Formation of phase structure in the superficial layer of iron nitrides and iron carbonitrides

The incomplete image regarding knowledge of all factors and mechanisms forming the composition and phase structure of nitrides layer limits our abilities of influence the functional qualities of this layer and narrows the range of controlling over diffusion layer kinetics of growth. Apart from few remarks, this other aspect has not been discussed in the literature so far, however, as it comes out from the research performed by the authors, it is just the composition and the phase structure of nitrides layer, which have their essential (not depending on nitrogen potential and temperature) share in formation of diffusion zone, and its effective thickness in particular. The problems mentioned above were a main impulse for better understanding of mechanism that are responsible for phase evolution taking place in nitrides layer on chosen alloys and carbon steels. In the research presented in the article concerning steels 20, N9 and 4340 it was proved that formation of nitrides layer happens along two sequences α → γ' → ε1 and α → ε2 → γ' → ε1. It was observed that domination of the first or second sequence of layer formation depends mainly on chemical composition of steel and nitrogen potential at given temperature. Characteristic phase changes taking place in the layer in the function of time have also been noticed. (author)