The influence of microstructural anisotropy on secondary cracking in ferritic ODS steels


The influence of microstructural anisotropy on secondary cracking in ferritic ODS steels

Das, A.; Viehrig, H.-W.; Altstadt, E.; Bergner, F.; Heintze, C.

ODS steels are known to exhibit anisotropic fracture behaviour owing to their anisotropic microstructure and form secondary cracks. Secondary cracks are observed more often in hot-rolled than in hot-extruded ODS steels. They tend to absorb energy and help in stabilizing primary crack propagation at low temperatures but initiate at lower loads than primary cracks. In this work, a correlation is made between the microstructural anisotropy of three ferritic ODS steels and the secondary cracking induced in these materials. Better understanding of these factors can lead to tailoring of improved ODS steels. Fracture toughness testing of three batches of ferritic ODS steels, one hot-rolled and two hot-extruded, were carried out using small C(T) specimens. 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 dominant role in the formation of secondary cracks in hot-rolled ODS steels. Secondary crack initiation at low loads in hot-rolled material is predominantly due to anisotropic grain morphology. At lower temperatures, secondary cracks occur via transgranular cleavage while at higher temperatures, the fracture mode changes to ductile and intergranular.

Keywords: secondary cracking; ODS steels; fracture; microstructure

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Publ.-Id: 29623