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

In this work, the development of a duplex nitriding (DN) surface treatment combining High Temperature Gas Nitriding (HTGN) and Low Temperature Plasma Nitriding (LTPN) is reported. The microstructure and hardness variation of the duplex treated steel is compared with the properties obtained during single HTGN and single LTPN of UNS S31803 stainless steel. Single LTPN of UNS S31803 was carried out in an Active Screen Plasma Nitriding reactor, at 400 °C for 20 h, in a 75% N2 + 25% H2 atmosphere. Single HTGN of UNS S31803 was carried out at 1200 °C, under a 0.1 MPa high purity N2 gas atmosphere, during 8 h. The developed duplex nitriding (DN) surface treatment consists of a combination of both, HTGN and LTPN treatments, carried out in the same conditions described above.
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.