Thermochemical treatments carried out in microwave plasma are applied to steel substrates to increasehardness and prevent corrosion and simultaneously improve wear resistance. They are efficient for steelcarburizing. Since they are electrodeless process, the steel is free from metallic sputtering. The reactivity inargon–methane plasma as well as the reactivity at the steel surface exposed to active gaseous species isinvestigated in low pressure plasma. Various reactive mechanisms are involved in argon–methane microwavedischarge. They are thoroughly studied in the discharge and in afterglow. The high carburizing ratearises chiefly from the rapid infusion of carbon from the plasma into the surface that leads to a zoneoversaturated in carbon in the vicinity of the surface which provides the steel with carbon. Atmosphericplasma-assisted chemical vapor deposition (APACVD) process has also been developed for steel carburizing.It is one of the fastest material processing techniques because of the enhancement of the reactionkinetic. Moreover, there is no need of expensive vacuum equipments.