PSI - Issue 54

Paulo Mendes et al. / Procedia Structural Integrity 54 (2024) 340–353 Mendes et al. / Structural Integrity Procedia 00 (2023) 000–000

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Fig. 11. Hardness distribution across each heat-a ff ected sub-zone.

size (zone 2). It is possible to conclude that the hardness increases with a more refined grain size. In Figure 9.c), it is possible to identify a di ff erent constituent, which is found in the grain boundaries and can be definitively identified as ferritic due to its significantly lower hardness values compared to those of the base material. Ferrite located at the grain boundaries can introduce weaker regions within the microstructure. It is softer than martensite and bainite, and as a result, localized stress concentrations can form at grain boundaries, which in turn increase their susceptibility to crack initiation. Additionally, it can act as a preferential site for the accumulation of impurities, such as sulfur and phosphorus, further compromising the steel’s strength and making it more prone to crack initiation. Under cyclic loading conditions, this can significantly reduce the fatigue life and increase the likelihood of failure of the S690QL steel component (Bhadeshia and Honeycombe (2017)). The base material hardness values align with the previously conducted macrohardness test results, providing further evidence that this region of the welded joint is composed of a tempered martensite-bainite matrix, as its hardness is significantly lower compared to the heat-a ff ected zone. The morphology, size, and density of dimples on the surface di ff er between the weld material and the base material, as does the presence of inclusions within each one. Figure 12 depicts the overload area of fractured specimens used in a rotating bending fatigue test campagin from di ff erent zones of the welded joint and the EDS results from an assessment of the inclusions inside the dimples, which exhibit varying morphologies and compositions. This analysis reveals variations in the nature and composition of inclusions across di ff erent zones, suggesting that both chemical composition and the temperature cycle imposed influence their existence. The EDS spectrum can be used to qualitatively evaluate several zones that are representative of inclusions for better understanding. In the base material, the fracture surface is typically free of inclusions, with only a few small ones that originate from the sheet metal manufacturing process. An EDS spectrum (Figure 12.c)) identifies one of these inclusions as manganese sulfide. This type of inclusion is common in steels and can a ff ect the mechanical properties of the material, particularly in materials subjected to hot working processes like rolling and forging.These inclusions are soft and deformable, so these processes can elongate and plastically deform them, leading to anisotropic microstructures and properties. In this case, the elongated shape of the MnS inclusion likely resulted from the hot rolling process used to manufacture the plate. The fracture surface of the weld material has a di ff erent appearance compared to that of the base material. The weld material contains numerous inclusions, which are spherical in shape. According to the EDS analysis, the inclu- 4. Microstructural characterization 4.1. Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS)