The nitriding kinetics of iron-chromium alloys; the role of excess nitrogen: Experiments and modelling

Abstract

To investigate the morphology and the growth kinetics of nitrided layers of Fe-Cr alloys, nitriding experiments were performed for alloys with 4.3, 7.7, 14.0, and 21.5 at. pct Cr. The precipitation morphology of the nitrided samples was investigated with light optical microscopy and scanning electron microscopy (SEM). The elemental compositional variation was determined with electron probe microanalysis. To describe the evolution of the thickness of the nitrided layers, a numerical model was developed 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. Fitting of the model to the experimental data demonstrated for the first time that mobile and immobile excess nitrogen is present in the nitrided layers and that the mobile excess nitrogen considerably influences the nitriding rate.

Pore Formation Upon Nitriding Iron and Iron-Based Alloys: The Role of Alloying Elements and Grain Boundaries

“…(i) The (metastable) equilibrium amount of dissolved nitrogen in the ferrite matrix of nitrided iron-based Fe-Al and Fe-Cr alloys can in principle be larger than that for nitrided pure iron, due to (a) a higher nitrogen solubility in iron-based Fe-Al [31] and Fe-Cr alloys [32] as compared to pure iron and (b) the presence of so-called excess nitrogen taken up in the strain fields surrounding the MeN-precipitates. [33][34][35] (ii) Moreover, the development of AlN and CrN precipitates in Fe-Al and Fe-Cr alloys upon nitriding competes with the development of N 2 gas containing pores. Hence, it is concluded from (i) and (ii) that the pore fractions in nitrided Fe-Al and Fe-Cr alloys can be smaller than in nitrided pure iron, as observed (cf.…”

Normal and excess nitrogen uptake by iron-based Fe–Cr–Al alloys: the role of the Cr/Al atomic ratio

“…Upon nitriding of iron-based binary alloys containing alloying elements with a strong affinity for nitrogen, such as Cr [10][11][12][13][14][15][16][17][18][19][20][21], Al [22][23][24][25][26][27], V [28][29][30][31][32][33] and…”

Nitrogen absorption by Fe-1.04at.%Cr alloy: uptake of excess nitrogen

“…the nitriding potential is sufficiently low [3,4]), (only) a diffusion zone containing chromium-nitride precipitates develops (''internal nitriding''). A number of investigations have been devoted to nitrided iron-chromium alloys [5][6][7][8][9][10][11]. It has been found that the nitriding of Fe-Cr alloys leads to two precipitation morphologies: in the initial stage, fine precipitates with coherent or partly coherent interfaces with the iron matrix develop (so-called ''continuous precipitation'').…”

“…more nitrogen than necessary for both (a) precipitation of all chromium as nitride, [N] CrN , and (b) equilibrium saturation of the ferrite matrix, ½N 0 a [6][7][8][9]). It has been suggested that a significant part of the excess nitrogen in nitrided binary iron-based alloys is adsorbed at the nitride/matrix interfaces, [N] interface [12][13][14][15][16].…”

“…It has been suggested that a significant part of the excess nitrogen in nitrided binary iron-based alloys is adsorbed at the nitride/matrix interfaces, [N] interface [12][13][14][15][16]. [N] interface ; this nitrogen does not take part in the diffusion process and therefore has been called immobile excess nitrogen [8]. The other part of excess nitrogen consists of nitrogen dissolved interstitially in the ferrite matrix (enhanced nitrogen lattice solubility), due to the positive volume misfit between the (coherent/ semi-coherent) nitride particles and the iron matrix which causes a dilatation of the matrix lattice, [N] strain [16].…”