Post-test corrosion analysis of high-temperature thermal energy storage capsules

Thermal energy storage capsules have been freeze-thaw cycled in vacuum at 1000 ± 100 K. The capsules were fabricated from Inconel 617^® (Inco Alloys International, Inc.) and contained eutectic fluoride mixtures of sodium, magnesium, lithium, and potassium. Samples that were thermal cycled for 20,000 and 30,000 hr were analyzed for corrosion effects. Radiography indicated neither flaws nor inhomogeneities, and there were no significant microstructural changes in the container alloy. The microstructural damage penetrating the inside surface was as deep as 120 m and that penetrating the outside surface was approximately 150 m. Microprobe results on the containers revealed a concentration gradient of alloying elements. The aluminum concentration was reduced from 1.34% in the original matrix to 0.4% at 10 m from the inside surface, and chromium was reduced from a nominal value of 23% to 10% at the outer surface. The depletion of aluminum and chromium from the outer surface was due to vacuum vaporization at elevated temperature. X-ray diffraction revealed the formation of possible protective films consisting of MgNaF₃ and MgF₂. The measured and theoretically predicted concentrations of aluminum and chromium were in good agreement. It was concluded that the corrosion process is a solid-state diffusion-dominated process and an expected lifetime of 5 to 7 years is a reasonable estimate.     

Inconel 617 and Stellite 6 alloys for tooling in thixoforming of steels

Thermal fatigue and high temperature wear are the two principle failure mechanisms for thixoforming dies. Samples of Inconel 617 and Stellite 6 alloys were submitted to thermal cycling under conditions which approximate thixoforming of steels and to sliding wear tests at 750 °C. The experimental results thus obtained were compared with those of the X32CrMoV33 hot work tool steel. The Inconel 617 and Stellite 6 samples are much more resistant to oxidation and to softening than the hot work tool steel, providing a superior resistance to thermal fatigue cracking. The wear resistance of the Inconel 617 and Stellite 6 alloys at 750 °C is also markedly superior. The adhesive oxides growing slowly on Inconel 617 and Stellite 6 alloys sustain the wear action without spalling and are claimed to be responsible for the superior wear resistance of these alloys at 750 °C.     

Dissimilar welding of AISI 310 austenitic stainless steel to nickel-based alloy Inconel 657

The current work was carried out to characterize welding of AISI 310 austenitic stainless steel to Inconel 657 nickel–chromium superalloy. The welds were produced using four types of filler materials; the nickel-based corresponding to Inconel 82, Inconel A, Inconel 617 and 310 austenitic stainless steels. This paper describes the selection of welding consumables for the joint. The comparative evaluation was based on hot-cracking tests (Varestraint test) and estimation of mechanical properties. According to Varestraint tests, Inconel A showed the least susceptibility to hot cracking. In tension tests, all weldments failed in the weaker parent metals (i.e., Inconel 657). Moreover, Inconel A weldment had the highest strength and total elongation. On the other hand, the weld metals failed by ductile fracture except Inconel 617, which exhibited mixed fracture mode. At last, it was concluded that Inconel A filler material offered the best compromise for the joint between Inconel 657 and 310 stainless steel.     

Inconel 617合金高温热变形行为与组织演变

为解决Inconel 617合金的高温加工问题,对锻造Inconel 617合金的高温热变形行为进行了研究。利用Gleeble-3500热模拟试验机研究了Inconel 617合金在900~1200 ℃、应变速率为0.001~10 s^-1范围内的热塑性行为。推导了该温度和应变速率下的本构方程,得到了该温度范围内的热加工图。用电子背散射衍射研究了合金压缩后的动态再结晶。确定了失稳区的位置,并表明在热变形条件下,确实发生了动态再结晶,获得了细小的晶粒。Inconel 617热处理的最佳温度范围为1075~1175 ℃,该温度范围处于材料的安全区。     

Chromium depletion profiles after preferential removal of chromium from alloys

Chromium depletion profiles and surface concentrations were determined from vacuum annealed and oxidized 18/8 type stainless steels by serial sectioning after neutron irradiation, energy dispersive X-ray analysis and Auger electron spectroscopy.It was established that with vacuum annealing and with selective oxidation of chromium in austenitic stainless steel the rate of chromium loss from the alloy is controlled by a balance of chromium diffusion in the metal and the rate of interchange of chromium from the alloy surface to the vacuum or oxide respectively. An analysis for the variation of the metal surface chromium concentration with time was derived. According to this analysis the metal surface chromium concentration decreases with increasing oxidation or vacuum annealing time, the rate of decrease depending on two parameters, chromium diffusivity in the metal and an interface transfer coefficient.     

Water Vapour Effect on the Oxidation Mechanism of a Cobalt-Based Alloy at High Temperatures (800–1,100 °C)

A cobalt-based Phynox alloy was oxidized in the 800–1,100 °C temperature range. The alloy oxidation was consistent with a growth mechanism limited by the diffusion process in a growing Cr₂O₃ oxide scale. Water vapour enhanced the alloy oxidation rate and scale porosity. Thermal cycling tests at 900 and 1,000 °C showed that water vapour reduces the outer Mn_1.5Cr_1.5O₄ subscale adherence, but the chromia scale adherence was not affected. These temperatures permited a rapid chromium supply from the substrate to form a continuous chromia scale. At 1,100 °C thermal cycling conditions led to scale spallation and chromium depletion in the alloy. In dry air, weight losses were recorded due to cobalt and molybdenum oxidation, giving CoCr₂O₄ and CoMoO₄. In wet air, the initial porous chromia scale permited nickel and cobalt oxidation, leading to Ni₅Co₃O₈ and CoCr₂O₄ formation and resulting in bad adherence during thermal cycling.     

Chromium transport through Cr2O3 scales. II. Changes in scale morphology during high vacuum treatment of oxidized chromium specimens

Chromium specimens oxidized at 1200 and 1300° O to give Cr₂O₃ scales with varying thicknesses have been high vacuum annealed for extended periods at temperature. During the high vacuum anneal chromium is transported through the scale and evaporates from the scale surface. Initially the rate of chromium evaporation decreases with time as a result of recrystallization and densification of the scale. On extended high vacuum treatment the rate of chromium evaporation again increases and major changes in scale morphology takes place. The outer scale surfaces develop hollows in the oxide grains while the grains protrude from the scale at the inner surfaces. The morphological changes are interpreted in terms of differences in diffusion rates along grain boundaries and through the lattice and resultant variations in surface energy along the surfaces.     

GH690合金管材热处理工艺优化

以国外Inconel 690成品管的显微组织为参照对象,对国产GH690合金管材在不同工艺条件下的固溶处理与TT处理工艺进行了研究;采用OM、SEM和TEM等表征手段分析了工艺参数对其晶粒度、晶界碳化物形貌和贫Cr区的影响。结果表明:国产GH690合金管在固溶处理过程中,随着固溶温度的提高,尺寸较大晶粒所占比例逐渐升高,长大激活能为265 kJ/mol。当固溶温度超过1100℃时,保温时间对晶粒尺寸影响显著。国产GH690合金管析出细小半连续晶界碳化物的TT处理工艺参数为680℃/10~20 h,715℃/10~20 h,750℃/5~15 h。经1090~1110℃/5 min固溶处理以及715℃/10 h或15 h的TT处理后,国产GH690合金管晶粒尺寸分布、晶界碳化物形貌特征和贫Cr区演化特征与国外Inconel 690成品管非常相似;而其TiN颗粒数量和尺寸明显少于和小于后者,贫Cr区的最低Cr浓度高于后者。通过对显微组织特征的综合评价,表明国产GH690合金管的显微组织总体优于国外Inconel 690成品管。同时,兼顾实际生产中的成本问题,提出国产GH690合金管热处理工艺优化的建议。     

Chromium depletion from stainless steels during vacuum annealing

During selective chromium Oxidation of stainless steels the changes in chromium concentration at the metal surface and in the metal have an important bearing on the overall oxidation performance. It has been proposed that an analogue of chromium behaviour during selective oxidation is obtained from volatilisation of chromium during high temperature vacuum annealing. In the present report the evaporation of chromium from 316 type of steel, vacuum annealed at 1000 °C, has been investigated by menas of energy dispersive X-ray analysis and by neutron activation analysis. It was established that chromium loss from austenitic stainless steels is rate controlled by interdiffusion in the alloy. As predicted the chromium concentration at the metal surface decreased with increasing vacuum annealing time. The chromium depletion profile in the metal was in good agreement with the previously derived model apart from an anomalous region near the surface. Here the higher resolution of the neutron activation technique indicated a zone within ～2μm of the surface where the chromium concentration decreased more steeply than expected.     

Model predictions of grain boundary chromium depletion in Inconel 690

A new three dimensional modelling technique has been developed to predict chromium depletion from grain boundaries in Ni-Cr-Fe alloys. The technique is based on precipitation kinetics represented by the Zener’s Equation and the error function solution of the diffusion law. Unlike former models, the model predicts a natural transition from sensitization to desensitisation and time delay in reaching the minimum chromium concentration at grain boundaries. In addition, the whole prediction can be made using a single Excel worksheet, therefore provides a simple and effective tool for sensitization/desensitisation predictions and studies. Grain size effects and temperature-time-concentration (TTC) maps can also be predicted. The model has been validated against experimental investigations reported in the literature on Inconel 690 aged at four temperatures for different time periods and good agreement between model predictions and experimental observations of chromium depletion has been achieved.     

Corrosion properties of inconel 617 alloy after heat treatment at elevated temperature

Inconel alloys find wide application in industry as high-temperature resistance materials. In the present study, refurbishment of the Inconel 617 alloy after 37,000 h of operation in the field is carried out through the heat-treatment process. The electrochemical response of the heat-treated alloy is determined through potentiodynamic testing of the surfaces. The heat-treatment process is carried out at 1175 °C for 1 and 2 h in an air free furnace. The corrosion rate is estimated from TAFEL and polarization measurements. The surface morphology after the electrochemical tests is studied using scanning electron microscopy (SEM), while the material characterization at the surface is carried out using energy disperse spectroscopy (EDS). It is found that the corrosion resistance improves considerably for the workpieces subjected to 1 h heat treatment. The depletion of Cr and Mo at grain boundaries results in excessive pitting in this region. Moreover, enrichment of Cr at the surface after 1 h heat treatment reduces the corrosion current.     

Tensile properties of haynes alloy 230 and inconel 617 after long exposures to LiF-22CaF2 and vacuum at 1093 K

As a part of a study of a space-based thermal energy storage system utilizing the latent heat of fusion of the eutectic salt LiF-20CaF₂ (mole%), the two wrought Ni-base superalloys Haynes alloy 230 and Inconel 617 were subjected to molten salt, its vapor, and vacuum for periods as long as 10,000 h at 1093 K. Following exposure, the microstructures were characterized, and samples from each superalloy were tensile tested between 77 and 1200 K. Neither the structure nor mechanical properties revealed evidence for additional degradation due to exposures to the salt. Although some loss in tensile properties was noted, particularly at 77 K, this reduction could be ascribed to the influence of simple aging at 1093 K.     

Evolution of high temperature stability of Inconel 617 by surface modification

A comparative study on the high temperature stability of bare Inconel 617 and Inconel 617 surface-modified by TiAlN overlay- and Al diffusion-coating was performed, and a remarkable improvement in high temperature stability was achieved by surface modification. Microstructural analysis revealed that the formation of brittle and unstable Cr₂O₃ crusts which makes Inconel 617 vulnerable at high temperatures was inhibited both by a TiO₂-rich top layer with an Al-containing subsurface layer (TiAlN overlay-coated), and by an Al_xNi_1?x (x=0.4?C0.6) alloy layer (Al diffusion-coated). Both TiAlN overlay- and Al diffusion-coated Inconel 617 also showed enhanced wear resistance before and after heat treatment at 1000 °C.     

真空热循环作用下LF6合金焊接接头的拉伸性能和微观组织

采用精密多功能低温静载材料实验机、热循环试验装置、透射电镜和扫描电镜，系统研究了真空热循环条件下LF6合金焊接接头的拉伸性能和微观组织，分析讨论了真空热循环对LF6合金焊接接头的影响规律。研究结果表明，在循环300周次范围内，真空热循环可改善LF6合金焊接接头的拉伸性能，使其强度和塑性均得到提高。热循环对焊接接头拉伸性能具有影响，并同循环次数有关。初始循环时，由于位错不断增殖，使得LF6合金焊接接头的强度和塑性增加较快；经热循环70周次后，接头的强度和塑性均达到最高值；循环次数继续增加，强度和塑性开始下降，这可能与晶内位错密度降低、晶界处形成位错墙有关。     

Degradation of TiN Coatings on Inconel 617 and Silicon Wafer Substrates Under Pulsed Laser Ablation

The degradation behavior of TiN coatings on Inconel 617 and silicon (Si) wafer substrates was compared following Nd:YAG pulsed laser ablation to apply thermomechanical stress. Surface cracks and pores were observed on the TiN coating on the Inconel 617 after five pulses, and melting of the coating was occurred over ten pulses. The TiN coating on the Si wafer also showed surface cracks and pores, but there was no surface melting. As the pulses were increased, the surface roughness of the TiN coating on Inconel 617 increased more than the TiN coating on the Si wafer, and interfacial cracking was the dominant degradation behavior on the Si wafer. The hardness of the TiN coating decreased below 50% of its initial value (2200 HK) after five pulses on the Inconel 617, whereas over 70% of the initial value (2400 HK) was maintained on the Si wafer. The TiN coating on Inconel 617 showed diffusion of substrate atoms to the surface, while Si was not found in the TiN coating on the Si wafer even after 25 pulses. It was determined that the decrease in hardness was influenced by the cracking behavior and the diffusion of atoms from the substrate.     

新型高热导率氮化铝基微波衰减陶瓷研究

针对传统氧化铍基微波损耗陶瓷存在毒性大、一致性差和焊接难度大的问题,发展了一种高热导率氮化铝基的AlN-FeSiAl微波衰减陶瓷。该材料在AlN基体中添加FeSiAl,属环境友好型材料。为获得高热导率和良好的电磁性能,开展了不同氮化铝粉末、不同FeSiAl粉末添加量以及烧结工艺对复合材料性能影响的研究。结果表明,采用日本东洋氮化铝粉末、添加10%(质量分数,下同)的FeSiAl,在1650℃和85 MPa下进行真空热压烧结,获得了衰减性能优异的微波衰减陶瓷,且材料热导率达到88.2 W/m·K,接近美国Ceradyne公司AlN基微波衰减陶瓷的热导率水平。     

Evaluation of the properties of anodized aluminum 6061 subjected to thermal cycling treatment using electrochemical impedance spectroscopy (EIS)

The corrosion resistance of anodized Al 6061 produced by two different anodizing and sealing processes was evaluated for 30 days during exposure to 3.5 wt% NaCl using EIS. Thermal cycling treatments at 120, 160 and 200 °C have been applied for the two types of samples. The degradation of the properties of the anodized layers has been determined by thorough analysis of the EIS data for control samples and samples that had undergone thermal cycling. Scanning electron microscopy has been used to evaluate the damage to the anodized aluminum layers due to thermal cycling. It was found that the thermal treatment produced considerable damage of both the porous layer and the barrier layer. The EIS data suggest that some cracks extended into the bare metal. The damage of the oxide layers increased with increasing thermal cycling temperature for both types of samples. Self-sealing of the porous layer and the barrier layer occurred during immersion in NaCl.     

Improvement of thermal and mechanical properties of graphite/copper composites through interfacial modification

Unidirectionally reinforced graphite/copper composites have been fabricated using a pressure infiltration casting procedure. T300 and T650 graphite fibers have been used to reinforce copper and copperchromium alloys. The effects of the chromium level in the copper matrix on the tensile strength, stiffness, and thermal expansion behavior of the composites have been evaluated through tensile and three-point bend testing, and thermal cycling. At the 0.5 wt% alloying level, chromium increases the stiffness and optimizes the thermal expansion behavior of graphite/copper composites. The longitudinal tensile strengths of these composites are above 1606 MPa, whereas the transverse tensile strengths are lower than 40 MPa due to incomplete infiltration during processing. Scanning electron microscopy analyses reveal that the unalloyed copper matrix composites experienced extensive fiber/matrix debonding under tensile loading. The addition of chromium to the copper increases the level of matrix bonding to the graphite fibers, as evidenced by observations of fractured tensile specimens. Auger electron spectroscopy analyses indicate that a chromium carbide phase present at the interface is responsible for the improved bonding.     

Preparation a nickel-aluminide bonded diamond tool by self-propagating high-temperature synthesis and strengthening by nickel-chromium-phosphorus alloy and copper

Metal bonded diamond tools are normally manufactured by hot-press sintering, which is a high energy consuming process and using a large number of graphite moulds as well. Graphite dust is hazardous to operator's health and environment in machining of graphite mould. In present study, a metal bonded diamond tool is manufactured by self-propagating high-temperature synthesis (SHS) without hot press sintering and consuming of graphite moulds. In order to improve bonding of diamond grit and flexural strength of the tool for this method, nickel‑chromium‑phosphorus alloy and copper are introduced into nickel‑aluminum SHS in present study. The influence of nickel‑chromium‑phosphorus alloy and copper content on the combustion processes, the thermal behaviors, and the microstructures of the synthesized composite bonds are examined in detail. A diamond tool based on the optimized composite is prepared, and the grinding performance is examined. The results reveal that the addition of nickel‑chromium‑phosphorus alloy and copper inhibited the SHS reaction of nickel‑aluminum and reduced the combustion velocity, combustion temperature and reaction exothermic enthalpy. Copper can significantly improve flexural strength of the nickel aluminide bond. However, the maximum of nickel‑chromium‑phosphorus alloy or copper is about 60 wt% to sustain the SHS. A homogeneously dispersed and interconnected nickel-chromium-phosphorus phase in the matrix of nickel aluminides can be obtained with addition of over 40 wt% nickel‑chromium‑phosphorus alloy. A continuous chromium rich layer, composed of chromium carbide, has been formed at the interface between diamond grit and matrix, which renders a chemical bond of diamond grit. On the basis of the addition of 40 wt% nickel‑chromium‑phosphorus alloy and 10 wt% copper, an interconnected nickel‑chromium‑phosphorus phase for strong bonding of diamond grit and a flexural strength up to 700 MPa are achieved, which is considered as an excellent candidate material for diamond tools. The grinding test shows that the diamond tool owns a sound machining ability on the granite, and the diamond grit can be well retained and protruded.     

Corrosion behaviour of Ni-based superalloys in molten FLiNaK salts

High-temperature corrosion tests of alloys, Nimonic 80A, Inconel 718 and Inconel C-276, were investigated at 680°C in molten alkali fluoride salt (LiF–NaF–KF: 46.5–11.5–42%) environment. In this work, techniques included were weight loss measurements and potentiodynamic polarisation curves measurements. Inconel C-276 and Inconel 718 showed better corrosion resistance, while Nimonic 80A exhibited comparatively lower corrosion resistance. The high-temperature corrosion behaviour was observed using measurements of the oxide morphology and thickness. The corrosion rates were determined by recording the weight changes of the sample alloys at different time intervals. Microstructural examination showed the depletion of Cr near the surface of the alloys and hence the significant weight loss in the early stages of corrosion tests. The corrosion mechanism of the alloys is discussed in detail.