The roles of sulphides, oxides and pearlite in the ductile fracture of a niobium microalloyed steel

Abstract

Void growth and final fracture is studied experimentally and theoretically in a niobium microalloyed steel. A detailed study is performed for tensile testing in the rolling direction. The development of hydrostatic pressure in the centre of necks is analysed with Bridgman's technique. The void structure is studied in axial sections of specimens from interrupted tensile tests. Void nucleation and growth is evaluated quantitatively at pearlite, sulphides and oxides. A growth model for voids at pearlite which assumes that voids are created at a finite strain and that the pearlite nodule thereafter elongates more slowly than the matrix is formulated. The growth model for voids at sulphides and oxides also assumes a finite nucleation strain but the particles are assumed to be stiff during deformation. A theoretical criterion for strain localization is used to model the final failure. The model takes into account the strain-hardening capability of the matrix and the strain-softening effect caused by void growth. The results for tensile testing in the rolling direction are compared with results in the short transverse direction.