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

890

5

initiation at inclusions was only found at low testing temperatures (compared to the transition temperature) or low values of the tempering parameter P as shown in figure 4.

Table 3. Impact toughness transition characteristics for each heat treatment condition. TXXJ: Temperature at an energy of XX J, TK0.9: Temperature at a lateral expansion of 0.9 mm. T50% Temperature at 50% crystallinity. USE: Upper Shelf Energy Cool. rate (°C / h) Temp. T (°C) Temp. length (h) P T56J (°C) Tk0.9 (°C) T50% (crys.) (°C) USE (J)

150 150 150 150

610 640 660 640 610 640 660 640

6 6 6 6 6 6

19.25 19.90 20.33 21.00 19.25 19.90 20.33 21.00

-43 -28 -24 -18 -74 -89 -65 -60

-37 -21 -21 -17 -69 -84 -65 -60

-6

180 192 206 201 207 230 238 245

8 6

20

14

10000 10000 10000 10000

-37 -41 -27 -25

20

4. Discussion

4.1. Impact toughness behavior

The DBTs showed the impact toughness properties strongly depend on the cooling rate. But, even though the tensile properties have an impact on the DBT through the di ff erence between the flow stress and the critical cleavage fracture stress, there is a significant microstructural e ff ect that still needs to be explored. Figure 6 shows that similar di ff erences in yield strength were also found in the instrumented Charpy curves, namely, in the general yield load Pgy, which is the load at which the whole section was plastically strained. On the other hand, for a given cooling rate, the four tempered microstructures exhibited a very similar strain hardening behavior in both tensile and impact toughness tests (figure 1 and figure 6). Thus, for a given value of the hammer displacement (corresponding to plastic bending and notch opening), the stress level in the Charpy specimen could be correlated to the yield strength, and thus, to Pgy. Thus, at given hammer displacement, the higher the cooling rate, the higher the corresponding load, the higher the local stress. In the lower part of the DBT (absorbed energy ≤ 80 J ), the Charpy specimens of various microstructures broke after comparable hammer displacement values (figure 6), and with comparable amounts of ductile crack advance, but with local stress levels increasing with the cooling rate. Consequently, the higher the cooling rate, the higher the critical cleavage fracture stress, in agreement with the finer bainitic microstructure.

Fig. 3. Ductile-to-brittle transitions of all eight microstructures.