Effect Of Long-term Thermal Aging Research Articles

Effect of long-term thermal aging on lead-bismuth eutectic corrosion behavior of 9Cr ferritic/martensitic steel

In lead-cooled fast reactor (LFR) systems, the liquid lead-bismuth eutectic (LBE) coolant provides a corrosive environment that damages the steel components during high-temperature operation. This study investigated the microstructural deterioration of 9Cr ferritic/martensitic (F/M) steel under thermal aging at 550 °C for 2,000, 10,000, or 20,000 h and its effect on oxidation corrosion in an LBE environment using multiscale characterization techniques. The results indicated that the thickness of the internal oxidation zone (IOZ) increased significantly with extended thermal aging, whereas that of the spinel layer remained relatively constant. The abundant subgrain boundaries that emerged during extensive thermal aging facilitated Fe diffusion, and the enlarged Cr-rich M23C6 carbides contributed to the formation of preferential oxidation regions, accelerating IOZ layer growth. The spinel layer formed from the IOZ was influenced by microstructural defects within the IOZ. A theoretical model describing the accelerated oxide layer growth due to thermal aging was developed. These findings support the advancement of LFR technology.

Effects of thermal aging at 873 K on the impact properties of an ODS ferritic steel

This investigation presents the effects of long-term thermal aging on the impact properties of an ODS reduced activation ferritic steel. The ODS ferritic steel, with composition Fe–14Cr–2W–0.4Ti–0.3Y2O3 (wt. %), was manufactured by mechanical alloying, compacted by hot isostatic pressing and hot cross rolled. A 1273 K annealing treatment was applied, followed by thermal aging at 873 K for 2000 h in Ar, to evaluate the thermal stability of the alloy. Charpy impact testing was performed from 77 to 473 K to investigate the mechanical response of the material under dynamic loads. The ductile-to-brittle transition temperature (DBTT) showed a slight increase with values of 255 ± 12 K before aging, and 270 ± 20 K after aging. The aging treatment softened the steep character of the Charpy impact curves inducing a wider transition regime with mixed brittle and ductile fracture mechanisms. This behavior was confirmed by the analysis of Force vs. Displacement Charpy curves and fractographic observations. Hot cross rolling induced a crack divider geometry and secondary cracks formation by delamination. Ductile fracture was less evident after aging due to the redistribution and transformation of Cr-W-rich precipitates along grain boundaries. Electron Backscattered Diffraction highlighted the features of transgranular brittle cracks at temperatures below the DBTT and showed misoriented and deformed regions near the fracture surface at temperatures above the DBTT.

Effect of Long-Term Thermal Aging at 680 °C on Microstructure and Mechanical Properties of P92 Steel

Effect of Long-Term Thermal Aging at 680 °C on Microstructure and Mechanical Properties of P92 Steel

Effects of long-term thermal aging on the microstructure and mechanical behaviors of 16MND5/Alloy 152 dissimilar metal weld

The effect of thermal aging on microstructural evolution and mechanical behavior was investigated via advanced instrumental analysis. The scanning electron microscope (SEM) results demonstrated that the microstructure transformed from martensite into martensite/bainite mixture in the coarse-grained heat-affected zone (CGHAZ) of 16MND5. The micro-hardness in the CGHAZ has a significantly positive correlation with thermal aging time owing to the change of microstructure and the increased number of M23C6 precipitates. Moreover, the tensile strength and elongation of the dissimilar metal welds have a positive correlation with thermal aging time. However, the fracture occurs on the Alloy 152 side and the fracture mode was the ductile fracture.

Characterization of Yb2SiO5-based environmental barrier coating prepared by plasma spray–physical vapor deposition

Due to the high-input power compared to atmospheric plasma spraying (APS), plasma spray-physical vapor deposition (PS-PVD) can primarily achieve a splat-like deposition, allowing for the preparation of high-density environmental barrier coatings (EBCs). In this paper, dense Yb2SiO5-based coatings are prepared by PS-PVD at different substrate temperatures. It was found that the coating deposited at the substrate temperature of 700 °C contained a large amount of silicon-rich amorphous phase. When the substrate temperature increased to 1100 °C and a slow cooling process after deposition was involved, a coating with high crystallinity of ∼77% and low porosity of less than ∼2% was achieved. Phase evolution of the coatings was studied by a semi-in-situ high-temperature X-ray diffractometer. During the heating process, metastable phases X1-Yb2SiO5 and α-Yb2Si2O7 emerged and transformed into stable phases following high-temperature treatment. Furthermore, the effects of long-term thermal aging at 1300 °C on the microstructure, phase composition, thermal conductivity, and hardness of the coating prepared at the substrate temperature of 1100 °C were found to be limited.

Effects of long-term thermal aging on elevated temperature deformation behaviors of wrought 316LN stainless steel by small punch test

The deformation behaviors of as-received and thermal aged wrought 316LN austenitic stainless steel (ASS) were investigated at 350 °C by small punch test (SPT) in order to evaluate the effects of long-term thermal aging on the elevated temperature deformation behaviors of the wrought ASS. The results showed that long-term thermal aging at 400 °C had obvious influences on the density and structure of dislocations, hence affecting the deformation mechanism and mechanical behaviors. There were many planar slip bands and tangled dislocations in the as-received wrought 316LN ASS, while, a low-density dislocation multiples, coplanar dislocation arrays, stacking faults and extended dislocations were formed due to the microstructure recovery during the long-term thermal aging. Correspondingly, both dislocations slip and deformation twinning played an important role in plastic deformation in the as-received wrought 316LN ASS, while interaction of dislocations and stacking faults inclined to dominate the plastic deformation process of the thermal aged 316LN ASS under the elevated temperature. As a result, long-term thermal aging resulted in the change of the mechanical properties of the wrought 316LN ASS. The hardness and SPT maximum load of 316LN ASS had a slight increase, while the SPT total energy exhibited a reverse trend with increasing aging time, indicating that the 316LN ASS showed a tendency of hardening after the long-term thermal aging.

A comprehensive study of the effects of long-term thermal aging on the fracture resistance of cast austenitic stainless steels

Loss of fracture resistance due to thermal aging degradation is a potential limiting factor affecting the long-term (80+ year) viability of nuclear reactors. To evaluate the effects of decades of aging in a practical time frame, accelerated aging must be employed prior to mechanical characterization. In this study, a variety of chemically and microstructurally diverse austenitic stainless steels were aged between 0 and 30,000 h at 290–400 °C to simulate 0–80+ years of operation. Over 600 static fracture tests were carried out between room temperature and 400 °C. The results presented include selected J-R curves of each material as well as K0.2mm fracture toughness values mapped against aging condition and ferrite content in order to display any trends related to those variables. Results regarding differences in processing, optimal ferrite content under light aging, and the relationship between test temperature and Mo content were observed. Overall, it was found that both the ferrite volume fraction and molybdenum content had significant effects on thermal degradation susceptibility. It was determined that materials with >25 vol% ferrite are unlikely to be viable for 80 years, particularly if they have high Mo contents (>2 wt%), while materials less than 15 vol% ferrite are viable regardless of Mo content.

Degradation of impact toughness in cast stainless steels during long-term thermal aging

Cast austenitic stainless steels (CASSs) have been extensively used for the large components of nuclear reactor primary coolant systems. Since the cast steels inevitably contain degradable metastable phases and replacement of the large coolant system components is impractical, the thermal embrittlement of CASS components has been a serious concern in the extended-term operation of nuclear power plants. This study aimed to systematically measure and analyze the effect of long-term thermal aging on the Charpy impact toughness to provide a comprehensive understanding of thermal degradation behavior and a practical aging model to predict the degree of thermal degradation in the cast stainless steels. The materials tested in the research include eight CASS alloys (two CF3s, one CF3M, three CF8s, and two CF8Ms) and two reference wrought materials (304L and 316L), in which the nominal δ-ferrite content ranges from ~2% to 33%. These stainless steels have been thermally aged at two light water reactor (LWR) temperatures (290 and 330 °C) and at two accelerated-aging temperatures (360 and 400 °C) for up to 30,000 h; these include both under-aged and over-aged conditions relative to the extended service lifetime (80 years). Charpy impact testing was performed for aged and non-aged specimens, and the impact (absorbed) energy parameters were correlated with a new aging parameter (A). Both the reduction of impact fracture toughness and the shift of ductile-brittle transition temperature were strongly dependent on the δ-ferrite content and degree of thermal aging. A linear relationship was found between the increasing rate of the index transition temperature T41J and aging parameter A; base on which an empirical model was proposed for prediction of the transition temperature as a simple function of the aging parameter (A) and δ-ferrite content (F). Finally, the critical aging parameter for embrittlement (AC) was evaluated and compared with the existing δ-ferrite content criteria.

Effect of long-term thermal aging in heat exchange equipment of fast neutron switchgears on the structure and properties of austenitic chromium-nickel steel

The influence of long-term operation at 515 °C on structure and properties of 09Cr18Ni9 steel was investigated. Structure and phase composition were obtained using optical and scanning electron microscopy. The phase composition of the steel in equilibrium state was determined by thermodynamic modeling in the software package Fact-Sage. As a result of the study, it was found that during the operation at 515 °C with a duration of 195,000 h, the structure changes occurred in the 09Cr18Ni9 steel with the formation of secondary phases, initiated by the release of elements with limited solubility from the supersaturated solid solution. The following secondary precipitates in structure of the solid solution of austenite presented: Cr23C6 chromium carbide, ferrite (a), G-phase. Based on comparison of the thermodynamic modeling results and on experimental determination of the phase composition, it was established that the steel structure is in a state close to equilibrium. The mechanism of structural transformations course and sequence of the secondary phases’ formation were revealed and described. At the initial stage, chromium carbide is formed, then a-ferrite is formed near the carbides, and then G-phase is formed. Results of the tests for impact strength and static elongation have shown that the change in phase composition in process of thermal aging leads to embrittlement of the steel - a reduction in ductility and impact energy. Fractografic studies of fracture surfaces of the samples have shown that the decrease in plasticity during long-term high-temperature operation is associated with softening of the grain body and strengthening of the boundaries due to secondary precipitations of the carbide phase. As a result of this process, plastic deformation is localized in the weakened volume of the body of grain surrounded by strong boundaries. The structure evolution during prolonged heat aging has the greatest effect on impact strength. At the same time, the change in ultimate and yield stress is insignificant. The main contribution to the change in mechanical characteristics of steel is made by the secondary precipitates of the carbide phase.

Effects of long-term thermal aging on bulk and local mechanical behavior of ferritic-austenitic duplex stainless steels

Ferritic-austenitic cast duplex stainless steels (CDSS) undergo significant thermal aging embrittlement in service in light water nuclear reactors. Cast CF–3 and CF–8 duplex stainless steels have been laboratory aged for up to 17,200 h at four different temperatures (280 °C, 320 °C, 360 °C, 400 °C) in order to characterize the evolution of microstructure and mechanical properties. Standard Charpy V-Notch (CVN) impact testing and tensile testing have been conducted to measure the progression of embrittlement of the steels. Concurrently, nanoindentation measurements were performed to measure the hardening of the individual ferrite and austenite phases. Atom probe tomography (APT) analysis of the constituent phases is used to investigate the relationship between spinodal decomposition of the ferrite and the mechanical property evolution during aging. The effects of the local microstructure and property changes in the ferrite phase on the overall bulk mechanical property degradation of these steels are discussed.

Effect of long-term thermal aging on magnetic hysteresis for low-alloy pressure vessel steel

Long-term thermal aging experiments have been performed at 290 and 550°C up to ∼ 10000 h for low-alloy pressure vessel steels with low and high Ni contents, in order to elucidate effects of thermal embrittlement on a structure-sensitive property of minor B-H loops. While a minor-loop property obtained from a power-law scaling of minor-loop parameters exhibits a small but steady increase after aging at 290°C, that exhibits a decreasing trend after aging at 550°C. The observations suggest that the evolution of nanometer precipitates plays a crucial role for a magnetic property change at 290°C, while relaxation of lattice strain in a matrix associated with precipitation may dominate at 550°C.

Thermal Aging Effect on the Joint Strength between an SOFC Glass-Ceramic Sealant and LSM-Coated Metallic Interconnect

Lanthanum strontium manganite (LSM) has been practically coated on the metallic interconnect to prevent Cr poisoning in SOFC. The aim of this study is to investigate the joint strength between a glass-ceramic sealant and an LSM-coated interconnect steel at room temperature (RT) and 800 °C. Effect of long-term thermal aging is also characterized. Results indicate that LSM-coated specimens have a lower joint strength than the uncoated ones. It is attributable to the existence of pores around the interface between glass-ceramic sealant and LSM coating. However, the shear strength of LSM-coated joint specimen is enhanced by 52% at RT and by 200% at 800 °C after 1000-h thermal aging at 800 °C. The given thermal aging treatment also improves the tensile strength of LSM-coated joint specimen by 50% at 800 °C. It is attributed to a self-healing effect of the glass-ceramic sealant during thermal aging to reduce the pore size.

Characterization of Plastic Deformation Behavior of a Thermally Aged Duplex Stainless Steel

In situ tensile tests at room temperature have been conducted on a duplex stainless steel (DSS) thermally aged at 400 °C for 10,000 h to investigate both the plastic deformation mechanisms and the effect of long-term thermal aging on crack initiation. After thermal aging, the ultimate tensile strength of the DSS increases and the plasticity significantly decreases. The fracture morphology changes from ductile fracture with shallow dimples to a mixture of cleavages in ferrite and tearing in austenite. Electron backscattered diffraction (EBSD) technique has been used to determine the crystallographic orientations of austenite and ferrite grains on three areas deformed differently. The EBSD analysis results indicate that high strain occurs near grain boundaries and phase boundaries. The localized strain incompatibility is considered to be responsible for high stress concentration and crack initiation. The long-term thermal aging affect on the crack initiation and the cleavage cracks are found to be initiated in aged ferrite grains.

Investigation of effects of long-term thermal aging on magnetization process in low-alloy pressure vessel steels using first-order-reversal-curves

We have investigated effects of long-term thermal aging at 550°C up to 10000 h on major-loop coercivity, hysteresis scaling of minor loops, and first-order reversal curves (FORCs) for low-alloy pressure vessel steels with low and high Ni contents. While major-loop coercivity and minor-loop coefficient of the scaling exhibit a gradual decrease with aging for high-Ni steel, those for low-Ni one are very weakly dependent on aging time. On the other hand, we found that FORC distribution becomes steep along both axes of interaction and switching fields and the peak shifts toward a lower switching field for both steels. Considering that there is no significant development of nanoscale precipitates during the aging as revealed with small-angle neutron scattering experiments, a relaxation of lattice strain in a matrix, possibly associated with diffusion of Ni atoms, may dominate magnetic properties at 550°C.

니켈계 합금 용접부의 미세조직 및 기계적 특성에 대한 장기 열적 시효의 영향

To investigate the effect of long-term thermal aging on the microstructural and mechanical characteristics of weldment made of nickel base alloy and its weld metal, an accelerated heat treatment was applied to simulate the process of long-term thermal aging in the operating condition of nuclear power plant. A representative nickel-based weldment with Alloy 600 and Alloy 182 was fabricated and heat-treated at <TEX>$400^{\circ}C$</TEX> for 1,713 h and 3,427 h to simulate the thermal aging for the period equivalent to 15 and 30 years in operating pressurized water reactors, respectively. The microstructural and mechanical characteristics were analyzed by using optical microscopy, scanning electron microscopy and Vickers microhardness measurement. Changes were observed in precipitation behavior and microhardness of each specimen, and these changes were mainly attributed to the change in precipitated morphology and residual stress across the weld during the thermal aging process.

Microstructural evolution and stress-corrosion-cracking behavior of thermally aged Ni-Cr-Fe alloy

To understand the effect of long-term thermal aging in power plant systems, representative thick-walled Alloy 600 was prepared and thermally aged at 400°C to fabricate samples with thermal aging effects similar to service operating conditions. Changes of microstructures, mechanical properties, and stress corrosion cracking susceptibility were investigated mainly through electron backscatter diffraction, nanoindentation, and high-temperature slow strain rate test. The formation of abundant semi-continuous precipitates with chromium depletion at grain boundaries was observed after thermally aged for 10 equivalent years. Also, alloys thermally aged for 10 equivalent years of thermal aging exhibited the highest susceptibility to stress corrosion cracking.

Fusion boundary precipitation in thermally aged dissimilar metal welds studied by atom probe tomography and nanoindentation

In this study, microstructural and mechanical characterizations were performed to investigate the effect of long-term thermal aging on the fusion boundary region between low-alloy steel and Nickel-based weld metal in dissimilar metal welds used in operating power plant systems. The effects of thermal aging treatment on the low-alloy steel side near the fusion boundary were an increase in the ratio of Cr constituents and Cr-rich precipitates and the formation and growth of Cr23C6. Cr concentrations were calculated using atom probe tomography. The accuracy of simulations of thermal aging effects of heat treatment was verified, and the activation energy for Cr diffusion in the fusion boundary region was calculated. The mechanical properties of fusion boundary region changed based on the distribution of Cr-rich precipitates, where the material initially hardened with the formation of Cr-rich precipitates and then softened because of the reduction of residual strain or coarsening of Cr-rich precipitates.

Effects of long-term thermal aging on the stress corrosion cracking behavior of cast austenitic stainless steels in simulated PWR primary water

The stress corrosion cracking (SCC) behavior of cast austenitic stainless steels of unaged and thermally aged at 400 °C for as long as 20,000 h were studied by using a slow strain rate testing (SSRT) system. Spinodal decomposition in ferrite during thermal aging leads to hardening in ferrite and embrittlement of the SSRT specimen. Plastic deformation and thermal aging degree have a great influence on the oxidation rate of the studied material in simulated PWR primary water environments. In the SCC regions of the aged SSRT specimen, the surface cracks, formed by the brittle fracture of ferrite phases, are the possible locations for SCC. In the non-SCC regions, brittle fracture of ferrite phases also occurs because of the effect of thermal aging embrittlement.

Effects of thermal aging on the microstructure of Type-II boundaries in dissimilar metal weld joints

In order to investigate the effects of long-term thermal aging on the microstructural evolution of Type-II boundary regions in the weld metal of Alloy 152, a representative dissimilar metal weld was fabricated from Alloy 690, Alloy 152, and A533 Gr.B. This mock-up was thermally aged at 450°C to accelerate the effects of thermal aging in a nuclear power plant operation condition (320°C). The microstructure of the Type-II boundary region of the weld root, which is parallel to and within 100μm of the fusion boundary and known to be more susceptible to material degradation, was then characterized after different aging times using a scanning electron microscope equipped with an energy dispersive X-ray spectroscope for micro-compositional analysis, electron backscattered diffraction detector for grain and grain boundary orientation analysis, and a nanoindenter for measurement of mechanical properties. Through this, it was found that a steep compositional gradient and high grain average misorientation is created in the narrow zone between the Type-II and fusion boundaries, while the concentration of chromium and number of low-angle grain boundaries increases with aging time. A high average hardness was also observed in the same region of the dissimilar metal welds, with hardness peaking with thermal aging simulating an operational time of 15years.

Effects of thermal aging on microstructures of low alloy steel–Ni base alloy dissimilar metal weld interfaces

In this study, the advanced instrumental analysis has been performed to investigate the effect of long-term thermal aging on the microstructural evolution in the fusion boundary region between weld metal and low alloy steel in dissimilar metal welds. A representative dissimilar weld mock-up made of Alloy 690-Alloy 152-A533 Gr. B was fabricated and aged at 450°C for 2750h. The micro- and nano-scale characterization were conducted mainly near in a weld root region by using optical microscopy, scanning electron microscopy, transmission electron microscopy, and three dimensional atom probe tomography. It was observed that the weld root was generally divided into several regions including dilution zone in the Ni-base alloy weld metal, fusion boundary, and heat-affected zone in the low alloy steel. A steep gradient was shown in the chemical composition profile across the interface between A533 Gr. B and Alloy 152. The precipitation of carbides was also observed along and near the fusion boundary of as-welded and aged dissimilar metal joints. It was also found that the precipitation of Cr carbides was enhanced by the thermal aging near the fusion boundary.