Focused ion beam (FIB) microscopy, in combination with scanning electron microscopy (SEM) has been used to characterise the porosity through the oxide scale of an experimental martensitic 9Cr-1Mo steel exposed for 3325 h to a CO2-rich environment at 600 °C. A typical magnetite outer layer forms with a small fraction of spherical pores, but no interconnectivity between them. A complex mixed middle layer is observed which consists of larger magnetite grains, smaller spinel grains and some pore interconnectivity, but typically the porosity is present in localised regions, inhibited from coalescence by the different grain types. A spinel layer also shows greater pore interconnectivity with three large pores spanning the total distance through this layer, but no single pore spans the total oxide scale thickness. A mechanism linking the substrate microstructure to the variation in oxide structure between ferritic and martensitic material is proposed which explains the formation of the magnetite, the complex oxide and spinel oxide layers.

While the tempered martensitic microstructure of Creep Strength Enhanced Ferritic (CSEF) steels provides remarkable creep resistance, long-term stability remains a challenge. Microstructural risk factors in CSEF steels caused by nitrides, inclusions and coarsening of particles compromise mechanical integrity, accelerating crack initiation and propagation. Tantalum (Ta) additions promote the formation of nanoscale MX carbonitrides that hinder dislocation recovery and stabilise the microstructure, extending creep life. However, the unintended formation and coarsening of large Ta-containing particles could potentially counteract these benefits by serving as preferential sites for creep cavity nucleation. To better understand the direct correlation between creep damage and coarse Ta-containing particles, 3D Focused Ion Beam (FIB) serial sectioning, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and machine learning (ML) enhanced image segmentation was performed on a novel CSEF steel. High-resolution SEM imaging of sequential 100 nm thick slices enabled 3D reconstruction and quantification of particle distributions. ML enhanced image segmentation significantly accelerated the phase separation process across the large dataset, enabling efficient and consistent analysis that would be impractical with manual methods. The results reveal a close spatial association between the coarse Ta-containing particles and creep damage, with progressive accumulation of these particles toward the fracture surface (volume percentage increasing from 0.038% in as-received material to 0.093% near creep fracture), suggesting these particles serve as preferred sites for cavity nucleation with substantial impact on creep behaviour. This study paves the way for microstructural risk factor assessment in next-generation high-temperature materials and highlights the advantages of combining FIB-SEM serial sectioning with ML to analyse complex microstructures.

Stainless steel alloys can be difficult to etch to reveal the intricate features of the microstructure due to their resistance to chemical attack. This can be particularly true of duplex stainless steels as its major phases (ferrite and austenite) can etch differently due to their chemical differences and crystal structures. This becomes increasingly complicated as secondary (sometimes detrimental) phases, for example sigma phase, chi, nitrides and carbides, form in the material during exposure to high temperatures. This paper demonstrates a thermal etching technique (heat tint) to reveal the microstructures and discusses potential issues and advantages. The resulting colours are related to the oxide formed on the surface of the phases in the material giving interference colours. The paper discusses the advantages of the technique in observing the complex structures as they exhibit clear differences in colour which can be used to quantitatively measure the phase fraction. Using this technique, it may be possible to remove some of the ambiguity seen in prediction of the formation of these phases.

Energy storage technologies such as lithium-ion batteries (LIBs) incorporate carbon components key to their function. Graphite and carbon binder components in LIB electrodes are engineered to deliver critical electrical and mechanical properties, as are the surface chemistry and morphology of carbon blacks (CBs) in LIBs and catalysts. The challenge of relating surface chemistry to morphology is complicated by the numerous forms of carbon bonding and potential for surface functional groups. Furthermore, materials processing can influence bonding and structure of carbon at multiple length scales, as seen in mechanochemical functionalization of CBs. To understand the nature of carbon surfaces, secondary electron hyperspectral imaging (SEHI) is introduced as a spatially resolved analysis bridging the nano to microscale. The ability to provide novel insights is demonstrated three example applications: observation of nanoscale "satellite" particles of amorphous hydrogenated carbon on graphitic CB particles, differentiation between graphitic and amorphous hydrogenated nano-thickness carbon coatings on particles of lithium iron phosphate, and differentiation between graphitic carbon active material and carbon binder domain in a LIB anode material. SEHI analysis using peak fitting models for graphitic and disordered carbons is developed based on reference materials and standard spectroscopic methods: Raman spectroscopy and X-ray photoelectron spectroscopy.

About 9 wt.% Cr steels are widely used in the design and fabrication of thick section components in combined cycle or coal-fired applications for working temperatures of 600 similar to 650 degrees C. This family of materials possesses a martensitic microstructure stabilised by precipitates. The presence of nitrides, inclusions or evolution of second-phase particles may increase the metallurgical risk to creep. The chemical composition and microstructural evolution of 9 wt.% Cr steels contribute to thermal stability and long-term performance. In some specialist alloys, Ta is added to the composition which causes the formation of fine MX precipitates which are only present at the nanometre scale in tempered martensite, which hinders the recovery of dislocations and the migration of laths to extend creep life. However, the presence of large Ta-containing particles or inclusions in the 9 wt.% Cr steels may have a detrimental effect on its creep performance, as they may act as preferred sites for cavity nucleation. To fully appreciate the development of damage in these steels, it is necessary to link the pre- and post-test conditions, evaluate damage in the parent metal, develop procedures that provide consistency of results, and obtain statistically relevant data. The evolution of the Ta-containing phase has been tracked and quantified using a variety of correlative characterisation approaches. Utilising focused ion beam microscopy and two-dimensional electron-based microscopic characterisation, three-dimensional tomography has identified a strong relationship between creep cavities and Ta-containing phases from the early stages of creep.

A detailed investigation has been carried out of the matching composition filler metal for welding MarBN steel to explore the influence of welding procedure and post-weld heat treatment (PWHT) on the microstructure and properties of the weld metal and a MarBN steel weld joint. The filler metal was in the form of a shielded metal arc welding (SMAW) electrode. With different interpass temperatures and PWHT procedures applied, the microstructure and mechanical properties, particularly toughness, of the all-weld metal deposits and welded MarBN alloy joints were examined. The differences in microstructure (grain structure, precipitates, etc.) were evaluated to provide a better understanding of how the PWHTs influence the starting conditions of the matching weld metal and welded MarBN steel joints before service. The current work found that for a 30-mm-thick all-weld metal coupon, reasonable impact toughness was achieved after PWHT of 735 °C × 8 h; longer heat treatment durations (16 h and 36 h) did not further increase the toughness. For thicker cast blocks, a PWHT of 735 °C × 16 h was needed for weld toughness comparable to the base metal, hence was considered more appropriate for actual engineering applications. Evenly distributed Laves phase precipitation was observed after 735 °C × 8 h; the precipitates gradually increased in size and area fraction as the duration increased, and appeared to be stable when approaching 36 h. Differences in the precipitation were observed between different weld regions. The weld cap region had a lower density of Laves precipitates compared to the lower weld area. From the current investigation, aspects for further investigation are also identified.

Abstract MarBN steels, originally developed by Professor Fujio Abe at NIMS Japan, have undergone significant advancement in the UK through a series of government-funded collaborative projects (IMPACT, IMPEL, INMAP, IMPULSE, and IMPLANT). These initiatives have achieved several major milestones, including operational power plant trials, full-scale extruded pipe production, matching welding consumable development, and most notably, the creation of IBN-1—a new steel demonstrating 30-45% higher creep strength than Grade 92. However, like other creep strength-enhanced ferritic steels, IBN-1 shows reduced creep ductility under the lower stress conditions typical of operational use. Since adequate creep ductility is essential for component damage tolerance and effective in-service monitoring, this study investigates the effects of an alternative normalizing and tempering heat treatment on cast IBN-1. The research presents creep rupture test results showing improved ductility and analyzes the microstructural mechanisms responsible for this enhancement.

Electromagnetic pulse welding is a high-velocity impact joining process employed with the intention of forming fast and effective solid-state bonds. Electron microscopy techniques, including SEM and TEM, revealed that bonding was not fully accomplished in the solid state; instead, local melting can occur. These locally melted areas likely occur around the point of first contact during the welding process and are associated with a debonded region that runs alongside or through the centre of melted zones. Microstructural characterisation showed dispersoid-free regions, columnar grains, epitaxial growth, and localised increases in O, Fe, Si, and Mn content in locally melted areas. This region contrasts with the solid-state bonded region, in which the interface exhibited sub-micron grains.

Abstract Since the reports of promising creep properties of MARBN alloys (a short name for MARtensitic 9Cr steel strengthened by Boron and Nitrides), nearly two decades ago, significant effort has been input in examining the base alloys, development and validating of the matching filler metals. One of the main targeted applications of MARBN alloys is the castings for thermal power plant turbine components. These applications will involve repair welding of castings and joint welding between castings and other structures, such as joints between casted valves and steam pipe works, etc. For weld joints with satisfactory microstructure and properties, there are practical needs for developing appropriate welding procedures and post weld heat treatment (PWHT) parameters. Particularly, a relatively lower PWHT peak temperature, i.e. 735°C, is preferred for the fabrication of casting components. Within the UK collaborative projects, IMPLANT, a detailed investigation has been carried out to the matching composition filler metal for welding MARBN alloys to explore the influence of welding procedure and PWHT on the microstructure and properties of the weld metal and MARBN alloy weld joint. The filler metal was in form of shielded metal arc welding (SMAW) electrode. All-weld metal assemblies and weld joints made with IBN-1 base alloy plate were prepared. For the weld joint welding, different inter-pass temperatures were applied. After welding, post weld heat treatments at 735°C with different soaking durations were employed. Ambient temperature mechanical test was conducted. All-weld metal and cross-weld transverse tensile, Charpy impact toughness, hardness and bend properties were assessed. The microstructure of the all-weld metals and weld joint after different PWHT procedures were examined using optical microscopy and scanning electron microscopy (SEM). The differences in microstructure (grain structure and precipitations, etc) are evaluated in order to determine the possible reasons for the differences in mechanical properties, particularly in toughness at ambient temperature. This paper reports the results obtained so far. Initial conclusions are summarised. Aspects for further investigation are identified.

Microstructural characterisation was performed on the gauge section of two creep rupture samples manufactured from Super 304H; an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The samples had been exposed to uniaxial creep tests (Sample A: 700 degrees C and 75Mpa, Sample B: 650 degrees C and 120Mpa) where both exhibited low creep ductility. Six phases were identified: MX (Nb-rich), sigma, Cu-rich, M23C6, modified Z phase and Fe (BCC). Quantification of Nb-rich and Cu-rich particles revealed similar to 10% and similar to 16% more Nb-rich and Cu-rich particles respectively in Sample A versus Sample B. The amount of sigma and the average particle size for the Nb and Cu-rich particles was similar for both samples. There appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation which could have contributed to the low creep ductility exhibited by these two creep samples.

Focused ion beam microscopy and scanning electron microscopy have been used to characterise the porosity of the oxide scale of an experimental 9Cr–1Mo steel sample exposed for 4580 h in a CO 2 -rich environment. The magnetite shows a high frequency of spherical pores (~ 1 µm 3 ) with no interconnectivity. The Cr-rich spinel layer shows greater interconnectivity, but no single pore spans the total oxide scale. A mechanism for the formation of the different morphologies observed across the scale is proposed, linking porosity changes across the oxide scale to the carburisation and elemental segregation of Cr within the substrate. Graphical abstract

To understand the effect of microstructure on the oxidation characteristics of 9Cr-1Mo steel, experimental material with two different starting microstructures (ferritic and martensitic) were exposed to a CO2 rich atmosphere at 600 degrees C and 640 degrees C for up to similar to 7000 h. The microstructure influences the size and distribution of carbides forming within the 9Cr-1Mo steel exposed to a CO2 rich atmosphere. These differences lead to elemental segregation during carburisation and oxidation. This elemental segregation influences the oxidation characteristics of the 9Cr-1Mo steel and the morphology of the internal oxidation zone, and subsequently the spinel structure shows ghosts of the prior substrate microstructure.

To better understand oxidation and carburisation processes taking place during exposure of 9Cr-1Mo steel experimental samples have been exposed at 600?degrees C and 620 degrees C to a CO2 rich atmosphere for various exposure times. Complex multi-elemental carbides form showing a range of compositions, with some showing coring/elemental segregation. Certain carbides show both M7C3 and M23C6 within a single multi-component carbide. At the oxidation front some carbides resist oxidation through growth of a Cr rich oxide shell. Carbides within both substrate and oxide have been characterized with mechanisms being proposed for the complex interactions taking place within both the substrate and oxide.

In order to quantify precipitates such as Laves phase, M23C6 carbides and modified Z-phase in 9-12 wt% Cr power plant steels, a range of microscopy techniques, including scanning electron microscope (SEM) and focussed ion beam (FIB) imaging, are currently used. This paper's key finding reports on the novel application of a field emission SEM (FESEM) in-column upper secondary electron detector (USD) for imaging and quantification of precipitates in a thermally aged 11-12 wt% Cr steel sample. The USD produces images from first order secondary electrons generated directly from primary electron beam interaction. These images were found to not only show significant contrast between the matrix and the precipitates to enable quantification, but between particles which otherwise have identical greyscale intensities in FIB images. Previously assumed to be the same phase, some of these precipitates appeared significantly darker than others. The greyscale differential between these precipitate types was sufficient for separate quantification. Energy dispersive x-ray spectroscopy and selected area diffraction analysis identified the darker particles as modified Z-phase and the lighter grey particles as M23C6 carbides. When analysing all particles together, comparison of data from FESEM USD and FIB images confirmed that both techniques produce statistically comparable precipitate analysis results. However, as modified Z-phase and M23C6 carbide distributions can be analysed separately from a FESEM in-column USD image at low accelerating voltages, it was found that this technique provides for more reliable precipitate quantification than FIB imaging alone in these steels. With future work, this could enable faster characterisation and may lead to automated larger area mapping.

The weldments made from the 9-12% Cr tempered martensitic steel are associated with a complex micro-structure arising from complicated thermal histories of the fusion and heat affected zones. The complicated microstructural and micro-mechanical states in these critical regions provide a challenge for the determination of creep failure mechanisms. Based on detailed metallographic examination, the microstructural distribution in the heat affected zone of the welds constructed using a recently developed 9% Cr MarBN steel, IBN-1, has been identified and classified into Equiaxed Zone (EZ), Duplex Zone (DZ) and Over-tempered Zone (OZ). Cross-weld testing performed at 650 degrees C has revealed a significant reduction in creep life as compared to bulk material. Creep rupture has been shown to occur in the parent metal region with a ductile manner at a high stress, whereas creep rupture initiates in the DZ region in an intergranular manner at a low stress. Detailed metallographic investigation has further revealed a higher damage susceptibility in the regions along the pre-existing Prior Austenite Grain Boundaries (PAGBs). The diffusional reaustenitisation of local microstructure along the PAGBs leads to a lower strength of matrix in combination with a lack of intergranular precipitates as compared to the surrounding microstructure formed after displacive reaustenitisation.

Extensive research and development has been undertaken in the UK on MarBN steels. These were first proposed by Professor Fujio Abe from NIMS in Japan. Within the UK, progress has been made towards commercialisation of MarBN-type steel through a series of Government funded industrial collaborative projects (IMPACT, IMPEL, INMAP and IMPULSE). As part of the IMPACT project, which was led by Uniper Technologies, boiler tubes were manufactured from the MarBN steel developed within the project, IBN1, and installed on the reheater drums of Units 2 and 3 of Ratcliffe-on-Soar Power Station. The trial tubes were constructed with small sections of Grade 91 tubing on either side of the IBN1 to allow direct comparison after the service exposure. This is the world’s first use of a MarBN steel on a full-scale operational power plant. In September 2018 the first tube was removed having accumulated 11,727 hours operation and 397 starts. This paper reports microstructural and oxidation analysis, that has been undertaken by Loughborough University as part of IMPULSE project, and outlines future work to be carried out.

For VM12-SHC 11-12 wt. % Cr steel, there have been no systematic investigations to define the regions or characterise the microstructures within the heat-affected zone (HAZ) of weldments. In similar steels, these regions relate to the Act and Ac3 transformation temperatures and can affect weldment performance. In this study, controlled thermal cycles were applied to VM12-SHC parent metal using a dilatometer and the Act and Ac3 temperatures were measured for various heating rates. The Ae(1) and Ae(3) temperatures were also calculated by thermodynamic equilibrium modeling. Through dilatometry, thermal cycles were then applied to simulate the microstructures of the classically defined HAZ regions. The microstructural properties of each simulated material were investigated using advanced electron microscopy techniques and micro-hardness testing. It was found that the simulated HAZ regions could be classified as; (1) the completely transformed (CT) region, with complete dissolution of pre-existing precipitates and complete reaustenitisation; (2) the partially transformed (PT) region, exhibiting co-existing original martensite with nucleating austenite microstructures with partial dissolution of precipitates; and (3) the over tempered (OT) region, with no phase transformation but precipitate coarsening and decreased hardness.

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 characterise the microstructure of ex-service reheater and superheater tubes as this will help to understand 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 degrees C), has been characterised. The characterisation techniques used were electron microscopy based and include imaging and chemical analysis techniques. Seven phases were observed as a result of the characterisation work. The phases observed were, MX carbo-nitrides rich in niobium, copper rich particles, M23C6, sigma, Z phase, a cored phase and a BCC phase.

Martensitic steels strengthened by Boron and Nitrogen additions (MarBN) were developed for high temperature/high stress service in power plant for periods of many years and are being considered as a promising candidate for the replacement of the more conventional Grade 91/92 steels. In the present study, extensive microstructural observation of physically simulated Heat Affected Zone (HAZ) MarBN material has been carried out after dilatometry simulations to link the variation in microstructure with weld thermal cycles. The microstructure in the MarBN HAZ has been observed to vary from a refined equiaxed morphology to a duplex microstructure consisting of refined grains distributed on the pre-existing Prior Austenite Grain Boundaries (PAGBs) as the peak temperature of the weld thermal cycle decreases. The temperature range corresponding to the formation of the duplex grain structure coincides with the temperature regime for the dissolution of the pre-existing M23C6 carbides. An even distribution of the M23C6 carbides within the martensitic substructure was also observed after Post Weld Heat Treatment (PWHT), which is beneficial for the creep performance of the weld HAZ. The MX precipitates are more resistant to thermal exposure and are not completely dissolved until the peak temperature reaches 1573 K (1300 degrees C). The Nb-rich MX precipitates are the predominant type observed both after weld simulations and PWHT. The hardness between the materials experienced with the thermal cycles with different peak temperature does not significantly vary after PWHT conducted in an appropriate condition, which is likely to mitigate an unfavoured stress condition in the localised area within the HAZ.

This paper reports the short-term creep behaviour at elevated temperatures of a MarBN steel variant. Creep tests were performed at three different temperatures (625 degrees C, 650 degrees C and 675 degrees C) with applied stresses ranging from 160 MPa to 300 MPa, and failure times from 1 to 350 h. Analysis of the macroscopic creep data indicates that the steady-state creep exhibits a power-law stress dependence with an exponent of 7 and an activation energy of 307 kJ mol(-1), suggesting that dislocation climb is the dominant rate-controlling creep mechanism for MarBN steel. Macroscopic plastic instability has also been observed, highlighted by an obvious necking at the rupture region. All the macroscopic predictions have been combined with microstructural data, inferred from an examination of creep ruptured samples, to build up relations between macroscopic features (necking, damage, etc.), and underlying microstructural mechanisms. Analysis of the rupture surfaces has revealed a ductile fracture mode. Electron Backscatter Diffraction (EBSD) analysis near to the rupture surface has indicated significant distortion and refinement of the original martensitic substructure, which is evidence of long-range plastic flow. Dislocation pile-ups and tangles from TEM were also observed near substructure boundaries and precipitate particles. All of these microstructural observations suggest that creep is influenced by a complex interaction between several elements of the microstructure, such as dislocations, precipitates and structure boundaries. The calculated stress exponent and activation energy have been found to agree quantitatively with the highlighted microstructural features, bearing some relationships to the true observed creep microstructures.