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Identification and quantification of martensite in ferritic-austenitic stainless steels and welds
University West, Department of Engineering Science, Division of Welding Technology. (PTW)ORCID iD: 0000-0002-6820-4312
University West, Department of Engineering Science, Division of Welding Technology.ORCID iD: 0000-0001-6242-3517
University West, Department of Engineering Science, Research Enviroment Production Technology West. University West, Department of Engineering Science, Division of Welding Technology. (PTW)ORCID iD: 0000-0001-8822-2705
2021 (English)In: Journal of Materials Research and Technology, ISSN 2238-7854, Vol. 15, p. 3610-3621Article in journal (Refereed) Published
Abstract [en]

This paper aims at the phase identification and quantification in transformation induced plasticity duplex stainless steel (TDSS) base and weld metal containing ferrite, austenite, and martensite. Light optical microscopy (LOM) and electron backscatter diffraction (EBSD) analysis were employed to analyze phases. Samples were either mechanically or electrolytically polished to study the effect of the preparation technique. Mechanical polishing produced up to 26% strain-induced martensite. Electrolytic polishing with 150 g citric acid, 300 g distilled water, 600 mL H3PO4, and 450 mL H2SO4 resulted in martensite free surfaces, providing high-quality samples for EBSD analysis. Martensite identification was challenging both with LOM, due to the similar etching response of ferrite and martensite, and with EBSD, due to the similar lattice structures of ferrite and martensite. An optimized Beraha color etching procedure was developed that etched martensite distinctively. A novel step-by-step EBSD methodology was also introduced considering grain size and orientation, which successfully identified and quantified martensite as well as ferrite and austenite in the studied TDSS. Although here applied to a TDSS, the presented EBSD methodology is general and can, in combination with knowledge of the metallurgy of the specific material and with suitable adaption, be applied to a multitude of multiphase materials. It is also general in the sense that it can be used for base material and weld metals as well as additive manufactured materials.

Place, publisher, year, edition, pages
Elsevier Editora Ltda , 2021. Vol. 15, p. 3610-3621
Keywords [en]
Duplex stainless steel, Mechanical polishing, Electrolytic polishing, Phase analysis, Martensite, Electron backscatter diffraction
National Category
Metallurgy and Metallic Materials Other Materials Engineering
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-17790DOI: 10.1016/j.jmrt.2021.09.153ISI: 000712162500008Scopus ID: 2-s2.0-85117073816OAI: oai:DiVA.org:hv-17790DiVA, id: diva2:1622443
Note

 This study received support from the EU-project H2020-MSCA-RISE-2018 Number 823786, i-Weld, and the Swedish Agency for Economic and Regional Growth through the European Union–European Development Fund

Available from: 2021-12-22 Created: 2021-12-22 Last updated: 2024-02-20
In thesis
1. Laser Welding and Additive Manufacturing of Duplex Stainless Steels: Properties and Microstructure Characterization
Open this publication in new window or tab >>Laser Welding and Additive Manufacturing of Duplex Stainless Steels: Properties and Microstructure Characterization
2022 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Duplex stainless steels (DSS), with a ferritic-austenitic microstructure, are used ina wide range of applications thanks to their high corrosion resistance and excellent mechanical properties. However, efficient and successful production and joining of DSS require precise control of processes and an in-depth understanding o frelations between composition, processing thermal cycles, resulting microstructures and properties. In this study laser welding, laser reheating, and laser additive manufacturing using Laser Metal Deposition with Wire (LMDw) ofDSS and resulting weld and component microstructures and properties are explored.

In the first part a lean FDX 27 duplex stainless steel, showing the transformation induced plasticity (TRIP) effect, was autogenously laser welded and laser reheated using pure argon or pure nitrogen as shielding gas. The weld metal austenite fraction was 22% for argon-shielding and 39% for nitrogen-shielding in as-welded conditions. Less nitrides were found with nitrogen-shielding compared to argonshielding. Laser reheating did not significantly affect nitride content or austenite fraction for argon-shielding. However, laser reheating of the nitrogen shieldedweld removed nitrides and increased the austenite fraction to 57% illustrating the effectiveness of this approach.

Phase fraction analysis is important for DSS since the balance between ferrite and austenite affects properties. For TRIP steels the possibility of austenite tomartensite transformation during sample preparation also has to be considered. Phases in the laser welded and reheated FDX 27 DSS were identified and quantified using light optical microscopy (LOM) and electron backscatter diffraction (EBSD) analysis. An optimized Beraha color etching procedure was developed for identification of martensite by LOM. A novel step-by-step EBSD methodology was also introduced, which successfully identified and quantified martensite as well as ferrite and austenite. It was found that mechanical polishing produced up to 26% strain-induced martensite, while no martensite was observed after electrolytic polishing.In the second part a systematic four-stage methodology was applied to develop procedures for additive manufacturing of standard 22% Cr duplex stainless steel components using LMDw combined with the hot wire technology. In the four stages, single-bead passes, a single-bead wall, a block, and finally a cylinder with an inner diameter of 160 mm, thickness of 30 mm, and height of 140 mm were produced. The as-deposited microstructure was inhomogeneous and repetitive including highly ferritic regions with nitrides and regions with high fractions ofaustenite. Heat treatment for 1 hour at 1100 ̊C homogenized the microstructure, removed nitrides, and produced an austenite fraction of about 50%. Strength, ductility, and toughness were at a high level for the cylinder, comparable to those of wrought type 2205 steel, both as-deposited and after heat treatment. The highest strength was achieved for the as-deposited condition with a yield strength of 765 MPa and a tensile strength of 865 MPa, while the highest elongation of 35% was found after heat treatment. Epitaxial growth of ferrite during solidification, giving elongated grains along the build direction, resulted in anisotropy of toughness properties. The highest impact toughness energies were measured for specimens with the notch perpendicular to the build direction after heat treatment with close to 300 J at -10oC. It was concluded that implementing a systematic methodology with a stepwise increase in the deposited volume and geometrical complexity can successfully be used when developing additive manufacturing procedures for significantly sized metallic components.

This study has illustrated that a laser beam can successfully be used as heat source in processing of duplex stainless steel both for welding and additive manufacturing. However, challenges like nitrogen loss, low austenite fractions and nitride formation have to be handled by precise process control and/or heat treatment.

Abstract [sv]

Duplexa rostfria stål (DSS) är viktiga konstruktionsmaterial tack vare derasutmärkta mekaniska egenskaper och goda korrosionsbeständighet. Vid svetsningoch additiv tillverkning krävs noggrann styrning av parametrar och kunskap om processernas inverkan på mikrostrukturen för att uppnå önskade egenskaper.Lasersvetsning, värmebehandling med laser och additiv tillverkning i form av lasermetalldeponering med tråd (LMDw) har därför studerats för DSS.

Det duplexa stålet FDX 27 lasersvetsades utan tillsatsmaterial och med argon ellerkväve som skyddsgas. Kvävgasskydd gav mer austenit och färre nitrider änargonskydd. En efterföljande laservärmebehandling löste upp nitriderna då kväve användes som skyddsgas och austenithalten ökade till 57%. Austeniten i FDX 27kan vid deformation omvandlas till martensit. Två metoder för identifiering av martensit utvecklades därför: en färgetsmetod för ljusoptisk mikroskopi samt en metod som utnyttjar bakåtspridda elektroner (EBSD) vid elektronmikroskopi.Som mest bildades 26% martensit vid mekanisk provpreparering medan elektropolerade prover endast innehöll austenit och ferrit.

Procedurer togs fram för additiv tillverkning av komponenter, i 22% krom duplexa rostfria stål, med LMDw kombinerat med varmtrådsteknik. Slutprodukten var en 140 mm hög cylinder med 160 mm inre diameter och tjocklek av 30 mm. Mikrostrukturen var inhomogen med periodiskt omväxlande ferritiska områden med nitrider, och områden med stor andel austenit.Värmebehandling under 1 timme vid 1100oC eliminerade nitriderna och gav en homogen struktur med ca. 50% austenit. De mekaniska egenskaperna var, både före och efter värmebehandling, jämförbara med de typiska för motsvarande stål. Högst hållfasthet uppmättes före värmebehandling med sträckgränsen 765 MPa och brottgränsen 865 MPa, medan den största förlängningen var 35% efter värmebehandling. Slagsegheten var upp till 300 J vid -10oC men varierade med hur provstavens brottanvisning var orienterad relativt byggriktningen.Laser är en lämplig energikälla vid svetsning och additiv tillverkning av duplexa rostfria stål. Utmaningar som kväveförlust, låga austenithalter och nitridbildning kan hanteras med noggrann processkontroll och/eller värmebehandling.

Place, publisher, year, edition, pages
Trollhättan: Högskolan Väst, 2022. p. 174
Series
Licentiate Thesis: University West ; 38
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-18126 (URN)978-91-89325-19-7 (ISBN)978-91-89325-18-0 (ISBN)
Presentation
2022-03-08, J111, Gustava Melins Gata 2, Trollhättan, 10:00 (English)
Supervisors
Available from: 2022-03-08 Created: 2022-02-07 Last updated: 2022-03-02Bibliographically approved
2. Directed Energy Deposition Additive Manufacturing and Welding of Duplex Stainless Steel using Laser Beam
Open this publication in new window or tab >>Directed Energy Deposition Additive Manufacturing and Welding of Duplex Stainless Steel using Laser Beam
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Duplex stainless steels (DSSs), with a ferritic-austenitic microstructure, are used in a wide range of applications thanks to their high corrosion resistance and good mechanical properties. However, efficient and successful production and joining of DSS require precise control of processes and an in-depth understanding of the relations between composition, processing thermal cycles, resulting microstructures and properties. In this study welding and direct energy deposition of DSS using a laser beam, resulting weld and component microstructures, and properties are explored.

In the first part a lean FDX 27 DSS, showing the transformation-induced plasticity (TRIP) effect, was autogenously laser welded and laser reheated using pure argon or pure nitrogen as shielding gas. The weld metal austenite fraction was 22% for argon-shielding and 39% for nitrogen-shielding in the as-welded conditions. Less nitrides were found with nitrogen-shielding compared to argon-shielding. Laser reheating did not significantly affect nitride content or austenite fraction for argon-shielding. However, laser reheating of the nitrogen shielded weld removed nitrides and increased the austenite fraction to 57% illustrating the effectiveness of this approach.

Phase fraction analysis is important for DSS since the balance between ferrite and austenite affects the properties. For TRIP steels the risk of austenite-to-martensite transformation during sample preparation also has to be considered. Ferrite, austenite and martensite were identified and quantified using light optical microscopy (LOM) and electron backscatter diffraction (EBSD) analysis. It was found that mechanical polishing produced up to 26% strain-induced martensite, while no martensite was observed after electrolytic polishing.

In the second part a systematic four-stage methodology was applied to develop procedures for additive manufacturing of standard 22% Cr DSS components employing direct energy deposition using a laser beam and wire feedstock (DED-LB/w) combined with the hot wire technology. In the four stages, single-bead passes, a single-bead wall, a block, and finally a cylinder with an inner diameter of 160 mm, thickness of 30 mm, and height of 140 mm were produced. Implementing this methodology with a stepwise increase in the deposited volume and geometrical complexity can successfully be used when developing additive manufacturing procedures for significantly sized metallic components. The as-deposited microstructure was inhomogeneous and repetitive including highly ferritic regions with nitrides and regions with high fractions of austenite. Heat treatment for 1 hour at 1100°C homogenized the microstructure, dissolved the nitrides, and almost balanced the ferrite and austenite phase fractions. Strength, ductility, and toughness were at a high level for the cylinder, comparable to those of wrought type 2205 steel, both as-deposited and after heat treatment. The pitting corrosion resistance revealed that microstructural differences, including ferrite-to-austenite ratio, alloying element distribution in ferrite and austenite , and the presence of nitrides, affected the corrosion resistance of DED-LB/w DSS. It was also shown that alongside the decomposition of ferrite into Fe-rich (α) and Cr-rich (αʹ) phases, clustering of Ni, Mn, and Si atoms are contributing to the 475°C -embrittlement of DSS manufactured by DED-LB/w.

This study has illustrated that a laser beam can successfully be used as heat source in processing of DSS both for welding and additive manufacturing. However, challenges like nitrogen loss, low austenite fractions and nitride formation have to be handled by precise process control and/or heat treatment.

Abstract [sv]

Duplexa rostfria stål (DSS) med en ferritisk-austenitisk mikrostruktur används inom ett brett spektrum av tillämpningar tack vare hög korrosionsbeständighet och goda mekaniska egenskaper. Effektiv och framgångsrik produktion och sammanfogning av DSS kräver noggrann kontroll av processer och en djupgående förståelse av sambanden mellan kemisk sammansättning, termiska cykler, resulterande mikrostrukturer och egenskaper. I detta arbete studerades svetsning och metalldeponering (direct energy deposition) av DSS med hjälp av laseroch resulterande mikrostrukturer samt egenskaper utvärderades.

I den första delen svetsades ett lägre legerat FDX 27 duplex rostfritt stål, som har en TRIP-effekt (transformation-induced plasticity), med laser och laseruppvärmdes med ren argon eller ren kvävgas som skyddsgas. Svetsgodsets austenitandel var 22% för argonskydd och 39% för kvävgasskydd under svetsningen. Färre nitrider observerades med kvävgasskydd jämfört med argonskydd. Laseruppvärmning påverkade inte signifikant nitrid- eller austenitandelen för argonskydd. Dock resulterade laseruppvärmningen av svetsen med kvävgasskydd i minskad nitridandel samtidigt som austenitandelen ökade till 57%, vilket visar effektiviteten av detta tillvägagångssätt.

Analys av fasfraktion är viktig för DSS eftersom balansen mellan ferrit och austenit påverkar egenskaperna. För TRIP-stål måste risken för martensitomvandling av austenit under provberedningen också beaktas. Ferrit, austenit och martensit identifierades och kvantifierades med hjälp av ljusoptisk mikroskopi (LOM) och analys med hjälp av diffraktion av bakåtspridda elektroner (EBSD electron backscatter diffraction). Det visade sig att mekanisk polering gav upp till 26% deformationsinducerad martensit, medan ingen martensit observerades efter elektrolytisk polering.

I den andra delen användes en systematisk metodik i fyra steg för att utveckla procedurer för additiv tillverkning av standardkomponenter i 22% krom DSS med metalldeponering och laser med svetstråd som tillsatsmaterial (DED-LB/w), kombinerad med varmtrådteknologi. I de fyra stegen tillverkades enkelsträngar, enkelväggar, ett block och slutligen en cylinder med en inre diameter på 160 mm, tjocklek på 30 mm och höjd på 140 mm. Genom att implementera denna metodik med en stegvis ökning av den deponerade volymen och geometrisk komplexitet kan additiva tillverkningsprocedurer framgångsrikt användas för utveckling av metallkomponenter med betydande storlekar. Den deponerade mikrostrukturen var ojämn och innehöll upprepade områden med hög ferrithalt och nitrider samt områden med hög andel av austenit. Värmebehandling i 1 timme vid 1100°C homogeniserade mikrostrukturen, löste upp nitriderna och jämnade nästan ut ferrit- och austenitandelarna. Hållfasthet, duktilitet och seghet var goda för cylindern, jämförbara med de av smidda typer av 2205 DSS, både som deponerad och efter värmebehandling. Gropfrätning och korrosionsmotstånd visade att mikrostrukturella skillnader, inklusive förhållande ferrit till austenit, fördelning av legeringselement i ferrit och austenit och närvaro av nitrider, påverkade korrosionsmotståndet för DED-LB/w DSS. Det visades också att, tillsammans med sönderfallet av ferrit till Fe-rika (α) och Cr-rika (αʹ) faser, bidrar kluster av Ni, Mn och Si-atomer till sprödhet vid 475°C hos DSS tillverkade av DED-LB/w.

Detta arbete har visat att en laser framgångsrikt kan användas som värmekälla vid tillverkning av DSS både för svetsning och additiv tillverkning. Utmaningar som kväveutarmning, låga austenitandelar och bildning av nitrider måste dock hanteras genom noggrann processtyrning och/eller värmebehandling.

Place, publisher, year, edition, pages
Trollhättan: University West, 2024. p. 90
Series
PhD Thesis: University West ; 63
Keywords
Duplex stainless steel; Laser welding; Additive manufacturing; Direct Energy Deposition using a Laser Beam; Microstructure characterization, Duplexa rostfria stål; Lasersvetsning; Additiv tillverkning; Metalldeponering med laser; Mikrostruktur
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-21251 (URN)978-91-89325-71-5 (ISBN)978-91-89325-70-8 (ISBN)
Public defence
2024-04-22, C118, Gustava Melins gata, Trollhättan, 10:00 (English)
Opponent
Supervisors
Note

All papers are CC BY 4.0

Paper F is  submitted.

Available from: 2024-03-11 Created: 2024-02-20

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