Crack propagation paths during the low-temperature brittle fracture of 18Cr-18Mn-0.7N austenitic steel were investigated by using scanning electron microscopy. The correspondence between fracture facets and microstructures was established by simultaneous observations of the fracture surface and of the microstructure of the adjacent surface. It was shown that annealing-twin boundary-cracking occurred during fracture. A large number of twins formed during solution treatment; with steps that were several μm-high on the twin boundaries. A considerable number of planar deformation structures developed on {111} planes during fracture. The fracture facets of the annealing twin boundary were quite flat and smooth; with bent steps of μm-scale height, and a pattern of 3 sets of parallel straight lines that intersected at 60°. There was no river pattern on the facets. The bent steps resulted from the partial propagation of cracks along steps that developed on annealing-twin boundaries during solution treatment. The line-pattern was made up of the intersections of the planar deformation structures with the fracture facet. It was suggested that the annealing-twin boundary-cracking was due to stress concentrations that arose at the intersections of planar deformation structures with annealing-twin boundaries.

Annealing Twin Boundary Cracking in the Low-Temperature Brittle Fracture of a High-Nitrogen Bearing Austenitic Steel. S.Liu, S.Liu, D.Liu: Journal of Materials Science, 2004, 39[8], 2841-8