PSI - Issue 60

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Chitresh Chandra et al. / Procedia Structural Integrity 60 (2024) 165–176 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Electricity generation, its storage, dissemination and efficient conversion to other forms are one of the most prominent pillars of industrial development. The sources from where this power is generated also plays a major role in determining the cost and ease of power production. Fossil fuels are the major source of power generation. Among the fossil fuels, coal accounts for over half (58.7%) of the electricity produced in the world (Statistics W. E. 2018). Due to the rising concerns for efficiency and greenhouse emissions, there has been a significant public outcry over the environmental consequences effected by power generating stations, which has in turn fuelled R&D attempts to reduce particulates and greenhouse gases in the exhaust emissions. One of the definitive methods to effectively curtail the number of particulates is to boost up the efficiency, and a definite way to attain it is by operating the powerplant at higher temperatures (A. Maxson et. al. 2011). The implementation of higher operational temperatures necessitates development of materials capable of maintaining structural and compositional integrity under thermo-mechanical loads at high temperatures. As the operational cycle generally comprises of start-up followed by long term operation at elevated temperatures under heavy load and eventual shut down, creep-fatigue considerations ought to be taken into account during design (Saxena A. 1993). The synergistic interaction between cyclic and static loads result in component failure. In addition to initiating the nucleation of new cracks, such loading regimes accelerate the coalescence of micro-cracks and propagation of already existing cracks, present within the components due to the manufacturing and processing phenomena undergone by the material (Zhao L. et. al. 2017, Saxena A. 2015, Lu Y. L. 2007) . ASTM “E -2760-10 was specifically developed in 2010 to evaluate and quantify the behaviour of samples with prior cracks under trapezoidal loading waveforms (ASTM E2760 2019). Modified 9Cr-1Mo steel is the preferred material for structural applications in power plants with increased efficiency (Shrestha T. et. al. 2015). This material is designated by the American Society for Testing and Materials (ASTM) as grade T91/P91 steel. The prefixes in this designation correspond to tubing and piping applications, respectively. This material is commonly used in components such as steam headers, superheater and reheater tubes in ultra-super critical power plants (Shibli A. et. al. 2007). P91 steels are a ferritic-martensitic (F-M) class of steel with superior creep strength. The first advantage of the use of 9-12% Cr is the high chromium content, which allows the formation of a protective and continuous scale in the case of dry oxidation in severe conditions (high temperature and steam) (Quadakkers W. J. et. al. 2010, Jianian S. et. al. 1997, Zhong X. et. al. 2012). F-M steels have been considered to be superior over austenitic stainless steel due to its low coefficient of thermal expansion, good thermal conductivity and high creep rupture strength (Babu S. H. et. al. 2013, Coussement C. et. al. 1991, Pandey C. et. al. 2017). Apart from high creep strength, P91 steels also possess ductility, toughness, high resistance to stress corrosion cracking and oxidation. Good weldability and microstructural stability at elevated temperature for a long time is also an attractive property of P91 steel (Ennis P. J. et. al. 2003). Creep fatigue crack growth tests for high temperature materials have been covered in several articles. Researcher have evaluated the CFCG behaviour of different materials (i.e., 316L, P22, P91 and 1CrMoV) under different test conditions like frequency, temperature and testing environment, and the result were compared with the available fatigue crack growth and creep crack growth data. It was inferred that, at the threshold period, CFCG rates vary directly with respect to the loading period irrespective of the loading waveform, signifying the presence of time dependent crack propagation mechanism. The kinetics of crack propagation shifted from trans granular to intergranular with increase in hold time. The production of micro-voids and microcracks beyond the fracture tips owing to the creep damage throughout the dwell period is the primary damage mechanism (Mehmanparast A. et. al. 2011, Narasimhachary S. B. et. al. 2013, Bassi F. et. al. 2015, Ab Razak et. al. 2018, Lu Y. L. et. al. 2006). Multiple articles are available wherein CFCG behaviour of the P91 steel has been evaluated at multiple temperatures and with different hold times but with a force ratio of only 0.1. There are only a very few articles available in the open literature on the influence of different force ratios on creep fatigue crack growth behaviour (Chandra C. et. al. 2022). This need to be further explored in detail. The present work describes the experimental analysis of CFCG on C(T) samples of P91 steel at 600 °C for a dwell period of 10 sec and 600 sec with force ratios of 0.1, and 0.5 with initial stress intensity factor of 20MPa√m. The crack