Effect of Nitrogen on Low Temperature Nitrided Stainless Steels for Steam Turbine Blades

Abstract

In the last few years, an increasing interest has been taken in AISI 300 series austenitic stainless steels for use in industrial applications. In these steels nickel is partly replaced by other austenite stabilizing elements like manganese, chromium, nitrogen. However, their chromium content is usually lower than that of largely used Fe-Cr-Ni based other grade of stainless steels, so that their corrosion resistance may be lower. For these AISI 300 series of stainless steels, low temperature nitriding treatments have been successfully employed to increase the corrosion resistance, due to the formation of modified surface layers consisting of chromium-nickel layers. In this research, a low nickel AISI 316LN austenitic stainless steels was subjected to gas nitriding treatments in the range of 450-5500C. The applications was found to be in steam turbine blades. The specimens were characterized by optical microscope results and scanning microscope results. The treatments produce modified surface layers having a double layer structure. When the treatments are carried out at these temperatures, the outer layer was modified by iron chromium nitrides, and their amount increases as the treatment time goes higher.