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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In order to detect the presence of potential chemical microsegregation (at a millimetric scale), Vickers microhardness maps have been obtained 5 mm beneath the crack propagation plane of one broken Charpy specimen of each material. For all maps, a load force of 0.20 kg was used, and the grid step between two measures was 150 µm, resulting in 1798 points for MM (5 mm x 10 mm reduced thickness cross section) and 3779 measuring points for HMM and IM (10 mm x 10 mm cross section). Electron BackScattered Diffraction (EBSD) maps have then been performed on similar planes in order to characterize the crystallographic microstructure of each material. The acquisition system was a Quantax system (Brucker) plugged in a JSM 7001F LV (JEOL) Scanning Electron Microscope (SEM). The scan step was 0.25µm, and the size of the maps was 300 * 225 µm², resulting in more than a million points. The Probability Density Function (PDF) of misorientation angles (discarding values smaller than 2.5°) was estimated to compute microstructural parameters that help differentiating such ferritic microstructures (granular, upper or lower bainite, martensite,…). Among these parameters, the ratio R of the PDF value at the misorientation angle peak observed at 60° over the one at 54° was estimated. Moreover, the average size of the ferritic grain was determined considering that such a grain corresponds to a region within which there is no misorientation angle between two neighbouring pixels larger than 15°. Fractographic analyses have been performed on HMM and IM Charpy V-notch and CT(0.5T) specimens to determine the brittle fracture mode and nature of brittle particles at onset of cleavage initiation. For this, broken specimens at temperatures close to the one for which a mean absorbed energy of 7 daJ/cm² (equivalent to 56 J for standard Charpy specimens) and a mean fracture toughness of 100 MPa.m 0.5 (Master Curve reference temperature T 0 ) were analysed. Whenever a precise microstructural phase was identified, its chemical composition was investigated using Energy-Dispersive X-ray spectroscopy (EDX) qualitative analyses on a Bruker XFlash 6-30 detector. To minimize the influence of matrix composition on the EDX spectra of small fracture initiators (< 1 µm), a sensitivity analysis was carried out, from which the following parameters were obtained: acceleration voltage of 10 keV, dead time of 20–40% and 1–2 million counts per spectrum. For this class of low-alloyed steels, the main microstructural constituents at the origin of brittle failure are indeed reported in the literature to be carbides (Chekhonin et al., 2023; Delattre, 2023; C. Li et al., 2016; Naylor, 1979; Zhang and Knott, 1999). Therefore, the population of carbides present in each material has been characterized by image analysis of the JEOL SEM images (x5000, 10nm pixel size) of polished and 2% nital etched metallographic cuts. The region analysed on each material contained several thousands of carbides in order to properly estimate the surface density of carbides Ns and the PDF of their sizes. This last function is then described by a Lee-Weibull distribution (Lee et al., 2002): = 2 ?2 − A ()$ D− ?2 − A ( E (5) in which α, β and γ are the distribution parameters, and r is the equivalent radius of the carbide. 3. Results of material characterization and discussions The comparisons between experimental and simulated stress-strain uniaxial tensile curves obtained at different temperatures are reported on Figure 1 for the three materials. Experimental tensile curves exhibit a Lüders plateau that is not taken into account in the present fit since the objective is to describe the constitutive behaviour for the large stress-strain levels present at the crack tip. Due to experimental limitations, the parameters governing the large stress strain behaviour (Q2 and b2) could only be determined for the MM (Ren et al., 2022) and are assumed to be identical for all materials.