In-situ XRD investigations during nitriding of duplex steel 318LN

Nitriding of duplex stainless steel 318LN was investigated using in-situ X-ray diffraction (XRD) to obtain details on the temporal evolution of the diffusion and the phase formation in the temperature range of 350–500 °C. Additionally, complementary results from ex-situ investigations are used obtain a comprehensive picture. In-situ XRD data show a fast nitrogen diffusion where the reflections of the underlying substrate are replaced by an expanded austenite phase. The diffusivities derived from in-situ XRD and Time-of-flight secondary ion mass spectrometry (ToF-SIMS) are nearly identical, yet the phase resolved data yield a slightly faster diffusion for ferrite than for the austenite matrix. Only for the highest temperature of 485 °C, a decay into CrN and a FeNi phase is observed rather early, starting within 25 min. For 450 °C, no such decay has been observed.

Furthermore, in-situ XRD results obtained during etching after nitriding at 450 °C again are consistent with a single expanded austenite phase on top of the duplex steel substrate. However, complementary scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction (EBSD) investigations point towards a mixture of expanded austenite and an X-ray amorphous, highly disordered ferritic structure as the nitrogen containing phase with this second phase being completely invisible in XRD.