Low-temperature plasma nitriding of AISI F51 duplex stainless steel

AISI F51 duplex stainless steel, with aligned ferrite and austenite stringers, was DC-plasma nitrided at 400 °C, in a 75% N2 + 25% H2 atmosphere. A modulated nitrided layer formed on the specimenʹs surface, the nitrided layer on the ferritic regions being thicker than on the austenitic regions of the microstructure. X-ray diffractometry and electron backscattered diffraction patterns (EBSD) revealed two phases in the nitrided layer: expanded austenite (γN) formed on the surface of austenite grains and expanded ferrite (αN) formed on the surface of ferrite grains. The nitrogen contents of both expanded phases were measured by wavelength dispersive X-ray diffractometry (WDX), being 3.8 wt.% in expanded austenite and 4.4 wt.% N in expanded ferrite. N supersaturation led to the strong hardening of the surface, up to 1350 HV. Microhardness profiles of the nitrided layer, measured on ferrite and austenite stringers, allowed estimating the nitrided layer thicknesses of each of the expanded phases. The expanded ferrite regions (αN) showed very fine deformation bands while the expanded austenite regions showed slip lines formed as a consequence of large compressive stresses developed in the nitrided layer.