Influence of the treatment atmosphere on the characteristics of glow-discharge treated sintered stainless steels

The surface hardening of sintered stainless steel components is a major goal, in order to extend the applications of these materials. The aim of the present study was to evaluate the influence of the treatment atmosphere on the microstructure and mechanical properties of AISI 316L austenitic and AISI 410 martensitic sintered stainless steels by performing glow-discharge nitriding and nitrocarburizing treatments. The treatments, performed at 773 K for 8 h, produced modified surface layers. The use of a nitrocarburizing atmosphere is able to promote the formation of epsilon carbonitride [Fe2-3(N, C)], especially on AISI 410 martensitic samples. The microhardness profiles of the treated samples show high hardness values in the modified layers and a steep decrease to matrix values. The AISI 316L samples have thinner hardened layers and lower hardness values, in comparison with AISI 410 ones. Thinner hardened layers with a steeper decrease to matrix values are obtained using a nitrocarburizing atmosphere, if compared with those of nitrided samples. On the modified layers of both the nitrided and nitrocarburized AISI 316L samples the metastable S phase is detected. The X-ray diffraction analysis has shown that its lattice parameter values, evaluated by fitting it with a face centred tetragonal lattice, are influenced by the treatment atmosphere and that the amount of this phase increases when a nitrocarburizing atmosphere is used.