Mechanism of low temperature brittle fracture in high nitrogen bearing austenitic steels

Abstract

Low temperature embrittlement is studied for three high-nitrogen bearing austenitic steels, 18Mn–18Cr–0.5N, 18Mn–18Cr–0.8N and 17Cr–13Ni–2Mo–0.5N (all in mass%) steels. These steels all show ductile-to-brittle transition behavior. They have fractured in a cleavage-like mode when tested at 77 K by a Charpy impact tester and the fractured surfaces have shown facets along {111} planes identified by using etch-pits and scanning electron microscopy. Slip traces have frequently been observed on flat facets and fracture has taken place along {111} planes by slipping-off, i.e. separation occurring along an active slip plane with a high density of dislocations. Transmission electron microscopy has revealed that there are many arrays of planar dislocations and dislocation bands just below the fractured surface. Neither twinning nor epsilon martensite has been observed. Fracture characteristics observed are primarily related to brittle fracture occurring along active slip planes by slipping-off, not by the cleavage fracture associated with occurrence of mechanical twinning.