Correlation between Microstructure and Microhardness of Nitrided Austenitic Stainless Steel

This paper will study on the effect of the duration of solution nitriding towards the microstructure and micro-hardness of AISI 316L stainless steel. The AISI 316L stainless steel will be exposed to the high temperature gas nitriding process at 1200oC by using the 50% of ammonia gas and 50% of nitrogen gas for different duration of 8, 15 and 20 hours. The nitrided AISI 316L was then undergone slow cooling process after the solution nitriding process. After that, the samples were polished and etched using Viella’s etchant in order to have a better image on the microstructure for the optical micrograph observation. It was observed that there were several formation of twins occurred on the microstructure of the nitrided samples. There were two types of twins indentified on the microstructure of the samples which were transgranular twin and suspended twin. It was also noted that the grain size is increasing with the increase of duration of nitriding due to the slow cooling process. Then, the nitrided samples were analyzed using the Electron Dispersive X-ray Spectroscopy (EDS) embedded in the Phenom ProX Scanning Electron Microscope (SEM). From the EDS analysis, it was found that the nitrogen content in the nitrided austenitic stainless steel is increasing as the duration of solution nitriding is increase. The highest nitrogen content was located in the nitrided samples for 20 hour which have about 8.5 wt% of nitrogen. After that, the micro-hardness of the nitrided samples were investigated using the Vickers hardness testing machine. The microhardness test were conducted on the cross-sectional area of the nitrided samples. The highest reading were acquired on the surface of each nitrided samples and the reading is decreasing towards the center of all the nitrided samples. From the hardness test, it shown that the nitrided samples for 20 hours have the highest surface hardness of about 245.1 HV which is about 70% higher than the surface hardness of as-received sample. All the results obtained from all the experiments and tests conducted were analyzed to find the correlation between the microstructure and micro-hardness of the nitrided austenitic stainless steel