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Corrosion resistance and microstructure analysis of additively manufactured 22% chromium duplex stainless steel by laser metal deposition with wire
University West, Department of Engineering Science, Division of Welding Technology. (KAMPT)ORCID iD: 0000-0002-6820-4312
Department of Materials Science and Engineering, The Ohio State University, Columbus (USA).
Department of Materials Science and Engineering, The Ohio State University, Columbus (USA).
University West, Department of Engineering Science, Division of Welding Technology. (KAMPT)ORCID iD: 0000-0003-3374-6282
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2023 (English)In: Journal of Materials Research and Technology, ISSN 2238-7854, Vol. 26, p. 6741-6756Article in journal (Refereed) Published
Abstract [en]

Microstructure characteristics and pitting corrosion of a duplex stainless steel (DSS) manufactured by laser metal deposition with wire (LMDw) were studied. The layer-by-layer LMDw process resulted in a mixed microstructure of predominantly ferrite with 2% austenite and chromium-rich nitrides, and reheated regions with ~33% austenite. The high cooling rate of LMDw restricted the distribution of Cr, Mo, and Ni, in ferrite and austenite, while N diffuses from ferrite to austenite. Subsequent heat treatment at 1100 C for 1 h resulted in homogenized microstructure, dissolution of nitrides, and balanced ferrite/austenite ratio. It also led to the redistribution of Cr and Mo to ferrite, and Ni and N to austenite. At room temperature, cyclic potentiodynamic polarization measurements in 1.0 M NaCl solution showed no significant differences in corrosion resistance between the as-deposited and heat-treated samples, despite the differences in terms of ferrite to austenite ratio and elemental distribution. Critical pitting temperature (CPT) was the lowest (60 C) for the predominantly ferritic microstructure with finely dispersed chromium-rich nitrides; while reheated area with ~33% austenite in as-deposited condition achieved higher critical temperature comparable to what was obtained after heat treatment (73 and 68 C, respectively). At temperatures above the CPT, selective dissolution of the ferrite after deposition was observed due to depletion of N, while after heat treatment, austenite preferentially dissolved due to Cr and Mo concentrating in ferrite. In summary, results demonstrate how microstructural differences in terms of ferrite-to-austenite ratio, distribution of corrosion-resistant elements, and presence of nitrides affect corrosion resistance of LMDw DSS.

Place, publisher, year, edition, pages
2023. Vol. 26, p. 6741-6756
Keywords [en]
Additive manufacturing, Duplex stainless steel, Laser metal deposition, Localized corrosion, Microstructure-property relation
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-20883DOI: 10.1016/j.jmrt.2023.09.037ISI: 001139454200001Scopus ID: 2-s2.0-85171616425OAI: oai:DiVA.org:hv-20883DiVA, id: diva2:1822865
Note

CC BY 4.0

Available from: 2023-12-28 Created: 2023-12-28 Last updated: 2024-04-12
In thesis
1. 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
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All papers are CC BY 4.0

Paper F is  submitted.

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

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Baghdadchi, AmirValiente Bermejo, María AsunciónAndersson, Joel

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