Low-temperature plasma carburizing of AISI 420 martensitic stainless steel: Influence of gas mixture and gas flow rate

Low-temperature plasma carburizing was studied aiming to determine the effect of the gas mixture and flow rate on the surface properties of AISI 420 martensitic stainless steel samples. Plasma carburizing was carried out for gas mixtures of 20% Ar + 80% H2 comprising CH4 contents between 0.25 and 1.00%, and gas flow rates ranging from 1.67 × 10− 6 to 6.68 × 10− 6 Nm3 s− 1. The modified layers were characterized by confocal laser scanning microscopy, X-ray diffractometry and microhardness measurements. The plasma was also characterized by optical emission spectrometry. Results indicate the presence of a hard and thin outer layer and a carbon-enriched martensite diffusion layer. It is shown that gas mixture composition plays an important role in the process kinetics. Spectroscopic characterization of the glow discharge shows that the variation of the CH4 content in the gas mixture leads to a variation of the emission lines intensity but does not significantly alter the relative peak intensities. It suggests a variation on the plasma density and no significant variation on the active species. It also indicates that, for the studied conditions, the emission spectroscopy cannot be applied as a tool for process control.