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Valiente Bermejo, M. A., Hurtig, K., Eyzop, D. & Karlsson, L. (2019). A New Approach to the Study of Multi-Pass Welds–Microstructure and Properties of Welded 20-mm-Thick Superduplex Stainless Steel. Applied Sciences, 9(6), Article ID 1050.
Open this publication in new window or tab >>A New Approach to the Study of Multi-Pass Welds–Microstructure and Properties of Welded 20-mm-Thick Superduplex Stainless Steel
2019 (English)In: Applied Sciences, ISSN 2076-3417, Vol. 9, no 6, article id 1050Article in journal (Refereed) Published
Abstract [en]

Type 2507 superduplex stainless steel 20 mm in thickness was multi-pass-welded with Gas Metal Arc Welding (GMAW) and Flux-Cored Arc Welding (FCAW) processes. Recommended and higher arc energies and inter-pass temperatures were used. Thermal cycles were monitored using a recently developed procedure involving the successive instrumentation of the multi-pass welds, pass by pass, by addition of thermocouples in each weld pass. The repeatability of temperature measurements and survival rate of more than 90% of thermocouples confirmed the reliability of the procedure. Reheating by subsequent passes caused a progressive increase in the austenite content of the weld metal. The as-deposited GMAW passes with higher-than-recommended arc energy showed the lowest presence of nitrides. Therefore, the cooling rate—and not the time exposed at the critical temperature range—seems to be the key factor for nitride formation. The welding sequence layout also plays an important role in the distribution of secondary phases. A larger amount and concentration of secondary austenite and σ-phase was found for a larger number of subsequent passes in the immediate vicinity of a specific weld pass. The impact toughness exceeded requirements for all welds. Differences in absorbed energies were related to the amount of micro-inclusions found with the FCAW weld showing the lowest absorbed energies and highest amount of micro-inclusions. Pitting corrosion preferentially initiated in locations with secondary austenite and σ-phase. However, in the absence of these secondary phases, the HAZ containing nitrides was the weakest location where pitting initiated. The results of this work have implications on practical welding for superduplex stainless steels: the current recommendations on maximum arc energy should be revised for large thickness weldments, and the importance of the welding sequence layout on the formation of secondary phases should be considered.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
multi-pass welding, nitrides, secondary austenite, superduplex stainless steel, thermal cycles, welding, sigma-phase
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13730 (URN)10.3390/app9061050 (DOI)000465017200015 ()
Funder
Knowledge Foundation, 20140046
Available from: 2019-03-20 Created: 2019-03-20 Last updated: 2019-05-16
Hosseini, V., Hurtig, K., Eyzop, D., Östberg, A., Janiak, P. & Karlsson, L. (2019). Ferrite content measurement in super duplex stainless steel welds. Welding in the World, 63(2), 551-563
Open this publication in new window or tab >>Ferrite content measurement in super duplex stainless steel welds
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2019 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 63, no 2, p. 551-563Article in journal (Refereed) Published
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.

Keywords
Ferrite fraction, Ferrite number, Image analysis, Round Robin, Super duplex stainless stee, l Point counting
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13198 (URN)10.1007/s40194-018-00681-1 (DOI)000462318600028 ()2-s2.0-85063276679 (Scopus ID)
Funder
Vinnova, 2016-02834Knowledge Foundation, 20140130
Note

First Online: 05 December 2018

Available from: 2018-12-19 Created: 2018-12-19 Last updated: 2019-06-11Bibliographically approved
Fahlström, K., Blackburn, J., Karlsson, L. & Svensson, L.-E. (2019). Low Porosity in Cast Magnesium Welds by Advanced Laser Twin-Spot Welding. Materials Sciences and Applications, 10(1), 53-64
Open this publication in new window or tab >>Low Porosity in Cast Magnesium Welds by Advanced Laser Twin-Spot Welding
2019 (English)In: Materials Sciences and Applications, ISSN 2153-117X, E-ISSN 2153-1188, Vol. 10, no 1, p. 53-64Article in journal (Refereed) Published
Abstract [en]

Porosity is reported to be a major issue when welding cast magnesium. Therefore, it is important to understand the pore formation mechanisms and find procedures that could be used to reduce porosity. This study investigated the possibility of using twin-spot optics for reducing the porosity in laser welded cast magnesium. Two twin-spot welding setups were compared using either a beam splitter or twin-spot welding with primary and secondary (placed in front of the primary optic) optics. The results showed that welding with a dual optic setup with a defocused secondary beam reduced the volumetric porosity in the weld to 5%. The highest levels of volumetric porosity were 30%, and were a result of using the dual optic setup, but with a defocused primary beam. No clear relation between the level of porosity and power or welding speed was found. It was found that the amount of porosity depended on the balance of the energy input (controlled by defocusing) between the two beams. Porosity formation can be reduced if the energy from the first beam results in the nucleation and initial growth of pores. Reheating by the second beam then allows the pores to grow and escape from the molten material without melting additional base material. Furthermore, twin-spot welding is shown to be a promising combination of a production friendly solution and high quality welding.

Keywords
Laser Welding, Cast Magnesium, Twin-Spot, Metallurgy, Porosity, Automotive, AM50 Alloy
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-13753 (URN)10.4236/msa.2019.101006 (DOI)
Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-01Bibliographically approved
Hosseini, V. & Karlsson, L. (2019). Physical and kinetic simulation of nitrogen loss in high temperature heat affected zone of duplex stainless steels. Materialia, 6, Article ID 100325.
Open this publication in new window or tab >>Physical and kinetic simulation of nitrogen loss in high temperature heat affected zone of duplex stainless steels
2019 (English)In: Materialia, ISSN 2589-1529, Vol. 6, article id 100325Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Diffusion simulation, Duplex stainless steels, High temperature heat affected zone, Nitrogen loss, Thermodynamic simulation, Welding
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13858 (URN)10.1016/j.mtla.2019.100325 (DOI)2-s2.0-85064890131 (Scopus ID)
Funder
Knowledge Foundation, 20140130
Available from: 2019-05-24 Created: 2019-05-24 Last updated: 2019-07-25Bibliographically approved
Hosseini, V., Högström, M., Hurtig, K., Valiente Bermejo, M. A., Stridh, L.-E. & Karlsson, L. (2019). Wire-arc additive manufacturing of a duplex stainless steel: thermal cycle analysis and microstructure characterization. Welding in the World, 63(4), 975-987
Open this publication in new window or tab >>Wire-arc additive manufacturing of a duplex stainless steel: thermal cycle analysis and microstructure characterization
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2019 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 63, no 4, p. 975-987Article in journal (Refereed) Published
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).

Keywords
Duplex stainless steels; Additive manufacturing; GMAW; Thermal cycles; Austenite fraction; Secondary phases
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13947 (URN)10.1007/s40194-019-00735-y (DOI)000468518900005 ()2-s2.0-85066099481 (Scopus ID)
Available from: 2019-06-11 Created: 2019-06-11 Last updated: 2019-06-11
Hosseini, V., Karlsson, L., Engelberg, D. & Wessman, S. (2018). 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). Welding in the World, 62(4), 893
Open this publication in new window or tab >>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)
2018 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 62, no 4, p. 893-Article in journal (Refereed) Published
Abstract [en]

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

National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12659 (URN)10.1007/s40194-018-0565-y (DOI)2-s2.0-85048496813 (Scopus ID)
Funder
Vinnova, 2016-02834Knowledge Foundation, DNr 20140130
Available from: 2018-07-04 Created: 2018-07-04 Last updated: 2019-05-28Bibliographically approved
Fahlström, K., Blackburn, J., Karlsson, L. & Svensson, L.-E. (2018). Effect of Laser Welding Parameters on Porosity of Weldsin Cast Magnesium Alloy AM50. Modern Approaches on Material Science, 1(2), 25-32
Open this publication in new window or tab >>Effect of Laser Welding Parameters on Porosity of Weldsin Cast Magnesium Alloy AM50
2018 (English)In: Modern Approaches on Material Science, ISSN 2641-6921, Vol. 1, no 2, p. 25-32Article in journal (Refereed) Published
Abstract [en]

Pores in the weld metal lower the mechanical properties of the weld. It is therefore important to understand the pore formation mechanisms and find procedures that could reduce porosity. This study focused on laser welding of 3 mm thick magnesium alloy AM50, investigating how different parameters affect porosity formation. Low levels of porosity content were achieved by either increasing the welding speed or using a two-pass welding approach. It was found that higher welding speeds did not allow pores,which were pre-existing from the die-casting process, to have sufficient time to coalesce and expand. In the two-pass welding technique, pores were removed as a result of a degassing process which occurred through the second pass.

Keywords
Laser welding; Magnesium, Cast; Metallurgy; Porosity; Automotive; AM50
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-13754 (URN)10.32474/MAMS.2018.01.000106 (DOI)
Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-01Bibliographically approved
Hosseini, V., Karlsson, L., Wessman, S. & Fuertes, N. (2018). Effect of sigma phase morphology on the degradation of properties in a super duplex stainless steel. Materials, 11(6), Article ID 933.
Open this publication in new window or tab >>Effect of sigma phase morphology on the degradation of properties in a super duplex stainless steel
2018 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 11, no 6, article id 933Article in journal (Refereed) Published
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.

Keywords
Austenite; Corrosion resistance; Heat resistance; Localized corrosion; Microscopic examination; Morphology; Scanning electron microscopy; Stainless steel; Steel testing; Temperature, Chemical compositions; Duplex stainless steel; Electron backscattered diffraction analysis; Sigma-phase precipitation; Steady-state temperature; Super duplex stainless steel; Thermodynamic calculations; Water-cooled chambers, Steel corrosion
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12703 (URN)10.3390/ma11060933 (DOI)000436500300066 ()2-s2.0-85047834414 (Scopus ID)
Funder
Knowledge Foundation, 20140130
Available from: 2018-07-05 Created: 2018-07-05 Last updated: 2019-05-27Bibliographically approved
Hosseini, V., Karlsson, L., Örnek, C., Reccagni, P., Wessman, S. & Engelberg, D. (2018). Microstructure and functionality of a uniquely graded super duplex stainless steel designed by a novel arc heat treatment method. Materials Characterization, 139, 390-400
Open this publication in new window or tab >>Microstructure and functionality of a uniquely graded super duplex stainless steel designed by a novel arc heat treatment method
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2018 (English)In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 139, p. 390-400Article in journal (Refereed) Published
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

Keywords
Austenite; Chromium compounds; Cold rolling; Embrittlement; Hardness; Heat treatment; Microstructure; Nitrogen; Nitrogen compounds; Phase equilibria; Schematic diagrams; Stainless steel, Chi phase; Functionally graded microstructures; Nitrogen loss; R phase; Sigma phase, Temperature
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12235 (URN)10.1016/j.matchar.2018.03.024 (DOI)000431469300042 ()2-s2.0-85044113030 (Scopus ID)
Funder
Vinnova, 2016-02834Knowledge Foundation, 20140130
Note

Available online 19 March 2018

Available from: 2018-04-03 Created: 2018-04-03 Last updated: 2019-05-27Bibliographically approved
Hosseini, V., Thuvander, M., Wessman, S. & Karlsson, L. (2018). Spinodal Decomposition in Functionally Graded Super Duplex Stainless Steel and Weld Metal. Metallurgical and Materials Transactions. A, 49A(7), 2803-2816
Open this publication in new window or tab >>Spinodal Decomposition in Functionally Graded Super Duplex Stainless Steel and Weld Metal
2018 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 49A, no 7, p. 2803-2816Article in journal (Refereed) Published
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.

National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12263 (URN)10.1007/s11661-018-4600-9 (DOI)000433974400022 ()2-s2.0-85045465335 (Scopus ID)
Funder
Knowledge Foundation, 20140130
Note

First Online: 17 April 2018

Available from: 2018-04-30 Created: 2018-04-30 Last updated: 2019-03-21Bibliographically approved
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