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Effect of $CO_2$ Content on the Growth and Corrosion Characteristics of the Compound Layers in Gaseous Nitrocarburized Carbon Steels  

Kim, Y.H. (Division of Metallurgical and Materials, Chemical Engineering, Dong-A University)
Kim, S.D. (Division of Metallurgical and Materials, Chemical Engineering, Dong-A University)
Yoon, H.J. (Division of Metallurgical and Materials, Chemical Engineering, Dong-A University)
Publication Information
Journal of the Korean Society for Heat Treatment / v.15, no.5, 2002 , pp. 219-227 More about this Journal
Abstract
This study has been performed to investigate the effect of $CO_2$ content on the growth characteristics of the compound layer, porous layer and corrosion characteristics of carbon steels after gaseous nitrocarburizing in $70%-NH_3-CO_2-N_2$ at $580^{\circ}C$ for 2.5 hrs. The results obtained from the experiment were the thickness of the compound and porous layers increased with increasing $CO_2$ contents. At the same fixed gas composition the thickness of the compound and porous layer increased with increasing carbon content of the specimens. X-ray diffraction analysis showed that compound layer was mainly consisted of ${\varepsilon}-Fe_{2-3}(N,C)$ and ${\gamma}^{\prime}-Fe_4N$ as the increased with $CO_2$ contents in atmosphere, compound layer was chiefly consisted of ${\varepsilon}-Fe_{2-3}(N,C)$ phase. With increasing $CO_2$ content and total flow rate in gaseous nitrocarburizing, the amount of ${\varepsilon}-Fe_{2-3}(N,C)$ phase in the compound layer was increased. The current density of passivity decreased with increasing $CO_2$ content due to the development of porous layer at the out most surface of ${\varepsilon}-Fe_{2-3}(N,C)$.
Keywords
Gaseous nitrocarburizing; $CO_2$ content; Total flow rate; ${\varepsilon}-Fe_{2-3}(N,C)$; ${\gamma}^{\prime}-Fe_4(N,C)$; Compound layer; Porous layer; Current density of passivity;
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