Characteristics of Low Temperature Brittle Fracture Surface of High Nitrogen Austenitic Steel

Fracture surface and crack propagation in low temperature brittle fracture (LTBF) of an 18Cr-18Mn-0.7N high nitrogen austenitic steel (HNAS) were examined by means of scanning electronic microscopy, and compared with behaviours of LTBF of low carbon steel. Similar to BCC low carbon steel, the HNAS experienced a typical ductile-to-brittle transition (DBT) with decreasing temperature, and the appearance of the fracture surface transited from fibrous to granular. Dual-surface observation revealed that there were three types of fracture modes in LTBF of the HNAS: annealing twin boundary fracture, intergranular fracture, and transgranular fracture. The annealing twin boundary fracture facets were parallel to {111} planes, and were fairly flat and smooth, with a pattern of three sets of parallel straight-lines intersecting at 60. There were also bent steps that originated and terminated at grain boundaries. The transgranular fracture facets were coarse and uneven, with uniformly distributed small pits and partially river pattern on them. The intergranular fracture facets were smoothly curved ones on which more than three sets of parallel deformation structure trace lines were observed. Careful observation on crack propagation demonstrated that during LTBF of the HNAS, microcracks formed firstly at grain boundary and annealing twin boundary, and then these microcracks came together and coalesced to induce crack propagation through grains, resulting in a fracture appearance with shiny facets distributing in dull facets.

The Structure of an Incoherent Annealing Twin Boundary in FCC Alloys

Because of the simple crystallographic relationship between matrix and twin in FCC materials, an incoherent twin boundary can be thought of in terms of an array of 1/6 <112> Shockley partials or 1/3 <111> Frank partials such as that shown schematically in Figure 1. Here the interface is made up of alternately spaced Frank partial s of opposite sign; however, these could be replaced with appropriate Shockley partials such that no far-reaching strain field is associated with the interface.