On the Effect of Nonmetallic Inclusions on Ductility and Toughness of Structural Steels

Abstract

Ductility and ductile fracture in relation to nonmetallic inclusions has been studied on laboratory melted commercially impure copper bearing steels in various aging conditions and also on low carbon steels with minor alloying additions.The results of light microscopy and scanning electronmicroscopy suggest that the first step of the fracture is cracking and decohesion of elongated inclusions in early stages of deformation.The damaged inclusions form internal necks (as opposed to external neck), and they, in turn, accelerate the final separation process which is formation and coalescence of microvoids associated with submicroscopic spherical oxide inclusions.This two stage mechanism explains general experience of harmful effect of elongated sulfide inclusions and observed duplex dimple pattern of the fractured surface.Change of inclusion shape from elongated to spherical improves ductility probably because of more difficulty for spherical inclusions to form internal necks.The damage of inclusions during deformation and also the final fracture seem to be controlled by dissipated plastic energy to some extent.Although the effect of non-metallic inclusions on toughness is complex, inclusions are generally noted to promote brittleness, but to less extent for the case of elongated inclusions where triaxial stress ahead of crack is relaxed by fissuring along their elongated direction.