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Low cycle fatigue life modelling using finite element strain range partitioning for a steam turbine rotor steel
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Solid Mechanics.
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0001-8306-3987
2020 (English)In: Theoretical and applied fracture mechanics (Print), ISSN 0167-8442, E-ISSN 1872-7638, Vol. 107, article id 102510Article in journal (Refereed) Published
Abstract [en]

Materials made for modern steam power plants are required to withstand high temperatures and flexible operational schedule. Mainly to achieve high efficiency and longer components life. Nevertheless, materials under such conditions experience crack initiations and propagations. Thus, life prediction must be made using accurate fatigue models to allow flexible operation. In this study, fully reversed isothermal low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done under strain control with different total strain ranges and temperatures (20 °C to 625 °C). Some tests included dwell time to calibrate the short-time creep behaviour of the material. Different fatigue life models were evaluated based on total life approach. The stress-based fatigue life model was found unusable at 600 °C, while the strain-based models in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life prediction, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges at 500 °C. This allows for the formulation of life curves based on either plasticity-dominated damage or creep-dominated damage. At 600 °C, creep dominated while at 500 °C creep only dominates for higher strain ranges. The deformation mechanisms at different temperatures and total strain ranges were characterised by scanning electron microscopy and by quantifying the amount of low angle grain boundaries. The quantification of low angle grain boundaries was done by electron backscatter diffraction. Microscopy revealed that specimens subjected to 600 °C showed signs of creep damage in the form of voids close to the fracture surface. In addition, the amount of low angle grain boundaries seems to decrease with the increase in temperature even though the inelastic strain amplitude was increased. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed of being all plastic strain, which need to be taken into consideration when constructing a life prediction model.

Place, publisher, year, edition, pages
Elsevier, 2020. Vol. 107, article id 102510
Keywords [en]
Creep-fatigue interaction, Creep-resistant steel, EBSD, Low cycle fatigue, Steam turbine steel, Strain range partitioning
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:liu:diva-164610DOI: 10.1016/j.tafmec.2020.102510ISI: 000528008200019Scopus ID: 2-s2.0-85079627150OAI: oai:DiVA.org:liu-164610DiVA, id: diva2:1417205
Note

Funding agencies: European UnionEuropean Union (EU) [764545]

Available from: 2020-03-26 Created: 2020-03-26 Last updated: 2023-09-29Bibliographically approved
In thesis
1. High-Temperature Fatigue in a Steam Turbine Steel: Modelling of Cyclic Deformation and Crack Closure
Open this publication in new window or tab >>High-Temperature Fatigue in a Steam Turbine Steel: Modelling of Cyclic Deformation and Crack Closure
2021 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Existing conventional thermal power plants are retrofitted for flexible operations to assist the transition toward more renewable energies. The deployment of many renewable energy power plants is necessary to achieve a clean environment with less pollution. However, the intermittent nature of renewable energies, due to weather changes, and the lack of efficient large energy storage systems put renewables at a disadvantage. Flexible operations of power plants imply fast and frequent start-ups. Thus, retrofitted power production plants can be utilised as an energy backup to satisfy the immediate demand during peak energy times or when renewable energies are suddenly limited. 

Large thermal power plants generally employ steam turbines with high inlet temperature and pressure steam conditions. Materials used for components at the high-temperature turbine sections are expected to withstand harsh environments. The use of 9-12 % Cr martensitic steels is desirable due to, among other things, their superior resistance to creep for temperatures up to 625 °C. Retrofitting for flexible operations put steam turbine components under high-temperature fatigue loading conditions different from how they were designed before. The flexible operations could lead to fatigue cracking at critical locations, such as grooves and notches at the inner steam turbine casing. Thus, fatigue behaviour understanding of steam turbine materials under such loading conditions is essential for components life prediction. Accurate and less conservative fatigue life prediction approach is necessary to extend the turbine components life, which reduces waste and provides economic benefits. This can be done by extending operations past crack initiation phase and allowing controlled propagation of cracks in the components. 

Within the 9-12 % Cr steel class, the martensitic steam turbine steel called FB2 is studied under high-temperature fatigue. This includes investigating high-temperature fatigue life behaviour, cyclic deformation behaviour, stress relaxation behaviour, and crack propagation behaviour along with crack closure behaviour. This was achieved by experimentally testing samples made from FB2 steel under isothermal low cycle fatigue, isothermal fatigue crack propagation, and thermomechanical fatigue crack propagation. 

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2021. p. 57
Series
Linköping Studies in Science and Technology. Licentiate Thesis, ISSN 0280-7971 ; 1900
National Category
Energy Engineering
Identifiers
urn:nbn:se:liu:diva-173354 (URN)10.3384/lic.diva-173354 (DOI)9789179296964 (ISBN)
Presentation
2021-03-12, Online via Zoom. Contact Robert Eriksson (Robert.eriksson@liu.se) or Ahmed Azeez (ahmed.azeez@liu.se) for ZOOM link, 10:15 (English)
Opponent
Supervisors
Funder
EU, Horizon 2020, 764545
Available from: 2021-02-17 Created: 2021-02-17 Last updated: 2021-03-30Bibliographically approved
2. High-Temperature Durability Prediction of Ferritic-Martensitic Steel
Open this publication in new window or tab >>High-Temperature Durability Prediction of Ferritic-Martensitic Steel
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Materials used for high-temperature steam turbine sections are generally subjected to harsh environments with temperatures up to 625 °C. The superior creep resistance of 9–12 % Cr ferritic-martensitic steels makes them desirable for those critical steam turbine components. Additionally, the demand for fast and frequent steam turbine start-ups, i.e. flexible operations, causes accelerated fatigue damage in critical locations, such as grooves and notches, at the high-temperature inner steam turbine casing. A durability assessment is necessary to understand the material behaviour under such high temperatures and repeated loading, and it is essential for life prediction. An accurate and less conservative fatigue life prediction approach is achieved by going past the crack initiation stage and allowing controlled growth of cracks within safe limits. Besides, beneficial load-temperature history effects, i.e. warm pre-stressing, must be utilised to enhance the fracture resistance to cracks. This dissertation presents the high-temperature durability assessment of FB2 steel, a 9-12 % Cr ferritic-martensitic steam turbine steel.

Initially, isothermal low-cycle fatigue testing was performed on FB2 steel samples. A fatigue life model based on finite element strain range partitioning was utilised to predict fatigue life within the crack initiation phase. Two fatigue damage regimes were identified, i.e. plastic- and creep-dominated damage, and the transition between them depended on temperature and applied total strain. Cyclic deformation and stress relaxation behaviour were investigated to produce an elastic-plastic and creep material model that predicts the initial and mid-life cyclic behaviour of the FB2 steel.

Furthermore, the thermomechanical fatigue crack growth behaviour of FB2 steel was studied. Crack closure behaviour was observed and accounted for numerically and experimentally, where crack growth rate curves collapsed into a single curve. Interestingly, the collapsed crack growth curves coincided with isothermal crack growth tests performed at the minimum temperature of the thermomechanical crack growth tests. In addition, hold times and changes in the minimum temperature of the thermomechanical fatigue cycle did not influence crack closure behaviour.

Finally, warm pre-stressing effects were explored for FB2 steel. A numerical prediction model was produced to predict the increase in the apparent fracture toughness. Warm pre-stressing effects can benefit the turbine life by enhancing fracture resistance and allowing longer fatigue cracks to grow within safe limits.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2023. p. 162
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2339
Keywords
Ferritic-martensitic steel, Low cycle fatigue, Thermomechanical fatigue, Crack propagation, Fracture mechanics, Finite element modelling
National Category
Applied Mechanics
Identifiers
urn:nbn:se:liu:diva-198206 (URN)10.3384/9789180753241 (DOI)9789180753234 (ISBN)9789180753241 (ISBN)
Public defence
2023-11-10, C3, C Building, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Note

Funding agency: The European Union's Horizon 2020 research and innovation programme under grant agreement No. 764545 as part of the project TURBO-REFLEX

Available from: 2023-09-29 Created: 2023-09-29 Last updated: 2023-10-02Bibliographically approved

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Azeez, AhmedEriksson, RobertLeidermark, DanielCalmunger, Mattias

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