Experimental determination of the carbon solubility limit in ferritic steels
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27/4/04
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Experimental Determination of the Carbon Solubility Limit in Ferritic Steels J. MERLIN, P. MERLE, S. GARNIER, M. BOUZEKRI, and M. SOLER Despite the existence of a number of published results, the data on the solubility of carbon in alpha iron are still inaccurate. An analysis of published experimental results shows that available values vary greatly (between 50 and 100 ppm by wt, for example, at 600 °C). These discrepancies make it difficult to optimize the metallurgical processes of low-carbon or ultralow-carbon alloys. An experimental methodology, using the measurement of the thermo-electric power (TEP) of the alloy, was set up. This enabled us to deduce the quantity of free interstitials in the matrix by measuring the amount of interstitials which segregate on dislocations after a deformation of the sample. This technique was used in the case of an Al-killed steel containing 0.2 pct Mn. The limit of solubility of carbon was determined with a precision of 2 ppm between 550 °C and 730 °C. This limit of solubility can be analytically described by the relation C(wt pct) 6.63 exp (11.8kcal·mol /RT), which is shown to be valid only for temperatures above 400 °C. We show experimentally that the residual concentration of carbon at low temperature is much greater than the value predicted by the extrapolation of this relation. Complementary studies on steels with various C and Mn contents allow us to verify the validity of the proposed methodology. 1
I.
INTRODUCTION
IT might be considered unnecessary to ask about the solubility limit of carbon in iron: the first determinations of Fe-C diagrams date back 100 years; a large number of studies have been published recently (refer, for example, to a well-documented internet site on this subject[1]); and many equilibrium diagrams have been published in various handbooks (there are about 20, for example, in The Encyclopedia of Materials[2]). However, one can observe that, in spite of the great number of published works, very little information is given about the particular case of the solubility limit of carbon in ferrite. Only one piece of precise data is given and concerns the maximal solubility (0.0210 0.0002 wt pct) at the eutectoid temperature (⬇725 °C) in pure iron. This is obviously not sufficient for a discussion concerning the metallurgy of the special kind of iron alloys which have recently been developed such as “low-carbon” or “ultralow-carbon” alloys, the carbon content of which is below or near this maximum solubility limit. In fact, since 1950, at least ten articles have been published to determine the limit of solubility of carbon in pure iron and in low-carbon steels.[3–13] This evaluation does not include the studies that have been done on the influence of other alloying elements on this limit of solubility. This analysis is restricted to the main works on the determination of the limit of solubility of carbon in alpha iron in the presence of cementite Fe3C. It concerns a temperature range going from the eutectoid temperatu
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