PSI - Issue 23

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ScienceDirect

Procedia Structural Integrity 23 (2019) 149–154 Structural Integrity Procedia 00 (2019) 000–000 Structural Integrity Procedia 00 (2019) 000–000

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9th International Conference on Materials Structure and Micromechanics of Fracture Low cycle fatigue modelling of a steam turbine rotor steel Ahmed Azeez a, ∗ , Robert Eriksson a , Mattias Calmunger b , Stefan B. Lindstro¨m a , Kjell Simonsson a 9th International Conference on Materials Structure and Micromechanics of Fracture Low cycle fatigue modelling of a steam turbine rotor steel Ahmed Azeez a, ∗ , Robert Eriksson a , Mattias Calmunger b , Stefan B. Lindstro¨m a , Kjell Simonsson a

a Division of Solid Mechanics, Linko¨ping University, 58183 Linko¨ping, Sweden b Division of Engineering Materials, Linko¨ping University, 58183 Linko¨ping, Sweden a Division of Solid Mechanics, Linko¨ping University, 58183 Linko¨ping, Sweden b Division of Engineering Materials, Linko¨ping University, 58183 Linko¨ping, Sweden

© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the ICMSMF organizers Abstract Materials in steam turbine rotors are subjected to cyclic loads at high temperature, causing cracks to initiate and grow. To allow for more flexible operation, accurate fatigue models for life prediction must not be overly conservative. In this study, fully reversed low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done isothermally, within temperature range of room temperature to 600 ◦ C, under strain control with 0.8–1.2 % total strain range. Some tests included hold time to calibrate the short-time creep behaviour of the material. Di ff erent fatigue life models were constructed. The life curve in terms of stress amplitude was found unusable at 600 ◦ C, while the life curve in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 ◦ C. To construct better life model, 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 di ff erent damage mechanisms at di ff erent strain ranges. This allowed the formulation of life curves based on plasticity or creep domination, which showed creep domination at 600 ◦ C, while at 500 ◦ C, creep only dominates for higher strain range. c 2019 The Authors. Published by Elsevier B.V. his is an open access article under the CC BY-NC-ND license (http: // creativecommons.org / licenses / by-nc-nd / 4.0 / ) eer-review under responsibility of the scientific committe of the IC MSMF organizers. Keywords: Low cycle fatigue; Creep-fatigue interaction; Strain range partitioning; FB2; Creep-resistant steel; Rotor steel Abstract Materials in steam turbine rotors are subjected to cyclic loads at high temperature, causing cracks to initiate and grow. To allow for more flexible operation, accurate fatigue models for life prediction must not be overly conservative. In this study, fully reversed low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done isothermally, within temperature range of room temperature to 600 ◦ C, under strain control with 0.8–1.2 % total strain range. Some tests included hold ti e to calibrate the short-time creep behaviour of the material. Di ff erent fatigue life models were constructed. The life curve in terms of stress amplitude was found unusable at 600 ◦ C, while the life curve in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 ◦ C. To construct better life model, 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 di ff erent damage mechanisms at di ff erent strain ranges. This allowed the formulation of life curves based on plasticity or creep domination, which showed creep domination at 600 ◦ C, while at 500 ◦ C, creep only dominates for higher strain range. c 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http: // creativecommons.org / licenses / by-nc-nd / 4.0 / ) Peer-review under responsibility of the scientific committee of the IC MSMF organizers. Keywords: Low cycle fatigue; Creep-fatigue interaction; Strain range partitioning; FB2; Creep-resistant steel; Rotor steel

1. Introduction 1. Introduction

The e ffi ciency potential of steam power plants depends on the materials used. This is due to the limitations imposed by the material’s mechanical properties at high temperatures. Raising the temperature and the pressure of the steam inlet to the turbine increases e ffi ciency but significantly shortens the life of the turbine components. A development of steels to withstand higher temperatures is important, and the steel class of 9–12 % Cr is a good candidate owing to its creep resistance at high temperatures. The enhancement of this steel class has achieved the requirements for Ultra-Supercritical (USC) power plants, which have steam inlet parameters of 600–620 ◦ C and 300 bar Augusto Di Gianfrancesco (2017); Holdsworth (2004). The e ffi ciency potential of steam power plants depends on the materials used. This is due to the limitations imposed by the material’s mechanical properties at high temperatures. Raising the temperature and the pressure of the steam inlet to the turbine increases e ffi ciency but significantly shortens the life of the turbine components. A development of steels to withstand higher temperatures is important, and the steel class of 9–12 % Cr is a good candidate owing to its creep resistance at high temperatures. The enhancement of this steel class has achieved the requirements for Ultra-Supercritical (USC) power plants, which have steam inlet parameters of 600–620 ◦ C and 300 bar Augusto Di Gianfrancesco (2017); Holdsworth (2004).

2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the ICMSMF organizers 10.1016/j.prostr.2020.01.078 ∗ Corresponding author. Tel.: + 46-13-28-1993. E-mail address: ahmed.azeez@liu.se 2210-7843 c 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http: // creativecommons.org / licenses / by-nc-nd / 4.0 / ) Peer-review under responsibility of the scientific committee of the IC MSMF organizers. ∗ Corresponding author. Tel.: + 46-13-28-1993. E-mail address: ahmed.azeez@liu.se 2210-7843 c 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http: // creativecommons.org / licenses / by-nc-nd / 4.0 / ) Peer-review under responsibility of the scientific committee of the IC MSMF organizers.