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Hardened austenite steel with columnar sub-grain structure formed by laser melting
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
2015 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 625, 221-229 p.Article in journal (Refereed) Published
Abstract [en]

Laser melting (LM), with a focused Nd: YAG laser beam, was used to form solid bodies from a 316L austenite stainless steel powder. The microstructure, phase content and texture of the LM stainless steel were characterized and compared with conventional 316L stainless steel. The crack-free LM samples achieved a relative density of 98.6 +/- 0.1%. The XRD pattern revealed a single phase Austenite with preferential crystallite growth along the (100) plane and an orientation degree of 0.84 on the building surface. A fine columnar sub-grain structure of size 0.5 mu m was observed inside each individual large grain of single-crystal nature and with grain sizes in the range of 10-100 mu m. Molybdenum was found to be enriched at the sub-grain boundaries accompanied with high dislocation concentrations. It was proposed that such a sub-grain structure is formed by the compositional fluctuation due to the slow kinetics of homogeneous alloying of large Mo atoms during rapid solidification. The local enrichment of misplaced Mo in the Austenite lattice induced a network of dislocation tangling, which would retard or even block the migration of newly formed dislocations under indentation force, turning otherwise a soft Austenite to hardened steel. In addition, local formation of spherical nano-inclusions of an amorphous chromium-containing silicate was observed. The origin and the implications of the formation of such oxide nano-inclusions were discussed.

Place, publisher, year, edition, pages
2015. Vol. 625, 221-229 p.
Keyword [en]
Laser melting, Austenite stainless steel, Solidification, Compositional fluctuation, Dislocations
National Category
Chemical Sciences
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-115683DOI: 10.1016/j.msea.2014.12.018ISI: 000349579000027OAI: oai:DiVA.org:su-115683DiVA: diva2:799690
Note

AuthorCount:4;

Available from: 2015-03-31 Created: 2015-03-27 Last updated: 2017-12-04Bibliographically 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: 2017-02-24Bibliographically approved

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