EFFECT OF NITROGEN ON THE TEMPERATURE-DEPENDENCE OF THE YIELD STRENGTH OF AUSTENITIC STEELS

The temperature dependence of the yield strength R-p0.2 in the range o f 4-300 K is studied on Cr18Ni16Mn10 steel alloyed with nitrogen or ca rbon or having no interstitials introduced purposefully. A striking in crease of the yield strength of nitrogen steel was observed at tempera tures below 100 K in accordance with previous results. The temperature dependence of stacking fault energy (SFE) in the range 300-130 K was measured by means of transmission electron microscopy and it was shown that nitrogen causes a sharp decrease of the SFE with decreasing temp erature while no marked temperature dependence of the SFE was observed in carbon steel. Samples of the studied steels were deformed by tensi on at 77 K and their dislocation structure was studied at room tempera ture. Tangled dislocations were observed in N(C)-free and C-alloyed au steniles while the obvious splitting of dislocations occurred in the n itrogen one. This result was attributed to the low SFE in nitrogen aus tenite at 77 K and an effect of interstitial atoms segregated on split dislocations during low temperature plastic deformation, keeping the splitting after deformation and heating to room temperature. The effec t of nitrogen on SFE is explained based on the results of the studies of electron structure of nitrogen austenitic steels and it is attribut ed to an increase in the density of states at the Fermi surface due to nitrogen. The results obtained enable one to interpret the nitrogen e ffect on the yield strength of austenitic steel within the framework o f Seeger's theory. (C) 1998 Acta Metallurgica Inc.