Abstract: Double-layer surfacing repair was conducted on worn surfaces of a mining sprocket of 40CrNiMoA medium-carbon quenched and tempered steel by argon-rich gas protection welding. The repairing process was as follows: preparing a 10 mm thick transition layer with JM100 structural steel wires first, and then treating the sample by tempering at 400 ℃ for 2-3 h; second, preparing a 10-15 mm thick wear-resistant layer with YD256 wear-resistant core electrode, and then treating the surfacing repair, sample by tempering at 350 ℃ for 3 h. The microstructure, hardness, tensile properties, impact properties and wear resistance of the surfacing repair sprocket were studied comparing with those of the new non-worn sprocket. The results show that the metallurgical bonding between the surfacing repair layer and the base material was good, without defects such as pores and cracks. The transition layer was composed of a large amount of needle-shaped ferrite and a small amount of lamellar pearlite, with fine and uniform grains. The wear-resistant layer was composed of ferrite, pearlite, tempered martensite and granular carbides. The hardness, tensile strength and impact energy of the surfacing repair sample were 57.2 HRC, 842 MPa and 110 J, which were 9.6%, 11.8% and 26.4% higher than those of the new non-worn sprocket, respectively. The friction wear mass loss and abrasive wear mass loss of the surfacing repair sample were lower, approximately 32% and 24% of those of the new non-worn sprocket, respectively. The mechanical properties and the wear resistance of the surfacing repair sprocket was good.