S. S. Hosmani

The kinetics of the nitriding of Fe–7Cr alloys; the role of the nitriding potential

Abstract The nitriding behaviour of the Fe–7Cr alloy was studied at 580°C in a gas mixture of ammonia and hydrogen. The nitriding potential was varied from 0.03 to 0.818 atm−1/2. Microstructural analysis of the nitrided specimens was performed by applying light microscopy, hardness measurements, X-ray diffraction, and electron probe microanalysis. The nitrided zone is composed of both regions with finely dispersed small chromium nitride (CrN) precipitates (continuous precipitates) in ferrite (α–Fe) grains and, mainly near the surface, regions where the precipitates have discontinuously coarsened leading to a lamellar CrN/α–Fe morphology. The nitrogen content within the nitrided zone is larger than expected on the basis of the chromium content and the solubility of nitrogen in (stress free) ferrite: excess nitrogen occurs. The hardness maximum in the nitrided zone and the nitriding depth increases with increasing nitriding potential as long as no iron nitride layer develops at the surface of the specimens. To describe the evolution of the nitrogen concentration depth profile of the nitrided layers, a numerical model was applied that has as important (fit) parameters: the surface nitrogen content, the solubility product of chromium and nitrogen dissolved in the ferrite matrix, and a parameter defining the composition of the precipitated chromium nitride. The nitriding depth depends roughly linearly on the square root of the nitriding potential. Analysis of the concentration depth profile data demonstrated that the amount of excess nitrogen considerably influences the nitriding kinetics.

The nitrogen-absorption isotherm for Fe-21.5 at. % Cr alloy : dependence of excess nitrogen uptake on precipitation morphology

Nitriding of Fe–21.5 at. % Cr alloy leads to a “discontinuously coarsened”, chromium-nitride/ferrite lamellar precipitation morphology in the nitrided zone. The nitrogen-absorption isotherm for this alloy with this precipitation morphology was determined at 560°C. To assure a constant precipitation morphology the Fe–21.5 at. % Cr specimen was first homogeneously pre-nitrided (at 580°C in an ammonia/hydrogen gas mixture of nitriding potential 0.103 atm−1/2) and then de-nitrided (at 470°C in hydrogen gas atmosphere). The amount of nitrogen remaining in the de-nitrided specimen indicated that the composition of the nitride precipitates is CrN and not (Fe, Cr)N. The measured nitrogen-absorption isotherm revealed the presence of excess nitrogen in the nitrided specimen, which is a surprise in view of the coarse, lamellar precipitation morphology. The occurrence of this excess nitrogen could be ascribed to an unexpected, minor fraction of the total chromium content in the alloy present as coherent, tiny nitride platelets within the ferrite lamellae of the “discontinuously coarsened” lamellar precipitation morphology, as evidenced by transmission electron microscopy. A possible kinetic background for this unusual phenomenon was discussed.

The emergence and disappearance of a high density of microcracks in nitrided Fe-4.65 at. % Al alloy

Abstract Nitriding of recrystallised and cold-rolled Fe-4.65 at.% Al alloy at 550 °C in an ammonia – hydrogen gas mixture of nitriding potential 0.104 atm− 1/2 indicated that nucleation of the thermodynamically (most) stable hexagonal (wurzite) AlN in the ferrite matrix of recrystallised specimen is difficult. The competition between the precipitation of nitrogen gas (dissociation of nitrogen supersaturated ferrite into pure ferrite and nitrogen gas) at the grain boundaries and the precipitation of hexagonal (wurzite) AlN led, via the coalescence of pores filled with nitrogen gas, to the emergence of microcracks along the grain boundaries and the formation of nitride-precipitate free zones near the grain boundaries of the recrystallised specimens. Upon continued nitriding the initially partially nitrided grains became fully nitrided and the microcracks disappeared.

Microstructure of the "white layer" formed on nitrided Fe-7 wt.% Cr alloys

Abstract The formation of a compound, “white” layer on the surface of Fe-7wt.% Cr alloy, by exposure to a gas mixture of ammonia and hydrogen at 580°C, was investigated. The microstructure of the surface layer was analysed employing light and scanning electron microscopy, X-ray diffraction and electron probe microanalysis. For the first time a quantitative (phase) analysis of such a white layer was carried out, demonstrating the presence of a dispersed CrN phase within a γ| iron-nitride matrix.

Interrelationships of defects, nitride modification and excess nitrogen in nitrided Fe-4.75 at.% Al alloy

Abstract In order to investigate the influence of crystal-lattice defects (dislocations) and of a relatively large Al content on the development of different modifications of aluminium nitride in ferrite and on the associated uptake of excess nitrogen, recrystallised and cold rolled specimens of Fe-4.75 at.% Al alloy were subjected to gaseous nitriding treatments at 550 °C employing a nitriding potential of 0.104 atm−1/2. In contrast with earlier results for an alloy of relatively low Al content, in both the nitrided cold rolled and the nitrided recrystallised specimens nanosized AlN precipitates developed, both of the metastable, cubic, NaCl type modification and of the stable, hexagonal, wurtzite type modification, which exhibit with the ferrite matrix a Baker–Nutting orientation relationship and a Pitsch–Schrader orientation relationship, respectively. A high-temperature hydrogen reduction treatment confirmed the stoichiometry of the precipitated nitrides as AlN. The fraction of cubic, NaCl type, AlN is significantly higher in the nitrided cold-rolled specimens because of its preferential development along dislocations. As compared to the nitrided recrystallised specimens, the higher dislocation density and the higher amount of cubic AlN in the nitrided cold-rolled specimens leads to a larger amount of excess nitrogen in the nitrided cold-rolled specimen.

Compound layer formation on iron-based alloys upon nitriding; phase constitution and pore formation

Abstract The microstructural development associated with the formation of the compound (so-called “white”) layer on the surface of ferritic iron and iron-based alloys by nitriding has been presented in a comparative way on the basis of results obtained in the recent past by nitriding model systems. A tentative explanation of the strikingly different layer-growth mechanisms for iron and iron-based alloys has been offered. Attention has been paid in particular to (i) the possible occurrence in the compound layer of “mixed” iron-based nitrides, with the alloying element Me with affinity for nitrogen (e.g. Me = Cr, Al, V) dissolved substitutionally on the iron sublattice of the nitride, and to (ii) the various operative mechanisms for pore formation in the compound layer.