Development and microstructure characterization of single and duplex nitriding of UNS S31803 duplex stainless steel

The microstructure of the as received material was composed by ferrite and austenitic stringers, aligned in the rolling direction. The results showed that LTPN of the UNS S31803 duplex stainless steel promotes the formation of a duplex modulated structure composed by 2.5 μm thick, 1510 ± 52 HV hard, expanded ferrite (αN) regions, and 3.0 μm thick, 1360 ± 81 HV hard, expanded austenite (γN) regions on ferrite and austenite grains, respectively. Intense coherent ε-Fe3N nitride precipitation inside expanded ferrite was observed. ε-Fe3N nitrides precipitated with an orientation relationship [111] αN//[120] ε-Fe3N, leading to increased microhardness of the expanded ferrite regions. After the first step of the duplex nitriding treatment (HTGN) a 550 μm thick, 330 HV hard, nitrogen rich, fully austenitic layer formed at the surface of the specimens, by transformation of ferrite stringers into austenite. The second nitriding step (LTPN) led to the formation of a homogeneous expanded austenite layer, 1227 ± 78 HV on top of the thick fully austenitic layer, formed during the first step. The duplex treatment resulted in a more homogeneous, precipitate-free, microstructure and a better transition between the mechanical properties of the hardened outermost layer and the softer substrate.