GASEOUS AND SALT BATH NITRIDIND OF Fe-6at.%Si ALLOY

Abstract

Gaseous and salt-bath nitriding of a ferritic Fe-6at % Si alloy has been carried out at different temperatures for varying time periods. Light microscopy, X-ray diffraction, scanning electron microscopy and hardness measurements have been employed to characterize the micro structure of the various nitrided samples. Gas nitriding was found to be ineffective in actually nitriding the specimens, i.e. neither iron nitride layers nor silicon nitride precipitates were observed, due to the presence of oxide layers on the sample surface. On the other hand, the problem of surface oxide layers could be overcome in case of salt bath nitriding, which was evident through the formation of iron nitride layers on the sample surfaces. Along with iron nitride layer (compound layer) formation at the surface, salt bath nitriding has also resulted in the formation of a diffusion zone of significant thickness, characterized by a typical hardness depth profile. The measured hardness values suggest the formation of silicon nitride precipitates, although they have not been directly revealed in the optical/electron micrographs.