Effect of External Factors on Diffusion Acceleration Mechanism During Steel Microarc Surface Impregnation

The considerable duration of steel chemical heat treatment (CHT) processes makes it necessary to intensify them. For this purpose, it is advisable to use microarc surface impregnation that provides acceleration of CHT in all stages. During microarc alloying steel products are immersed in a metal container filled with coal powder, followed by passage of an electric current in the circuit. Microarc discharges occur within the powder and are concentrated around the product with formation of a microarc halo and rapid heating to the diffusion saturation temperature. When the coal powder is heated a saturated atmosphere based on carbon monoxide is formed within the container that leads to steel product carburization. Preliminary application of a coating containing a diffusant to the surface provides preparation of multi-component coatings. This technology provides acceleration of CHT in all stages. The surface being treated is affected by microarc discharges that lead to accelerated formation of active diffusant atoms and ions and their movement towards a treated surface. On transfer into the metal there is an effect of electrotransport whose operating factors are a force for action of an electric field on the diffusant ions, and the strength of the interaction of atoms and diffusant ions with directional flow of conduction electrons that drives them in the direction of their own movement (“electron wind”) as well as heat transfer, whose impact is described by a vacancy mechanism (Wirts model), the mechanism of entrainment of ions by electrons under the action of a temperature gradient, as well as the mechanism of diffusing atom attraction by phonons of the crystal lattice (“phonon wind”). Experimental studies are conducted for the intensity of diffusion flow depending on the direction of the electric current for various alloying elements, as well as a calculated estimate of the values of these forces. It is found that the main factor determining intensification of diffusion saturation during microarc alloying is the “electron wind” strength. Calculated values of alloying element diffusion coefficients for various versions of microarc heating correspond to the scheme proposed for acting forces.

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Correspondence to M. S. Stepanov.

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Translated from Metallurg, Vol. 64, No. 9, pp. 41–46, September, 2020.

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Stepanov, M.S., Dombrovskii, Y.M. Effect of External Factors on Diffusion Acceleration Mechanism During Steel Microarc Surface Impregnation. Metallurgist 64, 894–901 (2021). https://doi.org/10.1007/s11015-021-01069-1

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Keywords

  • steel surface hardening diffusion surface impregnation
  • microarc surface alloying
  • diffusion impregnation acceleration
  • alloying element diffusion coefficients