries on the joint properties such as bending strength and the occurrence of porosity are studied. A 2 mm-thick Mg-25%Zn interlayer was placed between two AZ91 plates and the plate was heated up to 530°C before joining. At this temperature, when both the interlayer and the base metal were in the semisolid state, a stirrer was introduced into the joint. Drill-tip and round shape stirrer tools were employed at three different stirring rates. Welds produced with the two methods showed similar properties in the shear punch test. However, using the round tool geometry resulted in welds with excellent bending strength closely matching that of the base metal especially at the highest stirring rate. The improved properties when using the round tool was a result of the formation of a very fine and uniform microstructure with a low content of porosity.

  • 5.
    Hosseini, Vahid A.
    et al.
    University West, Department of Engineering Science, Division of Manufacturing Processes. Innovatum AB, Trollhättan, Sweden.
    Valiente Bermejo, María Asunción
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Gårdstam, Johannes
    Swerea KIMAB AB, Kista, Sweden.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Influence of multiple thermal cycles on microstructure of heat-affected zone in TIG-welded super duplex stainless steel2016In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 60, no 2, p. 233-245Article in journal (Refereed)
    Abstract [en]

    The influence of heat input and multiple welding cycles on the microstructure of the heat-affected zone in autogenously TIG-welded 6 mm 2507 type super duplex stainless steel plates was investigated. In order to produce multiple thermal cycles, one to four pass bead-on-plate welds were made with arc energies of 0.47 and 1.08 kJ/mm, corresponding to heat inputs of 0.37 and 0.87 kJ/mm. Several thermocouples were attached to record thermal cycles on the front and back sides of the plates. Finite element modelling was successfully done to map and correlate measured and calculated peak temperatures. Only minor changes were seen in the ferrite content at 1 and 2 mm from the fusion boundary. Nitrides formed in all passes of the low heat input samples in a region next to the fusion boundary, but only after the third and fourth passes of the high heat input samples. Sigma phase precipitated only in a zone heated to a peak temperature in the range of approximately 828 to 1028 °C. Multiple reheating was found to promote precipitation of sigma phase relatively more than slower cooling. A precipitation free zone was observed between the nitride and sigma phase bands. The precipitation behaviour could be understood from equilibrium phase diagrams, evaluation of local thermal cycles and by correlating results from the modelling and measurements of peak temperatures. It is suggested that the peak temperature, the accumulated time in the critical temperature range between approximately 828 and 1028 °C, and the number of thermal cycles are the most relevant criteria when evaluating the risk of sigma phase formation.

  • 6.
    Hosseini, Vahid A.
    et al.
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Wessman, Sten
    University West, Department of Engineering Science, Division of Manufacturing Processes. Swerea KIMAB AB, Kista, Sweden.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Nitrogen loss and effects on microstructure in multipass TIG welding of a super duplex stainless steel2016In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 98, no May, p. 88-97Article in journal (Refereed)
    Abstract [en]

    Nitrogen loss is an important phenomenon in welding of super duplex stainless steels. In this study, a super duplex stainless steel was autogenously TIG-welded with one to four bead-on-plate passes with low or high heat inputs using pure argon shielding gas. The goal was to monitor nitrogen content and microstructure for each weld pass. Nitrogen content, measured by wavelength dispersive X-ray spectrometry, was after four passes reduced from 0.28 wt% in the base metal to 0.17 wt% and 0.10 wt% in low and high heat input samples, respectively. Nitrogen loss resulted in a more ferritic structure with larger grains and nitride precipitates. The ferrite grain width markedly increased with increasing number of passes and heat input. Ferrite content increased from 55% in base metal to 75% at low and 79% at high heat inputs after four passes. An increasing amount of nitrides were seen with increasing number of weld passes. An equation was suggested for calculation of the final nitrogen content of the weld metal as functions of initial nitrogen content and arc energy. Acceptable ferrite contents were seen for one or two passes. The recommendation is to use nitrogen in shielding gas and proper filler metals.

  • 7.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology. Innovatum AB, Trollhättan, Sweden.
    Aashuri, H.
    Sharif Univ Technol, Mat Sci & Engn Dept, Azadi Ave, Tehran, Iran.
    Kokabi, A. H.
    Sharif Univ Technol, Mat Sci & Engn Dept, Azadi Ave, Tehran, Iran.
    Effect of welding parameters on semisolid stir welding of Mg-9Al-1Zn magnesium alloy2016In: Transactions of Nonferrous Metals Society of China, ISSN 1003-6326, E-ISSN 2210-3384, Vol. 26, no 10, p. 2586-2594Article in journal (Refereed)
    Abstract [en]

    Semisolid stir welding of AZ91 was investigated with focus on the joining temperature and rotational speed. An Mg-25% Zn interlayer was located between two AZ91 pieces and the system was heated up to the semisolid state of base metal and interlayer. The weld seam was stirred using a drill-tip at different joining temperatures and rotational speeds. Optical and scanning electron microscopes were employed to study microstructure, cavity formation, and segregation. Hardness profile and shear punch test were also employed to rank the welds based on their quality and homogeneity. Results showed that the lowest cavity content (2.1%) with the maximum ultimate shear strength (about 188 MPa) was obtained in weld with the joining temperature of 530 degrees C and the rotational speed of 1600 r/min. Low quality welds and a reduction of ultimate shear strength were observed at very high or low rotational speeds and joining temperatures. The process, in conclusion, produced close mechanical properties to those of the base metal and homogenous quality throughout the joint, when the intermediate temperature and rotational speeds were employed.

  • 8.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology. Innovatum AB, Sharif University of Technology, Tohoku University, Chalmers University of Technology.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Welding Technology.
    Eyzop, Daniel
    Outokumpu Stainless AB, Avesta Research Centre, Avesta, Sweden.
    Östberg, Agneta
    Sandvik Materials Technology, Sandviken, Sweden.
    Janiak, Paul
    Swerea KIMAB AB, Kista, Sweden.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Ferrite content measurement in super duplex stainless steel welds2019In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 63, no 2, p. 551-563Article in journal (Refereed)
    Abstract [en]

    Approaches to determining ferrite fraction (%) and ferrite number (FN) were examined for super duplex stainless steel (SDSS) welds. A reference sample was produced by bead-on-plate gas–tungsten arc welding of a type-2507 SDSS plate. By comparing different etchants and measurement practices, it was realized that etching with modified Beraha followed by computerized image analysis (IA) was the most accurate and quickest technique to measure ferrite fraction, which determined the same ferrite fraction (68.0 ± 2.6%) as that measured by electron diffraction backscattered analysis (67.6 ± 2.3%). A Round Robin test was performed on a reference sample at University West, Swerea KIMAB, Outokumpu Stainless, and Sandvik Materials Technology to investigate the repeatability of the technique. The ferrite fraction measurements performed at different laboratories showed very small variations, which were in the range of those seen when changing microscope in the same laboratory. After verification of the technique, the relationship between ferrite fraction and ferrite number (measured with FERITSCOPE®) was determined using 14 single (root) pass welds, including butt, corner, and T-, V-, and double V-joint geometries. The best-fit equation found in this study was ferrite number (FN) = 1.1 × ferrite fraction (%). To conclude, the ferrite fraction technique suggested in the present paper was accurate and repeatable, which made it possible to determine a ferrite fraction–ferrite number formula for SDSS single-pass welds.

  • 9.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Bead by bead study of a multipass shielded metal arc-welded super-duplex stainless steel2020In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669Article in journal (Refereed)
    Abstract [en]

    The present study aims at investigating bead geometry and the evolution of microstructure with thermal cycles in multipass shielded metal arc welding of a V-groove 13-mm type-2507 super-duplex stainless steel (SDSS) plate. The weld consisted of 4 beads produced with arc energies of 0.81-1.06 kJ/mm. The upper beads showed lower base metal (BM) dilution than the first bead. Thermal cycles were recorded with thermocouples, indicating that the cooling rate decreased in the as-deposited weld zone when adding a new bead. Ferrite fraction in the as-welded condition was lower for the upper beads. The austenite grain morphology in reheated passes varied depending on the local peak temperatures and the number of reheating passes. Sigma phase precipitated in a location reheated by the third and fourth passes that was subjected to a critical peak temperature for sigma precipitation. Ferrite content, measured using image analysis and Fisher FERITSCOPE technique, showed that the ferrite fraction moved toward 50/50% in the weld metal with an increasing number of reheating cycles. Finally, a schematic map showing an overview of the microstructure in the multipass SDSS weld was introduced.

  • 10.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology. Hogskolan Vast.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Effect of multipass TIG welding on the corrosion resistance and microstructure of a super duplex stainless steel2017In: Materials and corrosion - Werkstoffe und Korrosion, ISSN 0947-5117, E-ISSN 1521-4176, Vol. 68, no 4, p. 405-415Article in journal (Refereed)
    Abstract [en]

    This is a study of the effect of repetitive TIG (tungsten inert gas) welding passes, melting and remelting the same material volume on microstructure and corrosion resistance of 2507 (EN 1.4410) super duplex stainless steel. One to four weld passes were autogenously (no filler added) applied on a plate using two different arc energies and with pure argon shielding gas. Sensitization testing showed that multipass remelting resulted in significant loss of corrosion resistance of the weld metal, in base material next to the fusion boundary, and in a zone 1 to 4 mm from the fusion boundary. Metallography revealed the main reasons for sensitization to be a ferrite-rich weld metal and precipitation of nitrides in the weld metal, and adjacent heat affected zone together with sigma phase formation at some distance from the fusion boundary. Corrosion properties cannot be significantly restored by a post weld heat treatment. Using filler metals with higher nickel contents and nitrogen containing shielding gases, are therefore, recommended. Welding with a higher heat input and fewer passes, in some cases, can also decrease the risk of formation of secondary phases and possible corrosion attack.

  • 11.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Multipass Autogenous TIG Welding of Super Duplex Stainless2015In: 16th national conference of welding and inspection, Yazd, Iran: Proceedings, Yazd, 2015Conference paper (Refereed)
    Abstract [en]

    Multipass welding of super duplex stainless steels (SDSS) needs further characterization due to their growing applications inpetrochemical and offshore industries. This study, as a result, is aimed at investigating the effects of the number of passesand the arc energy on the microstructure and properties of 2507-type SDSS (UNS S32750). From one to four TIG weldpasses were autogenously applied on a plate using two different arc energies and with pure argon gas as the shielding gas.Chemical analysis showed increasing nitrogen loss with an increasing number of passes and increasing arc energy.Microstructural analyses revealed formation of nitrides in the weld metal and heat affected zone, and sigma phase at somedistance from the fusion boundary. Thermal cycle analysis in combination with Thermo-Calc calculations indicated thatexcessive reheating cause degradation of corrosion properties of multipass weldments, by reducing the pitting resistanceequivalent number of austenite to less than 40. Multipass welding resulted in a more ferritic weld metal microstructure and anincreased hardness.Recommendations, based on the present study, are as follows: 1) Corrosion attack can occur not only in the weld zone andnext to the fusion boundary, but also in a location at some distance from the fusion zone due to reheating in the sigma phaseformation temperature range. This should be considered in inspection procedures 2) Nitrogen loss degrades the mechanicaland corrosion properties of weldments even when welding with a low heat input. Using filler metals with higher nickelcontents and nitrogen containing shielding gases are therefore recommended. 3) It is often recommended to use a heat inputin the lower end of the recommended 0.3-1.5 kJ/mm range in multipass welding of super duplex stainless steels. However,welding with a higher heat input and fewer passes, in some cases, can decrease the risk of formation of secondary phases.

  • 12.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology.
    Högström, Mats
    University West, Department of Engineering Science, Division of Welding Technology.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Welding Technology.
    Valiente Bermejo, María Asunción
    University West, Department of Engineering Science, Division of Welding Technology.
    Stridh, Lars-Erik
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Wire-arc additive manufacturing of a duplex stainless steel: thermal cycle analysis and microstructure characterization2019In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 63, no 4, p. 975-987Article in journal (Refereed)
    Abstract [en]

    The evolution of microstructures with thermal cycles was studied for wire-arc additive manufacturing of duplex stainless steel blocks. To produce samples, arc energy of 0.5kJ/mm and interlayer temperature of 150 degrees C were used as low heat input-low interlayer temperature (LHLT) and arc energy of 0.8kJ/mm and interlayer temperature of 250 degrees C as high heat input-high interlayer temperature (HHHT). Thermal cycles were recorded with different thermocouples attached to the substrate as well as the built layers. The microstructure was analyzed using optical and scanning electron microscopy. The results showed that a similar geometry was produced with 14 layers4 beads in each layerfor LHLT and 15 layers3 beads in each layerfor HHHT. Although the number of reheating cycles was higher for LHLT, each layer was reheated for a shorter time at temperatures above 600 degrees C, compared with HHHT. A higher austenite fraction (+8%) was achieved for as-deposited LHLT beads, which experienced faster cooling between 1200 and 800 degrees C. The austenite fraction of the bulk of additively manufactured samples, reheated several times, was quite similar for LHLT and HHHT samples. A higher fraction of secondary phases was found in the HHHT sample due to longer reheating at a high temperature. In conclusion, an acceptable austenite fraction with a low fraction of secondary phases was obtained in the bulk of wire-arc additively manufactured duplex stainless steel samples (35-60%), where higher austenite fractions formed with a larger number of reheating cycles as well as longer reheating at high peak temperatures (800-1200 degrees C).

  • 13.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Physical and kinetic simulation of nitrogen loss in high temperature heat affected zone of duplex stainless steels2019In: Materialia, ISSN 2589-1529, Vol. 6, article id 100325Article in journal (Refereed)
    Abstract [en]

    High temperature heat affected zone (HTHAZ) of duplex stainless steels is prone to local corrosion attack due to a high ferrite fraction and nitride formation. Literature commonly attributes formation of this undesirable microstructure to rapid cooling from high peak temperatures. However, this study investigated the possible role of nitrogen loss in HTHAZ using a combination of physical and kinetics simulation. Applying a stationary gas-tungsten arc (GTA) on a water-cooled plate, a technique known as arc heat treatment, showed that considerable nitrogen loss occurred already after 0.5 min up to 150 µm from the fusion boundary. This zone was extended to 1300 µm after 600 min arc heat treatment. The results of bead-on-plate GTA welding and Gleeble testing replicating the thermal cycle in HTHAZ showed that the ferrite fraction of the real HTHAZ was 7% higher than that for Gleeble samples. This agrees with results from arc heat treatment, where ferrite fraction was found to increase due to nitrogen loss. Numerical and Dictra approaches were developed to simulate the kinetics of nitrogen loss in HTHAZ considering ferrite as the nitrogen rapid diffusion path towards the weld pool. Simulation showed good agreement with both welding and physical simulation. A combination of thermodynamic and kinetics simulations properly predicted the ferrite fraction at 1100 °C for different arc heat treatment times. In conclusion, the experiments (physical simulations and GTA welding) and kinetics simulation showed that nitrogen was lost from HTHAZ to the weld pool. © 2019 Acta Materialia Inc.

  • 14.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology. Innovatum AB., Trollhättan, Trollhättan, Sweden.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Engelberg, D.
    University of Manchester, School of Materials, M13 9PL, Manchester, United Kingdom.
    Wessman, Sten
    University West, Department of Engineering Science, Division of Welding Technology.
    Correction to: Time-temperature-precipitation and property diagrams for super duplex stainless steel weld metals (Welding in the World, (2018), 62, 3, (517-533), 10.1007/s40194-018-0548-z)2018In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 62, no 4, p. 893-Article in journal (Refereed)
    Abstract [en]

    Unfortunately due to typesetting mistakes, Tables 4-€“6 have been displayed erroneously in the article. © 2018, International Institute of Welding.

  • 15.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology. Innovatum AB.,Trollhättan, Trollhättan, Sweden.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Engelberg, Dirk
    The University of Manchester, School of Materials,Manchester,UK.
    Wessman, Sten
    University West, Department of Engineering Science, Division of Welding Technology.
    Time-temperature-precipitation and property diagrams for super duplex stainless steel weld metals2018In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 62, no 3, p. 517-533Article in journal (Refereed)
    Abstract [en]

    Super duplex stainless steel (SDSS) weld metal microstructures, covering the complete temperature range from ambient to liquidus, were produced by arc heat treatment for 1 and 10 min. Temperature modeling and thermodynamic calculations complemented microstructural studies, hardness mapping and sensitization testing. After 1 min, intermetallics such as sigma and chi phase had precipitated, resulting in moderate sensitization at 720–840 °C. After 10 min, larger amounts of intermetallics resulted in hardness up to 400 HV0.5 and more severe sensitization at 580–920 °C. Coarse and fine secondary austenite precipitated at high and low temperatures, respectively: The finer secondary austenite was more detrimental to corrosion resistance due to its lower content of Cr, Mo, and N as predicted by thermodynamic calculations. Increased hardness and etching response suggest that 475 °C embrittlement had occurred after 10 min. Results are summarized as time-temperature-precipitation and property diagrams for hardness and sensitization.

  • 16.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Welding Technology.
    Choquet, Isabelle
    University West, Department of Engineering Science, Division of Welding Technology.
    Engelberg, Dirk
    The University of Manchester, School of Materials, Manchester M13 9PL, UK.
    Roy, Matthew J.
    The University of Manchester, School of Mechanical, Aerospace and Civil Engineering,Manchester M13 9PL, UK.
    Kumara, Chamara
    University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing.
    A novel arc heat treatment technique for producing graded microstructures through controlled temperature gradients2017In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 121, no May, p. 11-23Article in journal (Refereed)
    Abstract [en]

    This paper introduces a novel arc heat treatment technique to produce samples with graded microstructures through the application of controlled temperature gradients. Steady state temperature distributions within the sample can be achieved and maintained, for times ranging from a few seconds to several hours. The technique reduces the number of samples needed to characterize the response of a material to thermal treatments, and can consequently be used as a physical simulator for materials processing. The technique is suitable for conventional heat treatment analogues, welding simulations, multi-step heat treatments, and heat treatments with controlled heating and cooling rates. To demonstrate this technique, a super duplex stainless steel was treated with a stationary TIG arc, to confirm the relationship between generated steady-state temperature fields, microstructure development, hardness, and sensitization to corrosion. Metallographic imaging and hardness mapping provided information about graded microstructures, confirming the formation of secondary phases and microstructure sensitization in the temperature range 850–950 °C. Modelling of temperature distributions and thermodynamic calculations of phase stabilities were used to simulate microstructure development and associated welding cycles.

  • 17.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Wessman, Sten
    University West, Department of Engineering Science, Division of Welding Technology. Swerea KIMAB AB, P.O. Box 7047, Kista, Sweden.
    Fuertes, Nuria
    Swerea KIMAB AB, P.O. Box 7047, Kista, Sweden.
    Effect of sigma phase morphology on the degradation of properties in a super duplex stainless steel2018In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 11, no 6, article id 933Article in journal (Refereed)
    Abstract [en]

    Sigma phase is commonly considered to be the most deleterious secondary phase precipitating in duplex stainless steels, as it results in an extreme reduction of corrosion resistance and toughness. Previous studies have mainly focused on the kinetics of sigma phase precipitation and influences on properties and only a few works have studied the morphology of sigma phase and its influences on material properties. Therefore, the influence of sigma phase morphology on the degradation of corrosion resistance and mechanical properties of 2507 super duplex stainless steel (SDSS) was studied after 10 h of arc heat treatment using optical and scanning electron microscopy, electron backscattered diffraction analysis, corrosion testing, and thermodynamic calculations. A stationary arc was applied on the 2507 SDSS disc mounted on a water-cooled chamber, producing a steady-state temperature gradient covering the entire temperature range from room temperature to the melting point. Sigma phase was the major intermetallic precipitating between 630 °C and 1010 °C and its morphology changed from blocky to fine coral-shaped with decreasing aging temperature. At the same time, the average thickness of the precipitates decreased from 2.9 Όm to 0.5 Όm. The chemical composition of sigma was similar to that predicted by thermodynamic calculations when formed at 800-900 °C, but deviated at higher and lower temperatures. The formation of blocky sigma phase introduced local strain in the bulk of the primary austenite grains. However, the local strain was most pronounced in the secondary austenite grains next to the coral-shaped sigma phase precipitating at lower temperatures. Microstructures with blocky and coral-shaped sigma phase particles were prone to develop microscale cracks and local corrosion, respectively. Local corrosion occurred primarily in ferrite and in secondary austenite, which was predicted by thermodynamic calculations to have a low pitting resistance equivalent. To conclude, the influence of sigma phase morphology on the degradation of properties was summarized in two diagrams as functions of the level of static load and the severity of the corrosive environment. © 2018 by the authors.

  • 18.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology. Innovatum AB., Trollhättan, Trollhättan, Sweden.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Örnek, Cem
    KTH Royal Institute of Technology, Department of Chemical Science and Engineering, Division of Surface and Corrosion Science, Stockholm, Sweden, Department of Corrosion in Energy and Processing Industry, Swerea KIMAB AB, P.O. Box 7047, Kista, Sweden.
    Reccagni, Pierfranco
    The University of Manchester, School of Materials, Manchester, United Kingdom.
    Wessman, Sten
    University West, Department of Engineering Science, Division of Welding Technology.
    Engelberg, Dirk
    The University of Manchester, School of Materials, Manchester, United Kingdom.
    Microstructure and functionality of a uniquely graded super duplex stainless steel designed by a novel arc heat treatment method2018In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 139, p. 390-400Article in journal (Refereed)
    Abstract [en]

    A novel arc heat treatment technique was applied to design a uniquely graded super duplex stainless steel (SDSS), by subjecting a single sample to a steady state temperature gradient for 10 h. A new experimental approach was used to map precipitation in microstructure, covering aging temperatures of up to 1430 °C. The microstructure was characterized and functionality was evaluated via hardness mapping. Nitrogen depletion adjacent to the fusion boundary depressed the upper temperature limit for austenite formation and influenced the phase balance above 980 °C. Austenite/ferrite boundaries deviating from Kurdjumov–Sachs orientation relationship (OR) were preferred locations for precipitation of σ at 630–1000 °C, χ at 560–1000 °C, Cr2N at 600–900 °C and R between 550 °C and 700 °C. Precipitate morphology changed with decreasing temperature; from blocky to coral-shaped for σ, from discrete blocky to elongated particles for χ, and from polygonal to disc-shaped for R. Thermodynamic calculations of phase equilibria largely agreed with observations above 750 °C when considering nitrogen loss. Formation of intermetallic phases and 475 °C-embrittlement resulted in increased hardness. A schematic diagram, correlating information about phase contents, morphologies and hardness, as a function of exposure temperature, is introduced for evaluation of functionality of microstructures. © 2018 The Authors

  • 19.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Manufacturing Processes.
    Shabestari, S.G.
    Iran University of Science and Technology (IUST), Center of Excellence for High Strength Alloys Technology (CEHSAT), School of Metallurgy and Materials Engineering, Narmak, Tehran, Iran.
    Study on the eutectic and post-eutectic reactions in LM13 aluminum alloy using cooling curve thermal analysis technique2016In: Journal of thermal analysis and calorimetry (Print), ISSN 1388-6150, E-ISSN 1588-2926, Vol. 124, no 2, p. 611-617Article in journal (Refereed)
    Abstract [en]

    Effect of non-equilibrium solidification conditions on the eutectic and post-eutectic reactions temperature and percentage of the phases were investigated using computer-aided cooling curve thermal analysis. In addition, hardness, secondary dendrite arm spacing, and maximum pore size were studied at different cooling conditions. Cooling curves were determined by setting thermocouples in the center of the molds. Solid fractions were calculated by Newtonian baseline technique. Results showed that increasing the cooling rate shifted the temperature of post-eutectic reaction upward, except final reaction. Higher cooling rate increased eutectic percentage about 4 %, but reduced total percentage of post-eutectic phases. Additionally, increasing the cooling rate shortened the maximum porosity diameter and secondary dendrite arm spacing and increased the hardness of the alloy. © 2015 Akadémiai Kiadó, Budapest, Hungary

  • 20.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology. Innovatum AB Trollhättan,Trollhättan,Sweden.
    Thuvander, Mattias
    Chalmers University of Technology, Department of Physics, Gothenburg,Sweden.
    Wessman, Sten
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Spinodal Decomposition in Functionally Graded Super Duplex Stainless Steel and Weld Metal2018In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 49A, no 7, p. 2803-2816Article in journal (Refereed)
    Abstract [en]

    Low-temperature phase separations (T < 500 °C), resulting in changes in mechanical and corrosion properties, of super duplex stainless steel (SDSS) base and weld metals were investigated for short heat treatment times (0.5 to 600 minutes). A novel heat treatment technique, where a stationary arc produces a steady state temperature gradient for selected times, was employed to fabricate functionally graded materials. Three different initial material conditions including 2507 SDSS, remelted 2507 SDSS, and 2509 SDSS weld metal were investigated. Selective etching of ferrite significantly decreased in regions heat treated at 435 °C to 480 °C already after 3 minutes due to rapid phase separations. Atom probe tomography results revealed spinodal decomposition of ferrite and precipitation of Cu particles. Microhardness mapping showed that as-welded microstructure and/or higher Ni content accelerated decomposition. The arc heat treatment technique combined with microhardness mapping and electrolytical etching was found to be a successful approach to evaluate kinetics of low-temperature phase separations in SDSS, particularly at its earlier stages. A time-temperature transformation diagram was proposed showing the kinetics of 475 °C-embrittlement in 2507 SDSS.

  • 21.
    Hosseini, Vahid
    et al.
    University West, Department of Engineering Science, Division of Welding Technology.
    Wessman, Sten
    University West, Department of Engineering Science, Division of Welding Technology.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Research Enviroment Production Technology West. University West, Department of Engineering Science, Division of Welding Technology.
    Predicting ferrite fractions in single pass super duplex stainless steel welds: thermal cycle analysis and phase transformation modeling2019In: ESSC and DUPLEX 2019: 10th European Stainless Steel Conference - Science and Market, 6th European Duplex Stainless Steel Conference and Exhibition, Wien: Austrian Society for Metallurgy and Materials (ASMET) , 2019, p. 180-197Conference paper (Refereed)
    Abstract [en]

    The relationship between welding process parameters, welding thermal cycle, and the final microstructure is of great importance for reliable fabrication of welded super duplex stainless steels (SDSS) structures. The present study was primarily aimed at investigating the relationship for root/single pass welding of type 2507 SDSS. Fourteen welds were produced using GMAW, GTAW, SAW, and SMAW with different joints geometries, plate thicknesses, and welding parameters. Thermal cycles were recorded using several thermocouples attached to the plates and thermocouples were also harpooned into the weld pool. Weld pool geometries and base metal dilution in the weld metal were determined for all welds. The general trend was that the ferrite fraction of the weld zone increased with increasing cooling rate and base metal dilution in the weld metal. The ferrite fraction was in the range 49-64% for all welds. Kinetics of austenite formation was modeled using computational thermodynamics (Thermo-Calc & DICTRA) to predict the ferrite fractions in the weld zone and calculated fractions were in good agreement with experimental results. Some conflicting results showed that in addition to dilution and cooling rate, the possible nitrogen loss must be taken into account when evaluating and predicting ferrite fraction. It was concluded that the above approach can be used for prediction of the ferrite fraction of super duplex stainless steel single pass welds. Â 2019 ESSC and DUPLEX 2019 - 10th European Stainless Steel Conference - Science and Market, 6th European Duplex Stainless Steel Conference and Exhibition. All rights reserved.

  • 22.
    Valiente Bermejo, María Asunción
    et al.
    University West, Department of Engineering Science, Division of Welding Technology.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Welding Technology.
    Hosseini, Vahid
    University West, Department of Engineering Science, Division of Welding Technology.
    Karlsson, Leif
    University West, Department of Engineering Science, Division of Welding Technology.
    Svensson, Lars-Erik
    University West, Department of Engineering Science, Division of Welding Technology.
    Monitoring Thermal Cycles in Multi-pass Welding2016In: The 7th International Swedish Production Symposium, SPS16, Conference Proceedings: 25th – 27th of October 2016, Swedish Production Academy , 2016, p. 1-5Conference paper (Refereed)
    Abstract [sv]

    Differently from any previous investigation in welding, this research work presents a novel development that allows temperature to be measured and recorded simultaneously with up to 32 thermocouples indifferent locations of a welding joint. Four experiments were designed to optimise the measurement technique by comparing the performance of three types of thermocouples (K, N, C) insulated with different materials and varying the insertion technique of the thermocouples in the joint. Results showed that type-K thermocouple had the best performance and proved that glass fibre insulation provided better protection than Inconel. The optimised measurement procedure developed in this work enables to monitor the thermal cycles in multi-pass welds. That information is essential in multi-pass welding of materials such as super duplex stainless steels, carbon steels or nickel alloys, as heating them repeatedly makes them susceptible to the formation of brittle phases and in turn it influences their mechanical and corrosion properties. This technique could be really important for future applications such as temperature modellingor prediction of mechanical properties and microstructure in relation to the thermal cycle experienced by alloys susceptible to the formation of undesirable phases.

1 - 22 of 22
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