Phase transformations in iron-nitride compound layers upon low-temperature annealing: Diffusion kinetics of nitrogen in ▓- and γ′-iron nitrides

Abstract

The /␥Ј-iron nitride (-Fe 3 N 1ϩx , ␥Ј-Fe 4 N) compound double layers with thicknesses of about 10 m were grown on pure ␣-Fe, by gas nitriding at 823 K, followed by quenching. The specimens were subsequently annealed at significantly lower temperatures, in the range of 613 to 693 K, for different periods of time. These heat treatments led to a redistribution of N, within the compound layer as well as between the compound layer and the adjacent ferrite, inducing thickness changes in the -and ␥Ј-sublayers. The microstructure and sublayer-thickness changes were analyzed by light microscopy and X-ray diffraction (XRD). The experimentally observed time and temperature dependences of the layer-thickness changes were compared with the results obtained from numerical simulations, by adopting a model based on volume diffusion in the -and ␥Ј-phases and on local equilibrium at the phase interfaces. In this manner, the intrinsic diffusion coefficient of N in the -phase and the integral diffusion coefficient of N in the ␥Ј-phase were determined for the applied range of annealing temperatures.

Microstructural development and crystallographic properties of decomposing Fe–N–C compound layers

“…7a nd 8. This is comparable to the back-growth of c ' sublayersu pona nnealing of e / c ' double layers formed by nitriding of pure Fe [52,53]. Even in this very late stage of e decomposition, the different growth rates of c ' grains formed by the coarsening of c ' plates within the original e grain and of the massive c ' grains can be observed (see above discussion and Fig.…”

Fe–N and Fe–N–C phase equilibria above 853 K studied by nitriding/nitrocarburising and secondary annealing

“…c. So-called secondary anneals under inert atmosphere applied to specimensp reviously nitrocarburised at temperatures identicalt oo rd ifferent from the annealing temperature [38,39,42]. Such anneals can level out concentration gradients and, because of this and/or an annealing temperatured ifferent from the nitrocarburising temperature, may induce new phase transformations in the compound layer.…”

Modelling and experiment of selective oxidation and nitridation of binary model alloys at 700 °C – The systems Fe, 1 wt.%{Al, Cr, Mn, Si}

“…Here, the iron surface remains unchanged throughout the heat-treatment thus allowing constant transport conditions for oxygen and nitrogen and the formation of all themodynamically stable phases of the alloying elements. This generates an extreme scenario for inner oxidation and nitride formation and allows to unravel the effects of precipitate formation in polycrystalline alloy compositions [24][25][26][29][30][31][32][33]. The temperature of interest in this study was 700°C, which corresponds to a typical temperature regime of the coiled material after hot-rolling in steel manufacture.…”