Microstructural evolution in two variants of NF709 at 1023 and 1073 K

Abstract

The microstructural state of two grades of the creep-resistant austenitic stainless steels NF709, a Fe20Cr-25Ni (wt pct) based steel, has been studied, in the as-received state, after an additional solution treatment and after static aging at 1023 and 1073 K. Although the two variants are chemically similar, they exhibit different microstructures following identical heat treatment. In particular, the Z phase and are found in largely different quantities. The study suggests that a seldom observed variant of the structure, Cr 3 Ni 2 SiX, is stabilized by nitrogen. Both steels exhibit a yet undocumented precipitation sequence when compared to more conventional austenitic steels (for which the precipitation sequences have been reviewed by Sourmail (Mater.

On Cr2N precipitation mechanisms in high-nitrogen austenite

“…This means that either precipitation has blocked the twin plate growth or this is an example of grain boundary migration and the two phases would subsequently grow cooperatively forming a discontinuous precipitation aggregate. This type of behaviour has been reported for M 23 C 6 [22]. However, to the authors' best knowledge, it has not been described for chromium nitride precipitation.…”

Effect of Alloying Additions on Phase Equilibria and Creep Resistance of Alumina-Forming Austenitic Stainless Steels

“…Such understanding is needed, because the high-temperature strength of AFA alloys cannot be further improved by nitrogen additions, which are typically used to form M(C, N)-type carbonnitrides (M: Nb, Ti, V)-strengthening precipitates in advanced austenitic stainless steel alloys such as alloy 709, [14][15][16] because of acicular, coarse AlN formation. [17] The AFA alloy compositions of interest are based on Fe- (12)(13)(14)Cr- (2.5-4) [18,19] Copper was added for further stabilizing the austenitic matrix, tungsten was added for solution hardening, and very small amounts of Ti and V (<~0.1 to 0.2 wt pct) were also added to enhance the MC-type carbide formation without the loss of Al 2 O 3 scale formability.…”

Advanced Austenitic Heat-Resistant Steels for Ultra-Super-Critical (USC) Fossil Power Plants

“…However, it broke down because of its poor high temperature performance and the unit must be shut down. At the efforts to pursuing for high www.intechopen.com temperature strength and good oxidation resistance by adding or optimizing alloying elements and increasing Cr, Ni content, conventional Cr-Ni austenitic steel has developed to modified 18Cr-9Ni type or 25Cr-20Ni type austenitic heat resistant steels (Yoshikawa et al, 1988;Sourmail & Bhadeshia, 2005). In order to improve high temperature creep rupture strength, not only solid solution strengthening elements such as W and Mo, but also precipitation strengthening elements such as Nb, Ti, V, are added to 18Cr-9Ni base austenitic steels to form carbon-nitrid MX for age hardening.…”