Search Results for niobium

High Cr ferritic steels have been developed for the large components of fossil power plants due to their excellent creep resistance, low thermal expansion, and good oxidation resistance. Development works to improve the operating temperature of these steels mainly focused on the high mechanical properties such as solid solution strengthening and precipitation hardening. However, the knowledge of the correlation between Laves phase precipitation and oxidation behavior has not clarified yet on 9Cr ferritic steels. This research will be focused on the effect of precipitation of Laves phase on steam oxidation behavior of Fe-9Cr alloy at 923 K. Niobium was chosen as the third element to the Fe- 9Cr binary system. Steam oxidation test of Fe-9Cr (mass%) alloy and Fe-9Cr-2Nb (mass%) alloy were carried out at 923 K in Ar-15%H 2 O mixture for up to 172.8 ks. X-ray diffraction confirms the oxide mainly consist of wüstite on the Fe-9Cr in the initial stage while on Nb added samples magnetite was dominated. The results show that the Fe-9Cr- 2Nb alloy has a slower oxidation rate than the Fe-9Cr alloy after oxidized for 172.8 ks

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.

The aspiration to deploy Nb-based alloys as viable upgrade for Ni-based superalloys is rooted in their potential for superior performance in high-temperature applications, such as rocket nozzles and next-generation turbines. However, realizing this goal requires overcoming formidable design hurdles, including achieving high specific strength, creep resistance, fatigue, and oxidation resistance at elevated temperatures, while preserving ductility at lower temperatures. Additionally, the requisite for alloy bond-coatings, to ensure compatibility with coating materials, further complicates the design process. QuesTek Innovations has its Integrated Computational Materials Engineering (ICME) technologies to design a superior performance high-temperature Nb-based superalloy based on solid solution and precipitation strengthening. Additionally, utilizing a statistical learning method from very limited available data, QuesTek engineers were able to establish physics-based material property models, enabling accurate predictions of equilibrium phase fraction, DBTT, and creep properties for multicomponent Nb alloys. With the proven Materials by Design methodology under the ICME framework, QuesTek successfully designed a novel Nb superalloy that met the stringent design requirements using its advanced ICMD materials modeling and design platform.

The complex nitride Z-phase, Cr(V,Nb)N, has recently been identified as a major cause for premature breakdown in creep strength of a number of new 9-12%Cr martensitic steels, especially the high Cr variants. A thermodynamic model of the Z-phase has been created based on the Thermo-Calc software. The model predicts the Z-phase to be stable in all of the new 9- 12%Cr martensitic steels, and this has generally been confirmed by experimental observations. Z-phase precipitation seems then to be a kinetic problem, and driving force calculations, using Thermo-Calc with the developed model, have been used to predict steel compositions, which could delay Z-phase precipitation. The model also predicted the existence of a new niobium free Z-phase variant, which has since been discovered in a niobium free 12CrMoV steel.

The paper deals with microstructural evolution in the AISI 316LN + 0.1 wt.% Nb steel during long-term creep exposure at 600 and 625°C. The following minor phases formed: Z-phase (NbCrN), M 23 C 6 , M6X (Cr3Ni2SiX type), η-Laves (Fe2Mo type) and σ-phase. M6X gradually replaced M 23 C 6 carbides. Primary Z-phase particles were present in the matrix after solution annealing, while secondary Z-phase particles formed during creep. Precipitation of Z-phase was more intensive at 625°C. The dimensional stability of Z-phase particles was excellent and these particles had a positive effect on the minimum creep rate. However, niobium also accelerated the formation and coarsening of σ-phase, η-Laves and M6X. Coarse particles, especially of σ-phase, facilitated the development of creep damage, which resulted in poor long-term creep ductility.

In this work, the effects of phosphorus addition on the creep properties and microstructural changes of wrought γ’-strengthened Ni-based superalloys (Haynes 282) were investigated, focusing on the effects of carbides precipitation. In an alloy with a phosphorus content of 8 ppm, precipitation of M 23 C 6 carbides was observed in both grain boundaries and the grain interior prior to the creep tests. Grain boundary coverage by carbide increased with phosphorus content up to approximately 30 ppm. On the other hand, the amount of M 23 C 6 in the grain interior decreased with phosphorus content. The results of the creep tests revealed the relationship between the time to rupture and the grain boundary coverage by carbides. The microstructure of the crept specimens showed the existence of misorientation at the vicinity of grain boundaries without carbides, as demonstrated via electron backscattered diffraction (EBSD) analysis. These results suggest that the observed improvement in the time to rupture is due to a grain-boundary precipitation strengthening mechanism caused by grain boundary carbides and that phosphorus content affects the precipitation behavior of M 23 C 6 carbides in the grain interior and grain boundaries. These behaviors were different between alloys with the single addition of phosphorus and alloys with the multiple addition of phosphorus and niobium.

Advanced austenitic stainless steels, such as Super 304H, have been used in reheater and superheater tubes in supercritical and ultra-supercritical power plants for many years now. It is important to characterize the microstructure of ex-service reheater and superheater tubes as this will help researchers understand the long-term microstructural evolution and degradation of the material, which can impact the performance and lifetime of the components that are in service. In this research, the microstructure of an ex-service Super 304H reheater tube that has been in service for 99,000 hours at an approximate metal temperature of 873K (600°C) has been characterized. The characterization techniques used were electron microscopy-based and included imaging and chemical analysis techniques. Seven phases were observed as a result of the characterization work. The phases observed were MX carbonitrides rich in niobium, copper-rich particles, M 23 C 6 , sigma phase, Z phase, a cored phase, and a BCC phase.

A new nickel-base superalloy GH750 has been developed as boiler tube of advanced ultrasupercritical (A-USC) power plants at temperatures about and above 750°C in China. This paper researched the weld solidification of GH750 filler metal, microstructure development and property of GH750 welded joint by gas tungsten arc weld. Liquid fraction and liquid composition variation under non-equilibrium state were calculated by thermo-dynamic calculation. The weld microstructure and the composition in the dendrite core and interdendritic region were analyzed by SEM(EDX) in detail. The investigated results show that there is an obvious segregation of precipitation-strengthening elements during the weld solidification. Titanium and Niobium are the major segregation elements and segregates in the interdendritic region. It was found that the changing tendency of the elements’ segregation distribution during the solidification of GH750 deposit metal is agree with the thermodynamic calculation results. Till to 3,000hrs’ long exposure at 750°C and 800°C, in comparison with the region of dendrite core of solidification microstructure, not only the coarsening and the accumulation of γʹ particles are remarkable in the interdendritic region, but also the small quantity of the blocky and needle like η phases from. The preliminary experimental results indicate that the weakening effect of creep-rupture property of the welded joint is not serious compared with GH750 itself.

The aim of this work was to reveal the effects of trace elements on the creep properties of nickel-iron base superalloys, which are the candidate material for the large components of the advanced-ultrasupercritical (A-USC) power generation plants. High temperature tensile and creep properties of forged samples with seven different compositions were examined. No significant differences were observed in the creep rate versus time curves of the samples, of which contents of magnesium, zirconium, manganese and sulfur were varied. In contrast, the curves of phosphorus-added samples showed very small minimum creep rates compared to the other samples. The creep rupture lives of phosphorus-added samples were obviously longer than those of the other samples. Microstructure observation in the vicinity of grain boundaries of phosphorus-added samples after aging heat treatment revealed that there were fine precipitates consisting of phosphorus and niobium at the grain boundaries. The significant suppression of the creep deformation of phosphorus-added sample may be attributed to the grain boundary strengthening caused by the fine grain boundary precipitates.

There is a constant need for improved knowledge of the influence of non-standard processing on the expected performance of creep strength enhanced ferritic (CSEF) materials as the total installed tonnage of these materials is rapidly increasing across the power generation industry. Cr-Mo-V steel grades micro-alloyed with niobium and titanium designed for pressurized equipment operating in the supercritical steam range proved to be very sensitive to relative minor variations in the principal heat treatment parameters time and temperature, when compared to the traditional Cr-Mo-V grades. A key component for successful welds is optimised post weld heat treatment (PWHT). Under certain conditions premature failures of welds can occur when incorrect weld and heat treatment performance result in a reduction of specified mechanical properties and high temperature creep performance, it is therefore of significant importance to have a good understanding of actual material properties for effective operation and plant life studies. This study investigated the effect and impact variations of post weld heat treatment time and temperature on mechanical properties of tungsten inert gas (TIG) and manual metal arc (MMA) welds on Grade 91 pipes from a set of reference samples. This is in preparation of establishing a benchmark set of tests to determine the integrity and expected long-term performance of butt-welds from limited site sample volumes, providing a non-intrusive methodology to identify welds suspected to have received non-standard PWHT cycles on Grade 91 pipework systems.

Inconel alloy 740, a precipitation-hardenable nickel-chromium-cobalt alloy with niobium addition, has emerged as a leading candidate material for ultra-supercritical (USC) boilers due to its superior stress rupture strength and corrosion resistance at operating temperatures near 760°C. While derived from Nimonic alloy 263, alloy 740's unique chemistry necessitates comprehensive weldability studies to address potential challenges including heat-affected zone liquation cracking, ductility-dip cracking, and post-weld heat treatment cracking. This ongoing investigation examines the alloy's weldability characteristics through material characterization studies comparing its cracking sensitivity to established aerospace alloys like Waspalloy and Inconel alloy 718. The research applies aerospace industry expertise to boiler applications requiring sections up to three inches thick, with gas tungsten arc welding and pulsed gas metal arc welding identified as the most promising processes for producing sound, crack-free welds.

Inconel alloy 740 is a precipitation-hardenable nickel-chromium-cobalt alloy with niobium, derived from Nimonic 263, and is considered a prime candidate for the demanding conditions of advanced ultrasupercritical boilers. It offers an exceptional combination of stress rupture strength and corrosion resistance under steam conditions of 760°C (1400°F) and 34.5 MPa (5000 psi), surpassing other candidate alloys. Initially, Inconel alloy 740 was prone to liquation cracking in sections thicker than 12.7 mm (0.50 in), but this issue has been resolved through modifications in the chemical composition of both the base and weld metals. Current concerns focus on the weld strength reduction factor for direct-age weldments. This has led to further development in welding Inconel alloy 740 using Haynes 282, which has higher creep strength and may mitigate the weld strength reduction factor. This study details successful efforts to eliminate liquation cracking and compares the properties of Inconel alloy 740 and Haynes 282 filler materials using the gas tungsten arc welding process.

The microstructural evolution has been investigated for an 18Cr-12Ni stainless steel (347HFG) that has been subject to a thermo-mechanical treatment to obtain a fine grain size (ASTM 7-10). In particular, sigma phase precipitation and growth has been evaluated. Samples of 347HFG stainless steel have been isothermally heat treated to reproduce and accelerate the ageing conditions experienced in-service at temperatures between 600 and 750 °C for up to 10,000 hours. Results have shown that sigma phase is precipitated at triple points and along grain boundaries after as little as 1000 hours which is contrary to thermodynamic predictions. In addition X-ray diffraction (XRD) and image analysis has been carried out to semi-quantitatively measure the amount of sigma phase present. The area fraction of sigma has been found to be 2.77 and 2.23 percent at 700 and 750 °C respectively. This is a higher volume fraction of sigma phase than has been previously observed in regular 347H at these conditions. It is thought that this is due to the reduced grain size that has provided an increase in nucleation sites and diffusion paths that can enhance the precipitation and growth of sigma phase. The results from this study are discussed with regards to the effect of precipitation on the service life of a 347HFG stainless steel tube operating in advanced supercritical boilers.

Materials with a higher creep strength and sensitization resistance superior to those of 347AP are required in complex refinery such as delayed coker. To optimize material designing, the effect of B addition on the Z phase-strengthened steels has been investigated. B addition significantly improved the creep strength of steel containing Nb despite the absence of M 23 C 6 , whereas the creep strength of steel without Nb showed only a slight change with B addition. The size of Z phase was smaller in the Nb-containing steel with B content than that in B-free steel. It was suggested that the refinement of Z phase contributed to the creep strengthening by B addition in steels containing Nb.

The competitive effect of Nb and V additions on the high-temperature oxidation behavior of Ti- 30Al alloys were studied at 800°C in air. Oxidation performance increased with increasing Nb content, however, V additions eliminated the beneficial effect of Nb on oxidation performance, causing higher oxidation mass gains. In-situ high-temperature XRD by means of synchrotron source suggested dissolution of Nb 5+ but lower valence of vanadium ions in the TiO 2 oxide scale during oxidation. Dissolution of Nb and V ions with different valence in TiO 2 during oxidation could cause the beneficial and detrimental effects observed on the performance of high-temperature oxidation of Ti-30Al.

Creep deformation and rupture properties of several long-term used Super 304H steel boiler tubes were presented in this paper. The aged superheater tubes that have been in service for about 140,000 hours at the approximate metal temperature ranged from 550°C to 640°C, were investigated. Creep tests were conducted at 650°C and 700°C using standard and miniature specimens taken from the axial and circumferential directions of tubes, and effects of specimen size, sampling direction and position on creep properties were discussed. Creep deformation of long-term used materials with significant microstructural evolution accelerated earlier than that of virgin material, and the time to creep rupture and the fracture ductility were also smaller. The degradation of rupture properties of the long-term used material was discussed in relation with microstructural evolution. In addition, there was little effects of specimen size and sampling direction on creep deformation and rupture time, whereas the time to creep rupture changed significantly due to the sampling position.

Inconel 718 is a nickel-based superalloy known for its excellent combination of high-temperature strength, corrosion resistance, and weldability. Additive Manufacturing (AM) has revolutionized traditional manufacturing processes by enabling the creation of complex and customized components. In this work, three prominent AM techniques: Laser-Based Powder Bed Fusion (PBF), Wire Direct Energy Deposition (DED), and Binder Jet (BJ) processes were explored. A thorough metallographic analysis and comparison of samples was conducted after short-term creep testing originating from each of the three aforementioned techniques in addition to wrought material. Detailed electron microscopy unveiled equiaxed grains in both BJ and wrought samples while PBF samples displayed elongated finer grain structures in the build direction, characteristic of PBF. The DED samples revealed a more bimodal grain distribution with a combination of smaller equiaxed grains accompanied by larger more elongated grains. When assessing the three processes, the average grain size was found to be larger in the BJ samples, while the PBF samples exhibited the most significant variation in grain and sub-grain size. Number density, size, and shape of porosity varied between all three techniques. Post-creep test observations in PBF samples revealed the occurrence of wedge cracking at the failure point, accompanied by a preference for grain boundary creep void formation while BJ samples exhibited grain boundary creep void coalescence and cracking at the failure location. In the DED samples, void formation was minimal however, it seemed to be more prevalent in areas with precipitates. In contrast, the wrought sample showed void formation at the failure site with a preference for areas with primary carbide formation. Despite BJ samples demonstrating similar or even superior rupture life compared to other AM techniques, a noteworthy reduction in rupture ductility was observed. While a coarse, uniform grain size is generally linked to enhanced creep resistance and rupture life, the combination of pre-existing voids along grain boundaries and the formation of new voids is hypothesized to accelerate rapid fracture, resulting in diminished ductility. This research shows careful consideration is needed when selecting an AM technology for high- temperature applications as creep behavior is sensitive to the large microstructural variations AM can introduce.

Understanding of the thermomechanical processing that affects microstructures is important to develop new alloys, because the mechanical properties of Ti alloys depend on the microstructures. In our previous study, we found Sn deteriorated the oxidation resistance, while Nb improved the oxidation resistance. Then, we have focused on Ti-Al-Nb-Zr alloys which Nb was added instead of Sn. Zr was added for solid solution strengthening. In this study, the formation of microstructures by thermomechanical processing and the effect of microstructure on the mechanical properties were investigated using the Ti-13Al-2Nb-2Zr (at%) alloy. The samples heat-treated in the β+α phase followed by furnace cooling after processed in the β+α phase formed the equiaxed or the ellipsoid α phase surrounded by the β phase. On the other hand, the sample heat-treated in the β+α phase followed by furnace cooling after processed in the β phase formed the lamellar microstructure. The compression strengths of the equiaxed α structure processed at two temperatures in the β+α phase were almost the same. While creep life of the bi-modal structure was drastically changed by processing temperature.

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.

Recent years high strength 9Cr1MoNbV steel developed in USA has been major material in boiler high temperature components with the increase of steam parameters of coal fired thermal power plants. As the microstructure of this steel is tempered martensite, it is known that the softening occurs in HAZ of the weldment. In the creep rupture test of these welded joints the rupture strength is lower than that of the parent metal, and sometimes this reduction of strength is caused by TypelV cracking. To develop an effective method to improve the rupture strength of welded joint, advanced welding procedure and normalizing-tempering heat treatment after weld was proposed. 9Cr1MoNbV plates with thickness of 40-50mm were welded by 10mm width automatic narrow gap MAG welding procedure using specially modified welding material. After normalizing at 1,050°C and tempering at 780°C, material properties of the welded joints were examined. Microstructure of HAZ was improved as before weld, and rupture strength of the welded joints was equal to that of the parent metal. The long term rupture strength of the welded joints was confirmed in the test exceeded 30,000hours. This welding procedure has been applied to seam weld of hot reheat piping and headers in USC boilers successfully.