Investigation of Chromium Nitride Precipitation in UNS S39274 Stainless Steel.

Chromium nitrides precipitation in a W-alloyed superduplex stainless steel grade UNS S39274 was studied. A microstructure of ferrite, austenite and quenched-in nitrides was produced by solution treatment at 1150 °C followed by water quenching. Although the ferrite to austenite ratio was close to 1:1 after solution treatment, the steel contained a high density of chromium nitrides in the ferrite phase. This intense precipitation could be detected by optical microscopy (LOM), scanning electron microscopy (SEM) and atom force microscopy. Different features were observed comparing specimens electrolytically etched with 40%HNO₃ solution and with 10% oxalic acid solution. The nitrides were also investigated by transmission electron microscopy (TEM), where two types of chromium nitrides were observed: larger CrN/Cr₂N particles and needle-like nanosized CrN particles. The larger nitrides were those observed by SEM and LOM. The nanosized nitrides could only be studied by TEM, where three variants of nanosized precipitates named V1, V2 and V3 were identified. They were found to be coherent with the ferritic matrix and respects the Baker–Nutting orientation relationship with ferrite ((100)_CrN//(100)_δ−Fe; [110]_CrN//[001]_δ−Fe). These fine precipitates provoke pinning of dislocations which is responsible for some hardening of the ferrite phase. [ABSTRACT FROM AUTHOR]