PSI - Issue 42

Jean-Baptiste Delattre et al. / Procedia Structural Integrity 42 (2022) 886–894 Jean-Baptiste Delattre / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1. (On the left) 150°C / h; (On the right) 10000°C / h. Experimental stress-strain curves for all room temperature tensile tests. The engineering strain was computed from the crosshead displacement measurements.

Fig. 2. Yied Strength and Ultimate Tensile Strength for all heat treatment conditions

As shown in figure 3 there was approximately a 50°C di ff erence in the T56J transition temperature and a 40J di ff erence between the upper shelf energies of the 150°C / h and 10000°C / h microstructures (table 3). This is all the more important as these variations were induced by the heat treatments, starting from the same as-received material. For a given temper, the transitions with di ff erent cooling rates di ff ered significantly between the two cooling rates. For a given cooling rate, the behaviours after various tempering conditions were close to one another. This shows that, while the cooling rate has a significant impact on the microstructure, it does not seem possible to homogenize the impact toughness through tempering. In order to link this mechanical property to microstructural features, all impact toughness samples below 80 J were observed with a SEM. All these specimens fractured by cleavage and, in some case, after some ductile tearing depending on the fracture energy. Figure 5 gives typical examples of fracture initiation sites. All initiation sites were located very near the edge of the first cleavage facet where cleavage starts propagating. Most initiation sites did not present any particular mi crostructural feature like inclusions (“interface only” labels in figure 4). This had also been already reported elsewhere (Rosenfield et al.). In these cases, the EDS analysis did not reveal any inclusions at the initiation site. In some cases, carbides were observed in the vicinity of the initiation site but their role in the fracture initiation remains unclear. Fracture initiated from inclusions in 11 specimens out of 43. MnS and Ti(C,N) inclusions were identified. Fracture