Effect of nanostructured grains on martensite formation during plasma nitriding of AISI 321 austenitic stainless steel

In this work, the effect of nanostructured grains on austenite to martensite phase transformation in AISI 321 austenitic stainless steel during plasma nitriding was investigated. Repeated cold rolling and subsequent annealing were conducted to achieve nanostructured grains in AISI 321 stainless steel. Plasma nitriding was performed for various temperature ranges from 400 to 500 °C for 5 h. Microstructural evaluations were conducted by OM and TEM. The microstructure and composition of the nitrided layer were characterized by SEM and GDOES. The phase transformation evaluations during cold rolling and plasma nitriding were performed by ferritoscopy and XRD respectively. The surface texture and morphology of the nitrided layer was studied by roughnessmeter and AFM means. Results indicated that the surface nitrogen concentration decreased with decreasing substrate grain size. Decreasing austenite grain size also led to decreases of the S phase thickness and surface roughness. Results revealed that, shear bands were formed in the nitrided layer of micro grain substrate unlike nano-grain substrate because of instability of the austenitic micro grains. Furthermore, studies revealed that, the austenite phase was indicated on the nanostructured surface layer while the martensite phase was indicated on the micro grain one after plasma nitriding at 500 °C.