PSI - Issue 13

Hugo Wärner et al. / Procedia Structural Integrity 13 (2018) 843–848

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Hugo Wärner et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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Austenitic stainless steels have previously been used for components in ultra-super critical (USC) thermal power plants (Gibbons 2013;Blum et al. 2004;Viswanathana et al. 2006). Usually, advanced austenitic stainless steels are designed to withstand temperatures up to 650 °C. Sandvik Sanicro TM 25 (Sanicro 25) is a newly developed advanced high strength heat resistant austenitic stainless steel. Sanicro 25 possess good resistances to steam oxidation and flue gas corrosion and has higher creep rupture strength than other austenitic stainless steels available today (Rautio et al. 2004;Chai et al. 2010;Chai et al. 2013). Thus, it is an excellent candidate for superheaters and reheaters for A-USC power plants. Recently the low cycle fatigue (LCF), thermo-mechanical fatigue (TMF) and creep-fatigue interaction (CF) behavior at elevated temperature of Sanicro 25 have been reported by Chai et al. (2013), Polák et al. (2014), Petráš et al. (2016), Wärner (2017) and Wärner et al. (2018). This paper provides a combined discussion on LCF, TMF and CF behaviors of virgin and aged Sanicro 25 at elevated temperature. The influence of aging and dwell time on the fracture behavior and cyclic life is evaluated. Electron microscopy is used to characterize the different fracture and damage mechanisms. Austenitic stainless steel grade UNS S31035, Sanicro 25, was supplied by Sandvik, Sweden, with a nominal chemical composition of Fe-22.5Cr-25.0Ni-3.6W-3.0Cu-1.5Co-0.5Nb-0.5Mn-0.23N- ≤ 0.1C in wt.% in solution heat treated condition. The material was solution heat-treated at a temperature of 1250 °C for 600 s followed by water quenching (Sandvik AB, 2017). For the LCF testing cylindrical specimens with a 6 mm diameter and a 15 mm gage length with button ends were used. For the TMF testing cylindrical specimens with a diameter of 6 mm and a gauge length of 15 and 25 mm were used. Cylindrical specimens with a diameter of 10 mm and a gauge length of 15 mm were used for the CF testing. Some specimens were pre-aged at 650 °C, 700 °C and 800 °C for 2000 hours to simulate prolonged service degradation. The microstructure of Sanicro 25 prior to the deformation is shown in Fig. 1. 1.1. Sanicro 25

120 µm

Fig. 1: Microstructure of Sanicro 25 prior to deformation.

1.2. Mechanical testing The LCF testing was performed using an electrohydraulic MTS system. Fully reversed strain controlled cyclic loading with constant strain rate 2 x 10 -3 s -1 at temperature 700 °C was applied, for more details see Polák et al. (2014). The TMF testing were performed in strain control using standard servo hydraulic testing machines in the temperature range ( ΔT ) of 250 to 700 °C using in-phase (IP) cycles, and in the temperature range of 100 to 800 °C in IP with a dwell time (t d ) of 300 s. For more details about the TMF testing see Petráš et al. (201 6) and Wärner et al. (2017) respectively. For the CF testing an MTS servo hydraulic testing machine was used. Strain controlled cyclic loading was performed in tension with constant strain rate 2 x 10 -3 s -1 , and with different dwell times at maximum strain in load control at 700 °C. For more details about the CF testing see Wärner et al. (2018).