Precipitation in creep resistant austenitic stainless steels

Abstract

Austenitic stainless steels have for some time been used as superheater tubes in the electricity generation industries in harsh environments with temperatures as high as 650°C at pressures of some 200 atm; they are expected to provide reliable service for 30 years or more. Their detailed mechanical properties are dependent on the stability of the microstructure, particularly the formation, dissolution, and coarsening of precipitates. Although the precipitation processes have been studied extensively, there remain important discrepancies. It is known that small changes in the chemical composition or thermomechanical processing can profoundly influence the evolution of the microstructure. This review focuses on precipitation in creep resistant austenitic stainless steels, in particular wrought heat resistant grades containing niobium and titanium additions. Conventional alloys such as 18–8 and 16–10 are included together with the new NF709 (20–25) and other recent variants. Precipitates forming in age hardening austenitic stainless steels are only briefly presented. Many studies have shown that MX is not a stoichiometric phase, and that chromium can be incorporated in the metal sublattice. Furthermore, the reported compositions show considerable variation. These studies are assessed and an explanation is offered, in terms of the Gibbs–Thompson effect, for the variation. A rational consideration of all the results suggests a size dependence in line with capillarity considerations. The MX phase does not form in isolation; its stability also depends on the formation of M23C6. The literature reveals that NbC is more stable than M23C6 but the case for TiC is less certain. The formation of Z phase in nitrogen bearing steels is a further complicating factor, and it is concluded that its formation is not adequately understood. This is unlike the case for M23C6, where there is consistent reporting in the literature. Finally, work on the M6C carbide in austenitic stainless steels is critically assessed. It is found that its detailed composition is not well characterised and that there are no general rules apparent about its formation. The review also covers intermetallic compounds such as σ phase. It is clear that chromium concentrations in excess of 18 wt-%, combined with a low carbon concentration, promote the formation of σ phase. This has implications particularly for steels containing niobium and titanium, both of which getter carbon. Other compounds reviewed include χ and G phases, which form at high temperatures and during very long aging such as that encountered in service.