Creep behavior of a new cast austenitic alloy (original) (raw)
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Creep behavior of a new precipitation-strengthened 15Cr–15Ni austenitic stainless steel with optimized content of Nb, C, N and Mo, subjected to a special multicycled aging-quenching heat treatment process, was investigated at a temperature of 750 1C and stress range of 78–200 MPa for up to 10,000 h. The steel exhibited excellent creep rupture strength which exceeds even that of the commercial NF709 alloy. The value of the creep exponent n at the applied testing temperature/stress conditions was found to be of around 5.6, demonstrating that dislocation creep is the dominant deformation mechanism during the performed creep tests. The crept microstructures showed the presence of a high number of copper precipitates, and of fine dispersion of densely distributed intragranular nano-sized (Nb,Cr)N nitride precipitates which transformed to more coarsening-resistant Z-phase precipitates with the prolonging creep exposure time. The nitrides, occurring in dominant quantities in the microstructures, showed superior coarsening-resistance during creep exposure while copper precipitates exhibited a relatively high coarsening rate. A comparison between creep rupture strength of the studied steel and of its commercial equivalent grade SUS 304 JI HTB revealed that the nano-sized (Nb,Cr)N nitrides and/or Z-phase precipitates essentially played the key role in the observed improved resistance to dislocation creep of the material. Considerations over the observed creep behavior and aspects of high-temperature microstructural stability such as coarsening of strengthening precipitates, issues related to phase transformation processes suggested that the steel in the present study exhibits capability for applications in USC fossil power plants with steam parameters of around 700 1C/35–40 MPa.
Materials Science and Engineering: A, 2020
This work focusses on the link between microstructure and creep properties for heat-resistant austenitic alloys with high aluminum content (3-5 wt. %). An emphasis was put on the coupling of thermodynamic simulations, microstructural characterizations by scanning electron microscopy and transmission electron microscopy, and creep testing. The phase predictions performed by the calculation of phase diagrams method are in good agreement with the observed microstructure after creep at 1000°C and 1050°C. Correlation between creep properties and microstructure characterizations at 1000°C and 1050°C revealed that NiAl and α' (chromium-rich base centered cubic phase) phases are deleterious for the creep properties at service temperature. Several high Al-content alloys are selected in order to replace the chromia-forming alloys standardly used in cracking furnaces.
Assessment of High-Alloyed Creep Resistant Materials
2012
Austenitic steels and nickel alloys are used in power plants with higher working parameters (temperature, pressure). These materials are applied especially at the outcome levels producing over-heated or supercritical steam. Creep strength is achieved by comprehensive strengthening of materials given by their fundamental constitution and proceeding even under operation temperatures. Under these conditions, oxidation resistance depends predominantly on both the content of chromium and its ability to transfer towards the metal-oxide interface. The paper compares different high-alloyed materials mostly in the subsurface area, which interacts with steam during the application.
Creep-Resistant, Al2O3- Forming Austenitic Stainless Steels
Science, 2007
A family of inexpensive, Al 2 O 3 -forming, high-creep strength austenitic stainless steels has been developed. The alloys are based on Fe-20Ni-14Cr-2.5Al weight percent, with strengthening achieved through nanodispersions of NbC. These alloys offer the potential to substantially increase the operating temperatures of structural components and can be used under the aggressive oxidizing conditions encountered in energy-conversion systems. Protective Al 2 O 3 scale formation was achieved with smaller amounts of aluminum in austenitic alloys than previously used, provided that the titanium and vanadium alloying additions frequently used for strengthening were eliminated. The smaller amounts of aluminum permitted stabilization of the austenitic matrix structure and made it possible to obtain excellent creep resistance. Creep-rupture lifetime exceeding 2000 hours at 750°C and 100 megapascals in air, and resistance to oxidation in air with 10% water vapor at 650°and 800°C, were demonstrated. O ne key to reducing emissions and increasing efficiencies in energy-conversion systems is the development of new structural materials with higher-temperature capability (1, 2). Areas of need range from thin metal foil for use in turbine recuperators to high-pressure steam tubing in fossil-fired ultrasupercritical steam plants. The dilemma is that higher operating temperatures shorten component lifetime by increasing the rates of degradation phenomena such as time-dependent deformation (creep) and oxidation, which result in loss of structural integrity and, ultimately, failure. The challenge is to improve both high-temperature creep strength and oxidation resistance, and yet achieve it at affordable cost for industrial use. There have been major advances in high-temperature alloy classes, ranging from oxide dispersion strengthened (ODS) alloys, intermetallic alloys, and Ni-base superalloys. However, all of these are either prohibitively expensive or have yet to exhibit the desired combination of mechanical properties, oxidation resistance, and manufacturability needed to make them viable for widespread use as heat-resistant components in energy-conversion systems.
Metallurgical Transactions A, 1983
The heat-to-heat variation in the creep strength and ductility of austenitic stainless steels was reviewed from the viewpoint of residual and trace element effects. Based on data reported in the literature, the creep strength of unstabilized alloys such as types 304 and 316 stainless steel increased with residual element and trace element content. Niobium appeared to be the most potent strengthener. There was no direct evidence that trace elements such as sulfur and phosphorus had a deleterious effect on either strength and ductility. It was assumed that the creep strength and ductility of the unstabilized grades of austenitic stainless steels are controlled by the precipitate characteristics. It follows from this that thermomechanical treatment or residual element additions that affect the precipitate characteristics influence subsequent time dependent mechanical properties. This view is consistant with most of the information in the literature. It was concluded that more systematic studies of trace and residual element effects would be beneficial to the improvement of steels. Incorporated into the studies should be quantitative characterization of evolving precipitate morphology and composition as they are influenced by residual elements. This information should be incorporated into modeling studies of nonequilibrium segregation. Ultimately, optimum elevated-temperature strength could be developed based on a materials science approach.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science - METALL MATER TRANS A, 2010
A family of creep-resistant, alumina-forming austenitic (AFA) stainless steel alloys is under development for structural use in fossil energy conversion and combustion system applications. The AFA alloys developed to date exhibit comparable creep-rupture lives to state-of-the-art advanced austenitic alloys, and superior oxidation resistance in the ~923 K to 1173 K (650 °C to 900 °C) temperature range due to the formation of a protective Al2O3 scale rather than the Cr2O3 scales that form on conventional stainless steel alloys. This article overviews the alloy design approaches used to obtain high-temperature creep strength in AFA alloys via considerations of phase equilibrium from thermodynamic calculations as well as microstructure characterization. Strengthening precipitates under evaluation include MC-type carbides or intermetallic phases such as NiAl-B2, Fe2(Mo,Nb)-Laves, Ni3Al-L12, etc. in the austenitic single-phase matrix. Creep, tensile, and oxidation properties of the AFA al...
Creep studies of Cold Worked Austenitic Stainless Steel
Procedia Structural Integrity, 2019
During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Creep and tensile behaviour of austenitic Fe–Cr–Ni stainless steels
Materials Science and Engineering: A, 2009
The control of creep behaviour during service of reformer tubes made of HP-40 austenitic stainless steels is still limited by the knowledge of creep mechanisms in these alloys. Two different HP-40 alloys modified with a low-level addition of Nb were studied. Creep tests were carried out at 980 and 1050 • C with different stress levels, in the range of 20-50 MPa, and their results were plotted in a Norton-type diagram. Also, low strain rate tensile tests were performed at temperature of 950, 980 or 1000 • C. As low strain rate tensile tests showed a plateau at nearly constant stress for a given strain rate, they could be somehow linked with creep tests. Accordingly, tensile and creep results were plotted together on a Larson-Miller (LMP) diagram. The fracture modes of tensile and creep samples were investigated and the effect of different parameters such as sample dimensions, temperature and atmosphere, was also studied.
Microstructure Evolution During Creep of Cold Worked Austenitic Stainless Steel
IOP Conference Series: Materials Science and Engineering, 2018
The 14Cr-15Ni austenitic stainless steel (SS) with additions of Ti, Si, and P has been developed for their superior creep strength and better resistance to void swelling during service as nuclear fuel clad and wrapper material. Cold working induces defects such as dislocations that interact with point defects generated by neutron irradiation and facilitates recombination to make the material more resistant to void swelling. In present investigation, creep properties of the SS in mill annealed condition (CW0) and 40 % cold worked (CW4) condition were studied. D9I stainless steel was solution treated at 1333 K for 30 minutes followed by cold rolling. Uniaxial creep tests were performed at 973 K for various stress levels ranging from 175-225 MPa. CW4 samples exhibited better creep resistance as compared to CW0 samples. During creep exposure, cold worked material exhibited phenomena of recovery and recrystallization wherein new strain free grains were observed with lesser dislocation network. In contrast CW0 samples showed no signs of recovery and recrystallization after creep exposure. Partial recrystallization on creep exposure led to higher drop in hardness in cold worked sample as compared to that in mill annealed sample. Accelerated precipitation of carbides at the grain boundaries was observed during creep exposure and this phenomenon was more pronounced in cold worked sample.
Journal of Material Science and Technology Research, 2014
This study concerns three alloys based on cobalt, nickel or iron, all containing chromium (25 wt.%), carbon and hafnium. The contents in the two last elements were chosen high enough to promote the formation of HfC carbides. All alloys were elaborated by casting and their microstructures preliminarily characterized. They were selected to be tested in three-points flexural creep at 1200°C under 20 MPa in inert atmosphere. All alloys contain hafnium carbides, together with chromium carbides in some cases. The HfC carbides are generally of two types: script-like eutectic and blocky pre-eutectic. The creep deformation is fast for the nickel-based and iron-based alloys, especially for the later one. In contrast the HfC-containing cobalt-based alloy behaves much better. It displays a creep resistance at 1200°C significantly higher than another cobalt-based alloy strengthened by chromium carbides added to the study for comparison. All alloys were also briefly tested in oxidation by air at 1200°C. The creep behaviors of the cobalt-based and iron-based alloys are not good and significantly worse than the nickel-based alloy's one. The oxidation resistance of the HfC-strengthened cobalt-based alloy must be improved to take benefit of its superior creep strength.