S phase formed on the surface of austenitic stainless steels at low temperature plasma nitriding has excellent wear and corrosion resistance.

The gas nitriding reaction of austenitic stainless steels is enhanced with mechanical pre-treatment such as shot peening and the same phenomenon seems to occur in plasma nitriding and hence the formation of S phase will be influenced by mechanical pre-treatment.

In this paper, AISI 304 stainless steel was pre-treated by peening using several media (SiO₂, SiC, 50Ni–50Cr and AISI M2) and plasma nitrided in 80%N₂–20%H₂ gas mixture at 673K for 28.8 ks under a pressure of 6.67 × 10² Pa. The structure of the plasma nitrided layers was investigated by means of optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and glow discharge spectrometry (GDS). Corrosion wear resistance was also examined.

The thickness of the nitrided layer increases by peening before nitriding in the following order: grinding with emery papers only < (peening by) 50Ni-50Cr < SiO₂ < SiC < AISI M2.

Accordingly, the nitriding reaction is enhanced with peening as a mechanical pre-treatment and the degree of the enhancement depends on the material of the medium used for peening.

Peening by SiO₂, 50Ni–50Cr and M2 before nitriding did not provide any improvement in corrosion wear resistance. However, the nitrided layer after peening with SiC shows excellent corrosion wear resistance compared with the S phase layer only.

It has been proven that peening as a mechanical pre-treatment influences the formation and characteristics of S phase.