Long-term High-temperature Exposure Research Articles

Effect of long-term high-temperature exposure in a controlled atmosphere on mullite-corundum ceramics for advanced applications in nuclear power technology

This work investigates the behavior and properties of corundum and mullite ceramics at high temperatures and during very long exposures in a controlled atmosphere. The motivation of the work lies primarily in advanced applications in nuclear energy technology, specifically in the search for materials for helium-cooled reactors of the 4th generation. The studied ceramic samples Al63, Al70, Al76, Al96, Al100, where the number indicates the mass percentage of Al2O3, were exposed for 1000 h at a temperature of 900 °C in gaseous atmospheres of helium, nitrogen and air. The properties of the exposed and unexposed samples were evaluated by measuring the bending strength at room temperature (CMOR), hot modulus of rupture at high temperature (HMOR), and isothermal creep, BET method, and using electron microscopy. The color changes induced by the reducing atmosphere and temperature were also interpreted using a simple thermochemical model. The CMOR measurement results show that all ceramic samples do not significantly change their properties due to long-term exposure. The detected deviations in the exposed samples from the unexposed ones can be considered random caused by the inherent heterogeneous structure. A similar conclusion also applies in the case of the HMOR measurement results, except for the results for the Al63 sample, where the drop in the strength of the exposed samples by approx. 37 MPa (20 %) may be related with the highest proportion of polyvalent oxide admixtures of all studied ceramics, i.e. 1.01 wt % Fe2O3 and 0.52 wt % TiO2. The change in color indicates that the iron and titanium present are being reduced. However, none of the conducted experiments indicate that changes in the redox state associated with the formation of oxygen vacancies would significantly reduce strength at high temperatures or increase creep in mullite ceramics.

Microstructural evolution in the vicinity of Mo–W interfaces after long-term high-temperature exposure

Microstructural evolution in the vicinity of Mo–W interfaces after long-term high-temperature exposure

Long-term high-temperature exposure effects on mechanical properties and structure of the 42XNM alloy after neutron irradiation in the VVER-1000. Part 1. Mechanical tests

The paper presents the results of mechanical tests of ring specimens made of the 42XNM alloy after irradiation as part of the control and protection system of the VVER-1000 reactor to a damaging dose of ~12 dpa at a temperature of ~350°C and subsequent isothermal annealings in the temperature range of 400– 1150°C (heating and holding for ~2 h). A model was constructed and validated by the finite element method, it describes the mechanical characteristics of irradiated and non-irradiated samples from the 42XNM alloy during tests in the temperature range from 500 to 1000°C. The model was used to plot the temperature dependences of the maximum local plastic deformation and the yield strength of the material under study.

Long-term high-temperature exposure effects on mechanical properties and structure of the 42XNM alloy after neutron irradiation in the VVER-1000. Part 2. Structural studies

The paper presents the results of structural studies of ring specimens made of the 42XNM alloy after irradiation as part of the control and protection system of the VVER-1000 reactor to a damaging dose of ~12 dpa at a temperature of ~350°C and subsequent isothermal annealings in the temperature range of 400– 1150°C (heating and holding for ~2 h). It is shown that during long-term isothermal annealing, a change in the phase composition of the alloy is observed, dislocation structures and grain-boundary segregations are annealed, and porosity evolves. It has been confirmed that decrease in the plastic properties of the 42XNM alloy after irradiation and subsequent isothermal annealing in the temperature range of 400–1000°C could be explained by the formation of precipitates of the second phases (zones of discontinuous decomposition of the solid solution with the release of α-Cr particles along the grain boundaries) and pores at the grain boundaries.

The study of sigma and carbide in cast austenitic stainless-steel grade HH after 24 years of high-temperature service

Cast austenitic stainless steel (CASS) has been widely used for long-term service periods in high-temperature applications. Nonetheless, the behavior of CASS after high-temperature and long-term service is insufficiently researched alongside the microstructural analysis for sigma phase and carbide. Here, intermediate pipe support made from CASS grade HH was investigated after 24 years of service at 700 °C and compared with the solution-treated specimen at 1100 °C for 2 h. The chemical composition was analyzed by optical emission spectroscopy (OES), while the microstructure was observed using optical microscopy and scanning electron microscope (SEM). The confirmation of phase composition and lattice parameters were further analyzed by energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction spectroscopy (XRD). The mechanical properties were assessed by tensile tests, hardness tests, and impact tests, whereas the corrosion property was analyzed using potentiodynamic polarization. Based on experiment and analysis, the effects of 24 years of exposure on the sigma phase are spheroidization and Cr depletion, and even partial transformation to ferrite. The sigma phase significantly increases the hardness, but Cr depletion slightly reduces the hardness to 96.86 HRB. Meanwhile, the carbide will increase in quantity and develop an irregular interface at long-term high-temperature exposure. Microstructure evolution of the sigma phase and carbide decreased tensile strength to 46%, elongation to 3%, and impact value to 1.6% of the minimum specifications, respectively, while the corrosion rate increased 10 times (about 7.35 μm per year).

Heating appliance used to study vibration strength of the structures at a temperature near 1300 °C

Studying the vibration strength of various structures, parts and load-bearing elements used in aviation engineering, oil-and-gas and metallurgical industry at elevated temperatures suggests using of special heating appliances. We present the heating appliance developed to study the vibration strength of the structures used as part of vibration stands capable of sustaining the temperature mode close to 1300°C during long periods of time. A framed structure of the heating appliance is presented, having neither bottom, side or end walls nor cover, all of them substituted with a roll heat-retaining material. The frame design of the heating appliance provides for rather quick manufacture of a frame of the desired dimensions. The use of heat-retaining material makes it possible to remove temperature sensors from the heating appliance, as well as pipelines for conducting gas-dynamic studies. The analysis of frequently used heat-resistant alloys (superalloys) is carried out. A wire made of Kh23Yu5T-N alloy was used as a material for heating elements to maintain the desired temperature conditions. The design, shape, mode of fixing the heating elements in the heating device, as well as the method of connection to the power supply and measuring systems are proposed. The results of experimental study are presented to confirm the long-term operability of the appliance with heating elements made of the wire 6 and 7 mm in diameter at a temperature up to 1300°C. The results obtained revealed the necessity of replacing the heating elements after each long-term high-temperature exposure. The proposed designs and material of the heating elements provide the possibility of their rapid manufacture and replacement. It should be noted that all the components of the heating device are economically efficient. The developed heating appliance was used in studying the strength of various structures under the effect of vibration and gas-dynamic loads at temperatures close to 1300°C.

КОМБИНИРОВАННАЯ СИСТЕМА ОЗОНИРОВАНИЯ ПРИ СТЕРИЛИЗАЦИИ МОЛОКА

The main disadvantage of the existing facilities for antibacterial milk treatment is its low quality due to the lack of ozonation. Milk sterilization requires a combined system of ozonation that includes ozonation and ultraviolet irradiation. In this system, ozone is prepared in a sealed chamber, and ozone saturation of milk occurs impulsively. In each feeding period, the amount of ozone is increased step-by-step, and its concentration can be varied according to the required level of microbial contamination of milk. At the initial stage, ozone at a concentration of 24 g/m3 is applied in pulses for 10 s; then, based on the readings of the bacterial concentration sensor, the operating mode is adjusted according to the program. An operational algorithm of the control program of the combined ozonation system for milk sterilization is developed to operate in two modes: the fi lling mode and the milk processing mode. Microbiological analysis of sterilized milk shows that the maximum eff ect is observed in the fi rst 15 seconds of the system operation. On average, the concentration of pathogenic microfl ora decreases by 24%. Within 60 seconds, the microbial contamination of milk decreases by 49%. The combined ozonation system makes it possible to improve the quality of milk sterilization, taking into account the required level of microbial contamination in milk and reducing the adverse effects of long-term high-temperature exposure. The authors have experimentally proved the feasibility of the combined ozonation system using the developed control program algorithm.

Mechanical properties and phases evolution in T91 steel during long-term high-temperature exposure

Scanning and transmission electron microscopy was used to characterize quantity, pattern, size, and distribution of microstructure and precipitated phases in T91 steel exposed to high temperature in the subcritical unit superheater of a specific power plant. The materials were mechanically and electrochemically tested using a universal testing machine and an electrochemical workstation. The structure and performance of the original material and after the long-term service time were compared. The hardness and strength of T91 steel initially increased before they decreased during long-term service. It took 4000 h for the T91 steel to transform from the original to the service state, forming (Cr, Fe, V, Mo)23C6 multi-component mixed phase, which delayed or inhibited the growth of the M23C6-type carbides during the long-term high-temperature service. The MX-type carbide formation would pin dislocations and increase the strength. When the service time was between 4000 and 130,000 h, the carbide coarsening was obvious, and the effect of solid solution strengthening was reduced, the dislocation density was decreased, reducing the material’s strength below the original state level. Electrochemical tests showed that longer service time degraded corrosion resistance of the T91 steel.

The mechanisms of scale and subsurface diffusion zone formation of heat-resistant HP40NbTi alloy at long-term high-temperature exposure

Surface multilayer scale and the subsurface diffusion zone of heat-resistant HP40NbTi alloy were characterized by SEM and X-ray mapping after high-temperature exposure for different durations. After long-term exposure, conditional equilibrium is established in the alloy subsurface area: scale formation and internal oxidation deplete the diffusion zone of Cr and Si, which is compensated for by their diffusion along with Ni and Fe diffusion from the bulk of the metal to the surface. The process slows over time owing to the increase in the diffusion zone depth and the formation of a silicon oxide “barrier” layer.

Optimization of A354 Al-Si-Cu-Mg Alloy Heat Treatment: Effect on Microstructure, Hardness, and Tensile Properties of Peak Aged and Overaged Alloy

The aim of the present work is the study of T6 heat treatment of A354 (Al-Si-Cu-Mg) casting alloy. The heat treatment was optimized by maximizing mechanical strength of the alloy while keeping the treatment cost effective, reducing treatment time and temperature. Due to the presence of low melting compounds, a double stage solution treatment was proposed. The first stage was aimed at the homogenization and dissolution of the low melting phase while a second stage at a higher temperature was evaluated to foster dissolution of Cu/Mg rich intermetallics and keep the solution time and temperature as low as possible. Microstructural investigations were performed through optical and electronic microscopy, which allowed the assessment of the evolution of intermetallic phases during the solution treatment. Artificial aging was studied at different temperatures from 160 °C to 210 °C where the peak aging condition was identified. Over aging of the heat treated alloy was evaluated by soaking T6 samples at 200, 245, and 290 °C for up to 168 h. Tensile behavior of the T6 and over-aged alloy (i.e., after soaking at 210 °C for 50 h) was evaluated at room temperature. Results showed that the proposed treatment allowed the enhancement of mechanical properties of the alloy in comparison to industrial practice treatment, maintaining a good level of ductility and conferring a superior resistance to long-term high temperature exposure.

Ultrahigh stability and strong precipitation strengthening of nanosized NbC in alumina-forming austenitic stainless steels subjecting to long-term high-temperature exposure

For precipitation-strengthened heat-resistant steels, property degradation resulted from phase coarsening is always one of the most critical challenges, and therefore, knowledge regarding the long-term influence of temperature on mechanical properties and microstructure is prerequisite for their applications. In this work, long-term thermal aging up to 10,000 h of typical alumina-forming austenitic (AFA) stainless steels was conducted, and evolution of microstructure and mechanical properties of these promising heat-resistant alloys were studied systematically. It was found that mechanical performance of the steel strengthened by secondary nanosized NbC showed not only much enhanced creep resistance, superior to that of most traditional heat-resistant steels, but also stable high-temperature strength even after long-term aging. The spheroidal secondary NbC particles showed extreme coarsening resistance with a low coarsening kinetic constant (almost six orders of magnitude smaller than that of the Laves Fe2Nb phase). The low interfacial energy of nanosize NbC, which was resulted from the cube-on-cube orientation relationship and semi-coherent interfacial structure with the matrix, gave rise to its sluggish coarsening kinetics. Our current findings not only confirm that the AFA alloys strengthened by dense precipitated, nanosized semi-coherent NbC is a promising structural material for long-term high-temperature applications, but also sheds new insights into understanding precipitation hardening mechanism for high-temperature materials in general.

Kinetics of the Formation of Intermetallic Phases in HP-Type Heat-Resistant Alloys at Long-Term High-Temperature Exposure

The kinetics of formation and morphology of the intermetallic phases in the structure of heat-resistant as-cast HP40NbTi alloys in the course of long high-temperature exposure have been studied with the help of light and electron microscopy, electron microprobe, and X-ray diffraction. During exposure of 2 to 1000 hours at 1423 K (1150 °C), intermetallic phase with conditional formula Cr7Ni5Si3N3FeNb is formed in the alloy. The analysis of the kinetics of intermetallic phase’s growth for an impact assessment of certain metal substitutional elements (niobium, chromium, silicon) on the size of the formed particles was performed. Formation and growth of the intermetallic phases with high silicon content in the alloy structure on the boundaries between niobium and chromium carbides (NbC and M23C6) and matrix γ-phase provide a diffusion barrier for oxygen in oxidizing environment. This may create partial protection against oxidation of hardening carbide phases in the structure and promote increasing of the serviceability of the HP series alloys under operating conditions in the petrochemical industry.

High-Temperature Storage Testing of ACF Attached Sensor Structures.

Several electronic applications must withstand elevated temperatures during their lifetime. Materials and packages for use in high temperatures have been designed, but they are often very expensive, have limited compatibility with materials, structures, and processing techniques, and are less readily available than traditional materials. Thus, there is an increasing interest in using low-cost polymer materials in high temperature applications. This paper studies the performance and reliability of sensor structures attached with anisotropically conductive adhesive film (ACF) on two different organic printed circuit board (PCB) materials: FR-4 and Rogers. The test samples were aged at 200 °C and 240 °C and monitored electrically during the test. Material characterization techniques were also used to analyze the behavior of the materials. Rogers PCB was observed to be more stable at high temperatures in spite of degradation observed, especially during the first 120 h of aging. The electrical reliability was very good with Rogers. At 200 °C, the failures occurred after 2000 h of testing, and even at 240 °C the interconnections were functional for 400 h. The study indicates that, even though these ACFs were not designed for use in high temperatures, with stable PCB material they are promising interconnection materials at elevated temperatures, especially at 200 °C. However, the fragility of the structure due to material degradation may cause reliability problems in long-term high temperature exposure.

High temperature exposure of in-situ thermocouple fixed-point cells: stability with up to three months of continuous use

To categorise thermocouples in batches, manufacturers state an expected operating tolerance for when the thermocouples are as-new. In use, thermocouple behaviour can rapidly change and the tolerance becomes invalid, especially when used at high temperatures (i.e. above 1000 °C) as the processes leading to de-calibration, such as oxidation and contamination, can be very fast and lead to erroneous readings. In-situ thermocouple self-validation provides a method to track the drift and correct the thermocouple reading in real-time, but it must be shown to be reliable. Two miniature temperature fixed-point cells designed at NPL for in-situ thermocouple self-validation, the first containing a Pt-C eutectic alloy and the second containing a Ru-C eutectic alloy, have been exposed to temperatures close to their melting point for 2200 h and 1570 h, respectively, and continuously, for up to three months. Recalibration after this long-term high-temperature exposure, where a tantalum-sheathed thermocouple was always in place, is used to show that no significant change of the temperature reference point (the melting temperature) has occurred in either the Pt-C ingot or the Ru-C ingot, over timescales far longer than previously demonstrated and approaching that required by industry for practical use of the device.

Thermal Degradation Evaluation of HP40Nb Alloy Steel After Long Term Service Using a Nonlinear Ultrasonic Technique

Nonlinear ultrasonic analysis has been proposed to characterize the thermal degradation of HP40Nb steel associated with microstructure evolution. Measurements of ultrasonic nonlinear signals were performed in the thermally degraded damaged HP40Nb specimens and the results showed a clear change of increase-plateau-decrease tendency of the normalized nonlinear parameter with respect to the different thermal loading time. Based on metallographic studies, the variation in the measured acoustic nonlinearity reveals that the normalized acoustic nonlinearity increases due to the increases of the second phase precipitates and dislocation density in the early stage and it decreases as a combined result of the reduction of dislocation density, coarsening of precipitates and initiation of micro-voids after long-term high temperature exposure. Moreover, an analytical model calculation of precipitate–dislocation interaction has been performed to interpret the correlation between microstructural evolutions and the measured ultrasonic nonlinearity. Consequently, ultrasonic nonlinearity is found to be strongly sensitive to the microstructure evolutions during thermal degradation of HP40Nb steels.

Effect of Sb additions on the microstructural stability and mechanical properties of cast Mg–4Zn alloy

The effect of 0.15, 0.4, and 0.7 wt.% Sb additions on the microstructure, thermal stability, and mechanical properties of a cast Mg–4% Zn was investigated. Microhardness and shear punch tests (SPT) were used to evaluate the mechanical properties of the alloys in both as-cast condition and after long-term annealing. Addition of Sb resulted in the refinement of the as-cast structure and also the formation of Mg 3Sb 2 intermetallic particles. According to the microstructural features, the presence of Mg 3Sb 2 particles caused excellent stability during long-term high-temperature exposure, resulting in superior thermal stability of the Sb-containing alloys in comparison with the base alloy. The retention of mechanical properties after long annealing treatment is believed to be caused by the strengthening effects of Mg 3Sb 2 intermetallic particles and also by the role of these particles in impeding the growth of α-Mg grains during high-temperature exposure. The highest mechanical properties, obtained for 0.4% Sb addition, can be attributed to the favorable combination of small size and high volume fraction of the Mg 3Sb 2 intermetallic particles in the Mg matrix. At the higher Sb level of 0.7%, however, strength and hardness decrease due to the coarsening of Mg 3Sb 2 particles.

Evolution of Carbide Precipitates in 2.25Cr-1Mo Steel during Long-Term Service in a Power Plant

Carbide precipitation from the steel matrix during long-term high-temperature exposure can adversely affect the fracture toughness and high-temperature creep resistance of materials with implications on the performance of power plant components. In the present work, carbide evolution in 2.25Cr-1Mo steel after long-term aging during service was investigated. Boiler pipe samples of this steel were removed from a supercritical water-cooled coal-fired power plant after service times of 17 and 28 years and a mean operational temperature of 810 K (537 °C). The carbide precipitation and coarsening effects were studied using the carbon extraction replica technique followed by analysis using transmission electron microscopy and energy dispersive X-ray spectroscopy. The carbides extracted using an electrolytic technique were also analyzed using X-ray diffraction to evaluate phase transformations of the carbides during long-term service. Small ball punch and Vickers hardness were used to evaluate the changes in mechanical performance after long-term aging during service.

High-temperature fatigue damage mechanisms in near-α titanium alloy IMI 834

The high-temperature fatigue and creep behaviour of near-α titanium alloy IMI 834 was studied for temperatures up to 650°C. In order to identify the relevant damage mechanisms, the test program involved continuous cycling, dwell tests with hold periods in tension and compression, and asymmetrical strain-time cycles. The dominant damage mechanism was found to change at a temperature of about 600°C. At low test temperatures fatigue life is largely dependent on the maximum stress of the fatigue cycle. At high temperatures a brittle oxygen-enriched subsurface layer forms, and thus, environmental degradation governs fatigue life in high-temperature low frequency tests. Lamella boundaries present in the bimodal microstructure were seen to deflect small fatigue cracks, and thus, this microstructure has superior fatigue properties as compared to an equiaxed one. During long-term high-temperature exposure, however, significant degradation of the lamella boundaries was observed, and then both microstructures display similar fatigue properties.

Interfacial properties of a SiC fibre-reinforced Ti alloy after long-term high-temperature exposure

The effect of long-term high-temperature exposure on the interfacial properties of a typical SiC continuous fibre-reinforced titanium alloy was investigated. Specimens were annealed in air and in vacuum under simulated operating conditions using temperatures up to 500 °C for times up to 700 h. The interfacial chemistry and fracture morphology of the specimens were determined using Auger and scanning electron microscopy. It was shown that high-temperature long-term exposure to air resulted in embrittlement of the fibre-matrix interface through combined oxidation and attack of the Ti matrix on the fibres and fibre coating. Exposure to operating temperatures in vacuum did not lead to a significant degradation of the interfaces. The degradation of the interfaces in high temperature service can lead to a reduction of the service life of components made of reactive system metal-matrix composites such as SiC-reinforced Ti alloys.

The effect of long-term high temperature exposure on the structure and properties of the titanium alloy Ti 5331S

Samples of oil-quenched and air-cooled Ti 5331S and of two experimental titanium alloys have been exposed at 575°C for times up to 1000h. A reduction in ductility occurred in both Ti 5331S and the experimental alloy which contained silicon. No such reduction was observed in the silicon-free experimental alloy. Examination using optical, scanning and analytical transmission electron microscopy has shown that fracture occurs on (0001), and is nucleated by fracture or decohesion of the suicide particles which are precipitated during high temperature exposure. These observations are discussed in terms of earlier work.