Search Results for high angle annular dark field imaging

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.

In the present study a creep resistant, ferritic steel, based on the chemical composition of Crofer 22 H, was analysed regarding microstructure and particle evolution. Because of the preceding hot-rolling process formation of sub-grain structures was observed, which disappears over time. Additionally formation of particle-free zones close to high angle grain boundaries was observed. These zones are considered to be responsible for long-term material failure by lacking particle hardening and thus a concentration of deformation. Therefore in-depth analyses by transmission and scanning electron microscopy were performed to investigate dislocation behaviour in these areas

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.

Precipitation of Z-phase, Cr(V,Nb)N, is known to negatively affect creep properties of 9-12%Cr steels for power plant applications as it dissolves finely distributed MX particles, (V,Nb)N, especially in high Cr steels. As the Z-phase precipitates slowly as large particles, this causes a net drop in precipitation strengthening. Two model alloys containing 9 and 12%Cr, but otherwise having similar composition, were produced in order to quantify the difference in Z-phase precipitation speed at different Cr levels. The nitride precipitation behavior was followed at different temperatures using TEM and XRD, allowing for a quantification of the Z-phase precipitation. The Z-phase was found to precipitate 20-50 times faster in the 12%Cr steel compared to 9%Cr steel in the temperature range 600- 650°C. The transformation of MX into Z-phase was followed in a Ta containing alloy without V or Nb. In this alloy the Z-phase precipitates very quickly, and thus appears as finely distributed particles which have the same strengthening effect as MX particles. Investigations using atomic resolution microscopy showed how Cr diffuses from the matrix into the TaN MX particles and gradually transforms them both chemically and crystallographically into Z-phase CrTaN particles.

Components such as tubes, pipes and headers used in power generation plants are operated in a creep regime and have a finite life. During partial replacement, creep exhausted materials are often welded to virgin materials with superior properties. The aim of this study was to identify a suitable weld filler material to join creep aged X20CrMoV12-1 to a virgin P91 (X10CrMoVNbV9-1) steel. Two dissimilar joints were welded using the gas tungsten arc welding (GTAW) process for the root passes, and manual metal arc (MMA) welding for filling and capping. The X20 and the P91 fillers were selected for joining the pipes. The samples were further heat treated at 755°C to stress relief the samples. Microstructural evolution and mechanical properties of the weld metals were evaluated. The average hardness of X20 weld metal (264 HV10) was higher than the hardness measurement of P91 weld metal (206 HV10). The difference in hardness was attributed to the high carbon content in X20 material. The characterisation results revealed that the use of either X20 or P91 weld filler for a butt weld of creep aged X20 and virgin P91 pipes material does not have a distinct effect on the creep life and creep crack propagation mechanism. Both weld fillers (X20 and P91) are deemed to be suitable because limited interdiffusion (<10 μm) of chromium and carbon at the dissimilar weld interface was observed across the fusion line. The presence of a carbon ‘denuded’ zone was limited to <10 μm in width, based on the results from local measurements of the precipitate phase fractions using image analysis and from elemental analysis using EDS. However the nanoindentation hardness measurements across the fusion line could not detect any ‘soft’ zone at the dissimilar weld interface. The effect of the minute denuded zone was also not evident when the samples were subjected to nanoindentation hardness testing, tensile mechanical testing, Small Punch Creep Test (SPCT) and cross weld uniaxial creep testing.

Research has demonstrated that creep damage in power plant steels is directly linked to grain boundary precipitates, which serve as nucleation sites for cavities and micro-cracks. The formation of M 23 C 6 carbides along grain boundaries creates chromium-depleted zones vulnerable to corrosion and significantly reduces creep life due to rapid coarsening. Through combined Monte Carlo grain boundary precipitation kinetics and continuum creep damage modeling, researchers have predicted that increasing the proportion of MX-type particles could enhance creep performance. This hypothesis was tested using hafnium-containing steel, which showed improved creep and corrosion properties in 9% Cr steels. Ion implantation of Hafnium into thin foils of 9 wt% Cr ferritic steel resulted in two new types of precipitates: hafnium carbide (MX-type) and a Cr-V rich nitride (M 2 N). The hafnium carbide particles, identified through convergent beam diffraction and microanalysis, appeared in significantly higher volume fractions compared to VN in conventional ferritic steels. Additionally, Hafnium was found to eliminate M 23 C 6 grain boundary precipitates, resulting in increased matrix chromium concentration, reduced grain boundary chromium depletion, and enhanced resistance to intergranular corrosion cracking.

Small punch creep testing (SPCT) is a small-scale, accelerated creep test that allows for the determination of creep data using a limited amount of material. The question, however, remains how the data generated by this technique correlate to more established techniques such as uniaxial testing and ultimately to predictions regarding the remaining service life of a plant component. This empirical study investigated the microstructure-to-property relationship of welded 9-12%Cr steels as measured using SPCT. Virgin P91 (X10CrMoVNb9-1) steel was joined to service exposed X20 (X20CrMoV12-1) steel using two different filler materials (X20 and P91) via fusion welding. Site-specific samples were extracted from the parent plates, heat affected zones and weld metals using electro-discharge machining. Small punch creep testing were performed using a 276 N load at a temperature of 625°C. The untested sample microstructures were quantitatively characterized using a range of electron microscopy techniques to determine the precipitate (M 23 C 6 , MX) spacing, subgrain sizes and dislocation densities for each region of the weldments. Multiple linear regression analysis found that the subgrain size (λsg) played the largest contribution to the SPCT rupture life. The heat affected zones had the lowest SPCT rupture times (49-68 hours), which corresponded to the largest subgrain sizes (1.1-1.3 μm). The P91 parent plate material had the longest SPCT rupture time (349 hours), which corresponded to the lowest subgrain size (0.8 μm). The P91 weld metal sample showed lower initial deflection rates during the SPC testing, however the presence of non-metallic SiO 2 inclusions in this zone contributed to accelerated brittle failure.

The Advanced Materials and Manufacturing Technologies (AMMT) program is aiming at the accelerated incorporation of new materials and manufacturing technologies into nuclear-related systems. Complex Ni-based components fabricated by laser powder bed fusion (LPBF) could enable operating temperatures at T > 700°C in aggressive environments such as molten salts or liquid metals. However, available mechanical properties data relevant to material qualification remains limited, in particular for Ni-based alloys routinely fabricated by LPBF such as IN718 (Ni- 19Cr-18Fe-5Nb-3Mo) and Haynes 282 (Ni-20Cr-10Co-8.5Mo-2.1Ti-1.5Al). Creep testing was conducted on LPBF 718 at 600°C and 650°C and on LPBF 282 at 750°C. finding that the creep strength of the two alloys was close to that of wrought counterparts. with lower ductility at rupture. Heat treatments were tailored to the LPBF-specific microstructure to achieve grain recrystallization and form strengthening γ' precipitates for LPBF 282 and γ' and γ" precipitates for LPBF 718. In-situ data generated during printing and ex-situ X-ray computed tomography (XCT) scans were used to correlate the creep properties of LPBF 282 to the material flaw distribution. In- situ data revealed that spatter particles are the potential causes for flaws formation in LPBF 282. with significant variation between rods based on their location on the build plate. XCT scans revealed the formation of a larger number of creep flaws after testing in the specimens with a higher initial flaw density. which led to a lower ductility for the specimen.

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.