EPSILON-GAMMA PRIME TRANSFORMATION IN NITRIDED Fe AND STEEL: STRUCTURE CHARACTERIZATION. P. S. Schabes-Retchkiman, G. Hinojosa* and J. Oseguera*, Instituto de Fisica, U.N.A.M., Apdo. Postal 20-364, Mexico, D. F. 01000, MEXICO, *ITESM-CEM,DGI, Apdo. Postal 18, Atizapan, Mexico 529926, MEXICO.

Thin layers formed by means of thermochemical nitriding treatments, of the surface of metals, particularly iron and steel, produce big enhancements in their mechanical and tribological properties. The origin of the improvement in iron and steel stems from the formation of a compact nitride compound surface layers and a diffusion zone of nitrogen interstitially dissolved in ferrite [1]. The top layers may be composed of epsilon and gamma' (carbo)nitrides. Above the eutectoid transformation point, an epsilon compact nitride layer is formed, and a transformation of the nitride into epsilon+gamma' during the sample's cooling occurs. In this work we have set out to study the epsilon to gamma' transformation, particularly by high-resolution transmission electron microscopy.

Glow discharge plasma nitriding was performed. In these experiments, pure iron and steel samples were nitrided. The results obtained in this work show that desaturation of the epsilon nitride during slow cooling produces equilibrium between the epsilon and gamma' phases. This reaction results in the formation of alternating plates of the given phases. HREM of the structures observed has confirmed that the transformation epsilon to gamma' is displacive confirming the model suggested by Gerardin et al [2].

1. J. Groseh, J. Morral and D. Schneider, editors: 1995 Carburizing and Nitriding with Atmospheres, Conf. Proc. 6-8 Dec 1995, ASM International, Materials Park OH, USA.1995.

2. D. Gerardin, H. Michel and M. Gantois, Script. Met 11(1977)557.

We acknowledge support by CONACyT grant 3334-A. Technical support by L. Rendon is also appreciated.