Effects of alloying elements on microstructure, hardness and growth rate of compound layer in gaseous-nitrided ferritic alloys

Pure iron and FeM (M = Mo, Si, Mn, Cr, Al, V) binary ferritic alloys were nitrided at 843 K for various times. Phase constituent, hardness distribution and growth rate of the compound layer are investigated by means of X-ray diffraction, EBSD, EPMA, 3DAP and nanoindentation. In pure iron, ¾ and £ A (beneath ¾) form on the surface, and voids form along the interface. The hardness of £ A is higher than ferrite matrix, while void formation causes softening of compound layer. Although alloying effects on the hardness of compound layer are small, Si, V and Al additions suppress the formation of voids, and therefore the softening is also contained. All the elements investigated increase growth rate of compound layers. Especially the Si, V and Al additions significantly accelerate their growth rate due to the presence of excess nitrogen in compound layer. However, in the case of Al-added specimen, the growth layer becomes sluggish at longer nitriding time, presumably because of the extensive precipitation of AlN into the diffusion layer.