Martensitic Stainless Steels Low-temperature Nitriding: Dependence of Substrate Composition

Low-temperature plasma assisted nitriding is a very promising technique to improve surface mechanical properties of stainless steels, keeping unaltered or even improving their surface corrosion resistance. During treatment, nitrogen diffuses into the steel surface, increasing its hardness and wear resistance. In the present work the nitriding process of different martensitic stainless steels was studied. As-quenched AISI 410, 410NiMo, 416 and 420 stainless steel samples were plasma nitrided at 300, 350, 400, 450, and 500 degrees C, for 4 h, at 3 Torr, in a gas mixture of 70% N-2 + 20% H-2 + 10% Ar, and flow rate of 3.33x10(-6) Nm(3)s(-1). The study of the nitrogen-rich layer thickness indicates two different activation energies, one for low (<400 degrees C) and another one for high temperature (>= 400 degrees C), except for the AISI 410NiMo. The surface hardness increases with temperature for all steels between 300 and 450 degrees C. For 500 degrees C, except for the AISI 410NiMo, the surface hardness decreases if compared to that obtained at 450 degrees C. This hardness decrease is related to chromium nitride precipitation, also associated to the treated surface sensitization. Finally, the CrN precipitation in treated surfaces is dependent on the steel composition, with the AISI 410NiMo steel presenting the lower average chromium nitride precipitation rate.