Diffusion in iron, iron solid solutions and steels

Systematic experimental and theoretical studies of the diffusion in iron, iron solid solutions and steels have been carried out at the Institute of Physical Metallurgy, Czechoslovak Academy of Sciences, Brno, for several years. The aim of this review is to present some results achieved in the Institute and to complement them by the results published in journals and monographs, in a complete well-organized format that gives a basic idea of the problem and enables certain technological applications to be made. In the four sections we deal with self-diffusion and heterodiffusion in iron and in iron solid solutions. The investigations have shown, as well as other facts, that three types of bulk diffusion in iron-based materials should be distinguished: (i) diffusion in the ferromagnetic b.c.c. structure, (ii) diffusion in the paramagnetic b.c.c. structure and (iii) diffusion in the f.c.c. structure. In Section 3 we demonstrate that the effect of magnetic order on diffusion, creep and high temperature yielding has a common base: the atomic spin-spin interaction which influences the variation in the elastic constants with temperature. The results concerning substitutional diffusion in steels are presented in Section 6. It can be concluded from the compiled data that the self-diffusion and heterodiffusion values measured in f.c.c. iron may be used as a good approximation in austenitic steels, if the data for these particular materials are not available. Examples of diffusion applications in related material branches are introduced in Section 7 on the homogenization of alloys and steels and in Section 8 on diffusion and creep. In Section 9 we deal with the diffusion of interstitial atoms in iron and steels and particularly with carbon diffusion and redistribution in steel weldments. It is shown that the concept of the carbon activity gradient as a chemical "driving force" in carbon diffusion and redistribution is well established and has been applied successfully.