Abstract: Causes, corrective methods and preventive measures were described for carbon black appearing on the surface of 18CrNiMo7-6 steel shaft workpieces carburized and quenched in pit furnace by chemical composition analysis and physical and chemical detection. By shot blasting and re-quenching the workpieces with surface carbon black, it is confirmed that the carbon black originates from the quenching process. The results show that the problem of surface carbon black can be effectively solved by adjusting the staged increase of quenching carbon potential to 0.65% C_p for 2 h and then to 0.83% C_p for 5 h, increasing the temperature of enriched gas droplets to 830 ℃, and increasing the gap between workpieces to 30 mm. It is verified by tests that the carbon black on the surface of the steel is generated during the quenching and holding stage. By adjusting the carbon potential and the furnace gap of the workpiece, the generation of carbon black on the surface of the steel can be avoided.

Key words: 18CrNiMo7-6 steel workpiece, pit furnace, carburizing and quenching, surface carbon black, carbon potential, gap