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 650³C 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 in¯uence 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 brie¯y 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 M 23 C 6 . The literature reveals that NbC is more stable than M 23 C 6 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 M 23 C 6 , where there is consistent reporting in the literature. Finally, work on the M 6 C 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 s phase. It is clear that chromium concentrations in excess of 18 wt-%, combined with a low carbon concentration, promote the formation of s phase. This has implications particularly for steels containing niobium and titanium, both of which getter carbon. Other compounds reviewed include x and G phases, which form at high temperatures and during very long aging such as that encountered in service.MST/4721

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Much recent work has been devoted to characterize the microstructure and mechanical properties bainitic nanostructured steels. The microstructure is developed by isothermal heat treatment at temperatures as low as 125-350ºC and adapted steel grades typically contain high carbon contents to achieve sufficient depletion of the B S-M S temperature range, and above 1.5 Si wt.% to suppress carbide formation during isothermal holding. On the latter, most of the published literature agrees on a limit of around 1.2-1.5 wt.% to suppress cementite in high carbon steels. For this reason perhaps, additions of Si significantly above this limit have not been investigated systematically in the context of nanostructured bainitic steels. The present work is concerned with the effect of up to ~3 Si wt.% in a steel grade otherwise adapted to low temperature bainitizing. Tensile properties as compared to similar grades, though with lower Si contents, exhibited unrivalled combinations of strength and ductility, with above 21% total elongation for a UTS above 2GPa. An attempt is made to explain the mechanical properties of this microstructure in terms of some of its most relevant and unique morphological and microstructural features.

The present study is concerned with the potential of high carbon, high silicon steel grades isothermally transformed to bainite at low temperature (< 300 °C). A first part gives an overview of design principles allowing very high strength and ductility to be achieved, while minimising transformation duration. Wear and fatigue properties are then investigated for over ten variants of such material, manufactured in the laboratory or industrially. The results are discussed against published data. Tensile strength above 2 GPa are routinely achieved, with, in one case, an exceptional and unprecedented total elongation of over 20%. Bainite plate thickness and retained austenite content are shown to be important factors in controlling the yield strength, though additional, non negligible parameters remain to be quantified. Rolling-sliding wear performances are found to be exceptional, with as little as 1% of the specific wear rate of conventional bainitised 100Cr6. It is suggested that this results from the decomposition of retained austenite in the worn layer, which considerably increases hardness and presumaby introduces compressive residual stresses. Fatigue performance were slightly improved over 100Cr6 for one of the two industrially produced material, but significantly lower otherwise. Factors controlling fatigue resistance require further investigations.

Specially designed steels with carbon contents from 0.6 to 1.0 wt.% were isothermally transformed at very low temperatures, between 220 and 270°C, in order to obtain a nano-structured bainitic microstructure. It is shown that the wear resistance in dry rolling-sliding of these nano-structured steels is significantly superior to that of bainitic steels transformed at higher temperatures with similar hardness values. In addition to the highly refined microstructure, the transformation under strain to martensite (TRIP effect), contributes to the plasticity of the nanoscaled steels, increasing surface hardness during testing, thus reducing the wear rate.