Search Results for scanning transmission electron microscopes

Martensitic 9Cr steels have been developed which are strengthened by boron in order to stabilize the microstructure and improve their long-term creep strength. Boron plays a key role in these steels by stabilising the martensitic laths by decreasing the coarsening rate of M 23 C 6 carbides, which act as pinning points in the microstructure. In this work two modified FB2 steel forgings are compared. Both forgings have similar compositions but one underwent an additional remelting process during manufacture. Creep tests showed that this additional processing step resulted in a significant increase in time to failure. In order to investigate the effect of the processing route on microstructural evolution during aging and creep, a range of advanced electron microscopy techniques have been used including ion beam induced secondary electron imaging and High Angle Annular Dark Field (HAADF) imaging in the Scanning Transmission Electron Microscope. These techniques have enabled the particle population characteristics of all the second phase particles (M 23 C 6 , Laves phase, BN and MX) to be quantified for materials from both forging processes. These quantitative data have enabled a better understanding of how the processing route affects the microstructural evolution of FB2 steels.

Prediction of long-term creep strength is an important issue for industrial plants operated at elevated temperatures, although the creep strength of high Cr ferritic steels depends on their microstructural evolution during creep. The state of microstructure in metallic materials can be expressed as numerical values based on a concept of system free energy. In this study, in order to evaluate long term creep strength of 9Cr ferritic steel containing B, change in the system free energy during creep of the steel is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. The system free energy decreases with creep time. Change in the energy is expressed quantitatively as a numerical formula using the rate constants which depend on applied stress. On the basis of these facts, long term creep strength of the steel can be evaluated at both 948K(675°C) and 973K(700 °C).

In order to evaluate long term creep strength of modified 9Cr ferritic steels, the system free energy of creep ruptured specimens at both 650 and 700 °C is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. Change ratio of the system free energy and creep stress showed the relationship with one master curve irrespective of creep conditions, indicating that the steel ruptures when the applied stress exceeds a limited stress depending on the microstructural state expressed by the change ratio of system free energy. Furthermore, it was found that dominant factor of the change ratio was the chemical free energy change. On the basis of these results, long term creep strength of the steel was evaluated at 700 °C, for example, 19MPa at 700 °C after 10 5 h. It is concluded that long term creep strength of modified 9Cr ferritic steels can be predicted by the system free energy concept using the ruptured specimens with various creep conditions.

This paper investigates the cause of premature failures in certain Grade 91 steel components used in UK power plants. The failures were linked to both low material hardness and specific chemical compositions that fell within ASTM specifications but had a low nitrogen-to-aluminum ratio (N:Al). The investigators examined eight material batches, including those involved in failures, new stock, and in-service components with similar properties. Testing confirmed these materials had lower creep resistance compared to standard Grade 91 steel. Microscopic analysis revealed the presence of large aluminum nitride precipitates, which limited the formation of beneficial vanadium nitride precipitates, leading to reduced creep strength. These findings suggest that even within the ASTM specification limits, a low N:Al ratio can negatively impact the performance of Grade 91 steel.

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.

Additive manufacturing (AM) is a process where, as the name suggests, material is added during production, in contrast to techniques such as machining, where material is removed. With metals, AM processes involve localised melting of a powder or wire in specific locations to produce a part, layer by layer. AM techniques have recently been applied to the repair of gas turbine blades. These components are often produced from nickel-based superalloys, a group of materials which possess excellent mechanical properties at high temperatures. However, although the microstructural and mechanical property evolution during the high temperature exposure of conventionally produced superalloy materials is reasonably well understood, the effects of prolonged high temperature exposure on AM material are less well known. This research is concerned with the microstructures of components produced using AM techniques and an examination of the effect of subsequent high temperature exposures. In particular, the paper will focus on the differences between cast and SLM IN939 as a function of heat treatment and subsequent ageing, including differences in grain structure and precipitate size, distribution and morphology, quantified using advanced electron microscopy techniques.

This study investigates the mechanisms of temper embrittlement in 410 martensitic stainless steel, a material widely used in steam turbine blades due to its excellent corrosion resistance and high strength achieved through quenching and tempering heat treatments. While the material’s hardness and impact toughness strongly depend on tempering temperatures, significant embrittlement occurs around 540°C, manifesting as decreased Charpy impact energy alongside increased strength and hardness. To understand this phenomenon at the nanometer scale, high-resolution transmission electron microscopy (TEM) analysis was performed, focusing on electron diffraction patterns along the <110>α-Fe and <113>α-Fe zone axes. The analysis revealed distinctive double electron diffraction spots at 1/3(211) and 2/3(211) positions, with lattice spacing of approximately 3.5 Å—triple the typical α-bcc lattice spacing (1.17 Å). These regions were identified as metastable “zones” resembling ω-phase structures, potentially responsible for the embrittlement. While this newly identified phase structure may not fully explain the complex mechanisms of temper embrittlement, it provides valuable insights for developing improved alloying and heat treatment methods to mitigate embrittlement in martensitic steels.

Microstructural change of 10 % Cr steel trial forgings subjected to different heat treatment conditions which aim to improve the creep rupture strength and microstructural stability during creep was investigated. Creep rupture strength of the forging subjected to the quality heat treatment with the austenitizing temperature of 1090° C is higher than that of the forging solution treated at 1050°C, however, the difference of creep rupture strength is reduced in the long-term region around 40,000 h. Decrease in creep rupture ductility of the forging until 43,300 h is not observed. Progress of the martensite lath recovery in the forging solution-treated at 1090°C is slower than that in the forging austenitized at 1050°C. Higher temperature solution treatment suppresses the recovery of lath structures. Formations of Z-phase are found in the specimens creep-ruptured at 37,300 h in the forging solution-treated at 1050°C and at 43,400 h in the forging austenitized at 1090°C. Z-phase precipitation behavior in this steel is delayed in comparison with the boiler materials, regardless of austenitizing temperature.

Modified 9Cr-1Mo steel (ASTM Gr.91) is widely used in components of fossil fueled power plants around the world today. This grade of steel has however been shown to exhibit significant variations in creep life and creep ductility, which has led to premature in-service failures. The aim of this work is to define potential metallurgical risk factors that lead to this variation in performance. To achieve this, a set of creep test samples that represent a wide range in this variation of creep behavior in this steel grade have been studied in detail. As a first stage in this characterization the macro-scale chemical homogeneity of the materials were mapped using micro-XRF. Understanding the segregation behavior also allows quantification of microstructural parameters in both segregated and non-segregated areas enabling the variations to be determined. For example this showed a significant increase in the number per unit area of Laves phase particles in high compared with low Mo content areas. To study the effect of MX particles on segregation a methodology combining SEM and TEM was employed. This involved chemically mapping the larger V containing particles using EDS in the SEM in segregated and unsegregated areas and then comparing the results to site-specific TEM analysis. This analysis showed that although the average size of the V containing samples is in the expected 0-50 nm size range, these particles in some samples had a wide size distribution range, which significantly overlaps with the M 23 C 6 size distribution range. This together with the segregation characteristics has important implications for determining meaningful quantitative microstructural data from these microstructurally complex materials.

Microstructure in the gage sections of ruptured GX12CrMoWVNbN10-1-1 cast steel specimens was examined after creep tests under applied stresses ranging from 120 to 160 MPa at T=893 K. The microstructure after tempering consisted of laths with an average thickness of 332 nm. The tempered martensite lath structure was characterized by M 23 C 6 -type carbide particles with an average size of about 105 nm, and MX carbonitrides with an average size of about 45 nm. Precipitation of Laves phase occurred during creep test. The structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. MX carbonitrides and M 23 C 6 -type carbide particles increase in size slightly under long-term creep. Microstructural degradation mechanisms during creep in GX12CrMoWVNbN10-1-1 cast steel are discussed.

A 9% Cr steel containing cobalt and boron, X13CrMoCoVNbNB9-2-1, has been manufactured by electroslag remelting (ESR) to evaluate its performance and to compare its creep strength and microstructure to a forging made from electroslag hot-topping ingot. The evaluation results confirm that it is possible to produce rotor forgings with homogeneous composition and good properties by the ESR process. The results of creep rupture tests up to 5000 h indicate that the creep strength of the forging made from ESR ingot is similar to that of the forging produced by the electroslag hot-topping process. Martensitic lath microstructures with high density dislocations and the precipitations of M 23 C 6 , VX, NbX and M2X are observed after the quality heat treatments at the center portion of both forgings. There is no large difference in the martensitic lath widths, distributions, and sizes of those particles between both trial forgings.

Hitachi and Hitachi Metals have developed low thermal expansion Ni-base superalloy, Ni-20Cr-10Mo-1.2Al-1.6Ti alloy (USC141) for use as A-USC steam turbine material. The approximate 10 5 h creep rupture strength at 740° C is 100MPa, so USC141 can be expected to apply for blades and bolts. Now we have been studying to get better creep properties by microstructure controlling such as grain size or grain boundary morphology. In addition, the segregation test of USC141 shows good Freckle tendencies, it means that it would be easy to make a large ingot which could be used as rotors or pipes. From these calculation results, we have been tried to make an 850mmϕ ESR ingot of USC141.

Creep strength enhanced ferritic (CSEF) steels, particularly modified 9Cr steels Grade 91 and 92, are increasingly used in advanced coal-fired power plants for header and steam piping construction. While these materials typically enter service after receiving a standard high-temperature normalizing treatment followed by lower temperature tempering to achieve optimal microstructure, practical situations like welding operations may expose components to additional heat treatment exceeding the Ac 1 , and potentially the Ac 3 , temperature before returning to tempering temperature. This research examines the effects of simulated post weld heat treatments (PWHT) on Grade 91 and 92 materials using dilatometer-controlled heating and cooling rates, with peak temperatures below Ac 1 , between Ac 1 and Ac 3 , and above Ac 3 , followed by heat treatment at 750°C for 2 hours. Hardness measurements revealed significant reduction when exceeding the Ac 1 temperature, while advanced electron microscopy, including electron back scatter diffraction, was employed to analyze changes in martensite laths and grain structure, along with detailed carbide size distribution analysis using both scanning and transmission electron microscopy. The findings are discussed in terms of how such PWHT overshoots might affect mechanical properties during high-temperature service.

Research on low-alloyed, heat-resistant 12Cr2MoWVTiB steel, implemented in China to power plants in 50’s last century, was performed to investigate a possibility of its application for pressure elements of boilers, in particular for membrane walls. The microstructure of the as-received 12Cr2MoWVTiB tube, investigated by light microscopy, scanning- and transmission electron microscopy, consists of ferritic grains with some bainite areas between them as well as primary carbides (MC) and secondary carbides (VC, M 23 C 6 , M 6 C) formed during tempering of the steel. Results of mechanical tests of 12Cr2MoWVTiB welded joints (butt- and fillet welded joints) as well as microstructure analyses of are satisfactory.

Friction Stir Layer Deposition on a Cu-containing high-entropy alloy (HEA) has been performed for its suitability of the core component of nuclear materials. Excellent irradiation resistance in this Cu-containing HEA has been reported previously. Friction stir layer deposition (FSLD) offers a solid-state deformation processing route to metal additive manufacturing, in which the feed material undergoes severe plastic deformation at elevated temperatures. Some of the key advantages of this process are fabrication of fully dense material with fine, equiaxed grain structures. This work reports the detailed microstructure of the FSLD product, and it discusses the grain refinement and micro-hardness variation observed in FSLD product.

A 10%Cr martensitic steel with 3%Co and 0.008%B exhibits extremely long creep rupture time of approximately 40000 h under an applied stress of 120 MPa at a temperature of 650°C. The steel’s microstructure after creep tests interrupted at different creep stages was examined by transmission and scanning electron microscopy. It was shown that superior creep resistance of this steel was attributed to slow increase in creep rate at the first stage of tertiary creep whereas the rapid acceleration of creep rate took place only at the short second stage of tertiary creep. Transition from minimum creep rate stage to tertiary creep was found to be accompanied by coarsening of Laves phase particles, whereas M 23 C 6 – type carbides demonstrated high coarsening resistance under creep condition. Strain-induced formation of Z-phase does not affect the creep strength under applied stress of 120 MPa due to nanoscale size of Z-phase particles.

TAF steel is a Japanese high-boron 10.5% Cr martensitic stainless steel known for its exceptional high-temperature creep strength. Its high boron content (300-400 ppm) limited practical applications due to reduced hot workability in large turbine components. Recent research suggests that increasing boron content while adjusting nitrogen levels could enhance creep properties by promoting fine vanadium carbonitride formation while preventing boron nitride formation. This study presents microstructural investigations, particularly using transmission electron microscopy, focusing on precipitation characteristics and long-term precipitate evolution within the COST 536 framework.

Alumina-forming austenitic stainless steels (AFAs) are potential materials for boiler/steam turbine applications in next generation fossil fuel power plants. They display a combination of good high temperature creep strength, excellent oxidation resistance and low cost. A recently-developed AFA alloy based on Fe-14Cr-32Ni-3Nb-3Al-2Ti (wt.%) shows better creep performance than a commercially-available Fe-based superalloy. In this paper we used scanning electron microscopy and transmission electron microscopy to study the fracture surfaces and cracking behavior in relation to the precipitates present in creep failure samples of this alloy tested at either 750°C/100 MPa or 700°C/170 MPa. It was found that most cracks are formed along the grain boundaries with precipitate-free zones beside the grain boundaries potentially providing the path for propagation of cracks.

The powder metallurgy (P/M) process has been applied to a high strength turbine disk alloy. It is known that P/M alloys show characteristic microstructures such as prior powder boundaries (PPB) compared to microstructures of conventional cast and wrought (CW) alloys. High temperature tensile tests were conducted on CW and P/M processed alloy720Li in order to reveal the effect of temperature and strain rate on deformation behavior and to demonstrate the effect of microstructure derived from P/M process on deformability. The fracture mode of the P/M material changed from grain interior fracture to fracture around large PPB with an increment of strain rate. In addition, samples ruptured at higher temperature showed grain boundary fracture regardless of strain rate. On the other hand, the CW material showed good deformability with chisel point fracture in the entire temperature and strain rate condition range. In the P/M material, melting of grain boundaries occurred at super solvus temperature conditions. Large PPB acts as nucleation site of voids at higher strain rate conditions. Precipitation strengthening by γ’ phase degrades deformability at sub solvus temperature conditions. However, deformability near the solvus temperature and low strain rate condition in as HIPed P/M material increased with fine grain size distribution in spite of the presence of large grains resulting from PPB.

Miniature specimen tests are necessary to assess the mechanical properties of new fuel cladding alloys for next-generation nuclear reactors. The small specimen allows for extensive testing programs from limited volumes of material. However, there is a lack of testing equipment to perform high-temperature mechanical tests on the miniature specimen. This work presents the development of a high-temperature creep test system for miniature specimens with in situ scanning electron microscope (SEM) testing capability for real-time characterization. Here, we discuss the challenges of the development of the system, such as gripping the samples, loading, heating, cooling mechanisms, and strain measurement. The equipment is used to investigate the creep behavior of FeCrAl alloy Kanthal APMT, and the results are compared with conventional creep test data from the same batch of this material.