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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Tensile and impact toughness specimens were then machined from each blank, with the loading direction parallel to the axis of the initial shell. Six uniaxial tension samples of gage diameter 3 mm and gage length 15 mm were tested using an Instron 4507 tensile testing machine with a load capacity of 20kN. The tensile properties were evaluated at -120°C, 20°C and 100°C, with 2 specimens by temperature, under a prescribed elongation rate of 5 . 10 − 4 s − 1 . The testing temperatures were chosen to estimate the temperature dependence of the tensile properties and to estimate tensile properties over the whole temperature range of the ductile-to-brittle transition (DBT) of the material. The DBT behavior of each microstructure was also determined. 15 Charpy ISO-V impact toughness specimens (10 × 10 × 55 mm with a 2-mm-deep V-notch) were tested on an instrumented Roell Amsler RKP 450 Charpy impact facility at various temperatures. The device capacity was 300 J and the pendulum impact speed was 5.23 m / s. The specimens were put at temperature in a nitrogen-cooled chamber and they were automatically transferred to the testing machine within less than 2.4 seconds. During each test, the load vs. hammer displacement curve was recorded and the fracture energy, the lateral expansion and the crystallinity were determined (Testing was realized according to the ISO148-1 standard and the crystallinity was determined according to the ISO14556 standard). Then, the following equation 2 was fitted to the results and the transition temperatures were determined. Finally, to investigate the brittle fracture mechanisms, a systematic fractographic study was conducted in the scan ning electron microscope (SEM) on all Charpy samples in the lower part of the ductile-to-brittle transition (absorbed energy lower than 80 J). The brittle fracture initiation site was identified on both parts of each broken specimen. Its location on the fracture surface (distance to side surfaces and distance to the ductile tearing crack front), its mi crostructural features, if any (initiation at an inclusion, for example), the size of the first cleavage facet, the location of the site relative to the first cleavage facet were determined. Energy dispersion spectrometry (EDS) mapping was systematically conducted on the initiation site, together with point analysis if there was any noticeable feature near the site. Table 2 gives the average room temperature tensile properties for all eight heat treatment conditions. There were very little variations between the results of the two tests performed at each temperature. The results were in agreement with previously reported ones (Renevey; Marini et al.; Andrieu; Li et al.). Figure 1 presents the experimental stress strain curves at room temperature. The lower yield stress was determined even though the curves presented a yield point as shown in figure 4. The eight microstructures only exhibited slight di ff erences in plastic flow properties because of the tempering heat treatment. Figure 2 represents the evolution of the 0.2% proof stress and of the tensile strength with cooling rate and tempering parameter. Both the yield strength and the ultimate tensile strength increased with the cooling rate (by ca. 50 MPa from 150°C / h to 10000°C / h at given tempering conditions). Both of them tended to decrease with increasing the tempering parameter, especially for lower values of P . According to table 2, the fracture elongation tended to decrease with the cooling rate and to increase with P up to P = 20 . 33 then started to decrease. E ( T ) = E U + E L 2 + E U − E L 2 tanh T − T t T w (2) 3. Results

Table 2. Average room temperature tensile properties for each heat treatment conditions. YS : Yield Strength, UTS :Ultimate Tensile Strength Cool. rate (°C / h) Temp. T (°C) Temp. duration (h) P

Lower YS (MPa) UTS (MPa) Fracture elongation Reduction of area at fracture

150 150 150 150

610 640 660 640 610 640 660 640

6 6 6 6 6 6

19.25 494 19.90 464 20.33 433 21.00 421 19.25 553 19.90 503 20.33 494 21.00 472

650 621 593 576 687 638 635 608

0.24 0.24 0.26 0.23 0.21 0.23 0.25 0.24

0.72 0.73 0.73 0.73 0.73 0.77 0.77 0.78

20

10000 10000 10000 10000

20