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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As expected, the hardness values varied widely throughout the weldment. However, all values remained below the 450 HV10 threshold specified in ISO 15614-1:2017 (International Organization for Standardization (2017a)), confirming the accuracy of the parameters and the quality of each welded joint. The highest recorded value, 430 HV10, was observed in the HAZ, suggesting that this region may be susceptible to brittle fracture. The average hardness value in the base material region, at 282 ± 8 HV10, demonstrated greater consistency across all measurements, as it was una ff ected by temperature fluctuations. The lower HAZ hardness values in the thicker joint can be attributed to the slower cooling rate inherent in thicker materials. Thicker materials tend to cool more gradually than their thinner counterparts due to their higher thermal dissipation characteristics. Slower cooling rates can lead to larger grain sizes and potentially lower hardness values. On the other hand, thinner materials cool more rapidly, resulting in finer grain structures and higher hardness values.

3.2. Hardness test analysis - Low force Vickers hardness test

The microstructure of S690QL steel consists of three main zones: the weld material (WM), the heat-a ff ected zone (HAZ), and the base material (BM). In order to di ff erentiate the HAZ from the weld and base material, a portion of the welded joint containing these three zones was chemically etched with Nital 2%. Figure 8 provides an overview of the microstructure comprising the three main zones of the S690QL welded joint, along with three distinct sub-zones within the HAZ, as identified through optical microscopy. 1 RESULTS AND DISCUSSION

HAZ closer to fusion line

1

3

2

3

Fig. 8. S690QL welded joint microstructure main zones and HAZ sub-zones (5 × ). A complementary low-force hardness test was carried out to more precisely identify the constituents and sub zones within the heat-a ff ected zone. This section of the welded joint has a high degree of irregularity in morphologies and subsequent mechanical properties. This variability is attributed to the varying temperatures reached and cooling rates as one moves away from the transition section of the fusion zone. The heat-a ff ected zone is characterized by the gradual increase of grain size towards the transition zone, but also by several sub-zones with somewhat distinct morphologies and sizes, as illustrated in Figures 9.a)-c). Figure 10 is a schematic representation of these sub-zones, corroborating with the optical microscopy, previously performed in Figure 8. It is possible to establish a correlation between zone 1 and Figure 9.a), zone 2 and Figure 9.b) and zone 3 and Figure 9.c). The use of a low-force microhardness test allowed for more precise characterization of the heat-a ff ected zone through finer indentations in each sub-zone. This approach will determine whether the small fraction of the previously unidentified constituent present in the HAZ, heated to intercritical temperatures (between Ac 1 andAc 3 ), is whether or not actually composed of a ferritic phase, as indicated in the CCT diagram (Sey ff arth et al. (2001)) from

HAZ c