Abstract: This paper adopts flame cutting to machine large-module involute gears made of 40CrNiMoA alloy steel. Dye penetrant inspection and hardness testing on the cut workpieces show that no cracks are found on the gear surface. The hardness of gear addendum and tooth flank ranges from 30 to 37 HRC, and the tooth flank is slightly harder than the addendum. Metallographic examination and hardness gradient test on sampled specimens reveal that the depth of the coarse-grained layer on the flame-cut surface is approximately 1.0 mm, and the total heat-affected zone of the gear surface is about 4 mm. When flame cutting is applied as the rough machining process for the parts, a unilateral machining allowance of 4～5 mm is reserved on tooth flanks and tooth roots. The subsequent finish machining can completely remove the heat-affected zone, so that the mechanical properties of the parts will not be impaired.