Influence of Substitutional Atoms on the Solubility Limit of Carbon in B.c.c. Iron

No clear-cut information is available with regard to the effect of foreign atoms on the solubility limit of C in b.c.c. iron despite many previous studies. Against this backdrop, the influence of substitutional atoms (Mn, Cr, P, Si, Al) on the solubility limit of C in b.c.c. iron in equilibrium with cementite was investigated in low-carbon steels at a temperature of 700°C. In detail, the C solubility limit was determined from internal friction measurements combined with infrared analysis of C using a high-frequency combustion technique. It has been clarified that Mn, Cr, and Al hardly change the C solubility limit, whereas P and Si increase it. The thermodynamical calculation indicates that, under para equilibrium Si increases the C solubility limit and Mn hardly changes it, while under ortho equilibrium Mn and Si decrease it. However, the present experimental condition was verified to be close to ortho form. The discrepancy between the experiments and the calculations seems to come from the fact that: 1) single solute C atoms and the C atoms combined as Substitute-C complex are not distinguished experimentally, and 2) in the regular solution model, the non-uniform distribution of C atoms around alloying atoms is not introduced into the entropy term, which is something that should be studied further in the future.