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. / 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.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:hv-18126 |
| Date | January 2022 |
| Creators | Baghdadchi, Amir |
| Publisher | Högskolan Väst, Avdelningen för svetsteknologi (SV), Trollhättan : Högskolan Väst |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Licentiate thesis, comprehensive summary, info:eu-repo/semantics/masterThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | Licentiate Thesis : University West ; 38 |
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