Inconel 738LC Research Articles

Thermo-mechanical simulation of steam turbine blade with spark plasma sintering fabricated Inconel 738LC superalloy properties

The exponential increase in the demand for energy has placed tremendous pressure on the power generation companies and the components used for the power supply. The efficiency of such supply is dependent on the operating parameters of the plants but at the expense of the useful life of the components. Turbine blade is one of the components that contribute immensely to such efficiency and it should be made of high-strength material with an exceptional ability to withstand the harsh operation environment. The properties of spark plasma sintering technique produced Inconel 738 low carbon (IN738LC) superalloy was used in finite element analysis software, Abaqus CAE/2017 to simulate the thermo-mechanical behaviour of steam turbine blade. The maximum thermo-mechanical stress and strain were developed on the root of the turbine blade (along the tailing edge) and the value of the maximum stress developed is far below the yield stress of the superalloy. Hence, spark plasma sintered Inconel 738LC is suitable for turbine blade production as such blade will survive several thermo-mechanical stress and strain cycles prior to failure.

Spark plasma sintering of Inconel 738LC: densification and microstructural characteristics

The optimization of sintering temperature in the production of nickel-based superalloy was investigated. Mixed elemental powders of Inconel 738LC superalloy was fabricated at different sintering temperature ranging between 900 and 1200 °C, with a heating rate of 100 °C min−1, pressure of 50 MPa and 5 min holding time. The microstructures, phase analyses, densification and microhardness tests were conducted by using scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), x-ray Diffraction (XRD), polarized optical microscope (POM), micro-hardness testing machine, and Archimedes’ method respectively. Results revealed the formation of intermetallic phases in the microstructure, irrespective of the sintering temperature. The relative density increased with increasing sintering temperature, as the highest relative density of 98.4% was attained at 1200 °C and with a corresponding microhardness value of 362.25 Hv. There was a marginal decrease in grain size with increasing temperature, thereby resulted in reduced grain growth. This implies that the SPS technology is appropriate for the fabrication of nickel-based superalloy.

Influence of sintering temperature on the corrosion and wear behaviour of spark plasma–sintered Inconel 738LC alloy

Inconel 738 low carbon is a nickel-based superalloy that is widely used in the marine and petroleum industries, where it is subjected to an aggressive erosive and/or corrosive environment. Degradation due to erosion and corrosion often has a negative impact on the material’s long-term performance. This study aims to improve the mechanical properties and wear resistance of the material. This was achieved by using spark plasma sintering (SPS) technique to fabricate IN738LC superalloy. The elemental powders were prealloy in a tubular mixer for a period of 12 h. The samples were sintered at four different temperatures of 900, 1000, 1100 and 1200 °C under a pressure of 50 MPa, heating rate of 100 °C/min and holding time of 5 min. Wear test was conducted on the sintered alloys at 15, 25 and 35 N loads. Potentiodynamic polarization tests were performed in 3.65% NaCl and 0.5 M H2SO4 solutions. Worn surfaces and microstructural analyses of the sintered alloys were conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and polarized optical microscopy (POM). Microstructural evaluation of the alloys revealed that there was a homogenous dispersion of elemental composition, with less morphological defects. Wear results showed that the sample sintered at 1200 °C has the highest resistance to wear and corrosion attack. It also possesses superior hardness property. Therefore, the new material is suitable for application in a highly corrosive environment and also for high strength applications.

Premature damage of the second stage nozzle guide vanes of a gas turbine made of Inconel 738LC

In this work, premature damage of the second stage nozzle guide vanes (NGVs) of a gas turbine engine made of nickel-base Inconel 738LC superalloy has been studied. To determine the cause of damage, evaluations including visual inspections, microstructural investigations, and finite element method (FEM) analyses were carried out on the damaged nozzle. The microstructural investigations were performed through evaluation of the microstructure near the damaged area using optical and scanning electron microscopes. The FEM analyses performed to determine the temperature and stress distribution at different points of the nozzle. The results of visual inspections and microscopic investigations revealed the signs of microstructural degradations and hot corrosion near some of the cracks on the trailing edge of the NGVs. The results of the FEM analyses showed that due to the temperature gradient, thermal stresses are arisen in the nozzle components and caused thermal fatigue occurrence. Based on the analyses, it was concluded that the thermal fatigue is the dominant mechanism for premature damage of the second stage nozzle guide vanes of the gas turbine. Moreover, the simultaneous occurrence of microstructural degradations and hot corrosion on the NGVs, has a supplementary effect on the propagation of the cracks in addition to thermal fatigue.

Microstructural characteristics and thermophysical properties of spark plasma sintered Inconel 738LC

Nickel-based superalloys are used for turbine component parts in the power industry, aircraft engines, and in the marine sector. One such alloy that is available for these purposes is Inconel 738LC (IN738LC) nickel-based superalloy. Therefore, it is pertinent to have good structural and thermal stabilities for high-temperature applications, since these characteristics are important for its performance. In this study, spark plasma sintering technology was used to fabricate IN738LC nickel-based superalloy by using sintering temperature in the range of between 900 and 1200 °C. The fabricated products were characterized by using the scanning electron microscope and X-ray diffraction. There was the formation of the γ′(Ni3(Ti, Al)) intermetallic phase in the microstructure, which contributed to the material (Inconel 738LC) property. The solid solution elements present are chromium, cobalt, tungsten, and tantalum; these equally contribute to the structural stability of IN738LC. The density, hardness, and the predicted yield strength increased with increasing sintering temperature, while the porosity decreased with increasing sintering temperature. The thermal conductivity, which is an important thermophysical property of IN738LC, enables the evaluation of the usefulness and service life of IN738LC superalloy at high-temperature applications, was also examined. The thermal conductivity was measured by using a laser flash method in the temperature range of 100–600 °C. For the sintered alloy, the thermal conductivity was determined as a function of the thermal diffusivity, specific heat capacity, and density. Analysis showed inflection pattern with increase in temperature on the sample fabricated at 900, 1000, and 1100 °C, ; a monotonic increase with increase in temperature was observed on the sample fabricated at 1200°C, ; the inflections are associated with the γ′ precipitates dissolution and redistribution.

The Effect of Brazing Temperature on Shear Strength of Nickel Based Superalloy (Inconel 738LC) Joints

The Effect of Brazing Temperature on Shear Strength of Nickel Based Superalloy (Inconel 738LC) Joints

Finite Element Analysis to Verify the Structural Integrity of an Aeronautical Gas Turbine Disc Made from Inconel 713LC Superalloy

Gas turbines are very important because they can be used in several areas, such as aeronautics and electric power generation systems. The operation of a gas turbine can be done by less pollutant fuels when compared to traditional kerosene, for example, resulting in less degradation to environment. Gas turbines may fail from a variety of sources, with the possibility of serious damage results. In this work, the structural integrity of the hot disc of an aeronautical gas turbine is addressed. Several numerical analyses have been performed by the finite element method: Temperature Distributions, Thermal Stresses and Dilatations, Structural Stresses and Deformations, Modal Behaviors and Fatigue Analysis. Creep of blades has also been considered. These are the most important failure modes that can happen to the studied hot disc under operating service. All these analysis have been performed considering the boundary conditions at the design point with maximum rotational speed. The mesh of the problem has been strictly evaluated by adaptive refinement of nodes and elements combined with a convergence analysis of results. Then, the material and basic properties of the hot disc have been defined to assure a normal operation free from failures. Therefore, the mechanical drawings of the studied hot turbine disc have been released for manufacturing and the construction of the first prototype of the aeronautical gas turbine is in testing phase showing that the results presented in this work are consistent.

On the solidification behaviour and cracking origin of a nickel-based superalloy during selective laser melting

Inconel 738LC samples were fabricated by selective laser melting (SLM) using different laser powers and scanning speeds and investigated using a range of characterisation techniques. High melting point Al-, Si- and W-based oxide particles and γ cellular structure were observed in the as-fabricated samples, implying that the solidification of the alloy melt during SLM may have started with preferential nucleation of oxide particles, followed by the widespread nucleation and growth of γ cellular structure. Cracks in the as-fabricated samples are associated with pores, or Al-, Si- and W-based oxide particles and small grains along some large grain boundaries (GBs). Those pores with sharp corners or rugged inner surfaces are particularly prone to cracking development during SLM. Most of the cracks were developed at γ grain-oxide interfaces or along the central line of oxide clusters, which is attributed to solidification contraction and thermal shrinkage of the grains that are associated with oxides. Nano-indentation tests show that Si/W/O-rich GB regions demonstrate a significantly higher hardness as compared with γ matrix, indicating that the segregation of Si, W and O has caused embrittlement to the GBs. Large misorientation was observed between grains (large or small) that are either associated with cracks or not, suggesting that large grain misorientation does not inevitably lead to cracking development in the current material. Small grains present along large grain boundaries were found to act as crack initiation sites or affect crack propagation path.

Influence of sintering temperature on microstructural evolution of spark plasma sintered Inconel738LC

This work studied the effect of processing temperature on the microstructure of sintered Inconel 738LC nickel-based superalloy processed via spark plasma sintering technology. The processing parameters used were, 50Mpa pressure, 5minuts holding time and 100 ˚C/min heating rate, with the sintering temperature varied over three different temperatures at 1000, 1100 and 1200 °C. Inconel 738LC powder was mix at the same proportion for all the samples and under the same condition. Test samples were fabricated from the admixed powders and the influence of sintering temperature on the microstructure, porosity and fracture toughness was characterized. Scanning electron microscope (SEM-EDS) was used to investigate the bulk morphology of the sintered alloy, the phases present within the bulk volume of the fabricated samples was investigated, using energy dispersive X-ray diffraction spectroscopy (XRD). From the result analyzed, the increase in sintering temperature lead to a reduction in porosity, decrease in fracture toughness and increase in hardness value. Increase in sintering temperature had positive impact on the sintered alloy, by reducing the level of porosity present in the sintered samples. There is a reduction in the fracture toughness value of Inconel 738LC, this is expected due to the increase in hardness property.

Role of Aluminide coating degradation on Inconel 713 LC used for Compressor Turbines (CT) of Short-haul Aircrafts

ABSTRACTThis paper investigates the role degradation of protective diffusion aluminide coating on Inconel 713LC used for CT blades of short-haul aircraft fleet played in having the blades prematurely retired from service at 6378 hours, as opposed to their pre-set service time of 10000 hours. The blade samples were subjected to various examinations; X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyse at the; tips, airfoil, as well as the base, transverse and longitudinal, sectioned and unsectioned. As affirmed by both the transverse and longitudinal sections examinations, it was established that thermal attack leading to deterioration of the coating was greater at the tip and airfoils of the blades (the hotter zones) and lesser towards the bases (colder zones). As a result, severe degradation of the core material at the tips and airfoils compared to the bases and more prevalent at the leading edges than trailing edges at the tips. The results further suggest that both active outward Ni diffusion and inward Al diffusion can coexist during exploitation of the blades in service. The study illustrates the role played by the aluminide coating in early failure of CT blades with the aim of bettering the surface coatings and enhancing coating technologies, managing CT blade material monitoring as well as to give insights on advancing CT blades maintenance practices.

Low cycle fatigue performance of Ni-based superalloy coated with complex thermal barrier coating

Thermal barrier coatings (TBCs) are widely applied to protect high-temperature components against high temperatures in harsh environments. Nineteen cylindrical specimens of Inconel 713LC were manufactured using the investment castings technique, and 10 specimens were subsequently coated with a novel complex thermal barrier coating (TBC) system. The TBC system comprises a metallic CoNiCrAlY bond coat (BC) and a complex ceramic top coat (TC). The TC is a mixture of a near eutectic nanocrystalline ceramic made of zirconia (ZrO2), alumina (Al2O3), silica (SiO2) and conventional yttria stabilized zirconia (YSZ) ceramic in the ratio of 50/50 in wt%. Low cycle fatigue (LCF) tests were carried out in a symmetrical push-pull cycle under strain control at 900 °C. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves of the TBC-coated and uncoated material were assessed. Fatigue life curves in total strain representation showed transient behaviour. Fracture surfaces and polished sections parallel to the loading axis of the TBC-coated and uncoated specimens prior and after cyclic loading were observed by means of scanning electron microscopy (SEM) to study the degradation mechanisms during high-temperature LCF. TBC delamination was observed at the TC/BC interface, and rafting of precipitates occurred after high-temperature exposure. The microstructural investigations further the discussion of the differences in the stress-strain response and the fatigue life of the TBC-coated and uncoated superalloy.

Solidification behavior of inconel 713LC gas turbine blades during electron beam welding

In this study, a comprehensive microstructure analysis was carried out to determine the solidification behavior of Inconel-713LC gas turbine blades under a variety of cooling conditions during electron beam welding (EBW). An empirical correlation between the secondary dendrite arm spacing (SDAS) in the fusion zone and cooling rate was established. Additionally, liquation was observed in the heat affected zone (HAZ), which is induced by the segregation of elements at the grain boundaries.

High temperature creep properties of a linear friction welded newly developed wrought Ni-based superalloy

AD730™ Ni-based superalloy specimens in solution-treated conditions were linear friction welded. Then, post-weld heat treatment (PWHT), consisting of γ′ sub-solvus solution treatments followed by aging, was conducted on the linear friction welded samples. High temperature creep tests were performed on the as-welded and PWHTed joints at two different temperatures: 700°C under 600 and 750MPa stress levels, and 850°C under 100 and 200MPa stresses. The creep resistance of the PWHTed joints was higher than that of the as-welded samples. The PWHTed joints exhibited better ductility than that of the base material at 850°C, while they showed slightly lower creep life at 700°C in comparison to the base metal. Microstructure examination showed that cracks initiated at the interface of oxidized particles at 700°C. The decrease in creep resistance of the AD730™ Ni-based superalloy at 850°C was related to a combination of the formation of precipitate-free zones (PFZ) in the vicinity of the grain boundaries (GBs) and microcracking assisted by oxidation. The Larson-Miller Parameter (LMP) was used to correlate the creep strength, temperature and time to failure for the as-welded and PWHTed samples. LMP values varied between 21.5 × 103 and 24.5 × 103. It was found that in the investigated temperature range, the PWHTed AD730™ has similar creep characteristics as Udimet™720 Li and Inconel 738LC at low values of LMP and better creep properties than those of the Inconel 617 alloy at higher LMP values.

Hot corrosion behavior and near-surface microstructure of a “low-temperature high-activity Cr-aluminide” coating on inconel 738LC exposed to Na2SO4, Na2SO4 + V2O5 and Na2SO4 + V2O5 + NaCl at 900 °C

Hot corrosion is a serious problem in gas turbines due to poor quality fuels which contain Na, V, S and Cl. To resolve the problem, Cr-aluminide was coated on IN-738LC superalloy with a two steps pack cementation process. Oxidation behavior and near-surface microstructure of the coating showed consecutive increase in destruction by exposition to Na2SO4, 75Na2SO4+25V2O5 and 70Na2SO4+25V2O5+5NaCl (wt.%). Kinetic studies indicated parabolic corrosion rate in salt-less samples due to diffusion. Similar expression for salt-covered samples was assessed for oxide dissolution. Plate-like, broken-plate-like and cauliflower-like morphologies attributed to the corrosion products were observed after exposition to above three salts, respectively.

Hot corrosion behaviour of plasma-sprayed functionally graded thermal barrier coatings in the presence of Na2SO4 + V2O5 molten salt

Hot corrosion behaviour of plasma-sprayed functionally graded thermal barrier coatings (TBCs) was evaluated in this study. Two layer (duplex) and functionally graded ZrO2/NiCrAlY coatings (FGC) were deposited on the Inconel 738LC substrate by atmospheric plasma spraying (APS). The hot corrosion behaviour of coatings was assessed in the presence of a corrosive molten salt. The salt was composed of 45wt% Na2SO4+55wt% V2O5. The hot corrosion test was carried out at 800°C and samples were placed in an atmospheric air electrical furnace for 120h. The microstructure, phase composition and elemental analysis of the coatings were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and energy dispersive spectroscopy (EDS), respectively. Investigations revealed that the dominant hot corrosion mechanism of coatings in the presence of the molten sulphate-vanadate salt was destabilization of Yttria Stabilized Zirconia (YSZ), resulting in reactions between Y2O3 and the corrosive salt. YVO4, which contained the depleted Y2O3 of YSZ, was the main product of the reaction, motivating the destructive destabilization and transformation of YSZ. In this transformation, the structure of ZrO2 was changed from a tetragonal phase to a monoclinic one accompanied by volume change. Also, hot corrosion test results depicted the improved corrosion resistance of FGCs in comparison with the duplex coating due to the formation of AlV and CrVO4, which postponed the mentioned destructive transformation.

Effect of alumina-silica-zirconia eutectic ceramic thermal barrier coating on the low cycle fatigue behaviour of cast polycrystalline nickel-based superalloy at 900 °C

Cylindrical specimens of nickel-based superalloy Inconel 713LC in an as-cast condition and coated with a thermal barrier coating (TBC) system consisting of a CoNiCrAlY bond coat and an eutectic ceramic alumina-silica-zirconia top coat were cyclically loaded under strain control at 900°C in air. TBC was deposited using atmospheric plasma spray and water-stabilised plasma spray techniques for the bond coat and the top coat, respectively. The microstructure of the substrate material and the TBC was characterised and the hardness of TBC was measured. The cyclic stress-strain response and fatigue life of both materials were assessed. The stress response of the TBC-coated superalloy is lower than the uncoated superalloy. The detrimental effect of TBC on the Basquin curve is documented. The specimen section and fracture surface observations revealed fatigue damage mechanisms in both the TBC-coated and uncoated specimens, and that finding helps explain the differences in the fatigue behaviour of both materials.

Failure Assessment of the First Stage Blade of a Gas Turbine Engine

The gas turbine engine under investigation in this paper was in service after major overhaul for about 2 h at 9:47 A.M. on 18 October, 2014. The 60 MW gas turbine engine experienced a forced break down because of extremely high vibrations and subsequent output power reduction. The blade was made of Inconel 738LC nickel-based super-alloy. Evaluation of the microstructures of the root and tip of the damaged blade, showed no significant change in the microstructure. In closer observation of the fractured blade, some points affected by impact of the remaining airfoil were observed. Metallurgical investigations of the damaged zones of the fractured blade showed many iron rich zones near the fractured surface. The morphology of the fractured surface showed a semi-brittle fracture due to the impact. Finally, it was concluded that the main reason for the gas turbine failure was domestic object damage due to the impact of the liberated components of the turbine engine on the blades.

Corrosion of Inconel 738LC Alloy in Ar/0.2%SO2 Gas at 800–1000 °C

Corrosion of Inconel 738LC Alloy in Ar/0.2%SO2 Gas at 800–1000 °C

Comparison of Isothermal with Cyclic Oxidation Behavior of “Cr-Aluminide” Coating on Inconel 738LC at 900 °C

A two-step pack cementation technique was used to cover nickel-based IN-738LC superalloy with a Cr-aluminide coating layer. The IN-738LC/Cr-aluminide samples were oxidized at 900 °C for 350 and 700 h, isothermally, and 175 and 350 h, cyclically. Each cycle lasted for 25 h. Effects of the isothermal and the cyclic oxidation on (a) morphology, (b) new-phase formation, (c) weight gain, and (d) oxidation reactions were investigated by scanning electron microscopy, electron dispersive spectroscopy, X-ray diffraction, and weight gain measurements. From results, it was concluded that the oxygen attack was more severe during the cyclic oxidation than the isothermal procedure. Kinetics of the reactions indicated a parabolic rate law for the isothermal process, while the cyclic oxidation firstly followed a similar parabolic rate law which was trailed later on by a severe surface spallation.

Comparative Study of Microstructure and High Temperature Low Cycle Fatigue Behaviour of Nickel Base Superalloys Inconel 713LC and MAR-M247

The present work is focused on the study of microstructure and low cycle fatigue behavior of the first generation nickel-base superalloy IN 713LC (low carbon) and its promising second generation successor MAR-M247 HIP (hot isostatic pressing) at 900 °C. Microstructure of both alloys was studied by means of scanning electron microscopy (SEM). The microstructure of both materials is characterized by dendritic grains, carbides and casting defects. Size and morphology of precipitates and casting defects were evaluated. Fractographic observations have been made with the aim to reveal the fatigue crack initiation place and relation to the casting defects and material microstructure. Low cycle fatigue tests were conducted on cylindrical specimens in symmetrical push-pull cycle under strain control with constant total strain amplitude and strain rate at 900 °C in air. Hardening/softening curves, cyclic stress-strain curve and fatigue life data of both materials were obtained. Cyclic stress-strain curve of MAR M247 is shifted approximately to 120 MPa higher stress amplitudes in comparison with IN 713LC. Significantly higher fatigue life of MAR-M247 has been observed in Basquin representation. On the other hand IN 713LC shows prolonged lifetime compared with MAR-M247 in the Coffin-Manson representation. Results obtained from high temperature low cycle fatigue tests are discussed.