PSI - Issue 5

J. Belzunce et al. / Procedia Structural Integrity 5 (2017) 1275–1282 J.Belzunce et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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3.5. Fracture toughness tests Fig. 5 shows the J-R or J-  a curves obtained with these steel grades under different applied displacement rates. The J Q (kJ/ m 2 ) initiation parameter obtained for a crack growth of 0.2 mm, the embrittlement index (EI) for J Q , and the hydrogen contents in the specimens along the tests (derived from the desorption curves) can be seen in Table 2. 42CrMo4_500 with internal hydrogen behaved in a very brittle way and only a K max (or J max ) toughness parameter could be obtained. The effect of hydrogen was especially significant in these tests, since a very clear decrease of J Q value. J Q parameter decreased at least around 50 % in all these tests, although it is also remarkable that the J initiation values in the presence of hydrogen for these steel grades are considerably high, except for the steel with the highest strength (42CrMo4_500), which behaved in a total brittle manner with internal hydrogen. In accordance with this behaviour, important changes in the fracture mechanism were also observed. Fig. 6a and 6b correspond to the 2.25Cr1Mo_600 grade tested under a displacement rate of 0.01 mm/min. The typical ductile fracture mechanism, characteristic for uncharged specimens (Fig. 6a), changed to a brittle intergranular fracture (Fig 6b) when H is present.

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Fig. 5. J-R curves obtained for the different steel grades. (a) 2.25Cr1Mo_690, (b) 2.25Cr1Mo_600, (c) 42CrMo4_700 and (d) 42CrMo4_500.

In addition, the H-charged specimens of 42CrMo4_700, tested at a displacement rate of 0.01 mm/min (EI=52%), show a peculiar growth of microvoids. Fig. 7d shows the typical features observed in this steel when tested with hydrogen: quite large and flat areas, that are not cleavage facets, but large and shallow voids (black arrows), that have grown due to the enhanced and localized plasticity produced by hydrogen accumulation in these areas [7]. 2.25Cr1Mo_690 showed similar fracture mechanism.