Mechanical behaviour of high-nitrogen stainless steels

Abstract

The application of previously published equations describing the dependence of proof stress on the nitrogen content of austenitic stainless steels has been validated and also extended to higher nitrogen contents in an experimental study of the mechanical property–structure relationships in Rex 734 steel. The proof stress is increased by nitrogen alloying through a combination of solid-solution hardening and a grain-size effect. The effective grain size of these steels is influenced by the nitrogen content through changes in stacking-fault energy, which Increases the density of twinning. In the analysis of tensile test data this effect appears as an increase in the grain-size coefficient of the Hall–Petch equation with increasing nitrogen concentration. Compression–tension low-cycle fatigue properties are similarly enhanced by a combination of nitrogen in solid solution and the influence of nitrogen on stacking-fault energy. An increase in the extent of planar slip is observed with increasing nitrogen content, and is most pronounced at low levels of fatigue strain. The increase in fatigue life observed at the strain levels examined reaches a limit at about 0·12 wt-%N both in AISI 316 grade steels and in Rex 734.

MST/453