Search Results for localization

Main steam control valves are crucial components in power plants, as they are the final elements in the steam piping system before the steam enters the turbine. If any parts of these valves become damaged, they can severely harm the steam turbines. Recently, power plants have been required to operate under cyclical loading, which increases the risk of cracks in the control valve seats. This is due to the different rates of expansion between the Stellite surface and the underlying Grade 91 steel surface when exposed to high temperatures. To ensure a reliable power supply, power plants cannot afford long downtimes, making on-site service essential. This paper presents an on-site technique for post-weld heat treatment (PWHT) of Stellite seats. By using a heating pad arrangement and an induction heater, the required PWHT temperature of 740°C, as specified in the welding specification procedure (WPS), can be achieved. This method allows for on-site valve seat repair and can be applied to other power plants as well.

Local vacuum electron beam welding is an advanced manufacturing technology which has been investigated at Sheffield Forgemasters to develop as part of a cost-effective, reliable, agile, and robust manufacturing route for the next generation of civil nuclear reactors in the UK. A dedicated electron beam welding facility at Sheffield Forgemasters has been installed. This includes an x-ray enclosure, 100kW diode electron gun, 100T turntable, and weld parameter development vacuum chamber. A small modular reactor demonstrator vessel has successfully been manufactured with a wall thickness of 180 mm, including indication-free slope-in, steady- state and slope-out welding parameters. Electroslag strip cladding has also been investigated to demonstrate its viability in reactor pressure vessel manufacture. The electro-slag strip cladding method has been shown to produce high quality 60 mm strips on a 2600 mm inner diameter ring.

The localized creep failure in the heat-affected zone (HAZ) of Grade 91 steel weldments has been identified as one of the most important factors causing significantly shortened service lifetime and structural integrity issues of welded components in advanced fossil and nuclear power plants. To conduct a reliable creep lifetime assessment, a new engineering assessment approach has been developed by incorporating the experimentally determined local properties of the heterogeneous HAZ. By creep testing a purposely simulated HAZ specimen with in situ digital image correlation (DIC) technique, the highly gradient creep properties across the HAZ of Grade 91 steel was quantitatively measured. A physical creep cavitation constitutive model was proposed to investigate the local creep deformation and damage accumulation within the heterogeneous HAZ, which takes into account the nucleation of creep cavities and their growth by both grain boundary diffusion and creep deformation. The relationship among the local material property, creep strain accumulation, and evolution characteristic of creep cavities was established. The approach was then utilized to investigate the creep response and subsequent life for an ex-service 9% Cr steel weldment by incorporating the effects of pre-existing damages which developed and accumulated during long-term services. The predicted results exhibited quantitative agreement with the DIC measurement in terms of both nominal/local creep deformation as well as the subsequent life under the test conditions at 650 and 80 MPa.

The morphological evolution of secondary γ′ precipitates under the coarsening process was investigated for commercial wrought Ni-based superalloys, which can be classified into two processes, i.e. “localization process” and “aggregation process”. The localization process was defined as a phenomenon in which cuboidal γ′ precipitates were arranged in the <100> direction for superalloys. In contrast, the aggregation process was defined as a phenomenon in which neighboring spherical γ′ precipitates coarsen while overlapping their interfaces for superalloys. All the wrought Ni-based superalloys could be classified into the above two processes based on their volume fraction and lattice misfit. The coarsening of γ′ precipitates follow the aggregation process when the misfit is smaller than 0.05%, and it follows the localization process otherwise.

The current study proposed a new method that utilizes digital image correlation (DIC) techniques to measure in-situ full field strain maps of creep resistant material welds. The stress-rupture test is performed in a Gleeble thermal mechanical simulator. This technique successfully captured a significant difference in the local creep deformation between two Grade 91 steel welds with different pre-welding conditions (standard and non-standard). Strain contour plots exhibited inhomogeneous deformation in the weldments, especially at the heat-affected zone (HAZ). Standard heat-treated specimens had significant creep deformation in the HAZ. On the other hand, non-standard heat treated specimens showed HAZ local strains to be 4.5 times less than that of the standard condition, after a 90-hour creep test at 650°C and 70 MPa. The present study measured the full field strain evolution in the weldments during creep deformation for the first time. The proposed method demonstrated a potential advantage to evaluate local creep deformation in the weldments of any creep resistant material within relatively short periods of time.

In the present study, the Inconel 617B superalloy welded trial rotor was fabricated by narrow gap tungsten inert gas (NG-TIG) welding and the effects of temperature on fracture toughness of its welded joint were investigated at 650 ℃ and 730 ℃. Fracture toughness (J0.2) of the base metal was much higher than that of the weld metal at the same temperature, which was attributed to its excellent macroscopical plasticity and the interactions of strain localization, misorientation, and coincidence site lattice (CSL) boundaries. For the base metal, the value of J0.2 was higher at 730 ℃ than at 650 ℃, resulting from the appreciable increase in ductility and decrease in strain localization as the temperature increased. For the weld metal, higher temperature (730 ℃) reduced strength but hardly improved plasticity, and the regions of high strain localization uniformly distributed in the weld metal, resulting in completely tearing the whole interface apart and lower fracture toughness of the weld metal.

Material properties and damage mechanisms exhibit significant variation across weldments. Micro tensile (MT) testing of specimens machined from specific narrow weldment zones is one method to characterize local property variation. Although limited, the literature data on micro-tensile specimen testing reports on low-temperature behavior. However, cross-weld local material data at high service temperatures have not been reported yet. In the present study, MT tests are conducted across similar P22 and P91 steel welds at 550°C and 600°C, respectively. To study deformation mechanisms and the role of surface condition on properties, specimens with different surface conditions (machined, polished, and electropolished) are tested. Two different loading rates of 0.2 mm/min and 0.5 mm/min are used to study the effect of loading rate on deformation and mechanical properties. Variations in weldment material properties are presented as a function of specimen surface conditions and loading speeds. Deformation behavior is studied on the side surfaces of tested micro-tensile specimens using SEM. Deformation is correlated to the microstructural constituent observed on side surfaces.

Tensile deformation behavior of γ-TiAl based alloys consisting of α 2 -Ti 3 Al/γ lamellar colonies, β-Ti grains, and γ grains were investigated by in-situ scanning electron microscopy and digital image correlation technique, in order to identify the role of each microstructure constituents in deformation. The alloy with nearly lamellar microstructure, in which the volume fraction of β/γ duplex ( V DP ) is 10%, shows elongation of only 0.14%, whereas the alloy with nearly globular β/γ duplex microstructure with V DP of 94% shows elongation of 0.49%. In α 2 /γ lamellar microstructure, obvious strain localization occurs along lamellae and develops at specific regions with loading. In the case of β/γ duplex microstructure, strain localization is observed in γ grains and in β phase regions near the β/γ phase boundary, although no obvious deformation is observed in the β grains. β/γ phase boundaries enhances room temperature ductility of TiAl alloys by inducing multiple slip in γ phase and deformation of β phase.

In this work, two unique heats of 9Cr creep strength enhanced ferritic (CSEF) steels extracted from a retired superheat outlet header after 141,000 hours of service were evaluated. These two CSEF steels were a forging manufactured to SA-182 F91 (F91) reducer and a seamless pipe produced to SA-335 P91 (P91) pipe. Their creep deformation and fracture behavior were assessed using a lever arm creep frame integrated with in-situ high-temperature digital image correlation (DIC) system. Critical metallurgical and microstructure factors, including composition, service damage, grain matrix degradation, precipitates, and inclusions were quantitatively characterized to link the performance of the two service aged F91 and P91 CSEF steels. The creep test results show the F91 and P91 steels exhibit a large variation in creep strength and creep ductility. The F91 steel fractured at 572 hours while P91 steel fractured at 1,901 hours when subjected to a test condition of 650 °C and 100 MPa. The nominal creep strains at fracture were 12.5% (F91) and 14.5% (P91), respectively. The high-resolution DIC strain measurements reveal the local creep strain in F91 was about 50% while the local creep strain in P91 was >80%. The characterization results show that the F91 steel possessed pre-existing creep damage from its time in service, a higher fraction of inclusions, and a faster matrix grain coarsening rate. These features contribute to the observed reduction in performance for the F91 steel. The context for these findings, and the importance of metallurgical risk in an integrated life management approach will be emphasized.

Fossil fuels continue to be the primary source of energy in the U.S and worldwide. In order to improve the efficiency of fossil power plants, advanced structural materials need to be developed and deployed to meet the need of high temperature creep resistance and corrosion resistance. Examples include creep strength enhanced ferritic (CSEF) steels, austenitic stainless steels, nickel-based superalloys, and oxide dispersion strengthened alloys. Welding is extensively used in construction of fossil power plants. The performance of the weld region can be critical to the safe and economical operation of fossil power plants. Degradations in performance such as reduced creep strength and premature failure in the weld region (e.g. Type IV failure in ferritic steels) are examples of longstanding welding and weldability problems for boiler and other components. In the past, extensive studies have been carried out to characterize the different microstructures in different regions of a weld, and to a certain extent, to establish the correlations between the microstructure and the creep strength. However, the metallurgical or microstructural induced local stress/strain variations have been seldom quantified. In addition, it has been long recognized that, due to the sharp microstructure and property gradients in the weld and HAZ, the standard creep testing procedure for the base metal can produce erroneous results when used for weld testing.

Increasing demand on efficiency and power output of steam generators leads to new designs of welded rotors. The reason for rotor welding is the large size of rotors, which are difficult to produce in a single piece. Secondly, as there are varying operation conditions along the rotor length. In a heterogeneous rotor, several materials appropriate for local service conditions can be used. At the rotor service temperatures, creep properties are crucial for successful design. The weakest point of every welded component is the heat affected zone. Therefore, the creep properties of a heterogeneous weld are subject of the investigation herein the current study, a heterogeneous weld of COST F and COST FB2 materials is investigated. The welding was performed by multi pass technique with overlaying welding beads that applied several heating cycles to heat affected zone. Metallographic investigation of the weld was performed and the weakest microstructure spots were detected. With the use of FEM simulation, appropriate heating/cooling cycles were obtained for the detected weak points. The temperature cycles obtained were subsequently applied to both base materials under laboratory conditions by induction heating. Creep properties of these materials were investigated. The influence of the initial base material’s grain size was also considered in the investigation. Two heating/cooling schedules were applied to both base materials with two grain sizes. Altogether, 8 different microstructures were examined in short term creep tests and the results were summarized.

As the demand for worldwide electricity generation grows, pulverized coal steam generator technology is expected to be a key element in meeting the needs of the utility power generation market. The reduction of greenhouse gas emissions, especially CO 2 emissions, is vital to the continued success of coal-fired power generation in a marketplace that is expected to demand near-zero emissions in the near future. Oxycombustion is a technology option that uses pure oxygen, and recycled flue gas, to fire the coal. As a result, this system eliminates the introduction of nitrogen, which enters the combustion process in the air, and produces a highly-concentrated stream of CO 2 that can readily be captured and sequestered at a lower cost than competing post-combustion capture technologies. Oxycombustion can be applied to a variety of coal-fired technologies, including supercritical and ultra-supercritical pulverized coal boilers. The incorporation of oxycombustion technology in these systems raises some new technical challenges, especially in the area of advanced boiler materials. Local microclimates generated near and at the metal interface will influence and ultimately govern corrosion. In addition, the fireside corrosion rates of the boiler tube materials may be increased under high concentration oxygen firing, due to hotter burning coal particles and higher concentrations of SO 2 , H 2 S, HCl and ash alkali, etc. There is also potential to experience new fouling characteristics in the superheater and heat recovery sections of the steam generator. The continuous recirculation of the flue gases in the boiler, may lead to increasing concentrations of deleterious elements such as sulfur, chlorine, and moisture. This paper identifies the materials considerations of oxycombustion supercritical and ultrasupercritical pulverized coal plants that must be addressed for an oxycombustion power plant design.

Cast nickel-based superalloys used as structural materials for gas turbine parts need to withstand high temperatures and dynamic mechanical loads. When in contact with ambient air, the formation of protective oxide scales causes a depletion of γ’-precipitates in the surface-near region and leaves a weakened microstructure. This environmentally based degradation of the material might be accelerated under cyclic thermal exposure. In this paper, the cyclic oxidation behavior of two cast nickel-based superalloys and one single crystalline variant are investigated: C1023, CM-247 LC and M-247 SX. Exposure tests were carried out under both isothermal and cyclic conditions in air at 850 °C, 950 °C and 1050 °C for times up to 120 h to investigate the impact of thermal cycling. The differences in oxidation mechanisms are analyzed phenomenologically via light and electron microscopy and brought in correlation with the oxidation kinetics, determined based on net mass change and depletion zone growth. An assessment of the impact of precipitation loss on local mechanical strength is attempted via nano-indentation method. The found relations can be transferred onto an acceleration of crack growth under creep-fatigue and thermo-mechanical fatigue conditions.

The behavior of strain-induced abnormal grain growth (AGG) in superalloy 718 has been investigated using compression testing and subsequent heat treatment below the d-phase solvus temperature of 980 °C. The nuclei of AGG grains were slightly newly recrystallized grains by a nucleation because small grains without dislocation was observed in the as- deformed microstructure. AGG was caused by the difference in intragranular misorientation (related to the stored strain energy in a grain) between dynamic recrystallized grains and deformed matrix. The initiation of AGG was retarded with decreasing plastic strain and produced microstructures consisted of larger grains having more complex morphology. It was observed that grain boundary migrated locally in the direction perpendicular to, or mainly in the direction parallel to the S3 {111} twin boundaries along with the formation of high-order twins. As a result of multiple twinning, AGG grains seemed to evolve with the growing directions changed.

Modern gas turbines are operated with fuels that are very clean and within the allowances permitted by fuel specifications. However, the fuels that are being considered contain vanadium, sulfur, sodium and calcium species that could significantly contribute to the degradation of components in hot gas flow path. The main potential risk of material degradation from these fuels is “hot corrosion” due to the contaminants listed above combined with alkali metal salts from ambient air. Depending on the temperature regime hot corrosion can damage both TBC coatings and bond coat/substrate materials. Deposit-induced or hot corrosion has been defined as “accelerated oxidation of materials at elevated temperatures induced by a thin film of fused salt deposit”. For the initiation of hot corrosion, deposition of the corrosive species, e.g. vanadates or sulfates, is necessary. In addition to the thermodynamic stability, the condensation of the corrosive species on the blade/vane material is necessary to first initiate and then propagate hot corrosion. Operating temperatures and pressures both influence the hot corrosion damage. The temperature ranges over which the hot corrosion occurs depend strongly on following three factors: deposit chemistry, gas constituents and metal alloy (or bond coating/thermal barrier coating) composition. This paper reports the activities involved in establishing modeling and simulation followed by testing/characterization methodologies in relevant environments to understand the degradation mechanisms essential to assess the localized risk for fuel flexible operation. An assessment of component operating conditions and gas compositions throughout the hot gas paths of the gas turbines, along with statistical materials performance evaluations of metal losses for particular materials and exposure conditions, are being combined to develop and validate life prediction methods to assess component integrity and deposition/oxidation/corrosion kinetics.

The oxide exfoliation is one of the main problems that cause the explosion of superheater or reheater, which threaten the safety of power plant units, but there is no direct test method of the particle concentration of the scales in high temperature steam. Based on the study of ferromagnetic and optical characteristics of scales, the technology and equipment were developed for on-line measurement based on magnetic sensitivity and granularity behavior. Through numerical simulation and dynamic simulation experiments of scale movement under high temperature and high pressure steam, calculating method of the particle concertation of scales in the main steam or reheated steam pipeline was retrieved by local sampling concentration.

In a European ultra-supercritical (USC) power station repaired reheater bundle tubes made out of 25% Chromium stainless steels developed stress relief damages at the tube-to-tube butt welds, leading to leakages after only 8.500 hours of operation. Laboratory investigations of the leakages revealed common features of stress relief cracking (SRC) such as highly localized intergranular cracking in the heat affected zone (HAZ) near the fusion line, creep void formation at the crack tip and around the crack. At that time no other SRC damages were known for the employed 25% Chromium stainless steel boiler tubes. This article briefly describes the SRC damage found on the repaired reheater bundle tubes. It further provides insight on the several laboratory tests employed to assess the SRC behavior of welded joints of different creep resistant stainless steels. Among the selected test methods were Slow-Strain-Rate-Tests (SSRT), static 3-point bending tests derived from the Van Wortel approach and component tests. The results provided by the described tests methods have shown that the SRC behavior of a given material combination must be assessed by different techniques. This is especially the case for the evaluation of potential countermeasures and for the determination of the service conditions leading to the highest susceptibility.

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.

Metallurgical factors affecting the fusion boundary failure and damage mechanism of DMWs (Dissimilar Metal Welds) between the CSEF (Creep Strength Enhanced Ferritic) steels and austenitic steels were experimentally and theoretically investigated and discussed. Long-term exservice DMWs up to 123,000 hours were investigated; the precipitates near the fusion boundary were identified and quantitatively evaluated. Comparing with the other generic Ni-based weld material, MHPS original filler metal HIG370 (Ni bal.-16Cr-8Fe-2Nb-1Mo) showed superior suppression effect on fusion boundary damage of DMWs, which was verified by both of the microstructure observation and thermodynamic calculation. Based on the microstructure observation of crept specimen and ex-service samples of DMWs, temperature, time and stress dependence of fusion boundary damage of DMWs were clarified. Furthermore, fusion boundary damage morphology and mechanism due to precipitation and local constituent depletion was discussed and proposed from metallurgical viewpoints.

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.