Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Stainless steels fabricated by laser melting: Scaled-down structural hierarchies and microstructural heterogeneities
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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. , p. 115
Keywords [en]
Selective laser melting, Stainless steel, Structural hierarchies, Microstructure heterogeneity, Mechanical properties
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-129055ISBN: 978-91-7649-353-3 (print)OAI: oai:DiVA.org:su-129055DiVA, id: diva2:919300
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
List of papers
1. Hardened austenite steel with columnar sub-grain structure formed by laser melting
Open this publication in new window or tab >>Hardened austenite steel with columnar sub-grain structure formed by laser melting
2015 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 625, p. 221-229Article 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.

Keywords
Laser melting, Austenite stainless steel, Solidification, Compositional fluctuation, Dislocations
National Category
Chemical Sciences
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-115683 (URN)10.1016/j.msea.2014.12.018 (DOI)000349579000027 ()
Note

AuthorCount:4;

Available from: 2015-03-31 Created: 2015-03-27 Last updated: 2017-12-04Bibliographically approved
2. Transformation of austenite to duplex austenite-ferrite assembly in annealed stainless steel 316L consolidated by laser melting
Open this publication in new window or tab >>Transformation of austenite to duplex austenite-ferrite assembly in annealed stainless steel 316L consolidated by laser melting
Show others...
2015 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 633, p. 463-469Article in journal (Refereed) Published
Abstract [en]

Laser melting (LM), with a focused Nd:YAG laser beam, was used to form solid bodies from 316L austenite stainless steel powder and the laser melted samples were heat treated at various temperatures. The phase changes in heat treated samples were characterized using X-ray diffraction (XRD). Samples heat treated at 800 degrees C and 900 degrees C remained single austenite while in samples heat treated at 1100 degrees C and 1400 degrees C a dual austenite-ferrite phase assembly was formed. The ferrite formation was further verified by electron back scattering diffraction (EBSD) and selective area diffraction (SAD). Microstructural changes were studied by scanning and transmission electron microscopy (SEM, TEM). In samples heat treated up to 900 degrees C, coalescence of the cellular-sub grains was noticed, whereas in sample heat treated at and above 1100 degrees C the formation of ferrite phase was observed. The correlation between the microstructure/phase assembly and the measured strength/microhardness were investigated, which indicated that the tensile strength of the laser melted material was significantly higher than that of the conventional 316L steel even after heat treatment whereas caution has to be taken when laser melted material will be exposed to an application temperature above 900 degrees C.

Keywords
Laser melting, Phase transformation, Microstructure, Mechanical properties
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-116747 (URN)10.1016/j.jallcom.2015.01.249 (DOI)000350911800072 ()
Note

AuthorCount:5;

Available from: 2015-05-05 Created: 2015-04-27 Last updated: 2017-12-04Bibliographically approved
3. Austenitic stainless steel strengthened by the in situ formation of oxide nanoinclusions
Open this publication in new window or tab >>Austenitic stainless steel strengthened by the in situ formation of oxide nanoinclusions
2015 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 5, no 27, p. 20747-20750Article in journal (Refereed) Published
Abstract [en]

An austenitic stainless steel was prepared by laser melting. High resolution transmission electron microscopy with energy dispersive spectrometry confirmed the in situ formation of oxide nanoinclusions with average size less than 50 nm. Scanning electron microscopy examination revealed the homogeneous dispersion of the oxide nanoinclusions in the steel matrix. The tensile and yield strengths of the prepared specimens were 703 and 456 MPa respectively with high ductility which is significantly improved compared to its conventionally casted counterpart.

National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-115957 (URN)10.1039/c4ra16721j (DOI)000350221600018 ()
Note

AuthorCount:4;

Available from: 2015-04-09 Created: 2015-04-08 Last updated: 2017-12-04Bibliographically approved
4. Scaled down microstructure hierarchy and local heterogeneity in laser melted 316L stainless steel
Open this publication in new window or tab >>Scaled down microstructure hierarchy and local heterogeneity in laser melted 316L stainless steel
(English)In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189Article in journal (Refereed) Submitted
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-129053 (URN)
Available from: 2016-04-13 Created: 2016-04-13 Last updated: 2017-11-30Bibliographically approved
5. Novel ferritic stainless steel formed by laser melting from duplex stainless steel powder with advanced mechanical properties and high ductility
Open this publication in new window or tab >>Novel ferritic stainless steel formed by laser melting from duplex stainless steel powder with advanced mechanical properties and high ductility
2016 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 665, p. 59-65Article 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.

Keywords
laser melting, ferritic steel, dislocation loops, nitride precipitation, mechanical properties
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-126542 (URN)10.1016/j.msea.2016.04.027 (DOI)000376788000007 ()
Available from: 2016-02-05 Created: 2016-02-05 Last updated: 2017-11-30Bibliographically approved

Open Access in DiVA

Stainless steels fabricated by laser melting(4761 kB)1829 downloads
File information
File name FULLTEXT02.pdfFile size 4761 kBChecksum SHA-512
4644f0f1dd0e1bbebaad5dbe3f3c97cde145aaa6fb7d323c8c5d37966986cf6a26b096cdce1b27c10753136f08e03b8ae1cf738660a97ba16f406718086ea36a
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Saeidi, Kamran
By organisation
Department of Materials and Environmental Chemistry (MMK)
Materials Chemistry

Search outside of DiVA

GoogleGoogle Scholar
Total: 1829 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 1513 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf