[en] A novel high-vanadium based hard composite coating was synthesized from premixed powders (V, Cr, Mo, Ti, Nb) on ductile iron (DI) substrate via atmospheric plasma beam surface alloying process. The graded coating can be divided into three distinct zones: upper alloyed zone (AZ) rich with spherical primary and eutectic submicron carbides, middle melted zone (MZ) with fine white iron structure embedded with high-carbon martensite and lower heat affected zone (HAZ) where martensite/ledeburite double shells were substantially formed. Spherical or bulk-like primary carbides with diameter <1 μm in the middle AZ were formed via in-situ reactions between alloy powders and graphites in DI, presenting a refined microstructure similar to high vanadium-containing tool steel. Microstructural characterizations indicate the carbides are chiefly globular MC-type particles mixed with hard-phases such as M₂C, M₇C₃, M₂₃C₆, and martensite. EDS mappings show that MCs (M = V, Ti, Nb) form together while M₂C-type carbides tend to locate at the grain boundary or around the MC particles. Ti and Nb occur in the MC-type primary carbides besides V and TiC even features as nuclei. Though dependent upon the size, shape, type and distribution of carbides, the microhardness was obviously enhanced in both AZ and MZ. - Highlights: • A graded metal matrix composite coating was in situ produced on ductile iron by plasma surface alloying process. • Spherical, hard submicro metal carbides that replace soft graphite nodules render coating hard-yet-tough. • Microscopic characterizations verify the abundant ultrafine particles primarily to be vanadium carbides.