Incubation time for iron-nitride layer formation upon gaseous nitriding of iron-based alloys

Abstract

A model was developed to predict quantitatively the influence of alloying element (Me) dissolved in the ferrite (α) matrix on the incubation time for iron-nitride layer formation upon gaseous nitriding of iron-based alloys. The model incorporates the coupled, concurrent processes of inward diffusion of nitrogen and the depth dependency of the time dependency of the precipitation of alloying-element nitride particles in the α matrix. Experimental results were obtained by gaseous nitriding of an Fe-2.23 at.% V alloy. The incubation time for iron-nitride formation on Fe–Me alloy is generally much larger than that for iron-nitride formation on pure iron due to a pronouncedly lesser rate of increase of dissolved N content at the surface of Fe–Me alloy. The extent of segregation of N at the MeN/α-Fe interfaces has distinct influence on the incubation time.

Keywords:

• Incubation time
• compound-layer growth
• internal precipitation
• misfit stress
• iron-based alloys

Acknowledgement

We are grateful to Dipl.-Ing. P. Kress (Max Planck Institute for Intelligent Systems, Stuttgart, Germany) for assistance with the nitriding experiments.

Notes

1. Here, it is assumed that the desorption rate of dissolved nitrogen atoms as molecular nitrogen at the surface is negligible, which holds at low nitriding potentials and at the usual nitriding temperatures (500–580 °C) [1,2,10].

2. For the derivation of Equation (6(6) $$\frac{\text{d}R}{\text{d}t}=\frac{D_{\text{Me}}}{R}·\frac{X_{\text{Me}}-X_{\text{Me}}^{\text{int}}}{\mathit{\delta }{X}_{\text{Me}}^{\text{P}}-X_{\text{Me}}^{\text{int}}},$$(6) ), the diffusion coefficient D _Me is assumed to be a constant [16]; in the present case, the self-diffusion coefficient pertaining to the nominal concentration of M in the (binary) alloy is employed.