Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels?

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.