Characterization of Plasma Nitrided Duplex Stainless Steel: Influence of Prior Shot Peening and Nitriding Atmosphere

In this work, as-received and shot-peened SAF 2507 duplex stainless steel (DSS) is plasma nitrided (PN) at 400 degrees C for 6 h. Two different atmospheres are used: 75%N-2-25%H-2 and 25%N-2-75%H-2. Microstructural characterizations by optical and scanning electron microscopy and x-ray diffraction (XRD) are carried out. Also, residual stress, microhardness and roughness are measured. Finally, corrosion tests are conducted. XRD of PN samples indicates the presence of expanded austenite gamma(N), expanded ferrite alpha(N), iron nitride epsilon-Fe3N and chromium nitride CrN. Though all PN samples present a gamma(N) decomposition into CrN, alpha and gamma, this reaction is more pronounced in samples treated with lower N-2 atmosphere. Microhardness is significantly increased along the nitrided layers. Roughness is slightly increased after PN treatments due to the sputtering effect of the active species in the plasma. Though PN was carried out at 400 degrees C, that it is reportedly safe to sustain corrosion resistance, it causes CrN precipitation in the studied DSS, which deteriorates corrosion properties.