Structural characterization of plasma nitrided interstitial-free steel at different temperatures by SEM, XRD and Rietveld method

Plasma nitriding processes are widely used to improve surface properties of several steels and alloys. In this work, the formation of nitrides in the surface of plasma nitrided IF steels as a function of the temperature was investigated. Three cold-rolled IF steel plates were nitrided for 4 h after shot peening at three different temperatures: 450 degrees C, 475 degrees C, and 500 degrees C. The resultant nitrided layers were then characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Rietveld method, and hardness measurements. Through SEM images, it was possible to visualize two main sublayers: a compound layer and a diffusion zone. Through XRD, two phases were identified in the compound layer, which were s-Fe2_3N and gamma'-Fe4N. The diffusion zone presented a ferritic matrix with fine precipitates, possibly alpha(vertical bar vertical bar)-Fe16N2. By Rietveld, the calculated quantity of gamma'-Fe4N was 68 wt.% for the sample treated at 475 C and 58 wt.% for the one treated at 500 C. These values were consistent with the hardness measurements. Thus, it is suggested that higher nitriding temperatures facilitate the decreasing of gamma'-Fe4N and, consequently, the increasing of s-Fe2_3N in the compound layer. 2016 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda.