ON THE EFFECT OF NITROGEN ON THE DISLOCATION-STRUCTURE OF AUSTENITIC STAINLESS-STEEL

The strengthening and work hardening characteristics of high nitrogen austenitic steel have been investigated by TEM in order to explain the outstanding mechanical properties, which are very high yield strength and good toughness. It is shown that plastic deformation of those ste els always occurs by a combination of planar glide and mechanical twin ning. However, the critical stress/strain conditions for the onset of mechanical twinning depend strongly on the actual nitrogen content. Sp ecifically, as the nitrogen content is increased, the onset of deforma tion twinning is shifted to lower strains and higher stresses, i.e. th e more important becomes the contribution of deformation twinning to t he total strain. The observed behaviour is explained by the influence of nitrogen on internal friction and stacking fault energy. It is conc luded that the high yield strength of cold worked nitrogen-bearing ste el is essentially due to tight stackings of twins and stacking faults. Apart from regular structure evolution, inhomogeneous structures are observed, which can be explained in terms of texture formation and the dynamic properties of stacking faults under stress.