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.