PSI - Issue 68

Cainã Bemfica et al. / Procedia Structural Integrity 68 (2025) 1188 – 1195 Ludovic Vincent et al. / Structural Integrity Procedia 00 (2025) 000–000

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(Tomimatsu et al., 2015; Zinkle and Was, 2013). The initial DBTT is highly dependent on the microstructure of the material, which in turns depends on the chemical composition, solidification, forging and heat treatment conditions of the material (Lee et al., 2011; C. W. Li et al., 2016; Yan et al., 2017). In order to contribute to the understanding of the effect of forging and heat treatment conditions, three alloys with similar chemical compositions and yet different manufacturing routes - and subsequently different microstructures - have been mechanically tested in the DBT regime and microstructurally characterized. The key microstructural elements have been incorporated into a modified Microstructure Informed Brittle Fracture Model (Forget et al., 2016) in order to evaluate their influences on the predictions of fracture toughness. In the following, the three materials are briefly introduced followed by a description of the experimental methods used to characterize both their mechanical behaviour and their microstructure. Experimental results are then presented and compared to the predictions of the new version of the MIBF model, after one of its constitutive equations has been modified to incorporate the ferritic grain size, considered as a microstructural element of prior importance in this study. 2. Materials and methodology Three forged low alloy steels with similar chemical compositions (close to the ASME A508 Gr3 Cl2 steel, see Table 1), but different thermomechanical histories were studied in the present work: • A laboratory-scale Model Material (MM) forged from small ingots (15 kg), whose Heat Treatment (HT) consisted of austenitisation (870 °C for 20 minutes), quenching (around 2500 °C/h) and tempering (650 °C for 3h); • A second model material, also obtained from small ingots (100 kg), but submitted first to homogenization (1200 °C for 24h) prior to forging, and then to precaution HT (900 °C, 1h – air cooling – 645 °C, 6h), quality HT (870 °C, 1h – quenching at 2400 °C/h between 800 and 600 °C – 645 °C, 6h30) and finally Post-Weld simulated HT (615°C, 16h). This material will be henceforth labelled as Homogenized Model Material (HMM); • A coupon from the manufacturing control zone of a large forging part, hereafter called Industrial Material (IM). Tensile tests, Charpy V-notch impact toughness and CT(0.5T) Fracture Toughness tests have been performed in the DBT domain following standards recommendations (NF EN ISO 6892-3, NF EN ISO 148-1 and ASTM E 1820&1921 respectively), in order to evaluate the mechanical behaviour and fracture properties of the three alloys. The thickness of Charpy specimens was only 5 mm for MM (10 mm for HMM and IM). Finite Element (FE) simulations of the fracture toughness tests are performed in order to obtain the plastic strain and stress fields at the crack tip that are eventually used as inputs for the Microstructure Informed Brittle Fracture (MIBF) model presented later on. The mechanical behaviour introduced in the FE simulations is described by the following set of constitutive equations: ( ! ) = "# + $ [1 − exp(− $ ! )] + % [1 − exp(− % ! )] (1) where "# ( ) = & + ' exp(− / " ) (2) $ = − $ + % (3) $ ( ) = 26 + 0.095 (4) Table 1: Chemical composition of the three low alloy steels. Material C Mn Ni Mo Cr Si P S Fe MM 0.189% 1.386% 0.764% 0.182% 1.472% 0.762% 0.160% 1.400% 0.710% 0.484% 0.503% 0.480% 0.122% 0.177% 0.160% 0.188% 0.187% 0.200% 0.0040% 0.0077% 0.0070% 0.0013% 0.0012% 0.0024% Bal. Bal. Bal. HMM IM