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Novel ferritic stainless steel formed by laser melting from duplex stainless steel powder with advanced mechanical properties and high ductility
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
2016 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 665, 59-65 p.Article in journal (Refereed) Published
Abstract [en]

Stainless steel bodies with relative density of 99.5% (with the theoretical density being 7.8gr/cm3) were manufactured by laser melting (LM) of duplex 2507SAF steel powder. The crystalline phases of starting powder were fully ferrite with only a small trace of austenite. The chemical composition was unchanged during laser melting. A unique mosaic-type structure with mosaics of 100-150 μm size was formed after LM. Recrystallized grains with 1-5 μm was formed in between the mosaic boundaries. A great number of entangled dislocation loops resembling a loops with 100-200 nm size were also formed inside each of these mosaics and also within recrystallized micron size grains at the mosaic boundary zones. Nitrogen enriched areas and nitride phase were detected in the inner microstructure of the laser melted samples. The measured tensile strength, yield strength and microhardness were 1214 MPa, 1321 MPa and 450 HV, respectively, which is superior to that of conventional ferritic, austenitic and duplex stainless steels. The Enhanced mechanical properties are due to a number of nano- and microstructure factors such as the nano-sized dislocation loops restricting dislocation movements, different crystalline grain orientation of grains within the mosaics and boundary inclusions and precipitates that inhibit slip/slide effects. Despite of high strength and hardness, the laser melted ferritic steel was very ductile according to stress-strain curves and fracture analysis.

Place, publisher, year, edition, pages
2016. Vol. 665, 59-65 p.
Keyword [en]
laser melting, ferritic steel, dislocation loops, nitride precipitation, mechanical properties
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-126542DOI: 10.1016/j.msea.2016.04.027ISI: 000376788000007OAI: oai:DiVA.org:su-126542DiVA: diva2:901038
Available from: 2016-02-05 Created: 2016-02-05 Last updated: 2016-06-20Bibliographically approved
In thesis
1. Stainless steels fabricated by laser melting: Scaled-down structural hierarchies and microstructural heterogeneities
Open this publication in new window or tab >>Stainless steels fabricated by laser melting: Scaled-down structural hierarchies and microstructural heterogeneities
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing is revolutionizing the way of production and use of materials. The clear tendency for shifting from mass production to individual production of net-shape components has encouraged using selective laser melting (SLM) or electron beam melting (EBM). In this thesis, austenitic, duplex and martensitic stainless steel parts were fabricated by laser melting technique using fixed laser scanning parameters. The fabricated steel parts were characterised using XRD, SEM, TEM/STEM, SADP and EBSD techniques. Mechanical properties of the fabricated steel parts were also measured. The mechanism of the evolution of microstructure during laser melting as well as the mechanism of the effect of developed microstructure on the mechanical properties was investigated. It was found that the intense localized heating, non-uniform and asymmetric temperature gradients and subsequently fast cooling introduces unique high level structural hierarchies and microstructure heterogeneities in laser melted steel parts. A unique structural hierarchy from the millimetre scale melt pools down to the sub-micron/nano scale cellular sub-grains was observed. The cellular sub-grains were 0.5-1μm with Molybdenum enriched at the sub-grain boundaries in SLM 316L. The Mo enriched cell boundaries affected the chemical and microstructure stability of the post heat treated samples. Well dispersed and large concentration of dislocations around the cell boundaries and well distributed oxide nano inclusions, imposed large strengthening and hardening effect that led to relatively superior tensile strength (700 MPa), yield strength (456 MPa), and microhardness (325Hv) compared to those of HIP 316L steel. The in-situ formation of oxide nano inclusions provided a unique way for preparation of oxide dispersion-strengthened (ODS) steel in a single process. The formation of oxide nano inclusions in the very low oxygen partial pressure of laser chamber was thermodynamically explained. High concentration of nano size dislocation loops, formation of nitride phases along with nitrogen enriched islands and oxide nano inclusions lead to strong dislocation pinning effect which strengthened the laser melted duplex stainless steel with a total tensile strength of 1321 MPa, yield strength of 1214 MPa and microhardness of 450HV. The grade 420 stainless steel was laser melted in Ar and N2 atmosphere which also showed a two level hierarchy with nanometric martensite lathes embedded in parental austenite cellular grains. The Ar treated sample had relatively higher retained austenite, lower YS (680-790 MPa) and UTS (1120-1200 MPa) compared to those treated in N2 (YS= 770-1100 MPa, UTS=1520-1560 MPa). The mechanism of the effect of atmosphere on phase transformation was explained.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University, 2016. 115 p.
Keyword
Selective laser melting, Stainless steel, Structural hierarchies, Microstructure heterogeneity, Mechanical properties
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-129055 (URN)978-91-7649-353-3 (ISBN)
Public defence
2016-05-19, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

Available from: 2016-04-26 Created: 2016-04-13 Last updated: 2016-04-14Bibliographically approved

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Saeidi, KamranShen, Zhijian
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