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

Abstract

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)

Diffusion of carbon in bcc Fe in the presence of Si

“…Rather simplistically, we expect therefore to see broadening of the activation energy spectrum as the main effect of alloying with Si although reproducing, and in detail explaining, the Snoek peaks requires a more detailed analysis. 24 We believe that the 0.34 eV binding energy between Si and C as proposed by Walz et al 23 is much too high. The similarity they see with vacancy-C binding ͑at 0.41 eV͒ is incorrect because the inward relaxation of the nearest-neighbor Fe atoms around a vacancy is much larger than around a substitutional Si atom.…”

“…30 More recent Snoek-type measurements have given a similar activation energy of 0.84 eV. 5,23 For completeness, the energy of substitutional C was computed as well. It is found to be about 2.22 eV above that of octahedral C so that under near equilibrium conditions, the occurrence of substitutional C can be ruled out.…”

“…The experience that Si suppresses or retards carbide formation 22,23 now can be understood to have two aspects: ͑a͒ by attracting and trapping the C chemical potential is reduced, reducing the driving force for carbide precipitation and ͑b͒ by reducing the diffusivity of C, the precipitation of carbides is slowed down. Both effects are sensitive to temperature because Si traps C only weakly.…”

“…8, 13, and 21͒ and it is also known to influence the precipitation of carbides. 22, 23 We will not attempt to simulate the Snoek peaks themselves 12,24,25 although an atomistic understanding of carbon transition rates makes this feasible with high accuracy. 24 The paper is outlined as follows: first, the individual solute atoms, C and Si, in bcc Fe will be examined through density-functional electronic-structure calculations involving supercells.…”

Effects of dilute substitutional solutes on interstitial carbon inα-Fe: Interactions and associated carbon diffusion from first-principles calculations

“…and 0.84 eV10 .FIG. 13: The minimum-energy paths of the carbon migrating from the 1nn to the 5nn o-sites with respect to the solute Mn (a) or Si (b) in α-Fe.…”