Study of microstructure of low-temperature plasma-nitrided AISI 304 stainless steel

Abstract

The microstructure of the low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel was studied by transmission electron microscopy (TEM). The results show that the surface of the layer consists of a supersaturated solid solution (␥ Ј N ) based on the ␥ Ј-Fe 4 N phase whose electron diffraction pattern (EDP) has a strong diffuse scattering effect resulting from supersaturating nitrogen (above 20 at. pct) and ͗110͘ streaks arising from matrix elastic strain due to the formation of paired or clustered Cr-N. The latter is due to the N above the 20 at. pct ␥ Ј-Fe 4 N-phase value and leads to a lattice parameter that is greater than that of the ␥ Ј-Fe 4 N phase. The subsurface of the layer is composed of a supersaturated solid solution based on ␥ -austenite, which is an expanded austenite, ␥ N . Its morphology shows the basketweave or "tweedlike" contrast consisting of so-called stacking fault precipitates having twin relationships with the matrix whose EDP shows diffuse scattering streaks with certain directions. The martensite transformation was observed in the subsurface of the layer. The increase in stacking faults compared with the original stainless steel and formation of martensite in the subsurface of the layer indicate that nitrogen lowers the stacking fault energy of austenite.

Microstructural Characterization of Layers Produced by Plasma Nitriding on Austenitic and Superaustenitic Stainless Steel Grades

“…In the same micrograph it is possible to identify "colonies" (marked with white arrows in Fig. 6) on the surface of the grain "B" whose morphology is quite similar to that observed in nitrided layers investigated by Mitchell et al [21] and Xu and colleagues [22]. These colonies result from localized expanded austenite decomposition and they display a lamellar type microstructure composed by ferrite and cubic chromium nitride [13,21].…”

Thermal expansion and phase transformations of nitrogen-expanded austenite studied within situsynchrotron X-ray diffraction

“…The resulting depth variation of the composition has several implications for the observed X-ray diffractograms. The compositional inhomogeneity within the investigated depth range causes asymmetric broadening and the induced macrostress gradient over the expanded austenite zone causes hkl-dependent shifts of the diffraction peaks (Ö ztü rk & Williamson, 1995;Xu et al, 2000;Christiansen et al, 2010). Asymmetric broadening is also anticipated from texture gradients (Riviè re et al, 2007).…”

Atom probe tomography characterization of nitrogen induced decomposition in low temperature plasma nitrided 304L austenitic stainless steel

“…The formation of nitrides is believed to be almost suppressed at nitriding temperatures around 400 1C or below, due to sluggish chromium diffusion, thus preserving corrosion resistance [6]. However, for long nitriding times CrN does form, especially at the surface [7][8][9]. We have recently reported that nanometer-sized precipitates do form after nitriding for 30 h at 400 1C in AISI 304L [10].…”