PSI - Issue 60

B Shashank Dutt et al. / Procedia Structural Integrity 60 (2024) 690–699 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

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1. Introduction Modified 9Cr1Mo (or P91) steel (Bhaduri and Laha 2013) is the material selected for many steam generator components of prototype fast breeder reactor. These components are subjected to operating temperatures in the range of 370 to 550 °C. Characterization of fracture resistance ( J -R curves) and fracture toughness ( J 1c ) of this steel at various temperatures is required for assessment of structural integrity of these components. The authors and other investigators have previously determined tensile properties (Choudhary et al. 1994; Kishore et al. 1997; Keller et al. 2012; Choudhary 2013), fracture resistance (Dutt et al. 2011; Dutt et al. 2018) and low cycle fatigue properties (Nagesha et al. 2002; Mannan and Valsan 2006) of this steel at various temperatures. It is known that for experimental determination of plane strain fracture toughness ( J 1c ) of any steel requires that the tested specimen thickness satisfy minimum thickness criterion as per ASTM (E1820-17a) standard. Dutt et al. (2018) have reported that P91 steel tested in the temperature range of 300-550 °C, of 20 mm thickness satisfied this criterion. Baskes (1975) proposed an empirical equation for prediction of fracture toughness ( K 1c ) from tensile properties. A different K 1c estimation method was derived for various types of steels by Oh (2022). For estimation of fracture toughness, tensile properties and charpy impact resistance were considered. Krishnan et al. (2022) and Janulionis et al. (2021) have applied different numerical (finite element) methods to predict fracture resistance of P91 steels. In this investigation, previous model by Baskes (1975) was considered for estimation of K 1c . The objectives of this study are as follows. The first objective is to estimate K 1c from tensile properties in the range of 300-550 °C. Modulus of toughness and plastic strain energy density are determined from tensile data. From the J R curves determined at 300-550 °C, J 1c values and equivalent K j1c values are determined. The estimated and determined fracture toughness values at various temperatures are compared. The second objective is to estimate K 1c for similar P91 steels (Stratil et al. 2017), in the temperature range of 23-500 °C. Estimation of K 1c will also be carried out for P91 steel (Samant et al. 2020) at room temperature. The P91 steels (Samant et al.2020) were previously subjected to different thermo-mechanical (in the range 550-1050 °C) heat treatments, before tensile testing at room temperature. In this study, in addition to P91 steel, a different grade (SA 33 type) of steel, having different microstructure and tensile properties, is also considered for K 1c estimation. The estimation of K 1c will be carried out for SA 333 of steel (Kamat et al. 2011), previously tested in the temperature range 25-350 °C. The composition of P91 steel is given in Table 1. The steel was procured in the form of plate in the normalized and tempered condition. From the plate, specimen blanks having Transverse Longitudinal orientation were fabricated and were of size 70 mm length, 65 mm width and 25 mm thickness. The gauge length of the tensile specimen was parallel to the rolling direction of the plate. From the blanks, tensile specimens were fabricated having 28 mm gauge length and 4 mm gauge diameter. Tensile testing was carried out using a screw-driven testing machine and at a strain rate of ~ 2×10 -4 s -1 . The temperature during testing was controlled within ± 2 °C. Tensile testing was carried out at a strain rate of 1.2×10 -4 s -1 and different test temperatures of 300, 350, 400, 450, 500 and 550 °C. Tensile testing was carried out as per ASTM E8 standard (E8-16a). The average tensile test result for all the test temperatures was reported by testing 3 numbers of specimens for every test temperature. From the specimen blanks, compact tension C(T) specimens were fabricated having 65 mm length, 62.5 mm width and 20 mm thickness. The C(T) specimens were subjected to pre-cracking under fatigue loading conditions. The initial crack length including fatigue pre-cracking length was targeted as ~ 25 mm, corresponding to crack length to width ratio of 0.5. During pre-cracking, fatigue loading was applied by stress intensity factor ( K ) in decreasing mode. The minimum load applied was ~ 5 kN and maximum load applied was 12 kN during fatigue pre-cracking. The stress ratio of 0.1 was maintained during fatigue loading. The initial frequency of loading was 75 Hz and there was no significant variation in the frequency for the entire duration of fatigue loading. After pre-cracking, C(T) specimens were side-grooved by 10% of original specimen thickness. Fracture testing ( J -R curves) was carried out using a servo-hydraulic testing machine having automated test control and data acquisition system. Fracture tests were carried out at test temperatures of 300, 350, 400, 450, 500 and 550 °C and at load-line displacement rate of 2. Experimental