Creep-resistant Austenitic Stainless Steel Research Articles

A Study of Precipitation-Strengthened Creep-Resistant Austenitic Stainless Steel for Supercritical and Ultra-Supercritical Power Plants

Abstract The properties of precipitation-hardened austenitic stainless steels are adversely affected by the precipitation of unwanted brittle phases, like sigma (nonmagnetic intermetallic phase that is composed of iron and chromium) and Laves (Fe2Nb). These stainless steels are also susceptible to stress corrosion cracking. The induced residual stress in the material in turn reduces its creep strength. The strain beyond a certain limit affects its creep-resistant property appreciably, and hence, beyond a certain strain, these stainless steels are to be solution-treated for stress relief. Characterization studies on precipitation-strengthened creep-resistant austenitic stainless steel are presented in this article. Effects of strain and aging on microstructures are also evaluated. In order to understand the precipitation-strengthening behavior and effect of strain on Super304H (CC2328-2), the material was strained and aged at 650°C for a period ranging from 7 h to 4 weeks, and then the microstructures were studied using an optical microscope. Experiments were carried out on tubes with no strain and tubes with strain, which were aged at 650°C. The results show that beyond a threshold strain of ∼17 %, the material becomes deteriorated, and to regain its original microstructure, solid solution treatment is necessitated.

Microstructural evolution of Alloy 709 during aging

The creep-resistant austenitic stainless steel Alloy 709 (Fe-20Cr-25Ni (wt%) based steel) is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500–550 °C. However, the study of microstructural evolution of Alloy 709 during aging is lacking. In this study, thus, the microstructure of Alloy 709 has been investigated using electron microscopy in the as-received state and after static aging at 550, 650 and 750 °C. The results show that the prominent precipitate in the as-received Alloy 709 is Nb(CN), with the rod-like Z phase (CrNbN) observed very occasionally. After aging at 550 °C even up to 2000 h, no significant microstructure change was observed, which means that Alloy 709 is fairly stable at 550 °C. Aging at 650 °C produced globular M23C6 phase on grain boundaries, plate-like M23C6 carbides at twin boundaries and in the grain interior, and blocky M23C6 carbide nucleated on Nb(CN). Fine dispersoid Z phases were found on dislocations after aging at 650 °C for 500 h; their amount increases with aging time and temperature. (Cr,Mo)3(Ni,Fe)2SiN θ phase forms at grain boundaries after aging at 650 °C for 1000 h. After aging at 750 °C, θ phase nucleated on M23C6 carbide and a transformation of M23C6 to θ phase was found, which suggests that θ phase is the more stable. Aging at 550 °C promotes the segregation of Cr and Mo to grain boundaries whereas clear Cr depletion was observed at 650 °C due to the precipitation of Cr-rich M23C6 carbides at grain boundaries. Such depletion nearly disappears at 750 °C. The implications of aging for the subsequent mechanical behaviour of Alloy 709 are discussed briefly.

High temperature creep of Sanicro 25 austenitic steel at low stresses

Creep deformation of an advanced high-strength, creep-resistant austenitic stainless steel Sanicro 25 was investigated experimentally at temperatures from 700 °C to 800 °C and stresses below 40 MPa. Results show that the creep behaviour is qualitatively similar as that of the other creep-resistant steels, having much weaker stress and temperature dependencies of the creep rate than those at higher stresses. Creep performance of the Sanicro 25 steel allows temperatures of at least 50 °C higher than the classical AISI-316 H austenitic steel, even at low stresses.

Application of Impression Creep Technique for Development of Creep Resistant Austenitic Stainless Steel

The present paper highlights the usefulness of impression creep technique as applicable to alloy development studies of a creep resistant austenitic stainless steel which has been used as structural material for Fast Breeder Reactors. The effect of nitrogen on creep deformation behavior of type 316LN stainless steel has been studied using impression creep testing technique. Impression creep tests have been carried out at 923 K on 316LN stainless steel containing 0.07, 0.11, 0.14, and 0.22 wt. % nitrogen, in the punching stress range of 472-760 MPa. The impression creep curves were characterized by a loading strain, a primary creep stage and a secondary creep stage similar to uniaxial creep curves. The tertiary stage observed in uniaxial creep curves was absent. The steady state impression velocity was found to increase with increasing punching stress. It was observed that impression velocity was sensitive to the variation in nitrogen content in the steel. Steady state impression velocity was found to decrease with increasing nitrogen content as was observed in conventional uniaxial creep tests. A good correlation between the impression creep test and the conventional creep test results was observed which demonstrates the applicability of impression creep testing technique for alloy development programme.

Evaluation of Alumina-Forming Austenitic Foil for Advanced Recuperators

A corrosion- and creep-resistant austenitic stainless steel has been developed for advanced recuperator applications. By optimizing the Al and Cr contents, the alloy is fully austenitic for creep strength while allowing the formation of a chemically stable external alumina scale at temperatures up to 900°C. An alumina scale eliminates long-term problems with the formation of volatile Cr oxy-hydroxides in the presence of water vapor in exhaust gas. As a first step in producing foil for primary surface recuperators, three commercially cast heats have been rolled to ∼100 μm thick foil in the laboratory to evaluate performance in creep and oxidation testing. Results from initial creep testing are presented at 675°C and 750°C, showing excellent creep strength compared with other candidate foil materials. Laboratory exposures in humid air at 650–800°C have shown acceptable oxidation resistance. A similar oxidation behavior was observed for sheet specimens of these alloys exposed in a modified 65 kW microturbine for 2871 h. One composition that showed superior creep and oxidation resistance has been selected for the preparation of a commercial batch of foil.

Composition, Microstructure, and Water Vapor Effects on Internal/External Oxidation of Alumina-Forming Austenitic Stainless Steels

A family of creep-resistant austenitic stainless steels based on alumina (Al2O3) scale formation (AFA alloys) for superior high-temperature oxidation resistance was recently identified. Excellent oxidation behavior was observed at 650 and 700 °C in air with 10% water vapor. However, particularly at 800 °C, the presence of water vapor greatly increased the tendency for internal oxidation of Al. Water vapor also enhanced subscale Al depletion in some AFA alloys relative to dry air exposure. Increased levels of Nb additions were found to significantly improve oxidation resistance, as were reactive element additions of Hf and Y. Computational thermodynamic calculations of the austenitic matrix phase composition and the volume fraction of MC, B2-NiAl, and Fe2Nb base Laves phase precipitates were used to interpret oxidation behavior in terms of two-phase oxidation theory, reservoir effect, and the third-element effect of Cr. Of particular interest was the enrichment of Cr in the austenitic matrix phase by additions of Nb, which aided the establishment and maintenance of alumina. Higher levels of Nb additions also increased the volume fraction of B2-NiAl precipitates, which served as an Al reservoir during long-term oxidation. Implications of these findings for the design of AFA alloys with increased upper use temperature limits are discussed.

Effects of minor alloy additions and oxidation temperature on protective alumina scale formation in creep-resistant austenitic stainless steels

Niobium was found to be beneficial for alumina scale formation in newly developed creep-resistant austenitic stainless steels. Additions of both Ti and V degraded alumina scale formation, whereas alloys with individual additions of Ti or V were able to form protective alumina scales. The ability to form external alumina scales was lost in the lower Al- and Nb-containing alloys between 800 and 900 °C. Compositions with the potential to form alumina at 900 °C were identified.

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

The microstructural state of two grades of the creep-resistant austenitic stainless steels NF709, a Fe-20Cr-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 σ are found in largely different quantities. The study suggests that a seldom observed variant of the η structure, Cr3Ni2SiX, 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. Sci. Technol., 2001, vol. 17, pp. 1–14)).

Modelling simultaneous precipitation reactions in austenitic stainless steels

A physical model has been developed for simultaneous precipitation reactions in austenitic stainless steels, taking into account interactions between phases. Comparisons against experimental results show satisfying agreement, with the model successfully predicting the existence of transient phases that are often observed, but not predicted in equilibrium calculations. An analysis of σ-phase kinetics, especially the effect of grain size, supports the hypothesis that the formation of this phase depends more on the availability of high energy nucleation sites than on the driving force for its formation. The study underlines a lack of thermodynamic data for important phases in creep-resistant austenitic stainless steels.

Precipitation in creep resistant austenitic stainless steels

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.