Low-temperature plasma nitrocarburizing of the AISI 420 martensitic stainless steel: Microstructure and process kinetics

Low-temperature plasma nitrocarburizing was performed in order to study the effect of treatment temperature and time on the microstructure, mechanical properties and growth kinetics of the AISI 420 martensitic stainless steel treated layers. Plasma nitrocarburizing was carried out using DC-pulsed power supply in a 71% N2 + 18% H2 + 10% Ar + 1% CH4 gas mixture, at temperatures of 300, 350, 400 and 450 °C, and times of 2, 4, 6 and 12 h. The applied peak voltage, gas flow rate and pressure were kept constant at 600 V, 3.32 × 10− 6 Nm3 s− 1 and 400 Pa, respectively. The treated samples were characterized by optical microscopy, X-ray diffractometry and microhardness measurements. A maximum surface hardness of 1280 ± 16 HV0.3 was obtained for samples treated for 4 h at 400 °C. Grain boundary chromium carbide/nitride precipitation has started for treatments of 4 h at 400 °C, which can be avoided by using lower nitrocarburizing temperature or time. It was verified that nitride/carbide precipitation tends to occur at lower temperatures and times when compared with the low-temperature nitriding and carburizing treatments. Finally, the nitrocarburized layer growth is proportional to the square root of treatment time and follows an Arrhenius law for the treatment temperature, with an activation energy of 76 kJ mol− 1.