Kinetics of carbon precipitation and re-solution in low Si-content silicon iron

In low Si‐charged silicon iron (Fe + 0.1%Si) the kinetics of carbon diffusion, precipitation and redissolution have been carefully studied by means of the magnetic after‐effect (MAE) within the temperature range 200 K < Ta < 1100 K. The activation parameters of respective processes have been determined by means of least squares fitting the experimental data. In the presence of substitutionally alloyed Si, the C‐Richter MAE gives rise to two Debye‐type relaxation peaks – resulting from elementary steps of carbon diffusion combined with reorientation in the (i) unperturbed and (ii) Si‐modified Fe matrix – situated near 265 K and 320 K, with activation enthalpies, Qi, of 0.84 eV and 1.08 eV. Two‐stage carbon precipitation, obeying first order kinetics, occurs, intensively, near 390 K with the elementary C diffusion enthalpy of 0.84 eV and, rather weakly, near 540 K with an enthalpy of 1.18 eV. The resulting precipitates are discussed in terms of partly intra‐grain deposited iron carbide phases (Fe3C) and, mainly, grain‐boundary determined C trapping. Decomposition of these precipitates occurring, again, in two stages – situated near 740 K and 950 K, with activation enthalpies of 1.72 and 2.02 eV – leads to a restitution of the state of maximum interstitially dissolved C in the matrix. Of practical importance for silicon steel fabrication is the observation that, after complete C precipitation (Ta . 580 K), the material can be kept in a state of minimum dissolved carbon content – and hence of minimum ac‐losses – by not allowing it to warm up above Ta ≤ 650 K. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)