A rapid D.C. plasma nitriding technology catalyzed by pre-oxidation for AISI4140 steel

• D.C. plasma nitriding (PN) of AISI4140 steel catalyzed by pre-oxidation (PO) was investigated primarily.
• Pre-oxidation had significant catalysis effect on D.C. plasma nitriding.
• The maximum compound layer thickness, two times thicker than that without PO, was obtained under the optimum PO conditions.
• The surface under the optimum PO conditions has the highest SFE, which could be the main mechanism of the catalysis effect.

D.C. plasma nitriding (PN) of AISI4140 steel catalyzed by pre-oxidation was investigated primarily. The pre-oxidation was carried out in air prior to plasma nitriding. The results revealed that pre-oxidation had significant catalysis effect on plasma nitriding, the maximum compound layer thickness of 15 μm was obtained after nitriding at 500 °C for 4 h under the optimum pre-oxidation (PO) condition of 300 °C and 30 min, which was two times thicker than that without pre-oxidation. The possible catalysis mechanism is due to the formation of a uniform nano-iron oxide particle along with some nanocracks and nanopores on the surface, which has the highest SFE.