A STUDY ON FRACTOGRAPHY IN THE LOW-TEMPERATURE BRITTLE-FRACTURE OF AN18CR-18MN-0.7N AUSTENITIC STEEL

The fracture mode and crack propagation behavior of brittle fracture a t 77 and 4 K in an 18Cr-18Mn-0.7N austenitic stainless steel were inve stigated using optical and scanning electron microscopy. The fracture path was examined by observing the side surface in a partially rupture d specimen. The relationship of the fracture facets to the microstruct ures was established by observing the fracture surface and the adjacen t side surface simultaneously. Three kinds of fracture facets were ide ntified at either temperature. The first is a smoothly curved intergra nular fracture facet with characteristic parallel lines on it. The sec ond is a fairly planar facet formed by parting along an annealing twin boundary, a real {111} plane. There are three sets of parallel lines on the facet and the lines in different sets intersect at 60 deg. The third is a lamellar transgranular fracture facet with sets of parallel steps on it. Fracture propagated by the formation of microcracks on a grain boundary, annealing twin boundary, and coalescence of these cra cks. The observation suggests that the ease of crack initiation and pr opagation along the grain boundary and the annealing twin boundary may be the main reason for the low-temperature brittleness of this steel. A mechanism for grain boundary cracking, including annealing twin bou ndary parting, has been discussed based on the stress concentration in duced by impinging planar deformation structures on the grain boundari es.