Search Results for thermal phase stability

Ni-38Cr-3.8Al has high hardness and high corrosion resistance with good hot workability, and therefore, it has been applied on various applications. However, in order to expand further application, it is important to understand the high temperature properties. Then, this study focused on the high temperature properties such as thermal phase stability, hardness, tensile property, creep property and hot corrosion resistance. As the result of studies, we found that the thermal phase stability of (γ/α-Cr) lamellar structure and the high temperature properties were strongly influenced by the temperature. Although the high temperature properties, except for creep property, of Ni-38Cr-3.8Al were superior to those of conventional Ni-based superalloys, the properties were dramatically degraded beyond 973 K. This is because the lamellar structure begins to collapse around 973 K due to the thermal stability of the lamellar structure. The hot corrosion resistance of Ni-38Cr-3.8Al was superior to that of conventional Ni-based superalloys, however, the advantage disappeared around 1073 K. These results indicate that Ni-38Cr-3.8Al is capable as a heat resistant material which is required the hot corrosion resistance rather than a heat resistant material with high strength at high temperature.

Inconel alloy 740H is designated for boiler sueprheater/reheater tubes and main steam/header pipes application of advanced ultra-supercritical (A-USC) power plant at operating temperatures above 750°C. Microstructure evolution and precipitates stability in the samples of alloy 740H after creep-rupture test at 750°C, 800°C and 850°C were characterized in this paper by scanning electron microscopy, transmission electron microscopy and chemical phase analysis in details. The phase compositions of alloy 740H were also calculated by thermodynamic calculation. The research results indicate that the microstructure of this alloy keeps good thermal stability during creep-rupture test at 750°C, 800°C and 850°C. The precipitates are MC, M 23 C 6 and γ′ during creep-rupture test. The temperature of creep test has an important effect on the growth rate of γ′ phase. No harmful and brittle σ phase was found and also no γ′ to η transformation happened during creep. Thermodynamic calculations reveal almost all the major phases and their stable temperatures, fractions and compositions in the alloy. The calculated results of phase compositions are consistent with the results of chemical phase analysis. In brief, except of coarsening of γ′, Inconel alloy 740H maintains the very good structure stability at temperatures between 750°C and 850°C.

A new nickel-base superalloy, Inconel alloy 740, is being developed for ultra-supercritical (USC) boiler applications operating above 750°C, designed to meet critical requirements for long-term high-temperature stress rupture strength (100 MPa for 10 5 hours) and corrosion resistance (2 mm/2 × 10 5 hours). Experimental investigations revealed key structural changes at elevated temperatures, including γ coarsening, γ' to η transformation, and G phase formation. To enhance strengthening effects and structural stability, researchers conducted a systematic optimization process based on thermodynamic calculations, implementing small adjustments to several alloying elements and designing modified alloy compositions. Comprehensive testing examined the long-term structural stability of these modifications, with investigations conducted up to 5,000 hours at 750 and 800°C, and 1,000 hours at 850°C. Mechanical property and oxidation resistance tests compared the modified alloys with the original Inconel alloy 740, yielding preliminary results that demonstrate minimal modifications can improve stress rupture strength while maintaining corrosion resistance. Microstructural examinations further confirmed the enhanced thermal stability of the modified alloy, positioning Inconel alloy 740 as a promising candidate for USC boiler applications at 750°C or higher temperatures.

Effects of Ni content and heat treatment condition on impact toughness and creep strength of precipitation strengthened 15Cr ferritic steels were investigated in order to discuss a possibility of improvement in both mechanical properties. Both creep strength and impact toughness of the developing steels were improved drastically by solid solution treatment with water quenching. However, an addition of Ni reduced the long-term creep strength of the steels, though Ni was effective in improvement in impact toughness. It was found that water quenching suppressed formation of coarse block type particles and precipitate free zones around them, and precipitation of plate type fine particles and thermal stability of them within ferrite phase were promoted by solid solution treatment with water quenching. However, martensite phase with sparsely distributed coarse block type particles were formed in the Ni added steels, and such microstructure reduced the precipitation strengthening effect slightly. On the other hand, increase in impact values of the steel indicated no relation to volume fraction of martensite phase. It was supposed that the impact toughness of ferrite phase itself was improved by solid solution treatment and addition of Ni.

The creep resistance of 9-12% Cr steels is significantly influenced by the presence and stability of different precipitate populations. Numerous secondary phases grow, coarsen and, sometimes, dissolve again during heat treatment and service. Based on the software package MatCalc, the evolution of these precipitates during the thermal treatment of the COST 522 steel CB8 is simulated from the cooling process after cast solidification to heat treatment and service up to the aspired service life time of 100.000h. On basis of the results obtained from these simulations in combination with a newly implemented model for evaluation of the maximum threshold stress by particle strengthening, the strengthening effect of each individual precipitate phase, as well as the combined effect of all phases is evaluated - a quantification of the influence of Z-Phase formation on the long-term creep behaviour is thus made possible. This opens a wide field of application for alloy development and leads to a better understanding of the evolution of microstructural components as well as the mechanical properties of these complex alloys.

Due to a high degree of mixing between substrate and weld deposit, fusion welding of dissimilar metal joints functionally produce new, uncharacterized alloys. In the power generation industry, such mixing during the application of cobalt-based hardfacing has led to a disconcerting number of failures characterized by the hard overlay welds disbonding. Investigations into this failure mechanism point to the unknown alloy beneath the surface of the hardfacing layer transforming, hardening, and becoming brittle during service. This research describes a methodology for exploring a chemical space to identify alloy combinations that are expected to be safe from deleterious phase formation. Using thermodynamic modeling software and a stepped approach to potential chemistries, the entire phase stability space over the full extent of possible mixing between substrate and weld material can be studied. In this way diffusion effects – long term stability – can also be accounted for even in the case where mixing during application is controlled to a low level. Validation of predictions specific to the hardfacing system in the form of aged weld coupons is also included in this paper. Though the application of this methodology to the hardfacing problem is the focus of this paper, the method could be used in other weld- or diffusion- combinations that are expected to operate in a high temperature regime.

More efficient, sustainable, flexible and cost-effective energy technologies are strongly needed to fulfil the new challenges of the German “Energiewende”. For a reduction of consumed primary resources higher efficiency steam cycles with increased operating parameters, pressure and temperature, are mandatory. Hence, advanced materials are needed. The present study focuses on stainless, high strength, ferritic (non-martensitic) steel grades, regarding thermal treatment effects on particle evolution. The heat treatment includes variations, e.g. a two phase pre heat treatment. Effects of the treatment were analysed and connected to creep performance. Experiments at differently heat treated materials show promising improvement of creep performance. These results can be linked to the stability and evolution of strengthening Laves phase particles.

A new nickel-based superalloy, designated as GH750, was developed to meet the requirements of high temperature creep strength and corrosion resistance for superheater/reheater tube application of A-USC power plants at temperatures above 750°C. This paper introduces the design of chemical composition, the process performance of tube fabrication, microstructure and the properties of alloy GH750, including thermodynamic calculation, room temperature and high temperature tensile properties, stress rupture strength and thermal stability. The manufacturing performance of alloy GH750 is excellent and it is easy to forge, hot extrusion and cold rolling. The results of the property evaluation show that alloy GH750 exhibits high tensile strength and tensile ductility at room and high temperatures. The 760°C/100,000h creep rupture strength of this alloy is larger than 100MPa clearly. Microstructure observation indicates that the precipitates of GH750 consist of the precipitation strengthening phase γ’, carbides MC and M 23 C 6 and no harmful and brittle TCP phases were found in the specimens of GH750 after long term exposure at 700~850°C. It can be expected for this new nickel-based superalloy GH750 to be used as the candidate boiler tube materials of A-USC power plants in the future.

Advanced 700°C-class steam turbines demand austenitic alloys for superior creep strength and oxidation resistance beyond 650°C, exceeding the capabilities of conventional ferritic 12Cr steels. However, austenitic alloys come with a higher coefficient of thermal expansion (CTE) compared to 12Cr steels. To ensure reliability, operability, and performance, these advanced turbine alloys require low CTE properties. Additionally, for welded components, minimizing CTE mismatch between the new material and the welded 12Cr steel is crucial to manage residual stress. This research investigates the impact of alloying elements on CTE, high-temperature strength, phase stability, and manufacturability. As a result, a new material, “LTES700R,” was developed specifically for steam turbine rotors. LTES700R boasts a lower CTE than both 2.25Cr steel and conventional superalloys. Additionally, its room-temperature proof strength approaches that of advanced 12Cr steel rotor materials, while its creep rupture strength around 700°C significantly surpasses that of 12Cr steel due to the strengthening effect of gamma-prime phase precipitates. To assess the manufacturability and properties of LTES700R, a medium-sized forging was produced as a trial run for a turbine rotor. The vacuum arc remelting process was employed to minimize segregation risk, and a forging procedure was meticulously designed using finite element method simulations. This trial production resulted in a successfully manufactured rotor with satisfactory quality confirmed through destructive evaluation.

Constricted steam oxidation resistance and finite microstructural stability limits the use of 9 - 12 wt.-% chromium ferritic-martensitic steels to steam temperatures of about 620 °C. Newly developed 12 wt.-% Cr steels are prone to Z-phase precipitation, which occurs at the expense of the strengthening precipitates, and therefore suffer an accelerated decline in strength during longterm operation. While the concept of ferritic-martensitic chromium steels thus seems to hit technological limitations, further improvement in steam power plant efficiency necessitates a further increase of steam pressure and temperature. Furthermore increasing integration of intermitting renewable energy technologies in electrical power generation poses a great challenge for supply security, which has to be ensured on the basis of conventional power plant processes. Besides improved efficiency for resource preservation, load flexibility, thermal cycling capability and downtime corrosion resistance will play key roles in the design of tailored materials for future energy technology. Under these preconditions a paradigm shift in alloy development towards improvement of cyclic steam oxidation and downtime corrosion resistance in combination with adequate creep and thermomechanical fatigue strength seems to be mandatory. The steam oxidation, mechanical and thermomechanical properties of fully ferritic 18 - 24 wt.-% chromium model alloys, strengthened by the precipitation of intermetallic (Fe,Cr,Si)2(Nb,W) Laves phase particles, indicate the potential of this type of alloys as candidate materials for application in highly efficient and highly flexible future supercritical steam power plants.

Reducing emissions and increasing economic competitiveness require more efficient steam power plants that utilize fossil fuels. One of the major challenges in designing these plants is the availability of materials that can stand the supercritical and ultra-supercritical steam conditions at a competitive cost. There are several programs around the world developing new ferritic and austenitic steels for superheater and reheater tubes exposed to the advanced steam conditions. The new steels must possess properties better than current steels in terms of creep strength, steamside oxidation resistance, fireside corrosion resistance, and thermal fatigue resistance. This paper introduces a series of experimental 9%Cr steels containing Cu, Co, and Ti. Stability of the phases in the new steels is discussed and compared to the phases in the commercially available materials. The steels were tested under both the dry and moist conditions at 650°C for their cyclical oxidation resistance. Results of oxidation tests are presented. Under the moist conditions, the experimental steels exhibited significantly less mass gain compared to the commercial P91 steel. Microstructural characterization of the scale revealed different oxide compositions.

A global movement is pushing for improved efficiency in power plants to reduce fossil fuel consumption and CO 2 emissions. While raising operating temperatures and pressures can enhance thermal efficiency, it necessitates materials with exceptional high-temperature performance. Currently, steels used in power plants operating up to 600°C achieve efficiencies of 38-40%. Advanced Ultra Supercritical (A-USC) designs aim for a significant leap, targeting steam temperatures of 700°C and pressures of 35 MPa with a lifespan exceeding 100,000 hours. Ni-based superalloys are leading candidates for these extreme conditions due to their superior strength and creep resistance. Haynes 282, a gamma prime (γ′) precipitation-strengthened alloy, is a promising candidate for A-USC turbine engines, exhibiting excellent creep properties and thermal stability. This research investigates the microstructural evolution in large, sand-cast components of Haynes 282. Microstructure, referring to the arrangement of grains and phases within the material, significantly impacts its properties. The research examines the alloy in its as-cast condition and after various pre-service heat treatments, aiming to fully identify and quantify the microstructural changes. These findings are then compared with predictions from thermodynamic equilibrium calculations using a dedicated Ni alloy database. The research reveals that variations in heat treatment conditions can significantly affect the microstructure development in Haynes 282, potentially impacting its mechanical properties.

Advanced 700°C-class steam turbines require the use of austenitic alloys instead of conventional ferritic 12Cr steels, which are inadequate in creep strength and oxidation resistance above 650°C. While austenitic alloys offer improved performance, they traditionally possess a significantly higher coefficient of thermal expansion (CTE) compared to 12% Cr steels. Through extensive research, the authors systematically investigated the effects of various alloying elements on thermal expansion and high-temperature strength. As a result of these investigations, they developed "LTES700," an innovative nickel-based superalloy specifically designed for steam turbine bolts and blades. This novel alloy uniquely combines a coefficient of thermal expansion comparable to 12Cr steels with high-temperature strength equivalent to conventional superalloys like Refractaloy 26, effectively addressing the critical limitations of previous materials.

The development of materials technologies for piping and tubing in advanced ultrasupercritical (A-USC) power plants operating at steam temperatures above 700°C represents a critical engineering challenge. The 23Cr-45Ni-7W alloy (HR6W), originally developed in Japan as a high-strength tubing material for 650°C ultra-supercritical (USC) boilers, was systematically investigated to evaluate its potential for A-USC plant applications. Comparative research with γ-strengthened Alloy 617 revealed that the tungsten content is intimately correlated with Laves phase precipitation and plays a crucial role in controlling creep strength. Extensive creep rupture tests conducted at temperatures between 650-800°C for up to 60,000 hours demonstrated the alloy's long-term stability, with 105-hour extrapolated creep rupture strengths estimated at 88 MPa at 700°C and 64 MPa at 750°C. Microstructural observations after creep tests and aging confirmed the material's microstructural stability, which is closely linked to long-term creep strength and toughness. While Alloy 617 exhibited higher creep rupture strength at 700 and 750°C, the materials showed comparable performance at 800°C. Thermodynamic calculations and microstructural analysis revealed that the Laves phase in HR6W gradually decreases with increasing temperature, whereas the γ' phase in Alloy 617 rapidly diminishes and almost completely dissolves at 800°C, potentially causing an abrupt drop in creep strength above 750°C. After comprehensive evaluation of creep properties, microstructural stability, and other reported mechanical characteristics, including creep-fatigue resistance, HR6W emerges as a promising candidate for piping and tubing in A-USC power plants.

A new alloy design concept for creep- and corrosion-resistant, fully ferritic alloys was proposed for high-temperature structural applications in current/future fossil-fired power plants. The alloys, based on the Fe-30Cr-3Al (in weight percent) system with minor alloying additions of Nb, W, Si, Zr and/or Y, were designed for corrosion resistance though high Cr content, steam oxidation resistance through alumina-scale formation, and high-temperature creep performance through fine particle dispersion of Fe 2 (Nb,W)-type Laves phase in the BCC-Fe matrix. Theses alloys are targeted for use in harsh environments such as combustion and/or steam containing atmospheres at 700°C or greater. The alloys, consisting of Fe-30Cr-3Al-1Nb-6W with minor alloying additions, exhibited a successful combination of oxidation, corrosion, and creep resistances comparable or superior to those of commercially available heat resistant austenitic stainless steels. An optimized thermo-mechanical treatment combined with selected minor alloying additions resulted in a refined grain structure with high thermal stability even at 1200°C, which improved room-temperature ductility without sacrificing the creep performance. The mechanism of grain refinement in the alloy system is discussed.

Achieving long-term stability of the tempered martensite is considered crucial for increasing the creep resistance of steels at elevated temperatures above 700°C. It is noted that at low stress levels, the creep deformation of the tempered martensite proceeds heterogeneously around prior austenite grain boundaries, as excess dislocations inside the grain are difficult to rearrange. This paper presents a new approach using carbon-free martensitic alloys strengthened by intermetallic compounds. An iron-nickel-cobalt martensite matrix with Laves phase as the precipitate is selected. The creep characteristics are discussed across a wide range of testing conditions, and the thermal cycle test behavior is examined to evaluate the potential of these alloys for future ultrasupercritical power plants operating in severe environments.

In this study, the role of minor alloying additions in 347H stainless steels (UNS34709, ASTM A240/240M) on creep-rupture properties at 650-750°C and microstructure evolution during isothermal exposure at 750°C has been investigated, aiming to provide the experimental dataset as boundary conditions of physics-based modeling for material/component life prediction. Four different 347H heats containing various amounts of boron and nitrogen additions were prepared and evaluated. The combined additions of B and N are found to stabilize the strengthening secondary M 23 C 6 carbides and retarding the transition from M 23 C 6 to sigma phase precipitates during thermal exposure. The observed kinetics of microstructure evolution reasonably explains the improvement of creep-rupture properties of 347H stainless steels with the B and N additions.

In the present study, the precipitation kinetics of topologically close-packed (TCP) Fe 2 Nb Laves and geometrically close-packed (GCP) Ni 3 Nb phases is studied quantitatively in experimental alloys with different Ta / Nb+Ta ratio, to clarify the mec4hanism of the Ta effect. The microstructure of alloys is changed from Widmanstätten structure to lamellar structure due to discontinuous precipitation, with increasing Ta / Nb+Ta. It is confirmed that Ta partitions into both Fe 2 Nb Laves and Ni 3 Nb phases. However, two phases stability is changed by added Ta content. Ta accelerates the formation kinetics of the precipitates at grain boundaries, as well as γ“-GCP phase within grain interiors, due to increased supersaturation by Ta addition. Besides, Ta retards the transformation kinetics of metastable γ“-Ni 3 Nb to stable the δ-Ni 3 Nb phase. The results indicate that Ta decreases the driving force for the transformation of the δ-GCP phase.

This overview paper contains a part of structure stability study on advanced austenitic heat-resisting steels (TP347H, Super304H and HR3C) and Ni-base superalloys (Nimonic80A, Waspaloy and Inconel740/740H) for 600-700°C A-USC fossil power plant application from a long-term joint project among companies, research institutes and university in China. The long time structure stability of these advanced austenitic steel TP347H, Super304H, HR3C in the temperature range of 650-700 °C and Ni-base superalloys Nimonic80A, Waspaloy and Inconel740/740H in the temperature range of 600-800°C till 10,000h have been detailed studied in this paper.

Higher steam temperature in steam power plants increases their thermal efficiency. Thus there is a strong demand for new materials with better creep and corrosion resistance at higher temperatures, while retaining the thermal flexibility of martensitic steels. Z-phase strengthened 12% Cr steels have been developed to meet the 923 K (650°C) challenge in these power plants. Ta, Nb, or V forms Z-phase together with Cr and N. A new trial steel was produced based on combining Ta and Nb to form Z-phase. It was shown that Z-phase was formed with a composition corresponding to Cr1+x(Nb,Ta)1-xN. The Nb/Ta ratio in Z-phase precipitates was higher than that in MX precipitates. Z-phase precipitates based on Ta and Nb were coarser than precipitates in a similar trial steel based on Ta alone.