Search Results for electron backscatter diffraction

In thermal power plants, weldments of all currently used martensitic 9% chromium steels are prone to Type IV cracking in the fine-grained region of the heat-affected zone (HAZ). Japanese researchers have introduced a new martensitic steel for ultra-supercritical (USC) steam conditions that demonstrates resistance to Type IV cracking. This study compares a modified version of this boron-nitrogen balanced advanced 9Cr-3W-3Co steel with CB2, the most promising 9% Cr steel developed through the European research initiative COST, in terms of weldability. The HAZ was analyzed using the "Heat-Affected Zone Simulation" technique with a Gleeble 1500 thermo-mechanical simulator. Basic optical microscopy was complemented by advanced electron microscopy techniques, including energy-filtered TEM (EFTEM), electron energy loss spectroscopy (EELS), electron backscatter diffraction (EBSD), and energy-dispersive X-ray analysis (EDX). Phase transformations in the HAZ were directly observed using in situ X-ray diffraction with synchrotron radiation at the Advanced Photon Source (APS) of Argonne National Laboratory, IL, USA. Although both steels exhibited similar transformation behavior, their resulting microstructures after the weld thermal cycle differed significantly. At peak temperatures above 1200°C, delta ferrite formed and remained stable down to room temperature due to rapid cooling in both steels. While CB2 exhibited conventional coarse-grained (CG), fine-grained (FG), and intercritical HAZ regions, the boron-nitrogen balanced 9Cr steel did not develop a fine-grained HAZ. Since Type IV cracking primarily occurs in the FGHAZ, this alloy shows strong potential for eliminating Type IV cracking as a major life-limiting factor in heat-resistant steel weldments.

Damage in the grade 91 steel partially transformed zone of weld heat affected zones has historically been associated with many different types of microstructural features. Features described as being responsible for the nucleation of creep damage include particles such as laves phase, coarse M 23 C 6 , inclusions, nitrides, or interactions between creep strong and creep week grains, grain boundaries and potentially other sources. Few studies have attempted to link the observations of damage on scales of increasing detail from macro, to micro, to nano. Similarly, assessments are not made on a statistically relevant basis using 2D or 3D microscopy techniques. In the present paper, 2D assessment using scanning electron microscopy (SEM) and quantification techniques such as energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) are utilized in combination with 3D serial sectioning of large volumes using plasma focused ion beam milling (P-FIB) and simultaneous EDS to evaluate an interrupted cross-weld creep test. Moreover, the sample selected for examination was from a feature cross-weld creep test made using a parent material susceptible to the evolution of creep damage. The test conditions were selected to give creep brittle behaviour and the sample was from a test interrupted at an estimated life fraction of 60%. The findings from these evaluations provide perspective on the features in the microstructure responsible for the nucleation and subsequent growth of the observed damage.

The creep strength and ductility of Grade P22 steel (2¼ Cr) was measured at 600°C under standard uniaxial tensile conditions at 150MPa. Test specimens were prepared by solution heat treatment at austenitization temperatures ranging from 900°C - 1200°C followed by normalization at 900°C before continuous air cooling to room temperature. In addition to specimens tested in the solution treated state, creep tests were also performed after tempering. The variable austenitization temperatures gave rise to different prior austenite grain (PAG) sizes, which in turn influenced the crystallographic packet and block boundary misorientation angle distribution. The latter parameters were measured using electron backscattered diffraction which also allowed partial reconstruction of the PAG boundaries. The time to creep failure at 600°C increased as function of PAG size up to approximately 70µm, but significantly decreased when the average prior austenite grain size measured approximately 108 µm. However, the minimum creep rate decreased even up to the largest PAG size with corresponding decrease in creep ductility. The stability of the crystallographic packet and block boundaries influences the high strength-low ductility for the large PAGs in comparison to the dominant effect of PAG boundaries at the smallest grain size where extensive recovery and recrystallization reduces creep strength.

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.

Boron and nitride additions are emerging as a promising design concept for stabilizing the microstructure of creep-resistant martensitic high-chromium steels. This approach, known as MarBN steel (martensitic steel strengthened by boron and nitrogen), combines the benefits of solid solution strengthening from boron with precipitation strengthening from nitrides. However, initial welding trials revealed challenges in achieving a uniform fine-grained region in the heat-affected zone (HAZ), which is crucial for mitigating Type IV cracking and ensuring creep strength. Despite these initial hurdles, preliminary creep test results for welded joints have been encouraging. This study presents an improved MarBN steel formulation and its investigation through uniaxial creep tests. Base material and welded joints were subjected to creep tests at 650°C for up to 25,000 hours under varying stress levels. The analysis focused not only on the creep strength of both the base material and welded joints but also on the evolution of damage. Advanced techniques like synchrotron micro-tomography and electron backscatter diffraction were employed to understand the underlying creep damage mechanisms. By combining long-term creep testing data with 3D damage investigation using synchrotron micro-tomography, this work offers a novel perspective on the fundamental failure mechanisms occurring at elevated temperatures within the HAZ of welded joints in these advanced steels.

The current research adopts a novel approach by integrating correlative microscopy and machine learning in order to study creep cavitation in an ex-service 9%Cr 1%Mo Grade 91 ferritic steel. This method allows for a detailed investigation of the early stages of the creep life, enabling identification of features most prone to damage such as precipitates and the ferritic crystal structure. The microscopy techniques encompass Scanning Electron Microscopy (SEM) imaging and Electron Back-scattered Diffraction (EBSD) imaging, providing insights into the two-dimensional distribution of cavitation. A methodology for acquiring and analysing serial sectioning data employing a Plasma Focused Ion Beam (PFIB) microscope is outlined, complemented by 3D reconstruction of backscattered electron (BSE) images. Subsequently, cavity and precipitate segmentation was performed with the use of the image recognition software, DragonFly and the results were combined with the 3D reconstruction of the material microstructure, elucidating the decoration of grain boundaries with precipitation, as well as the high correlation of precipitates and grain boundaries with the initiation of creep cavitation. Comparison between the 2D and 3D results is discussed.

Grade 92 steel has been widely applied in the power generation industry for use as steam pipes, headers, tubes, etc. owing to a good combination of creep and corrosion resistance. For the welding of thick section pipes, a multi-pass submerged arc welding process is typically used to achieve sufficient toughness in the weld. To relieve the internal stress in the welds and to stabilise their microstructures, a post weld heat treatment (PWHT) is commonly applied. The heat treatment conditions used for the PWHT have a significant effect on both the resulting microstructure and the creep behaviour of the welds. In this study, interrupted creep tests were carried out on two identical Grade 92 welds that had been given PWHTs at two different temperatures: 732°C and 760°C. It was found that the weld with the lower PWHT temperature had a significantly reduced stain rate during the creep test. In addition, microstructural examination of the welds revealed that the primary location of creep damage was in the heat affected zone in the sample with the lower PWHT temperature, whereas it was in the weld metal in the sample with the higher PWHT temperature. To understand the effect of the different PWHT temperatures on the microstructure, initially the microstructures in the head portions of the two creep test bars were compared. This comparison was performed quantitatively using a range of electron/ion microscopy based techniques. It was apparent that in the sample subjected to the higher PWHT temperature, larger Laves phase particles occurred and increased matrix recovery was observed compared with the sample subjected to the lower PWHT temperature.

The relationship between the hot workability and the precipitation morphology of γ′ phase in the Alloy U520 was examined with a focus on the presence of γ′-nodule. To change the morphology of γ’ phase, forged bars of the Alloy U520 were solution treated followed by cooling process with the cooling rates of 5~100 K/h. After the heat treatment, both γ’ phases of intragranular particle and nodule along grain boundaries were observed, and the both sizes increased by slowing down the cooling rate. That is, the area fraction of γ’-nodule increased from about 0.1 % in the sample cooled at 100 K/h to about 70 % at 5 K/h. In Gleeble tension test, the slow-cooled samples basically exhibited higher ductility than water-quenched samples below the γ′-solvus temperature. However, the ductility was maximized in the sample cooled at 20 K/h, and excessive decrease of cooling rate resulted in a drop in ductility. EBSD analysis revealed that dynamic recrystallization (DRX) was often occurred in grain interior but suppressed at γ′-nodule area, indicating that presence of γ′-nodule had a negative influence on hot workability at subsolvus temperature.

The broken elbow of the final superheater tube (ASME SA213 TP304H) from a coal-fired power plant was evaluated. The root causes were identified by metallographic observation, sensitization evaluation, hardness measurement, and EBSD analysis. The analysis results reached the following conclusions. (1) The tube bending was not performed in accordance with ASME Code requirements—a solid-solution heat treatment was not performed after cold working. (2) The hardness at the elbow is greater than 260 HV, exceeding the ASME code limit. (3) The sensitization was 19%, showing a performance degradation. (4) There are no obvious corrosion elements in the oxide layers of the cracks. (5) Metallographic microstructure analysis shows that there are many intergranular cracks and carbides such as Cr-rich phase and Fe-Cr are precipitated at the grain boundaries, ultimately resulting in strain-induced precipitation hardening damage.

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 Ac 1 and Ac 3 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 Ac 1 and Ac 3 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.

Ductility dip cracking (DDC) is known to occur in highly restrained welds and structural overlays made using high chromium (Cr) nickel (Ni) based filler metals in the nuclear power generation industry, resulting in costly repairs and reworks. Previous work explored the role of mechanical energy imposed by the thermo-mechanical cycle of multipass welding on DDC formation in a highly restrained Alloy 52 filler metal weld. It was hypothesized that imposed mechanical energy (IME) in the recrystallization temperature range would induce dynamic recrystallization (DRX), which is known to mitigate DDC formation. It was not shown however that IME in the recrystallization temperature range (IMERT) induced DRX. The objective of the work is to discern if a relationship between IMERT and DRX exists and quantify the amount of DRX observed in a filler metal 52 (FM-52) groove weld. DRX was analyzed and quantified using electron beam scattered diffraction (EBSD) generated inverse poll figures (IPF), grain surface area and grain aspect ratio distribution, grain orientation spread (GOS), kernel average misorientation (KAM), and grain boundary (GB) length density. From the analysis, GOS was determined to be an unsuitable criterion for quantifying DRX in multipass Ni-Cr fusion welds. Based on the observed criteria, higher IMERT regions correlate to smaller grain surface area, larger grain boundary density, and higher grain aspect ratio, which are all symptoms of DRX. High IMERT has a strong correlation with the symptoms DRX, but due to the lack of observable DRX, creating a threshold for DRX grain size, grain aspect ratio, and GB density is not possible. Future work will aim to optimize characterization criteria based on a Ni-Cr weld with large presence of DRX.

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.

Effects of alloying additions of Ti or Mo to a simplified chemical composition of the γ′′-Ni 3 Nb strengthened type Ni-based alloy 718 on the precipitation mode of δ-Ni 3 Nb phase were investigated to aim at designing grain boundaries using the δ phase for raising temperature capability of the γ′′ strengthened Ni-based wrought alloys. In the base alloy of Ni-22Cr-16Fe-3.5Nb, the δ phase precipitated at the grain boundaries of the matrix phase in a platelet form by continuous precipitation mode at temperatures above 1273K (1000°C) but in a lamellar morphology by discontinuous precipitation mode below that temperature. The boundary temperature where the continuous/discontinuous precipitation mode changes was raised by addition of 1 % Ti and lowered by addition of 5% Mo. The increase in the boundary temperature by Ti addition can be considered to have occurred by an increase in the solvus temperature of γ′′ phase. The decrease in the boundary temperature by Mo addition can be interpreted by the reduction of the strain energy caused by the coherent γ′′ precipitates and/or the volume change by the formation of δ phase from the γ/γ′′ phases, which may promote the continuous precipitation with respect to the discontinuous precipitation.

In order to study the effect of precipitation strengthening by MX precipitates on the restriction of microstructure degradation in 9 mass% Cr ferritic heat-resistant steels, V, Nb additioned model steels were evaluated by microstructure analysis through TEM and EBSD with reference to the creep test and creep interrupting test. VN precipitation increased the creep strength if the content was higher than 0.02%. Simultaneous addition of Nb and V in the specimen resulted in the complex NbC-VN precipitates even in the as-heat-treated specimens. The coherent and fine-needle-type VN was also detected in the steel. These precipitates are expected to increase the creep strength according to the creep strain curves. V variation up to 0.02% did not affect the crystallographic character of the grain boundary in the as-heat-treated specimens. Nb variation affected the crystallographic character of the grain boundary significantly because of the grain refinement effect of NbC. VN precipitation during the creep test restricted the crystallographic misorientation-angle-profile degradation. Integrating all intragranular precipitates, VN, restricts the crystallographic degradation significantly. The long-term creep test results and the precise precipitation analysis will be disclosed by the presentation.

The present study presents a detailed investigation on the evolution of the microstructure during welding on virgin and long-term service exposed (creep aged 1 = 535°C; 16.1 MPa; 156 kh and creep aged 2 = 555°C; 17.0 MPa; 130 kh) 12% Cr (X20CrMoV11-1) martensitic steel. This study was carried out in order to understand the impact of welding on prior creep exposed Tempered martensite ferritic (TMF) steel and to explain the preferential failure of weldments in the fine grained heat affected zone (FGHAZ) of the creep aged material side instead of the new material side. Gleeble simulation (Tp = 980°C; heating rate = 200 °C/s; holding time = 4 seconds) of the FGHAZ was performed on the materials to create homogeneous microstructures for the investigation. Quantitative microstructural investigations were conducted on the parent plate and simulated FGHAZ materials using advanced electron microscopy to quantify: a) voids, b) dislocation density, c) sub-grains, and d) precipitates (M 23 C 6 , MX, Laves, Z-phase) in the materials. Semi-automated image analysis was performed using the image analysis software MIPARTM. The pre-existing creep voids in the creep aged parent material and the large M 23 C 6 carbides (Ø > 300 nm) in the FGHAZ after welding are proposed as the main microstructural contributions that could accelerate Type IV failure on the creep aged side of TMF steel weldments.

This study demonstrates the Electro-Thermal Mechanical Testing (ETMT) system's capability to analyze the thermo-mechanical behavior of Inconel 718 (IN718) at a heating rate of 5 °C/s, achieving temperatures up to 950 °C. The temperature profile peaks at the sample's center and is approximately 25 °C at the extremes. Upon reaching 950 °C, the sample was aged for 30 hours before being rapidly quenched. This process froze the microstructure, preserving the phase transformations that occurred at various temperatures across the temperature parabolic gradient, which resulted in a complex gradient microstructure, providing a comprehensive map of phase transformations in IN718. The integration of thermal measurement, COMSOL modeling, scanning electron microscopy enabled a thorough characterization of the microstructural evolution in IN718, linking observed phases to the specific temperatures which provided a rapid screening of the effect of using different heating treatment routes.

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.

To address current energy and environmental demands, the development and implementation of more efficient power plants is crucial. This efficiency improvement is primarily achieved by increasing steam temperatures and pressures, necessitating the introduction of new materials capable of withstanding these extreme conditions. Nickel-based alloys emerge as prime candidates for high-temperature and high-pressure applications, offering significant creep strength and the ability to operate at metal temperatures above 750°C. This research focuses specifically on steam header and pipework systems, which are critical components carrying steam from boilers to turbines under severe operating conditions. The study emphasizes the importance of selecting suitable materials for these components and developing methodologies to predict their safe operating lifetimes, thereby ensuring the reliable and efficient operation of next-generation power plants.

700°C advanced ultra-supercritical system and supercritical CO 2 turbine system are developed for high efficiency turbine systems for next generation. This study covered the feasibility of creep life assessment of γ’-Ni 3 (Al,Ti) precipitation strengthened Ni-based superalloy rotor material, TOS1X-2, a modified alloy of UNS N06617 for these systems, based on hardness measurement method. It was found that the hardness of TOS1X-2 was governed by the change in precipitation strengthening and strain hardening during creep. The clear relationship between hardness increase in crept portion and macroscopic creep strain was observed, suggesting that it might be possible to estimate the creep strain or initiation of acceleration from hardness measurement. Microstructure inhomogeneity and microstructure evolutions during creep especially focused on dispersion of creep strain were characterized by EBSD quantitative analysis. It was found that creep strain was accumulated along the grain boundary, while it was relatively absent in coarse grains with low Schmid factor of {111} <110> slip system in fcc structure. The upper limit of hardness scatter band is thought to be important, since it represents the local and critical creep damage of the alloy.

In this study, 25Cr2Ni2Mo1V filler metal was deposited to weld low pressure steam turbine shafts, which are operated in fossil power plants. A comparison experiment was conducted on the weld metals (WMs) before and after varied various aging duration from 200 hours up to 5000 hours at 350 ℃. Microstructure was characterized by means of scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) techniques. In addition, mechanical properties of corresponding specimens were evaluated, e.g. Vickers microhardness, Charpy V impact toughness and tensile strength. It is shown that the tensile strength remained stable while impact energy value decreased with increasing aging duration. Based on the experiment above, it was concluded that the variation of mechanical properties can be attributed to the redissolution of carbides and reduction of bainite lath substructure.