Commercial Austenitic Steels Research Articles

Preliminary Characterization of Alumina-Forming Austenitic–Type Advanced Alloys as Structural Materials for LFRs

The lead-cooled fast reactor (LFR) is one of the most promising Generation-IV nuclear designs currently under development in Europe, China, and the United States. LFRs can ensure enhanced performance and minimal waste production thanks to a closed fuel cycle, but they also have some issues that need to be addressed. One of the most critical is the long-term degradation process initiated in structural materials exposed to liquid Pb. The present state of the art has shown that commercial austenitic steels, such as American Iron and Steel Institute 316L and 15-15Ti can be adopted as structural materials in Pb environments up to 480°C, beyond which they start to experience the dissolution of constituting alloying elements (Ni, Cr, and Fe) if not protected by a coating or by surface modification. In more recent years, a lot of research effort has been done in order to develop new coating technologies and new base materials for operation with liquid Pb at higher temperatures. Among the newest alloys, alumina-forming austenitic (AFA) steels have gained interest in the research community because of their promising corrosion resistance results even at temperatures of 600°C. In this framework, an experimental campaign has been run at the Research Center ENEA of Brasimone that aims to characterize the behavior of two different AFA steels (with low and high Ni content in their composition) in static Pb at 650°C and 750°C with a moderate low oxygen concentration (10−6 wt %). After exposure, the AFA steels were characterized from the point of view of the morphology and composition, and the results are presented and discussed here.

Thermomechanical Fatigue of Heat Resistant Austenitic Alloys

Rising global energy consumption and the increase in emissions of greenhouse gases (e.g. CO2) causing global warming, make need for more sustainable power generation. This could be accomplished by increasing the efficiency of the biomass-fired power plants, which is achieved by increasing the temperature and pressure. In addition, flexible generation of power is critical if only renewable power generation is to be achieved and this will increase the number of start-and stop cycles. Cyclic condition in a long-term high temperature environment is an operation process that such materials must withstand, in order to satisfy the needs for future power generation.Commonly austenitic stainless steel are used for critical components of power plants. Because of future change in operating conditions, further investigations are needed to verify that the demands on safety for cyclic long-term usage is fulfilled. This work includes investigation of two commercial austenitic steels: Esshete 1250 and Sanicro 25. The materials were exposed to thermomechanical fatigue (TMF) in strain control under In-Phase and Out-of-Phase conditions and main testing temperature ranges of 100-650°C and 100-800°C respectively. Some of the specimens were pre-aged to simulate prolonged service condition. Mechanical test data were obtained and analysed in order to define the TMF performance of the investigated alloys. The differences in performance were discussed in relation to mechanical and microstructural characterization.

Morphology and elemental composition of a new iron-rich ferrite phase in highly irradiated austenitic steel

Elemental composition and morphology of a previously unidentified radiation-induced ferrite phase were investigated in a 300-series steel irradiated by neutrons in-service up to 57.6 dpa. Specimens of 18Cr-10Ni-Ti stainless steel (AISI 321 analog) were cut from a hexagonal wrapper of a fuel assembly irradiated in the BN-350 sodium-cooled fast reactor. An Fe-rich bcc-phase was observed primarily on grain boundaries. In this phase, the concentration of Cr is ∼8–12% (compared to ∼19% in the matrix), the concentration of Ni is ∼1.5–3% (∼9% in the bulk material), and the concentration of Mn is ∼0.23% (1.3% in the matrix). This Fe-rich phase is distinctly different from the retained-ferrite phase, commonly found in commercial austenitic steels. The extensive appearance of this Fe-rich ferrite on grain boundaries suggests that enhanced surface–intergranular corrosion may occur in water-cooled power reactors, arising from the low Ni, Mn, and Cr concentrations in this phase.

Creep-fatigue interaction in heat resistant austenitic alloys

This work includes an investigation of two commercial austenitic steels: UNS S21500 (Esshete 1250) and UNS S31035 (Sandvik SanicroTM 25). The materials were exposed to isothermal strain controlled fatigue with load controlled dwell time at maximum strain. The testing temperature used was 700°C and the test cycles were performed in tension. Mechanical test data were obtained and analysed in order to define creep-fatigue damage diagrams at failure for the investigated austenitic alloys. During the given conditions, Sanicro 25 showed superior creep-fatigue life, suffered less amount of creep elongation for the same amount of strain amplitude and dwell times compared to Esshete 1250. Both alloys showed creep-fatigue interaction damage for specific test configurations.

Enhancing Mechanical Properties and Formability of AISI 301LN Stainless Steel Sheet by Local Laser Heat Treatment

This study demonstrates applying local laser heat treatment to produce ultrafine-grained austenite (UFGA) structures in an AISI 301LN type commercial austenitic steel. Pieces of 50% cold-rolled sheets containing more than 90% strain-induced martensite were heated locally by a laser beam to various peak temperatures to obtain different degrees of martensite reversion to austenite. Mechanical properties and formability of grain-refined and coarse-grained structures were measured by tensile and Erichsen cup tests. In addition to standard Erichsen cup test, additional interrupted tests were carried out, where cups were first stretched close to the critical strain. Drawn cups were then heated locally by a laser beam to revitalize the structure and thereby enhance the formability in the following cupping test until failure. Results showed that local laser heat treatment is suitable for the reversion treatment to refine the austenite grain size. Various structures were produced: completely reverted microstructures (T > 700 °C) with grain sizes 0.9 - 2 µm in addition to partially reverted structure (T < 700 °C) containing nano- and ultrafine-grained austenite (0.6 µm) with some martensite. The grain refinement by local annealing improved the strength properties. The Erichsen cup tests showed that the formability was equal in the completely reverted ultrafine-grained structures to that of the coarse-grained sheets. It was demonstrated that the local laser treatment restored formability of the drawn cups, allowing stretching to be continued. The second forming step after the laser-treatment provided an enhancement of 19 and 14% in the cup depths in coarse-grained and ultrafine-grained steels, respectively, even though the laser-treatment parameters were not optimized yet.

Application of Martensitic Transformation Fundamentals to Select Appropriate Alloys for Grain Refining Through Martensite Thermomechanical Treatment

The main objective of the current work is to present a predictive model capable of assessing appropriation of a chemical composition for being used in the martensite thermomechanical treatment to develop nano/ultrafine-grained microstructures in austenitic steels. For this purpose, the correlation between stacking fault energy (SFE), driving force for transformation of martensite to austenite, fraction of deformation-induced α′-martensite, and saturation strain (e s) in some ternary Fe-Cr-Ni alloys and commercial austenitic steels was investigated. It was shown that the volume fraction of α′-martensite and e s (key parameters in the martensite treatment) are closely related to SFE and the driving force of the alloys. In addition, a nickel-free austenitic alloy was used to evaluate the discussions. A fairly good agreement was found between the calculated and the experimental results.

Solute cluster formation in austenitic and ferritic alloys under ion irradiation: a three‐dimensional atom probe characterization

AbstractVessel and internal core steels of nuclear power reactors are subjected to energetic neutron irradiation. The production of point defects in the material results from elastic collisions in displacement cascades. On a microscopic scale, changes in the microstructure and local microchemistry are observed in both steels. On a global scale, a shift in the transient ductile to brittle temperature of vessel steels and an increase in the irradiation‐assisted stress‐corrosion‐cracking susceptibility of internal core stainless steels are observed. Also, on a nanometre scale, radiation‐induced phase transformation occurs in both steels. A systematic approach has proved its efficiency here to study the specific role of displacement cascades on cluster formation in two different materials: ferritic steels (bcc structure) and austenitic steels (fcc structure). In order to produce the primary knock atom, model ferritic steels containing a small amount of copper as well as a commercial austenitic steel (316) were irradiated using Fe and Ni ions, respectively. The different resulting microstructures are then studied on a nanoscale using tomographic atom probe three‐dimensional reconstruction. Copyright © 2004 John Wiley & Sons, Ltd.

Investigation of a New Methodology in High Temperature Oxidation Application to Commercial Austenitic Steels

Avesta Sheffield R&D has evaluated a scaling temperature for heat resistant steels from laboratory gravimetric measurements of cyclic exposures. In this paper, this old method is described. A new methodology, based on the results of isothermal and cyclic tests, is proposed and discussed. It includes isothermal and cyclic oxidation kinetics of different commercial austenitic steels. Oxidation of Avesta Sheffield 153MA, 253MA and 353MA and standard commercial heat resistant steels 309S, 310S and A800 have been investigated under isothermal and cyclic conditions, in air, in the temperature range 800-1200°C. For all alloys, oxidation rates obey a parabolic law below a critical temperature. The activation energies have been calculated. Kinetics of the cyclic tests show that a critical mass change corresponding to a critical thickness of the oxide scale is responsible for the spallation start. This thickness depends on the composition of the alloys, and is very much increased by alloying minor elements like Ce.

Coal-ash deposit corrosion of some steels and nimonic alloys

AbstractThe corrosion behaviour of some commercial austenitic steels and nimonic aalloys has been studied at 650, 800, 900 and 1000°C in air and in the presence of ash residues obtained from coals ...

The creation and accommodation of extrinsic dislocations at grain boundaries

Abstract This paper considers the observation of extrinsic grain boundary dislocations in a commercial austenitic steel. It is shown that the stability of these non-equilibrium defects is a sensitive function of the precise thermo-mechanical history of the specimen material and is often controlled by the presence of active pinning centres in the grain boundary (e.g. solute and/or precipitate)