Some Micro-Observations of the Brittle Fractures in Mild Steel Arc Welds Containing Nitrogen

Abstract

Charpy specimens of mild steel weld metals containing comparatively large nitride precipitates were fractured by impact bending at various temperatures within the so-called transition temperature range. The lattice distortions in the brittle fractures were surveyed by the X-iay back diffraction, method. Angular correlations among fractured edges, nitride needles and slip lines, generated by static compression independently of the rupture, were measured statistically with micro-photographs.
From these observations, the following conclusions wgre obtained:
1. The fractures, which look similarly brittle to the naked eye, actually have different degrees of lattice distortions correspondihg to the testing temperature.
2. X-ray back reflection photography is useful for comparing brittleness of these fractures.
3. Crystallographic plane of cleavage fractures could not be hastily decided to be a {001} plane, but it was ascertained to be one of {h01} planes, in which h is an integer or zero.
4 Nitrides in weld metals inhibit. plastic flow. They contribute to stress concentration and reduce the cohesive strength of ferrite crystals, thereby promoting brittle fracture.