On the Nitrogen-Induced Lattice Expansion of a Non-stainless Austenitic Steel, Invar 36®, Under Triode Plasma Nitriding

Abstract

Chromium, as a strong nitride-forming element, is widely regarded to be an ''essential'' ingredient for the formation of a nitrogen-expanded lattice in thermochemical nitrogen diffusion treatments of austenitic (stainless) steels. In this article, a proprietary ''chrome-free'' austenitic iron-nickel alloy, Invar Ò 36 (Fe-36Ni, in wt pct), is characterized after triode plasma nitriding (TPN) treatments at 400°C to 450°C and compared with a ''stainless'' austenitic counterpart RA 330 Ò (Fe-19Cr-35Ni, in wt pct) treated under equivalent nitriding conditions. Cr does indeed appear to play a pivotal role in colossal nitrogen supersaturation (and hence anisotropic lattice expansion and superior surface hardening) of austenitic steel under low-temperature (£ 450°C) nitrogen diffusion. Nevertheless, this work reveals that nitrogen-induced lattice expansion occurs below the nitride-containing surface layer in Invar 36 alloy after TPN treatment, implying that Cr is not a necessity for the nitrogen-interstitial induced lattice expansion phenomenon to occur, also suggesting another type of c N .

The Anisotropic Stress-Induced Diffusion and Trapping of Nitrogen in Austenitic Stainless Steel during Nitriding

“…The depth profiles calculated with the trapping-detrapping model are in excellent agreement with the experimental ones [17,38,39]. However, recently it was shown for "chromium-free" austenitic iron-nickel alloy that Cr is not a necessity for the nitrogen-interstitial-induced lattice expansion phenomenon to occur [35]. The penetration of nitrogen without assistance of chromium and the driving force is internal stresses which create stress-induced diffusion [24][25][26][27][28][29][30].…”

“…High content of nitrogen penetrating the volume of austenite has been attributed to the trapping of nitrogen at octahedral interstitial sites, by alloying element atoms dissolved in the matrix with high affinity for N such as Cr [18,34]. The role of Cr has been investigated to have pointed to "trapping and detrapping" diffusion of nitrogen [35]. In the trapping-detrapping model, chromium atoms play a crucial role via a chemical bonding effect where Cr atoms form trap sites for N [36].…”

Vanadium‐Incorporated CoP2 with Lattice Expansion for Highly Efficient Acidic Overall Water Splitting

“…2b). The XRD pattern of V-CoP2 displays quite similar to that of CoP2, but the diffraction peaks slightly move to the lower angle region, illustrating that V incorporation causes a certain expansion of interplanar distance without changing the intrinsic structure of CoP2 35,36 . Raman spectra of V-CoP2 and CoP2 in Fig.…”

From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase

“…It is now known that Cr, which is also a nitride-/carbide-forming element, promotes the formation of the S phase. Fe-Ni or Fe-Mn alloys, having a fcc structure, have been reported to be not able to form S phase [64,65], but an expansion of the austenite fcc structure has been observed in ion-beam nitrided Invar (Fe-34Ni) [66] and in triode-plasma nitrided Invar 36 ® (Fe-36Ni) [67]. The presence of a fairly great content of Mn, like that present in AISI 200 series and Ni-free austenitic stainless steels, tends to promote nitride formation at temperatures at which AISI 300 series stainless steels do not produce nitride precipitation, since Mn is a nitride-forming element [55,68].…”