PSI - Issue 13

Ghassen Ben Salem et al. / Procedia Structural Integrity 13 (2018) 619–624

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Ben Salem Ghassen / 00 (2018) 000–000

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Figure 2. Optical micrograph of the dissimilar interface : (a) in the as-welded state (b) after PWHT [1]

Figure 3. (a) CT specimens composition and position of mechanical crack tip (b) Fracture toughness vs. test temperature for CT specimens of 16MND5 / (309L / 308L) classified by the austenite proportion in the fatigue precrack

3.2. Failure surface analysis

An important dispersion in the brittle-to-ductile transition domain is illustrated on the toughness-temperature curve (Fig.3(b)) and could be explained by the metallurgic singularity in the interface and also by the di ff erence in the crack tip position between specimens. Post-mortem observations of fracture surfaces revealed di ff erences in precracked zones between specimens : some (CT1, CT3, CT6) were mainly in the ferritic side whereas others (CT2, CT5, CT8) have a mixed precracked surface between austenitic and ferritic steel as illustrated for CT20 on Fig.4(a). The presence of austenitic zones in the fatigue precrack leads to lower toughness values (i.e CT2, CT20...) whereas specimens with the highest toughness values have homogeneous fatigue precrack tips mainly in the ferritic side. SEM observations of mixed precrack specimens (CT20 in Fig.4(b)) show that the specimen failure seems to initiate from the austenitic zones with an intergranular brittle fracture mode. The grain boundaries observed indicate an intergranular fracture originating from coarse grains, characteristic of the austenitic buttering, or of the former austenitic grains of the martensitic layer. We can conclude that the brittle mechanism initiated at the Martensite Austenite interface (MA interface). The failure mechanism responsible for the low fracture resistance of the CT specimens with mixed precrack seems to be an intergranular initiation at the MA interface after a ductile deviation of the crack from the austenitic starting front to the FL. In the case of a mainly ferritic precrack, the crack does not seem to deviate towards the MA interface and the initiation of the brittle fracture happens by cleavage in the ferritic HAZ, leading to higher toughness values. The embrittlement caused by the intergranular initiation on the MA interface confirms the metallurgical analysis of the previous section and the mechanical and microstructural heterogeneities existing at this interface. 3