No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Performance of Inconel Alloy 617 in Actual and Simulated Gas Turbine Environments
Materials and Manufacturing Processes
Abstract
INCONEL® alloy 617 is a solid solution, nickel-chromium-cobalt-molybdenum alloy with an exceptional combination of high-temperature strength, oxidation and carburization resistance and thermal stability. In this paper, the performance of alloy 617 in actual and simulated gas turbine environments will be presented. In addition some mechanical property data will be included to show the stability of the alloy after exposure to high temperatures for an extended period of time. For comparison the data obtained on such alloys as INCO® alloy HX, Haynes alloys HS188®, 214® and 230® are also reported. (INCONEL and INCO are trademarks of the Inco family of companies. HS188, 214 and 230 trademarks of Haynes International).
... Inconel 617 is a tungsten-free nickel-based superalloy that is applicable in power generation, aerospace and nuclear industries due to its excellent mechanical properties, corrosion resistance, creep strength and oxidation resistance at high temperatures [1,2]. Inconel 617 has been successfully used for hot gas paths in gas and industrial turbines, and aerospace products for over four decades [3]. Various types of welding and brazing techniques have been used for repairing the defects of industrial components [4,5]. ...
... This means that this time has been sufficient for enough diffusion of MPD elements from the interlayer toward the base metal for occurrence of isothermal solidification in the entire interlayer. 3 The thickness of filler metal is a very effective parameter on the quality and strength of the joints. Increasing the thickness of filler metal needs further holding time for enough diffusion of elements, and elimination of liquid phase and resultant eutectic structure, considering the longer diffusion distances for the larger amount of the MPD elements. ...
In the present paper, vacuum transient liquid phase bonding of Inconel 617 alloy was studied. The experiments were carried out at 1135 °C for two different holding times of 45 and 300 min, and using Ni-Cr-Si-Fe-B filler metals with 50 and 100 μm thicknesses. The microstructure, shear strength, and microhardness of the resultant joints were investigated. The results showed that the complete isothermal solidification occurred after 300 min for both the filler metal thicknesses. The presence of eutectic phases and cavities in the interlayer and non-dissolved carbides in the diffusion affected zone were the main reasons for the reduction in the shear strength. By increasing the holding time, the carbides were dissolved and the hardness of the diffusion affected zone was reduced. The highest shear strength was obtained for the 50 μm thick interlayer and 300 min holding time (with about 96% joint efficiency).
... under the trade name of Inconel 617 as a material with an exceptional combination of high temperature creep and oxidation resistance [35][36][37]. It is one of a few materials with allowable design stresses specified by ASME Boiler and Pressure Vessel Code for operation up to 980°C [9,38]. ...
... 37 Surface XRD of the sample exposed in environment 1(CO/CO 2 = 9) at 1000°C for 100h. The expected positions of the high intensity peaks of Cr 2 O 3 are marked on the plot with the diamond symbol in red color. ...
The objective of this research was to determine the mechanism of decarburization and carburization of alloy 617 by determining the gas-metal reactions. Binary gas mixtures containing only CO and CO2 as impurities were chosen to circumvent the complications caused by impurities H2, H2O, and CH4, normally, present in helium in addition to CO and CO2; and oxidation tests were conducted between 850 C-1000 C in six environments with CO/CO2 ratio varying between 9 and 1272.
A critical temperature corresponding to the equilibrium of the reaction
2Cr + 3CO Cr2O3 + 3Csolution
was identified. Below the critical temperature the alloy reacted with CO resulting in formation of a stable chromia film and carburization, whereas, above the critical temperature the decarburization of the alloy occurred via reaction between the chromia film and carbon in the alloy producing CO and Cr. In environment with CO/CO2 of 9 the critical temperature was between 900 C and 950 C, whereas, in environment with CO/CO2 ratio higher than 150, it was greater than 1000 C.
The decarburization of the alloy occurred via two reactions occurring simultaneously on the surface:
2Cr + 3/2O2 Cr2O3
Cr2O3 + 3Csolution 2Cr + 3CO.
At 1000 C, the rate liming step was the formation of chromia which prevented the growth of chromia film until the carbon in the sample was depleted. The time taken for this to occur was 300h.
The carburization of the alloy resulted in the formation of mixed Cr2O3 and Cr7C3 surface scale. The Cr7C3 was a metastable phase which nucleated due to preferential adsorption of carbon on the chromia surface. The Cr7C3 precipitates coarsened at the gas/scale interface via outward diffusion of Cr cations through the chromia scale until the activity of Cr at the reaction site fell below a critical value. Decrease in activity of Cr at the carbide/chromia interface triggered a reaction between chromia and carbide:
Cr2O3 + Cr7C3 9Cr + 3CO.
The CO so produced was transported through the oxide cracks and pores and was released into the environment. Chromium diffused outward from the reaction site to the gas/scale interface where it was re-oxidized
... Therefore, to understand creep resistance of the candidate superalloys, the oxidation behaviour and the stability of oxides should be properly understood. Accordingly, there have been several studies on the hightemperature oxidation of superalloys in various helium environments [7][8][9][10][11][12][13][14][15][16]. However, the evolution of oxides and microstructural changes of Alloy 617 during hightemperature oxidation have not been clearly identified. ...
... However, at 1000 1C, they observed initial weight loss caused by decarburization prior to stable weight gain. On the other hand, Ganesan et al. [9] reported similar behaviour to that observed in our study, although the turnaround time was about 3 times larger in air+10% water vapour at 1100 1C. Therefore, it could be said that the Cr-rich surface oxides would be stable in oxidizing environments at up to 1000 1C, but became unstable at 1100 1C. ...
Nickel-base superalloys are considered as materials for piping and structural materials in a very high temperature gas cooled reactor (VHTR). They are subjected to the environmental degradation caused by a continuous process for oxidation due to small amount of impurities in He coolant during long term operation. In the present study, the oxidation behaviors of several nickel-base superalloys such as Alloy617, Haynes214 and Haynes230 in particular, were studied at the temperature of 900°C and 100°C in air, the high purity He environment. Oxide layers were analyzed by SEM and EDX. The differences in oxidation behaviors of these alloys were mainly caused by different protective oxide layers on surface. In the case of Alloy617 and Haynes230, Cr 2O3 layer formed on the surface which is not stable at 1100°C. Therefore, the weight increased significantly due to oxidation at the initial stage, which followed by a decrease due to the spoiling and volatilization of Cr2O3 layer. On the other hand, since Haynes214 has mainly Al2O3 oxide layer on surface which is more stable and dense structure at the higher temperature, the weight gain eventually reaches to parabolic. Microstructural characteristics of internal carbides and carbide depletion zone were analyzed. With oxidation time, continuous grain boundary carbides of M23C6 type were getting thin or it disappeared partially. Especially, carbides on grain boundary disappeared entirely below oxide layer (carbide depletion zone). It was getting wide with oxidation time. For Haynes214, the size of carbide depletion zone was smaller than other alloys because Al2O3 layer acted as a diffusion layer prevented effectively to penetration of oxygen into base metal.
... Alloys for the hot sections of land-based gas turbines and aero-engines must have high temperature and cyclic oxidation resistance as well as an optimum combination of stress rupture and fatigue strength [1, 2]. Superalloys, especially nickel base superalloys, are excellent candidates for such applications as they exhibit outstanding strength and surface stability at temperatures up to 85% of their absolute melting points [3]. ...
... Superalloys, especially nickel base superalloys, are excellent candidates for such applications as they exhibit outstanding strength and surface stability at temperatures up to 85% of their absolute melting points [3]. Alloy 617 is a solid solution strengthened nickel base superalloy widely used in the aerospace and power generation industries because of its superior mechanical properties, corrosion, and oxidation resistance at elevated temperatures [1, 2,4567. The alloy is known for its good mechanical properties and a stable microstructure, especially after high temperature service when compared with Alloy 625 and HastelloyX [8]. ...
Transient liquid phase bonding was used to join Alloy 617 alloy using nickel-based filler metal (Ni–P 11 wt%) with P as the melting point depressant element. The influences of interlayer thickness, 25.4 and 38.1 μm, bonding temperature, 1,065 and 1,150 °C, and hold time, 5 min to 24 h, on microstructure evolution in the joint area, the diffusion zone and the base metal were investigated. Specifically, the composition, type, and crystallography of precipitates were determined and their evolution was related to process variables. Joints were characterized using optical and electron microscopy as well as microhardness testing. The results were analyzed in the framework of the diffusion, classical nucleation and growth theory, and the operating mechanisms identified. Based on the results of the investigation, uniform microhardness and microstructure across the joints was obtained with the 25.4-μm-thick interlayer bonded at 1,150 °C during 24 h.
... Therefore, to understand creep resistance of the candidate superalloys, the oxidation behavior and the stability of oxides should be properly understood. Accordingly, there have been several studies on the high temperature oxidation of superalloys in various helium environments [7][8][9][10][11][12][13][14][15][16]. However, the evolution of oxides and microstructure changes of Alloy617 during the high temperature oxidation have not been clearly identified. ...
... However, at 1000 o C, they observed initial weight loss caused by decarburization prior to stable weight gain afterward. On the other hand, Ganesan et al. [9] reported a similar behavior as observed in our study, though the turnaround time was about 3 times larger in air + 10% water vapor at 1100 o C. Therefore, it could be said the Crrich surface oxides would be stable in oxidizing environments at up to 1000 o C, but became unstable at 1100 o C. ...
The oxidation characteristics of Alloy 617, a candidate structural material for the key components in the very high-temperature gas-cooled reactor (VHTR), were investigated. High-temperature oxidation tests were conducted at 900 and 1100°C in air and helium environments and the results were analysed. Alloy 617 showed parabolic oxidation behaviour at 900°C, but unstable oxidation behaviour at 1100°C, even in a low oxygen-containing helium environment. The SEM micrographs also revealed that the surface oxides became unstable and non-continuous as the temperature or the exposure time increased. According to the elemental analysis, Cr-rich oxides were formed on the surface and Al-rich discrete internal oxides were formed below the surface oxide layer. After 100h in 1100°C air, the Cr-rich surface oxide became unstable and non-continuous, and the matrix elements like Ni and Co were exposed and oxidized. Depletion of grain boundary carbides as well as matrix carbides was observed during the oxidation in both environments. When tensile loading was applied during high-temperature oxidation, the thickness of the surface oxide layer, the internal oxidation, and decarburization were enhanced because of the increase in diffusion of oxidizing agent and gaseous reaction products. Such enhancement would have detrimental effects on the high-temperature mechanical properties, especially the creep resistance of Alloy 617 for the VHTR application.
... [21] [22] [23] [24] ...
... [17] [18] [19] [20] [21] [22] [23] [24] ...
This paper was presented at, and appears in the Proceedings of, the 2nd ECCC International Conference on Creep and Fracture in High Temperature Components: Design and Life Assessment Issues, 21 Apr 2009-23 Apr 2009, Zurich, Switzerland. It is available from http://www.iom3.org/events/2nd-eccc-international-conference-creep-and-fracture-high-temperature-components-design-and-l Inconel alloys are currently being investigated for high temperature applications such as HP and IP valve chest and rotor forgings in advanced steam power plant operating at temperatures of 700°C and above. One of the preferred alloys for these components is IN617. This is a solid solution strengthened austenitic Ni-based alloy containing ~23% Cr, 12% Co, and 9% Mo with small additions of Ti and Al which can contribute some additional precipitation strengthening. In the solution treated condition, the microstructure consists of equiaxed austenite containing M23C6 at the grain boundaries and occasional TiN particles within the matrix. Owing to high temperature exposure and the creep deformation processes that occur in-service, evolution of the microstructure occurs in the form of precipitation, precipitate coarsening and recovery effects. This paper discusses microstructural evolution occurring in this alloy in samples that have been exposed to temperatures up to 700°C and for durations up to 45,000 hours using advanced FEGSEM, TEM and XRD techniques.
... During cyclic oxidation tests of these MMC coatings conducted at 1100 • C with PAC applied between the cycles, a significant change in oxidation resistance was observed, primarily attributable to the presence of dispersed yttrium oxide particles (Fig. 12). As a reference, uncoated IN617 (Fig. 11a) showed mass loss after 80 cycles indicating characteristic chromia spallation [49]. Al diffusion-coated IN617 (Fig. 11a) showed a negative mass change after only 20 to 40 cycles, also due to significant oxide spallation [50]. ...
Incorporating reactive elements (RE) into turbine coatings is a well-established surface treatment. However, suboptimal RE concentrations can lead to compromised strength, heightened brittleness, and reduced adhesion. In contrast, RE oxides offer advantages of avoiding these detrimental effects, counteracting corrosion phenomena induced by V2O5 compounds and enhancing oxidation resistance. A notable challenge lies in optimizing RE oxide particle incorporation and understanding the influence of particles in coating microstructures. This study focuses on developing Nisingle bondAl and Ni-Cr-Al type metal matrix composite (MMC) coatings on Inconel 617 (IN617), containing up to 11 vol% of Yttria (Y2O3) nanoparticles. Y2O3 nanoparticles and Ni were co-electrodeposited on IN617 followed by either pack aluminizing or a two-step chromizing and aluminizing process. An even distribution of Y2O3 nanoparticles was observed throughout the entire 100 μm coating thickness, leading to significant grain refinement in the sub-micron to nano range in both coating types. Y2O3-strengthened coatings were subjected to oxidation at 1100 °C and hot corrosion at 700 °C and were compared to their Y2O3-free counterparts. Present at grain boundaries, Y2O3 markedly enhanced the oxidation and corrosion resistance by reducing interdiffusion, improving the oxide scale adherence and binding V2O5, highlighting the potential of this method for advanced turbine blade coatings.
... 617 alaşımı, yüksek sıcaklık dayanımı ve oksidasyon direncinin olağanüstü bir kombinasyonuna sahip katı çözelti, güçlendirilmiş nikel-krom-kobalt-molibden alaşımıdır (Hussain, Shadid, Khan, & Rahman, 1995), (HAYNES 617 alloy, 2022). Bu nedenle, oksitlerin oksidasyon davranışını ve kararlılığını anlamak için, alaşımların hava (Sharma, Ko, Li, & Kang, 2008), (Sharma, Li, Ko, & Kang, 2010), (Tung & Stubbins, 2012), (Al-Hatab, Al-Bukhaiti, & Krupp, 2014), (Jang, Lee, & Kim, Oxidation behaviour of an Alloy 617 in very high-temperature air and helium environments, 2008) ve çeşitli helyum atmosferlerinde (Jang, et al., 2011), (Hussain, Shadid, Khan, & Rahman, 1995), (Jang, Lee, & Kim, Oxidation behaviour of an Alloy 617 in very high-temperature air and helium environments, 2008), (Martins, Hosier, & Hassford, 1974), (Ganesan, Smith, & Yates, 1995), (CHIN, JOHNSON, & CHEN, 1982), (Kim, Lee, JEONG, KIM, & PARK, 2011), (Kim, Jang, & Ryu, 2009) yüksek sıcaklıkta oksidasyonu üzerine birkaç çalışma yapılmıştır. Simüle edilmiş bir VHTR atmosferindeki korozyon hasarı Bates (Bates, 1984), Christ ve diğerleri (Christ, Künecke, Meyer, & Sockel, 1988), (Christ, Künecke, Meyer, & Sockel, High-Temperature Corrosion of the Nickel-Based Alloy Inconel-617 in Helium Containing Small Amounts of Impurities, 1987), Jang (Jang, Lee, & Kim, Oxidation behaviour of an Alloy 617 in very hightemperature air and helium environments, 2008) tarafından incelenmiştir ve Cabet ve diğerleri (Cabet, Terlain, Lett, Guetaz, & Gentzbittel, 2006), (Cabet, et al., 2008), (Cabet & (Kim, Jang, & Ryu, 2009), Jang (Jang, Lee, & Kim, Oxidation behaviour of an Alloy 617 in very hightemperature air and helium environments, 2008) ve Wright (Wright, 2006) tarafından yüksek sıcaklıktaki helyum ve havada kısa süreli ve uzun süreli korozyonlar üzerine araştırmalar rapor edilmiştir. ...
... To achieve this, we propose a new processing schedule specifically targeted to combine both GBE and GBS approaches to attain an op-timized microstructure in Alloy 617, a Ni-based superalloy with lowto-medium stacking fault energy. This alloy is extensively employed in high-temperature applications like gas turbines [31] , thermal power plants [ 32 , 33 ], etc., where high-temperature hot corrosion (HTHC) phenomenon is a prime degradation mechanism responsible for the component failures. It is caused by the usage of low-grade fossil fuels which contain harmful impurities like Cl, S, Na, V, etc. [ 34 , 35 ]. ...
In this work, a novel idea of combining two established microstructural engineering approaches viz., grain boundary engineering (GBE) and grain boundary serration (GBS) through optimization of thermomechanical and thermal processing in Alloy 617 is investigated and the superior resistance of GBE+GBS microstructure to the high-temperature hot corrosion is demonstrated. To achieve the GBE+GBS microstructure, the GBS treatment was introduced as a part of the GBE processing schedule (i.e., incomplete GBE+GBS) and following the GBE treatment (i.e., complete GBE+GBS). The extent of GBS is found to be similar in all the processed specimens. However, the extent of GBE is observed to be lower in the specimens undergoing incomplete GBE+GBS. This is due to the occurrence of recrystallization and consequent infrequent multiple twinning. On the other hand, a higher extent of GBE is achieved in the specimens subjected to complete GBE+GBS owing to the retention of the optimized GBE microstructure following the GBS treatment. The synergistic influence of GBE and GBS on the hot corrosion behavior is assessed by exposing the as-received (AR) as well as optimized grain boundary engineered and serrated (GBES) specimens to a salt mixture at 1273K. The percolation depth after 24h and 48h exposure is significantly lower in the GBES specimen (∼55 µm and ∼115 µm, respectively) than the AR condition (∼305 µm and ∼630 µm, respectively). This is ascribed to the incorporation of Σ3ⁿ (n ≤3) and serrated boundaries in the GBES microstructure which obstructed the infiltration of harmful species into the alloy.
Texture analysis in nanocrystalline materials is a challenge. A simple technique as Selected Area Electron Diffraction (SAED) has been used to evaluate texture in a single nanocrystalline Alloy 617 ODS powder particle by carrying out Rietveld texture refinement using Material Analysis Using Diffraction (MAUD) software package and the results were compared with the ASTAR™/Precision Electron Diffraction (ASTAR™/PED) results. Extraction of texture from the SAED patterns involved image analysis and Rietveld refinement of radial plots divided into judiciously chosen angular spans. Inverse pole figure (IPF) from SAED pattern using MAUD revealed 〈110〉 texture parallel to the normal direction (electron beam direction) for unmilled and 6 h milled powders. IPF from ASTAR™/PED technique also shown 〈110〉 lying parallel to the normal direction (beam direction) in both unmilled and the 6 h milled powder. A one is to one correspondence of IPF obtained from MAUD to that of ASTAR™/PED was observed. Texture in unmilled powder could be attributed to the compressive force during rapid solidification in water atomisation process. During the milling process, inhomogeneous rolling and compression of the trapped powder particle in between the balls and between the balls and walls of the jar resulted in shear texture in 6 h milled powder. ASTAR™/PED technique also showed the presence of 〈100〉 //ND texture which was attributed to solidification in unmilled powder and 〈112〉 //ND type texture in milled powder attributed to twin mode of deformation during milling.
Finite element (FE) simulation of grained material with equiaxed grain distribution is of interest for the virtual prototyping of array structures and the assessment of signal processing algorithms. Construction of such models can be computationally intensive due to the large number of crystallographic orientations required to represent the material. This paper concentrates on analysis and processing of orientation data in order to establish a computationally efficient 2D FE model whilst maintaining appropriate accuracy of the grained structure. Two approaches for orientation processing are proposed and their performances are compared. Parametric studies show that the trade-off between computational overhead and model accuracy will reach the optimal point when Euler space is segmented with a bin size of 15 degree per Euler phase. A transducer array is then incorporated into the FE model to generate B-scan image of the material. The image is compared with experimental equivalent for FE model validation purpose. The minor difference of images proves that the constructed FE model is accurate, highlighting the potential of the proposed methods for application on other equiaxed-grain materials.
INCONEL@ alloy 625 has excellent fabricability because of its very high ductility (over 50% elongation) even at yield strengths as high as 414 MPa (60 ksi). Alloy 625 also has the unique combination of strength, ductility and versatile corrosion resistance that it has found many applications in the chemical process industry. It is melted in the electric arc furnace (EAF) and transferred to the argon oxygen decarburization (AOD) vessel for refining. It is further refined by electroslag remelting (ESR). Over the last 25 years many improvements have taken place in the AOD+ESR processing that it was decided to characterize the current production plate produced by the AOD+ESR process for its microstructure and mechanical properties. Also after fabrication it is important to restore the alloy's strength and corrosion resistance to its full potential. In addition to the characterization data, this paper will therefore also cover the results of a study conducted to develop an optimum post-weld heat treatment to retain high strength and corrosion resistance.
The role of temperature and load ratio (R) on the crack propagation rate (da/dN) of Alloys 276 and 617 under cyclic loading was investigated. The results indicate that the rate of cracking was gradually enhanced with increasing temperature when the R value was kept constant. However, the temperature effect was more pronounced at 100-150 degrees C. Both alloys exhibited maximum da/dN values at a load range of 4.5 kN that corresponds to an R value of 0.1. The number of cycles to failure for Alloy 276 was relatively higher compared with that of Alloy 617, indicating its slower crack-growth rate. Fractographic evaluation of the broken specimen surface revealed combined fatigue and ductile failures.
Ni base alloys such as IN617 are one of the preferred choices for steam turbine components used by fossil fuelled power generation plants. IN617 is a solid solution strengthened Ni based superalloy containing ∼23%Cr, 12%Co and 9%Mo with low content of precipitation strengthening elements Al, Ti and Nb. In the 'as received' (solution annealed condition), the microstructure consists of primary carbides (M23C6) and occasional TiN particles dispersed in a single phase austenitic matrix. Owing to high temperature exposure and the creep deformation processes that occur in service, evolution of the microstructure occurs. This results in secondary precipitation and precipitate coarsening, both on grain boundaries and intragranularly in areas of high dislocation density. The influence of creep deformation on the solution treated IN617 alloy at an operating condition of 650 ∼C/574 h, with emphasis on the morphology and distribution of carbide/nitride precipitation is discussed. The applied stress was at an intermediate level.
Anomalous increase in tensile yield strength (YS) of Alloy 617 at 800 and 900 °C has been investigated using different analytical tools. The enhanced YS has been attributed to the formation and accumulation of Cr23C6 and Mo23C6 precipitates, and development of pinned dislocations, dislocation pile-ups, and subgrains in the vicinity of the grain boundaries.
The influence of process parameters on microstructural characteristics of transient liquid phase (TLP) bonded Inconel 617 alloy was investigated. Experiments were carried out at 1065 °C using nickel based filler metal (Ni–4.5% Si–3% B) with B as the melting point depressant (MPD) element. Two different thickness of interlayer and various holding times were employed. The influence of these processing parameters on the characteristics of the joint area particularly size, morphology and composition of precipitates was investigated.The presence of MoB, Mo2B, M23C6, TiC, M23(B, C)6 and Ni3B precipitates in the diffusion layer and Ni3B, Ni3Si and Ni5Si2 precipitates in the interlayer at the interface between the base metal and interlayer were demonstrated using electron back scattered diffraction (EBSD), energy dispersive spectrometry (EDS) and TEM.
Micostructural characterization has been performed on a TLP (Transient Liquid Phase) bonded Inconel 617 superalloy. In the present work, the influences of holding time, bonding temperature and thickness and composition of the interlayer on microstructure, extent of the diffusion layer, morphology and composition of the precipitates in TLP bonded Inconel 617 were studied. Ni-11% P and Ni-4.5% Si-3 B% alloys with two different thiknesses were employed as the interlayer. Finally, two bonding temperatures, 1065°C and 1150°C, were used in the experiments. Microstructural examination was carried out using optical microscopy, SEM, FEG-SEM, and EBSD (electron back scattered diffraction) to identify the phases present, principal morphologies of the precipitates, and the characteristics of the diffusion layer with processing parameters. XRD was also employed to confirm the presence of precipitates in the joint area. Microstructural analysis showed that with increasing holding time the size of precipitates changed from coarse to fine. The growth of precipitates was observed to begin at grain boundaries and migrated to the interior of the grains via volume diffusion.
The study of the processes involved in the high-temperature reactions of elementary metals with gases dates from the early 1920's when Tammann, and also Pilling and Bedworth, reported that a number of metals absorb oxygen according to a parabolic rate law.
Oxide/substrate adhesion exerts a decisive influence on the oxidation resistance of a material. Poor adhesion can lead to oxide spallation in response to thermal cycling, resulting in enhanced oxidation rates. On the example of Ni-15. 1Ta-6. 7Al alloys, some of them doped with 0. 04% Y or 1. 5% Hf, it is shown that oxide spallation during thermal cycling is frequently preceded by the development at temperature of cavities between the oxide scale and the alloy substrate. Furthermore, ″pegging″ of the oxide scale to its substrate by internal oxidation of selected elements can result in improvement in oxide adherence and thereby in oxidation resistance.
The corrosion of iron-, nickel-, and cobalt-base alloys has been studied in atmospheres containing carbon and oxygen in the temperature range 894–1366 K. It was observed that preformed Cr2O3 films are not effective barriers to carbon transport in atmospheres in which the oxide is not stable but that stable, growing Cr2O3 films are excellent barriers to carbon penetration. The presence of Fe-containing oxides on Fe-Ni-Cr and Fe-Cr alloys cause the scales to be permeable to carbon. This phenomenon was found to be sensitive to alloy surface preparation. Carbon transport through oxide scales may occur by two mechanisms: diffusion or molecular transport through physical defects. The present work has evidence of the latter but cannot rule out the former in cases where the carbon activity is sufficiently large. In gases containing CO and CO2 in which Cr carbide is stable Cr2O3 was found to form at the carbide-alloy interface by oxygen transport through the carbide. In A-CH4 Fe-Ni-Cr were found to undergo graphitization attack. The results were consistent with the formation and subsequent decomposition of metastable carbides, as proposed by Hochmann.
The effect of small amounts of yttrium (up to 1 wt. %) and hafnium (up to 1.5 wt.%) on the oxidation behavior of Co-Cr-Al alloys in the temperature range 1000–1200C for times up to 1000 hr in air has been studied. The major portion of the study has been concerned with Co-10Cr-11Al base alloys. Both isothermal and cyclic tests have been carried out; the cycle used consisted of 20 hr at temperature, followed by cooling to room temperature. Both additions reduce the overall oxidation, Hf somewhat more so than Y. In part, this is due to the improved adhesion between scale and alloy reducing scale spallation at temperature, and in part due to possible modification of the Al2O3 grain size. The former factor is far more critical under thermal cycling conditions. Under isothermal conditions the oxidation rate increases with increasing Hf content with all but the 1.5 wt.% alloy oxidizing more slowly than the Hf-free alloy; increase in Y content has the reverse effect. Under thermal cycling conditions the 0.3 and 1.0 wt.% Hf alloys show the lowest overall weight gain. Metallographic evidence suggests that the improved scale adhesion is due principally to a pegging mechanism; the active elements promote the growth of intrusions of Al2O3 into the alloy. However, if the intrusions are too large, they can act as initiators of scale failure.
The oxidation behavior of aluminum-implanted Ni-25Cr and Ni-25Cr containing 1 wt.% Al has been studied at 1000C and 1100C in oxygen. As did Y alloying addition or Y-implantation, 1 wt.% Al added to Ni-25Cr prevented nodular formation of Ni-containing oxides, improved spalling resistance of the scale upon cooling to a similar degree, and eliminated the formation of large voids between the alloy and the scale at the oxidation temperature. However, the Al addition did not alter the rate of growth of the Cr2O3 scale, nor did it change the growth direction. Al-implantation produced no effect even when the maximum concentration and depth of penetration were adjusted to be identical with those of the yttrium in the Y-implanted alloy. The implications of these results concerning the reactive element effect are discussed.
High strength heat resistant alloys owe their high temperature corrosion resistance to the scales that form under the environmental
conditions to which they are exposed. Because of the trend to higher temperatures, six alloys—INCONEL alloy 617, INCO alloy
HX, NIMONIC alloy 86, INCOLOY alloy MA 956, and Haynes alloys 188 and 214—were selected for study in three environments: air
plus 5% water vapor, oxygen plus 4% SO2, and JP-5 fuel in a burner rig. The alloys were evaluated by conducting mass change analyses, and the chemical nature of
the scales was determined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy
(EDX). In this paper the mass changes produced by exposure to these environments are reported and the results are correlated
with the chemical nature of the scales formed during testing. The results indicate that alloy MA 956 performed exceptionally
well in all three environments. Its superior performance is attributed to the alumina scale adherence, which is promoted by
an yttria dispersoid that is distributed evenly by the mechanical alloying process.
The carburization of NiCr 32 20 and NiCrSi 60 16 has been studied in CH4-H2 mixtures in the temperature range 900–1100C. The methods included thermogravimetric measurements and studies on reacted specimens by X-ray diffraction, metallographic, and chemical analysis. Upon carburization internal carbides M7C3 and M23C6 are formed (M=mainly Cr); the rate of carburization is determined by carbon diffusion in the Fe-Ni matrix with carbide precipitations. The effect of the alloying elements Ni and Si on the carburization resistance of austenitic alloys is explained. By the same methods the oxidation and carburization in CO-H2O-H2 mixtures have been studied. The important role of a stable chromium oxide layer for the carburization resistance was confirmed. Creep tests at 1000C in a CO-H2O-H2 atmosphere where Cr2O3 is stable showed carburization occurring through cracks in the oxide layer. At high strain rates premature failure occurs by carburization, which is followed by internal oxidation and formation of cracks, voids, and holes.
“ INCOLOY alloy MA956 - A Material for Advanced Ethylene Production Processes
- R H Kane