Effect of Plasma Nitriding and Sensitization on the Microstructure and Microhardness of AISI 304 Austenitic Steel

Abstract

The article analyzes the influence of plasma nitriding and sensitization on the microstructure and microhardness of AISI 304 austenitic steel. The microstructure of AISI 304 austenitic steel in all states was observed on a Neophot 32 optical microscope. The initial structure consisted of polyhedral austenite grains of different sizes. The microstructure contained a large number of non-metallic MnS-based inclusions. After plasma nitriding, a continuous nitriding layer was formed under the steel surface, while the grain shape was preserved. Carbide chromium was excluded along the austenitic grain boundaries by the following processes: sensitization and plasma nitriding followed by sensitization. The nitriding layer began to deteriorate due to temperature and length of sensitization. Vickers microhardness measurement on a Zwick/Roell ZHµ machine proved that the nitriding layer is reaching the high hardnesses compared to the core of samples. After sensitization, the nitrided samples achieved only slightly increased microhardness compared to the core. Finally, EDX analysis of the nitriding layer was performed on the SEM and compared with the layer that was affected by the sensitization process.