Microstructural and electrochemical studies in thermally aged type 316LN stainless steels

Abstract

The effects of nitrogen (680 and 1600 ppm) on the microstructure and electrochemical behaviour of thermally aged type 316LN stainless steels is discussed. Electrochemical potentiokinetic reactivation tests indicated a decrease in chromium depleted regions with increasing nitrogen addition in austenitic stainless steels. Secondary precipitates developed in these alloys during aging at 873 K for 500 h were extracted by an electrochemical method. The precipitates were analysed by X-ray diffraction method. Further TEM investigation on 1600 ppm nitrogen steel was also carried out to help understand the precipitation behaviour. The presence of nitrogen resulted in precipitation of mostly Cr₂ N and χ (chi) phases in the alloy that contained 1600 ppm of nitrogen, in contrast to M₂₃C₆ precipitates in the alloy that contained 680 ppm of nitrogen. The influence of the microstructural evolution and its effect on chromium depletion observed in the present investigation is discussed.