In-situ deposition of silica layers during the operation of a micro-gas turbine and their effect on the oxidation of superalloy blades

Silica-based oxide layers were deposited in-situ on turbine blades made from Inconel 713 during the operation of a 13 kg_f-class gas turbine, and their effect on the ex-situ oxidation behavior of the blades at 1050 °C was examined. The two turbines were driven by burning liquid petroleum gas (LPG), one turbine at a rotation speed of 35,000 rpm for 4 h (TB04), and the other at 42,000 rpm for 8 h (TB08). For deposition, tetraethylorthosilicate (TEOS) was sprayed into the fuel line immediately ahead of the combustion chamber. The TEOS-to-LPG ratio for TB04 and TB08 was maintained at 5.4 wt. % and 2.3 wt. %, respectively. Directly after operation, the turbine blades were coated with silica layers to a thickness of ∼10 μm, independent of the operating conditions. These oxide layers on the blades provided excellent protection against oxidation during both operation and the ex-situ isothermal oxidation test.     

Effect of periodic in situ deposition of silica layer on the long term operation of a micro-gas turbine

Tetraethylorthosilicate (TEOS) diluted in methanol was periodically supplied during operation of a 13 kgf (max. thrust)-class small gas turbine to deposit silica layers on the turbine blades and its effect on the degradation of the turbine blades upon the long term cyclic operation (1 cycle: 30/10 min run/pause at 20,000 rpm) was tested. The exhaust gas temperature (EGT) was set at 800 °C. For comparison, two other gas turbines were also tested without feeding TEOS at the EGT of 800 °C and 700 °C, respectively. The blades were inspected optically and metallographically before and after the test. The supply of TEOS produced a white silica layer indicating a very low level of the apparent density all around the surface of the blades, some of which became dense at the end to a thickness of 5-10 μm. The underlying substrate Inconel 713 was effectively protected, while the blades operated at the EGT of 800 °C, but not in situ coated, were severely degraded in terms of the surface oxidation and microstructural evolution of the substrate. The microstructure of the protected substrate was comparable with that of the unprotected but operated at the EGT of 700 °C. This result was discussed based on the effect of the formation of dense silica layer on the blade by comparing with the previously obtained results.     

Laser cladding in repair of IN738 turbine blades

Abstract

Laser cladding with additive powder can be used for repairing high cost components such as industrial turbine blades. Repair of damaged blades is important for electrical power companies to reduce operating costs. The repair of IN738 gas turbine blades using laser cladding is reviewed. A laser cladding system for blade repair and the associated heat treatment procedures before and after repair are discussed. In general, laser cladding with IN625 powder does not require preheating and can be used for repairing IN738 blades in low stress areas. For laser cladding with a higher strength material, e.g. IN738 powder, preheating to high temperatures is necessary to avoid cracking. In a process simulation, test coupons were employed to compare the microstructures and mechanical properties of the IN738 clad region and base metal. Blade repairs with IN738 additive powder were found to be stronger but much more difficult to process than those with IN625 powder. Therefore, extreme caution is required when repairing IN738 blades in high stress areas using a matching filler metal.     

Condensation and condensation-diffusion coatings for turbine blades made of high-temperature alloys with a directed crystal structure

Results of an investigation of condensation (condensed) and condensation-diffusion coatings for protecting working turbine blades made of high-temperature nickel alloys from high-temperature gas corrosion are presented. The coatings were deposited on specimens of a high-temperature model nickel alloy by the ion-plasma method.Translated from Metallovedenie i Tetmicheskaya Obrabotka Metallov, No. 4, pp. 15 – 18, April, 1996.     

Novel instabilities in the oxidation of a Nimonic alloy when YBaCuO is deposited upon it

When an YBaCuO film is deposited onto a Nimonic 80A alloy substrate using a plasma technique there is extensive inter-reaction. The microstructural and compositional relationships between the layered products in this interfacial scaling process are described using data obtained by conventional edge-on TEM techniques. Our results demonstrate the way in which the segregation of chromium to the YBaCuO overlayer dominates the scaling behaviour that is seen, despite the formation of intermediate layers which would normally have been expected to act as diffusion barriers, and highlight the potential difficulties in making well behaved electrical contacts for this superconductor.     

Time-lapse,three-dimensional in situ imaging of ice crystal growth in a colloidal silica suspension

The freezing of colloidal suspensions is encountered in many natural and engineering processes such as the freezing of soils, food engineering and cryobiology. It can also be used as a bio-inspired, versatile and environmentally friendly processing route for porous materials and composites. Yet, it is still a puzzling phenomenon with many unexplained features, owing to the complexity of the system and the space and time scales at which the process should be investigated. This study demonstrates the interest in fast X-ray computed tomography for providing time-lapse, three-dimensional, in situ imaging of ice crystal growth in a colloidal silica suspension. The experimental measurements show that the local increase in colloid concentration does not affect the growth kinetics of the crystals until the colloidal particles become closely packed. For particles much smaller than ice crystals, the concentrated colloidal suspension is equivalent to a simple liquid phase with higher viscosity and a freezing point determined by the concentration of colloidal particles.     

电沉积CeO2薄膜对TP304H钢高温水蒸气氧化机制的影响

研究了TP304H钢表面电沉积CeO2薄膜在610-770℃水蒸气中的氧化行为的变化.结果表明,沉积CeO2薄膜可有效减缓氧化速度.SEM和EDX分析显示,氧化膜表面形貌由多层结构转化为单层,Cr元素扩散进入CeO2膜层而形成Cr-Ce复合氧化物膜层.表面形成的氧化膜中Cr浓度显著提高,CeO2膜处于外层氧化膜与基体之间,内氧化消失.根据CeO2特性和试验结果,由于CeO2中Ce离子具有处于富氧/贫氧环境中具有Ce^4＋/Ce^3＋两价和高氧空位浓度特性,认为沉积CeO2薄膜对氧向内扩散具有一定的阻挡作用,限制了CeO2薄膜/基体界面处的氧分压,使得Cr优先扩散穿过CeO2薄膜在CeO2薄膜/气体界面处氧化,从而推迟了Fe氧化物形成的时间.     

Substitution of hexavalent chromate conversion treatment with a plasma electrolytic oxidation process to improve the corrosion properties of ion vapour deposited AlMg coatings

In this paper plasma electrolytic oxidation (PEO) is examined as a potentially beneficial post-deposition treatment for Al-5 wt.%Mg coatings deposited onto Ti alloy substrates using Ion Vapour Deposition (IVD), with a view to replacing toxic hexavalent chromate conversion treatments and at the same time enhancing the barrier protection properties of such IVD coatings. The aqueous corrosion behaviour of PEO layers formed on IVD coatings was evaluated by means of potentiodynamic polarisation scans, open circuit potential measurements and electrochemical impedance spectroscopy. Normally, IVD aluminium-based coatings require a post-coat shot peening treatment to densify the coating structure; however it was found that PEO layers could be formed successfully on as-deposited IVD coatings deposited onto Ti alloy substrates, providing a cost effective process to improve corrosion behaviour. PEO treatment provides unique improvements in the corrosion resistance of IVD coatings; the PEO layer possesses effective anticorrosion properties in aqueous environments containing Cl⁻ ions. A more positive corrosion potential, lower corrosion current and increased polarisation resistance were recorded for PEO layers, compared to traditional chromate conversion treatments.     

Coalescence of wustite grains during iron oxidation in a hot-stage environmental SEM

The growth of wustite grains during the initial stages of iron oxidation at 1050°C and atm has been investigated in situ in a hot-stage environmental SEM (HSESEM). Adjacent wustite grains were seen to coalesce by annihilation of the grain boundary dislocations constituting original boundaries. Specifically, dissociation of a grain boundary into an array of dislocations followed by their annihilation either by glide or climb was observed. The grain boundaries exhibiting this coalescence are believed to be low-angle boundaries. A controlled extent of oxidation followed by rapid cooling in the HSESEM enabled the study of wustite grain boundaries. Dissociation of a grain boundary into dislocations without their annihilation was also observed in later stages of oxidation when the dislocation mobility was severely reduced.In this case, instead of grain coalescence, nucleation of a new grain with an orientation different from either of the two original grains took place.     

In situ deposition behavior of SiO2 on YSZ-TBC-coated IN738LC during a burner rig test

This work was planned as a preliminary test to obtain an optimal condition for in situ deposition of SiO₂ on zirconia-based thermal barrier coating (TBC)-coated IN738LC specimens using a burner rig. The effect of the in situ deposited SiO₂ on the long-term reliability of TBCs upon cyclic burner rig operations will be tested in the next study. Tetraethylorthosilicate (TEOS), the precursor for the deposition, was fed admixed with methanol into the combustion chamber of the burner rig at an exhaust flame temperature of 1530 °C. All five coating processes were selected by varying the concentration of TEOS in the mixture and applied to hollow type pin specimens infixed to the rotating carousel to face the downstream exhaust gas vertically. A total of 1500 cc of the mixture at a given vol.% TEOS was fed evenly to the burner rig for 54 min during the coating process.     

A study of the Cr-depleted surface layers formed on four Cr-Ni steels during oxidation in steam at 600° C and 800° C

Three commercial steels with 18% Cr-11% Ni, 19% Cr-25% Ni, and 19% Cr-35% Ni and a laboratory-made 20% Cr-35% Ni steel have been oxidized in steam at 600°C and 800°C. Two types of oxides are distinguished: a thin layer rich in Cr₂O₃ and patches of thick oxide of Fe, Ni, Cr spinels. The Cr concentration profile in the steels under the first type has been determined by microprobe analysis of ground and pickled or annealed specimens oxidized for various times. The variation of the Cr concentration in the alloy at the alloy-oxide interface with surface treatment, time, and temperature is discussed. It is concluded that one reason for the more extensive nodular growth on pickled or annealed specimens is their greater Cr depletion after short time oxidation. The interdiffusion coefficients in the Cr-depleted zones have been determined on ground and annealed specimens oxidized at 800°C. The acceleration of the diffusion rate due to grinding is clearly demonstrated. A fair agreement with diffusion data from diffusion couples is found.     

A thermo-gravimetric and microstructural study of the oxidation of Nbss/Nb5Si3-based in situ composites with Sn addition

The effect of Sn addition on the oxidation of the Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn (at.%) alloy (JG6) in the as cast (AC) and heat treated (HT) conditions was studied at 800 °C and 1200 °C in static air using thermo-gravimetry and microstructural analysis. The oxidation kinetics, morphology and microstructure of the oxide scale and the microstructure of the bulk of the oxidised alloy were investigated. Oxidation occurred by inward oxygen anion diffusion. The oxidation of JG6 at 800 °C and 1200 °C is compared with the oxidation of Sn-free Nb–Ti–Si–Cr–Al–Mo–Hf alloys and is found to have been improved by the addition of Sn. At 800 °C pest oxidation, which was exhibited by the heat treated Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloy (JG4-HT), was eliminated by alloying with 5 at.% Sn. The elimination of pesting at 800 °C is attributed to the nature of the Nb solid solution in the alloy which consists of Sn-rich, Si-rich and Ti lean solid solution usually surrounded by Sn-poor, Si-poor and Ti-rich solid solution. The oxide scales that formed at 1200 °C on JG6 did not separate from the base metal and consisted of Nb₂O₅, TiO₂, SiO₂, HfO₂ and TiNb₂O₇. TiN, instead of TiO₂, and the (Nb,Ti)₅(Sn_1−xSi_x)₃ phase, which is considered as a ternary phase based on Nb₅Sn₂Si, are formed in the diffusion zone of the alloys JG6-AC and JG6-HT after oxidation at 1200 °C. The formation of these phases in the oxidised alloys JG6-AC and JG6-HT controlled the penetration of oxygen into the base material. The better oxidation performance of JG6-AC compared to JG6-HT at 1200 °C is attributed to the formation of Nb₃Sn in the former. It is suggested that the presence of the Sn-poor, Si-poor and Ti-rich Nb_ss in the microstructure is a key to the formation of the Nb₃Sn phase at the scale/diffusion zone interface in the JG6-AC oxidised at 1200 °C.     

The improvement by a CVD silica coating of the oxidation behaviour of a 20% Cr/25% Ni niobium stabilized stainless steel in carbon dioxide

The improvement of the oxidation behaviour of the 20%Cr–25%NiNb stabilized (20/25/Nb) austentic stainless steel, in carbon dioxide, by a silica coating, formed by a vapour reaction upon the pre-oxidized steel surface, has been examined at 825°C. The exposure periods extended to 5975 h duration. The silica coating effectiveness increased with its thickness over the range examined, 0.5–27 μm. Coatings ? 2 μm thick reduced the extent of oxidation by at least a factor of five by providing a barrier to the diffusion of cations outwards and oxidant inwards responsible for oxide growth on the 20/25/Nb steel. In addition, they completely inhibited oxide spallation because oxidation of the steel constituents occurred primarily within the silica coating whose adherence was maintained throughout the exposure. During the prolonged heat treatment of ? 2 μm thick silica coated steel, the iron and nickel constituents in the precoating oxide film were reduced by solid-solid reaction with manganese and probably also chromium from the underlying steel.     

Micromechanical behavior of TiNb/NiTi composite during a pre-straining process:an in situ synchrotron investigation

The micromechanical behavior of a recently developed TiNb/NiTi composite during a pre-straining process was investigated to broaden the understanding of deformation mechanisms of the martensitic-transforming composites.It was found that during loading,besides inherent elastic elongation,the TiNb/NiTi composite also experienced two different categories of stress-induced martensitic transformations(SIMTs,including B2→ B19’ and β→α ) and slight plastic deformation.Upon the following unloading,this...     

Mössbauer spectroscopic studies of chromium depletion in a 310 stainless steel during air oxidation

The application of conversion electron Mössbauer spectroscopy (CEMS) to the oxidation of an ultra-thin 310 stainless steel foil is presented. It is shown that during oxidation the foil deviates from the expected oxidation behaviour, derived from thick film kinetics, due to the effects of chromium depletion. Evidence is presented for the observation, in the spectra, of chromium depletion. The experimentally derived time-to-failure is compared with that predicted by theory.     

Corrosion behaviour of Y2O3-ZrO2 coatings on IN713LC in a LiCl-Li2O molten salt

Hot corrosion studies were performed on superalloy specimens. The IN713LC superalloy were sprayed with an aluminized NiCrAlY-bond coat and then with an yttria-zirconia top coat. The bare superalloy reveals an obvious weight loss due to spalling of the scales by the rapid scale growth and thermal stress. The top coatings showed a much better hot corrosion resistance in the presence of LiCl-3 wt.% Li₂O molten salt when compared with those of the bare superalloy and the aluminized bond coatings. These coatings have been found to be beneficial for increasing hot corrosion resistance of structural materials for lithium molten salts.     

Determination of local strains in a monocrystalline turbine blade by microbeam X-ray diffraction with synchrotron radiation

X-ray peak profiles of the types {400} and {300} of monocrystalline nickel-based superalloy turbine blades were measured with a new microbeam synchrotron radiation X-ray diffraction technique. Under the coating of a turbine blade in the virgin state, a plastically deformed layer was found which is responsible for anomalous rafting. In a turbine blade subjected to an “accelerated mission test”, the phases γ and γ′ were found to be under elastic distortions arising from the superposition of counteracting internal stresses. It is concluded that the investigated section was under tensile creep load during service. In the regions near the surfaces, the lattice parameters of both γ and γ′ increase. The reason for this increase is the diffusion of Al from the NiAl coating into the bulk of the blade. Finally, the differences between the peak profiles measured with the microfocus technique and with a conventional diffractometer are discussed.     

术中静脉使用胰岛素对女性肥胖伴胰岛素抵抗患者糖代谢的影响

目的 了解女性肥胖伴胰岛素抵抗患者麻醉和手术中血糖和血浆胰岛素水平的变化,以及术中静脉滴注胰岛素对其影响。方法 20 例女性肥胖伴胰岛素抵抗的择期手术患者,ASAⅠ-Ⅱ级,随机分成2组,B组术中静脉滴注无糖溶液;C组使用含糖溶液加胰岛素静脉滴注,每4g葡苟糖用1U胰岛素,胰岛素总量控制在20U,其余液体使用无糖溶液;另设10例无胰岛素抵抗患者作为对照组(A组),分别于麻醉前、麻醉后、术中(手术进行30 min)和术毕测定血糖和血浆胰岛素水平。结果 女性肥胖患者均存在胰岛素 抵抗现象,表现为空腹血糖水平和血浆胰岛素水平较对照组明显升高,麻醉和手术应激均使血糖和血浆胰岛素水平升高,术中静脉滴注胰岛素可防止血糖升高。结论 女性肥胖患者存在胰岛素抵抗现象,术中适当使用胰岛素可纠正手术应激对其糖代谢的影响。