Abstract
Secondary cracks are known to absorb energy, retard primary crack propagation and initiate at lower loads than primary cracks. They are observed more often in hot-rolled than in hot-extruded ODS steels. In this work, the microstructural factors responsible for this observation are investigated. Better understanding of these factors can lead to tailoring of improved ODS steels. Fracture toughness testing of two batches of 13Cr ODS steel, one hot-rolled and the other hot-extruded, was carried out. The fracture behaviour of secondary cracks was investigated using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Crystallographic texture and grain morphology play a predominant role in preventing secondary cracks in hot-extruded ODS steels. At lower temperatures, secondary cracks occur predominantly via transgranular cleavage. The fracture mode changes to ductile and intergranular at higher temperatures.
Highlights
ODS steels are candidate materials for cladding tubes in advanced nuclear fission reactors and are candidate structural materials for fusion devices
At temperatures below the ductile to brittle transition temperature (DBTT), the occurrence of secondary cracks in hot-rolled materials is assisted by the {100} cleavage planes which are aligned parallel to the rolling plane
At temperatures higher than the DBTT, secondary cracking is more frequent in hot-rolled than in hot-extruded materials as the critical fracture stresses in secondary crack planes are lower due to the two dimensionally anisotropic elongated ‘pan-cake’ shaped grains which allows free crack propagation in two directions
Summary
ODS steels are candidate materials for cladding tubes in advanced nuclear fission reactors and are candidate structural materials for fusion devices. Nanostructured ferritic alloys (NFAs) are variants of ODS steels consisting of finer microstructures, with Cr content more than 12 wt.%, nano-particles in the size range of 1–10 nm and grain sizes in the sub-micron range [1,2,3]. The secondary cracks propagate in a plane perpendicular to the primary crack plane and are formed due to constraint induced stress. They may appear under certain conditions due to the phenomenon of delamination [4,5,6,7,8], known as splitting [4,6,9,10], which inhibits the primary crack propagation. ODS steels possess different microstructural regions, which behave like different layers of a composite. There is usually no sharp transition between these microstructural regions
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