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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4.2. Cleavage fracture initiation sites

Whatever the heat treatment condition examined here, and thus the resulting tempered microstructure, cleavage fracture initiation mechanisms as seen in SEM remained similar. All cleavage initiation sites were located next to (within 1 µ m) the edge of a cleavage facet, i.e., close to a high-angle boundary of the material (that could be either a packet boundary or a parent austenite grain boundary). Most fracture events occurred from a boundary where no particular constituent seemed to directly participate to the fracture initiation process. Thus, it seems that the first-order microstructural feature for the initiation of brittle cleavage fracture was the boundary. As no intergranular decohesion was found at any fracture initiation site, the boundary should be considered not as a brittle interface, but as a stress concentrator due to the incompatibility of the plastic deformation of the two misoriented grains. From the present experimental data, it was not possible to determine which kind of boundary was involved in each fracture process. The presence of a particle did not seem necessary but still could have an impact on local stresses. The present observations challenge the common hypothesis that brittle fracture is usually caused by carbides or inclusions (Beremin et al.; Forget et al.) and the observations reported in other studies (Lee et al.). While these findings have been evidenced here in the lower part of the DBT curve, the e ff ect of brittle constituents on brittle fracture further in the transition are still to be evaluated. It seems that the critical cleavage fracture stress increases with the cooling rate and decreases with increasing the tempering parameter. This result is in a good agreement with the evolution of the transition temperature. However, the origin of the dependence of the cleavage fracture stress on the microstructure remains to be investigated firstly from the di ff erences in misorientation distributions Starting from a single industrial material with low impurity and inclusion content, eight microstructures were obtained, which only di ff ered in the cooling rate at quenching after austenitization and in tempering conditions. Their impact toughness properties were determined in relation with microstructure and tensile properties, leading to the following results: • The e ff ects of cooling rate and tempering conditions on the ductile-to-brittle transition and on the tensile prop erties were quantified. Tempering tended to equalize some of the tensile properties (namely, tensile strength and ductility) but di ff erences clearly remained, for di ff erent cooling rates, in the impact toughness behavior. • Only one type of location for the cleavage initiation sites, common to all cooling rates, was identified. Fracture did not seem to initiate from large inclusions or carbides, allowing focusing on cleavage fracture properties of the matrix itself. • Cleavage crack initiation always occurred in the very vicinity of a boundary (supposedly a high disorientation angle boundary). MnS inclusions and Ti(CN) precipitates may play a role, but they did not seem to be necessary for cleavage initiation. The major contribution to stress concentration could be the local misorientation between ferrite crystals, not the presence of a brittle or hard particle, or, even, a brittle boundary. • The critical stress for cleavage was tentatively estimated from the instrumented impact testing curves. It in creased with the cooling rate and decreased with the tempering parameter. This is consistent with the observed di ff erences in the ductile-to-brittle transition behavior with the heat treatment conditions. As the fracture initia tion mechanism is always the same, this dependence remains to be investigated. 5. Conclusions

6. Acknowledgements

The authors thank Patrick Todeschini at EDF for his technical support Ian Zuazo from ArcelorMittal Industeel for the realisation of the heat treatments. They also thank Elodie Pons, Pierre Wident, Robin Le Coz, and Jean-Luc Flament at CEA (SRMA / LC2M) for the mechanical tests and Ame´lie Ganglo ff at CEA (SRMA / LA2M) for support for hardness testing, fractographic study, specimen preparation and metallographic observations.