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Effect of Trace Magnesium Additions on the Dynamic Recrystallization in Cast Alloy 825 after One-Hit Hot-Deformation
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process. Sandvik Materials Technology AB.ORCID iD: 0000-0003-0381-5494
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Structures.ORCID iD: 0000-0003-0533-6729
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process. (Powder metallurgy)ORCID iD: 0000-0002-6339-4612
Sandvik Materials Technology AB.
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2021 (English)In: Metals, ISSN 2075-4701, Vol. 11, no 1, article id 36Article in journal (Refereed) Published
Abstract [en]

Alloy 825 is widely used in several industries, but its useful service life is limited by both mechanical properties and corrosion resistance. The current work explores the effect of the addition of magnesium on the recrystallization and mechanical behavior of alloy 825 under hot compression. Compression tests were performed under conditions representative of typical forming processes: temperatures between 1100 and 1250 °C and at strain rates of 0.1–10 s−1 to a true strain of 0.7. Microstructural evolution was characterized by electron backscattered diffraction. Dynamic recrystallization was found to be more prevalent under all test conditions in samples containing magnesium, but not in all cases of conventional alloy 825. The texture direction ⟨101⟩ was the dominant orientation parallel to the longitudinal direction of casting (also the direction in which the samples were compressed) in samples that contained magnesium under all test conditions, but not in any sample that did not contain magnesium. For all deformation conditions, the peak stress was approximately 10% lower in material with the addition of magnesium. Furthermore, the differences in the peak strain between different temperatures are approximately 85% smaller if magnesium is present. The average activation energy for hot deformation was calculated to be 430 kJ mol−1 with the addition of magnesium and 450 kJ mol−1 without magnesium. The average size of dynamically recrystallized grains in both alloys showed a power law relation with the Zener–Hollomon parameter, DD~Zn, and the exponent of value, n, is found to be 0.12. These results can be used to design optimized compositions and thermomechanical treatments of alloy 825 to maximize the useful service life under current service conditions. No experiments were conducted to investigate the effects of such changes on the service life and such experiments should now be performed.

Place, publisher, year, edition, pages
MDPI AG , 2021. Vol. 11, no 1, article id 36
Keywords [en]
nickel alloys; alloy 825; magnesium; stress/strain measurements; grains and interfaces; texture
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-293866DOI: 10.3390/met11010036ISI: 000610505500001Scopus ID: 2-s2.0-85098666947OAI: oai:DiVA.org:kth-293866DiVA, id: diva2:1548858
Note

QC 20210517

Available from: 2021-05-03 Created: 2021-05-03 Last updated: 2022-06-25Bibliographically approved
In thesis
1. Thermomechanical Processing of Nickel-Base Alloy 825
Open this publication in new window or tab >>Thermomechanical Processing of Nickel-Base Alloy 825
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Alloy 825 material was studied using a Gleeble-3800 thermosimulatorby performing single-hot compression experiments.Optical microscopy and electron backscatter diffraction wereutilized to characterize the microstructure. Dynamicrecrystallization is not considerable in the as-cast alloy anddislocation recovery is deemed to be dominant. Based on thisfinding, the effect of adding trace amounts of alloying additionson the mechanical properties of cast alloy 825 was studied, withemphasis on whether or not dynamic recrystallization occurred.The results show that dynamic recrystallization was moreprevalent under all test conditions in samples containing a traceamount of magnesium, but not for the conventional alloy.However, alloying with trace magnesium did not lead to animprovement of the mechanical properties. Instead, processingmaps for hot forging of conventional Alloy 825 were required toidentify optimal working parameters and to achieve dynamicrecrystallization. The hot deformation behavior of cast Alloy 825was characterized by using dynamic materials modelling of hotcompression data. The results show that the maximum powerdissipation efficiency is over 35%. The highest efficiency isachieved in the temperature range of 1100 ℃ - 1250 ℃ and instrain rates in the range of 0.01 ≤ strain rate / s ≤ 0.1. The optimumprocessing parameters for good strain hardening are obtained inthe temperature range between 950 ℃ and 1100 ℃ with strainrates of 0.3 ≤ strain rate/ s ≤ 10.0. In addition, the influence of thedeformation level on the recrystallization and microstructuralchanges in Alloy 825 during hot forging operations attemperatures between 950 °C and 1200 °C was studied. Themaximum yield strength and ultimate tensile strength wereobtained after forging to achieve a true strain of 0.9 were 413 MPa and 622 MPa , respectively, with a ductility of 40%.However, Alloy 825 is often supplied as annealed bars.Therefore, the effect of the forging strain magnitude andsubsequent annealing on the microstructure, strengtheningmechanisms and room temperature mechanical properties wereinvestigated to assess the suitability of current industrialpractice. The results showed that the majority of strengtheningwas attributed to grain refinement, the dislocation densities thatarise due to the large forging strain, and due to solid solutionstrengthening. The results of calculations are in excellentagreement with experimental data, with less than 1% difference.These results can be used by future researchers and industry topredict the strength of Alloy 825 and similar alloys, especially inmaterial after a completed hot forging operation.

Abstract [sv]

denna avhandling studerades effekten av deformationsgrad pådynamisk rekristallisation av gjuten Alloy 825 genomexperiment i Gleeble-3800 termomekanisk simulator. Måletmed arbetet är att använda ljusmikroskop och EBSD för attstudera dynamisk rekristallisation i A 825 och dess effekt påmikrostrukturen. Dynamisk rekristallisation är inte betydande,men istället har en substruktur formats med lågvinkligakorngränser.Baserat på dessa resultat, studerades effekten av småmagnesiumtillsatser på de mekaniska egenskaperna av gjutenstruktur med fokus på huruvida dynamisk rekristallisation skereller inte. Resultaten visar att dynamisk rekristallisation varvanligare under alla testförhållanden i prover innehållandemagnesium, men inte i något fall av konventionell A 825.Legering med magnesium ledde emellertid inte till enförbättring av de mekaniska egenskaperna. Av denna anledningdrog man slutsatsen att en varmbearbetningskarta för smittkonventionell (magnesiumfri) A 825 krävdes för att identifieraoptimala processparametrar och uppnå dynamiskrekristallisation.Deformationen av gjuten A 825 undersöktes med hjälp av endynamisk materialmodell och data från kompressionsprover.Resultaten visar att den maximala effektöverföringen är över 35%. Den högsta verkningsgraden är vid ett temperaturintervall av1100 ℃ - 1250 ℃ och en töjningshastighet på 0.01 ≤  töjningshastighet / s ≤0.1 . De optimala varmbearbetningsparametrarna för goddeformationshärdning erhålls i temperaturområdet mellan 950℃ och 1100 °C med en töjningshastighet av 0.3 ≤  töjningshastighet/ s ≤ 10.0.Vidare undersöktes effekten av reduktionsgrad pårekristallisation och mikrostrukturutveckling vid smide inomtemperaturområde 950° C och 1200° C. Den maximala sträckochbrottgränsen erhölls efter smide till sann töjning av 0,9. Sträckgränsen var 413 MPa och brottgränsen 622 MPa, med enbrottförlängning på 40 %. Emellertid levereras materialet ofta isläckglödgat tillstånd. Därför undersöktes effekten av smidemed olika reduktionsgrad följt av mjukglödgning därhärdningsmekanismer och mekaniska egenskaper vidrumstemperatur undersöktes. Detta genomfördes för attbedöma lämpligheten av nuvarande industriell praxis.Resultaten visade att majoriteten av hårdnade tillskrevs småkorn, dislokationstäthet som uppstod på grund av den storasmides-deformationen och härdning genom fast lösning.Resultaten av beräkningarna överensstämmer medexperimentella data med mindre än 1 % skillnad. Resultaten kani framtiden användas av andra forskare och i industrin föroptimering av mekaniska egenskaperna för A 825 och liknadelegeringar.

Place, publisher, year, edition, pages
Stockholm, Sweden: Kungliga Tekniska högskolan, 2021. , p. 269: Kungliga Tekniska högskolan, 2021. p. 269
Series
TRITA-ITM-AVL ; 2021:32
Keywords
Alloy 825; Cast and wrought structures; Hot compression test; Microstructural evaluation, Modelling, Yield strength, Strengthening mechanisms
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-295594 (URN)978-91-7873-827-4 (ISBN)
Public defence
2021-06-14, https://kth-se.zoom.us/j/68764816247, Sockholm, 10:00 (English)
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Supervisors
Available from: 2021-05-25 Created: 2021-05-24 Last updated: 2022-06-25Bibliographically approved

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Al-Saadi, MunirMu, WangzhongHulme-Smith, ChristopherJönsson, Pär

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