Gas nitriding is a thermochemical surface treatment, which is frequently applied to Fe-C-base alloys in order to improve wear and corrosion resistance. During gas nitriding, N supplied by a NH3/H2 gas mixture is incorporated into the surface of the material. Iron (carbo)nitrides form in the interdiffusion zone between the surface in contact with the nitriding atmosphere and the base material. Typically, these (carbo)nitrides develop a single- or multi-layer structure denoted compound layer. In plain-carbon steel, the constitution of the compound layer correlates with a diffusion path in the Fe-C-N phase diagram.
Constitution and microstructure of steel and cast iron can be tailored by alloying with Si. Si hinders the precipitation of cementite during tempering, affects the spherodization and dissolution of cementite during annealing and promotes graphitization. During nitriding, Si is known to hamper formation and growth of iron (carbo)nitrides in α-Fe. Nevertheless, the precipitation of silicon nitride is very sluggish. Little is known about the compound layer development in Si steel and cast iron, which exhibit both high Si content and large fraction of cementite. Here, the effects of Si on nitride formation and cementite dissolution interrelate.
In this study, the effect of Si on the microstructure development during annealing (< 580 °C) and gas nitriding (540 °C) of hypoeutectic Fe-C and Fe-C-Si white cast iron has been experimentally assessed. The white cast iron exhibits a heterogeneous microstructure characterized by coarse eutectic cementite plates and large pearlite regions, which refer to decomposed primary austenite dendrites. Si accumulates in α-Fe, whereas cementite can be considered as Si-free. In Fe-C-Si, iron (carbo)nitride forms by the conversion of θ-cementite into ε-Fe3(C,N)1+x, while (carbo)nitride formation and growth is hampered in pearlitic α-Fe. The formation of γ’-Fe4(C,N) is hardly observed in the pearlite in Fe-C-Si, even though the employed nitriding conditions yield the formation of single-phase γ’ in contact with pearlite in Fe-C. Consequently, ε + α and ε + α + θ phase boundaries occur in Fe-C-Si, which do not comply with the ternary Fe-C-N phase diagram at 540 °C. The existence of a ε + α equilibrium is discussed in terms of both the indirect kinetic stabilization of the ε phase against its transformation into γ’ due to Si and hypothetical metastable Fe-C-N-Si phase diagrams.