Morphological features of the diffusion layer after microarc molybdenizing of steel
摘要
Microarc surface alloying provides rapid thermochemical surface treatment of steels, as multiple microarc discharges acting on the surface strongly intensify diffusion. When the part to be hardened is immersed in a carbon powder, the thermal decomposition of the powder releases carbon monoxide and carburizes the surface layer. Carbide-type diffusion coatings are produced by saturating the surface with chromium, molybdenum, vanadium, or tungsten; for this purpose a paste containing compounds of the respective carbide-forming elements is first applied to the surface.
This paper examines the fine structure of the surface layer formed on steel after microarc molybdenizing. The experiments used specimens of steel 20 coated with a paste of ammonium molybdate and an electrically conductive gel as a binder. During treatment, the current density at the specimen surface was 0.53 A/cm2 and the heating time was 6 min.
After diffusion saturation, a 50–55 μm-thick diffusion layer forms on the specimen surface. This layer has a multilayer structure and a complex phase composition. Its matrix is a fine ferrite–carbide mixture with a microhardness of 8–9 GPa that contains etch-resistant carbide particles up to 5 μm in size and up to 21 GPa in microhardness. Beneath this matrix there is a 10 μm-thick Mo-in-Fe solid-solution layer containing about 3% Mo, followed by a 200 μm-thick carburized layer with a eutectoid structure that gradually passes into the original ferrite–pearlite structure of steel 20. The increased hardness of the diffusion-layer matrix is attributed to nanoscale carbide particles with a volume fraction of up to 70%, distributed within the ferrite–carbide eutectoid mixture.