Growth Kinetics of Iron Carbide Layers during Gas Carburization

Abstract

The growth kinetics of carbide layers on iron surfaces has been studied by gas carburization in CO-H 2 -H 2 O-H 2 S gas mixtures at 500°C. Small additions of H 2 S have been added to the atmosphere to prevent graphite formation and carbide decomposition (metal dusting). The carburization was performed with CO-H 2 -H 2 O-H 2 S mixtures at carbon activities in the range 12 < a C < 4600. At a C ≤ 100 only cementite (Fe 3 C) is formed, at higher carbon activities an outer layer of Hägg carbide (Fe 5 C 2 ) grows on the inner layer of cementite. Parabolic rate constants have been determined for the growth of cementite and of Hägg carbide. From the dependence of the rate constant on carbon activity the self diffusion coefficient D C * for carbon in both carbides has been derived. IntroductionCementite Fe 3 C is the most important carbide in steels. It is a part of the microstructure of pure and low alloyed steels. Despite its high importance some essential properties of cementite like thermodynamics and diffusion are almost unknown. This fact was the motivation for conducting the investigations presented in this paper.There are only two studies about carbon diffusion in cementite available in the literature. Hillert [1] investigated carbon diffusion in cementite and determined an approximate diffusion coefficient at 865°C. Simkovich et al. [2][3][4] measured growth rates of cementite on fine iron particles carburized at 450 °C and obtained diffusion coefficients in the range D C * = 2Â -15 cm 2 /s. The catastrophic carburization process called metal dusting attacks iron, low and high alloyed steels and Co-or Ni-base alloys in strongly carburizing gas atmospheres at a C >1 in the temperature range 400 to 700°C. The metal dusting of iron and low alloyed steels in CO-H 2 -H 2 O atmospheres has been explained by the following reaction mechanism [5-9]:• rapid supersaturation of the metal phase with dissolved carbon by transfer from the gas phase (a C >a C(Fe/Fe3C) ), (ii) growth of a cementite layer at the surface acting as a barrier for carbon transfer, • graphite deposition which lowers a C → 1 rendering cementite unstable, • subsequent cementite decomposition Fe 3 C → 3 Fe + C into graphite and fine metal particles, which • act as catalysts for coke deposition.Interface Controlled Materials. Edited by M. Rühle and H. Gleiter

Metal dusting

“…But, more important, the nucleation of graphite is impeded and reaction step (iii) may not take place at all or is delayed for a long time. In this way, cementite is stabilized and its properties concerning thermodynamics, diffusion and stoichiometry could be studied [22,23]. Suppression of metal dusting depends on carbon and sulfur activity in the atmosphere, carburizing gas CO or CH 4 , temperature and time [22 -25], see chapter 7.1.…”

Growth of massive cementite layers; thermodynamic parameters and kinetics

“…The experimental values for the (temperature dependence of the) parabolic growth constant were determined. These results were evaluated in terms of an ''apparent'' activation energy for cementite-layer growth and discussed with reference to previous work on the kinetics of cementite (layer) growth [4,[13][14][15][16][17][18][19]. The discrepancy between the results of the present work and the literature data are discussed in terms of thermodynamic consequences of the different gas atmospheres used.…”

“…As compared to the present work, the available experiments reported in the literature [ 13 – 15 , 19 ] were performed at lower treatment temperatures. 8 Therefore, the present experimental data (growth constants for four different temperatures) can only be compared with these literature data (growth constants for various carbon activities in the solid at the surface) by extrapolation of the present experimental data to these lower temperatures.…”

“…In the literature, the growth kinetics of pure iron nitride layers [ 23 – 26 ] but also of cementite layers [ 4 , 15 , 16 ] was described by a parabolic growth law. A comparison of the rates of cementite-layer growth and γ′-Fe 4 N 1− y -layer growth indicates that, at the same temperature, γ′-Fe 4 N 1− y -layer growth proceeds faster than cementite-layer growth, which can be ascribed to nitrogen volume diffusion in γ′-Fe 4 N 1− y being faster than carbon volume diffusion in cementite.…”

“…2 to the experimental data obtained in this work was extrapolated to lower temperatures (dashed line). In [ 13 , 14 ] and [ 15 , 19 ], carbon activities in the solid at the surface were adjusted at various values at 723 and 773 K, and the corresponding k (growth constant) values were determined. These k values, corresponding to specific carbon activities as indicated in the figure, lie below the extrapolated dashed line corresponding to the data of the current work (cf.…”