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Hot Deformation Behaviour and Processing Map of Cast Alloy 825
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. R&D Metallurgy, AB Sandvik Materials Technology, SE-811 81 Sandviken, Sweden.ORCID iD: 0000-0003-0381-5494
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.ORCID iD: 0000-0002-6339-4612
R&D Metallurgy, AB Sandvik Materials Technology, SE-811 81 Sandviken, Sweden.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.ORCID iD: 0000-0001-9775-0382
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Alloy 825 is a nickel-based alloy that is commonly used in applications where both high strength and corrosion resistance are required. Applications include tanks in the chemical, food and petrochemical industries and oil and gas pipelines. Components made from Alloy 825 are often manufactured using hot deformation. However, there is no systematic study to optimise the processing conditions reported in literature. In this study, a processing map for as-cast Alloy 825 is established to maximise the power dissipation efficiency of hot deformation and correlate the processing conditions to final materials properties. The hot deformation behaviour of equiaxed Alloy 825 is characterized on the basis of the dynamic materials model and compression data in the temperature range of 950 °C to 1250 °C at an interval of 50°C and strain rate range of 0.01 s-1 to 10 s-1 to a true strain of 0.7 using a Gleeble-3500 thermomechanical simulator. Flow stress is modelled by the constitutive equation based on a hyperbolic sine function. The deformed material is characterized using Vickers hardness, optical microscopy and scanning electron microscopy, including electron backscattered diffraction. The true stress-true strain curves exhibit peak stresses followed by softening due to occurrence of dynamic recrystallization. The value of stress exponent in the hyperbolic sine-based constitutive equation, n=5.0. This suggests that the rate-limiting mechanism of deformation is climb (diffusion)-mediated dislocation glide. The activation energy for plastic flow in the temperature range tested is about 450 kJ mole-1, and the relationship between flow stress and temperature-compensated strain rate (via the Zener-Hollomon parameter) was found to be valid across this temperature range. The maximum power dissipation efficiency is over 35%. The highest efficiency is observed over temperature range of 1100 °C – 1250 °C and a strain rate of 0.01 s-1 – 0.1s-1. These are the optimum conditions for hot working. The optimum processing parameters for good strain hardening are obtained in the temperature range of between  950 °C  and  1100 °C with a strain rate between  0.3/s  and 10.0/s. 

Keywords [en]
Alloy 825, Hot-deformation; Stress-strain curve; Activation energy; Dynamically recrystallized; grain size
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-295474OAI: oai:DiVA.org:kth-295474DiVA, id: diva2:1556289
Note

QC 20210525

Available from: 2021-05-21 Created: 2021-05-21 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)
Opponent
Supervisors
Available from: 2021-05-25 Created: 2021-05-24 Last updated: 2022-06-25Bibliographically approved

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Al-Saadi, MunirHulme-Smith, ChristopherSandberg, FredrikJönsson, Pär

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