Inconel 740H Research Articles

Microstructure and Mechanical Properties of Inconel 740H after Long-Term Service

Inconel 740H is a nickel-based alloy for pressure components of ultra-supercritical boilers. Its chemical composition and strengthened matrix, as well as corrosion resistance, provide the highest creep resistance among the materials recommended for use in high-performance pressure components of power units. This paper investigates the changes in the microstructure and mechanical properties after ageing at 700 and 750 °C for 1000, 10,000, 20,000, and 30,000 h. Observation of the microstructure was performed using scanning and transmission electron microscopy. The identification of existing precipitates was conducted by X-ray phase analysis. The effects of time and ageing at elevated temperatures on the mechanical properties and precipitation process in the test alloy are discussed. The presented results are part of the material characteristics of the new-generation alloys to be used in the design of pressure equipment for steam boilers, as well as in diagnostic work during operation.

Influence of the high-temperature aluminizing process on the microstructure and corrosion resistance of the IN 740H nickel superalloy

The article discusses the effect of high temperature aluminizing processes on microstructure and corrosion resistance in 0.1 M Na2SO4 solution at room temperature of the IN 740H nickel superalloy. The aluminizing processes were carried out by chemical vapor deposition (CVD): (a) in AlCl3 vapors, (b) in a mixture of AlCl3+ZrCl3 vapors under a hydrogen atmosphere as carrier gas under the same conditions (process temperature −1040 °C, time-7h under reduced pressure of 150 hPa). Observations of the surface of the produced layers showed that the additional modification of the aluminizing process with zirconium affects the morphology of the layers formed as compared to the unmodified aluminizing process. It was also found that the prepared aluminide layers unmodified and modified additionally with zirconium improve corrosion resistance of the Inconel 740H nickel superalloy. On the basis of the conducted tests, it was also found that after high-temperature aluminizing processes in the IN 740H substrate precipitates are formed at the grain boundaries containing carbide forming elements such as: Cr, Ti, Nb, which may form secondary carbides, e.g. M6C, M7C3.

On the identification of eigenstrain sources of welding residual stress in bead-on-plate inconel 740H specimens

The main source of welding residual stress is identified by investigating the distribution of permanent plastic strains in as-welded and post-weld heat treated specimens of Inconel 740H. For this purpose, inverse eigenstrain problem is solved using experimental displacement data obtained by high precision coordinate measuring machine measurements from the surface of transversal wire-cut of electric discharge machining. A new multi-component iterative inverse eigenstrain model is developed and, in total, three different inverse eigenstrain models are analysed to get a high-quality fit with experimental data and have a reasonable prediction of residual stresses in the whole body of nonuniform bead-on-plate specimen design. In order to determine the boundaries of permanent plastic strains, which are the main source of welding residual stress, eigenstrain distribution size is analysed in terms of mean squared error using three models. The distribution size that provides the best match between the calculated displacements and the measured deplanation on the surface of cut is determined. The multi-component iterative eigenstrain reconstruction model validated itself by providing a good agreement with experimentally determined displacement and residual stress profiles, and this model is used to predict residual stress distribution in the whole body.

Characterization of Microstructures and Creep Fracture Characteristics of Dissimilar Metal Welds between Inconel 740H Ni-Based Superalloy and Gr.92 Ferritic Steel

Characterization of Microstructures and Creep Fracture Characteristics of Dissimilar Metal Welds between Inconel 740H Ni-Based Superalloy and Gr.92 Ferritic Steel

Transition of creep damage region in dissimilar welds between Inconel 740H Ni-based superalloy and P92 ferritic/martensitic steel

Characterization of microstructures and creep fracture characteristics of dissimilar welds between Ni-based Inconel 740H superalloy and ferritic/martensitic P92 steel has been investigated. The welds were produced by shielded metal arc (SMA) welding process with the AWS A5.11 Class ENiCrFe-3 filler metal, commonly known as Inconel 182 superalloy. Postweld heat treatment (PWHT) at 760 °C for 4 h was conducted to temper the martensite in heat-affected zone (HAZ) of P92 and to form γ′ strengthener in Inconel 740H. The deposited Inconel 182 austenitic weld metal had a columnar microstructure, and grew epitaxially from the Inconel 740H base metal. The weld interface between P92 and weld metal was characterized by the discrete line with a minimal inter-diffusion, where the fresh untempered martensite was found to be formed during PWHT due to higher Ni contents diffused from Inconel 182 into P92. The P92 base metal and Inconel 182 weld metal displayed low hardness values (~240 Hv), and the sharp hardness increase was detected at coarse-grained HAZ of P92 while the hardness minimum occurred at the fine-grained HAZ (FGHAZ). Fracture location after creep was found to shift from the P92-sided fusion line to the FGHAZ of P92 with increasing creep temperature. The carbon migration from the P92 into the weld metal was not significant at lower creep temperature (600 °C), where the crack initiation at the P92-sided fusion line appeared to be related with a significant strain incompatibility across the fusion line. At both 650 °C and 700 °C where diffusion-controlled creep was more pronounced, the type IV cracking was observed. This may be attributed to the strain localization at the weak grain boundaries of FGHAZ due to the smaller grain size as well as the lack of grain boundary strengthening by carbides.

High Temperature Corrosion of Ni Based Alloy Inconel 740H in Simulated Coal-ash and Flue-gas Environments

The corrosion behavior of the Ni-based superalloy Inconel 740H exposed to synthetic coal-ash/flue-gas at both 750°C and 780°C for 5000h was studied. The microstructures of samples were characterized by using XRD, SEM with EDS. The results indicate that the 740H alloy exhibits stable microstructure after long time aging. The overall corrosion of specimens was divided into two successive stages. In initial stage, the Cr2O3 protective film formed on the surface of the samples and became thicker slowly and had a good adherence to the matrix. In propagation stage, the Cr2O3 was dissolved by the composite sulfate. This resulted in the low temperature hot corrosion of alloy. The corrosion layers were composed of inner sulfide layer, oxide layer and outer loose layer. The alloy corrosion at 780°C was more severe than that of at 750°C.

Inconel 740H 니켈기 초내열합금과 TP316H 스테인리스강의 이종금속 SMA 용접부의 미세조직과 크리프 파단 특성

The microstructures and the creep rupture properties of dissimilar welds between the Ni-based superalloy Inconel 740H and the non-stabilized austenitic stainless steel TP316H have been characterized. The welds were produced by shielded metal arc (SMA) welding process with the AWS A5.11 Class ENiCrFe-3 filler metal, commonly known as Inconel 182 superalloy. Postweld heat treatment at 760 °C for 4 hours was conducted to form γ΄ strengthener in Inconel 740H. The austenitic weld metal produced by Inconel 182 had a dendritic microstructure, and grew epitaxially from the both sides of Inconel 740H and TP316H base metals. Since both Inconel 740H and TP316H did not undergo any solid-state transformation during welding process, there were no heat-affected-zone (HAZ) sub-regions and the coarsoned grains near the weld interface were limited to a narrow region. The hardness of Inconel 182 weld metal was ~220 Hv. The gradual hardness decrease was detected at HAZ of TP316H, and the TP316H base metal displayed the lowest hardness value (~180 Hv) whilst the Inconel 740H showed the highest hardness value (~400 Hv). Fracture after creep occurred at the center of weld metal, regardless of creep condition. It was found that during creep the cracks initiated and propagated along interdendritic regions and grain boundaries at which Laves particles enriched in Nb, Si and Cr were present. The appropriate design of weld metal was discussed to suppress the creep-induced cracking of the present dissimilar weld. Key words: Superalloy, Stainless steel, Dissimilar weld, Microstructure, Creep

Microstructure and phase precipitate behavior of Inconel 740H during aging

The microstructure and phase precipitate behavior and their effects on the room temperature hardness and impact toughness of Inconel 740H aged at 750 °C for 10000 h were investigated by SEM, TEM and mechanical analysis. The as-received alloy shows a low hardness value of HB 168 and a highest toughness value of 96 J. After an aging treatment at 800 °C for 16 h and cooled in air (standard heat-treated condition), fine γ′ phase particles precipitate within the grains and small carbide particles are located at the grain boundaries. The hardness increases to HB 304 and the impact toughness decreases to 15 J after standard heat treatment. A maximum hardness value of HB 331 is achieved for the alloy aged at 750 °C for 300 h. With increasing the aging time from 300 to 10000 h, a decrease of the hardness and toughness is observed along with an enhanced quantity of M23C6 particles and the coarsening of γ′ phase.

Corrosion Resistance of the Inconel 740H Nickel Alloy after Pulse Plasma Nitriding at a Frequency of 10 kHz

The microstructure and corrosion resistance of the CrN, Cr2N layer produced by pulse plasma ion nitriding at a frequency of 10 kHz on the Inconel 740H nickel superalloy were examined. The microstructure of the layers were characterized by light microscope and scanning electron microscopy. The corrosion resistance was examined by the impedance and potentiodynamic methods in a 0.1 M Na2SO4 solution and an acidified (pH = 4) 0.1 M NaCl solution at room temperature. The layer obtained, about 4.5 μm thick, contains CrN, Cr2N phases. It is continuous on the entire cross-section of the sample and adheres well to the substrate. However, in the chloride environment it constitutes an inhomogeneous cathodic coating for corrosion and thereby intensifies pitting corrosion of the material. In chloride-less environment, on the other hand, the layer has no essential effect on the corrosion rate of the Inconel 740H nickel alloy.

Research on Effect of Alloy Elements on Equilibrium and Properties of Ni-Cr-Co Type Nickel-Based Superalloy

Ni-Cr-Co type Nickel-based super alloy Inconel 740H was studied. The effect of Nb, Al and Ti on the equilibrium of this alloy was analyzed by JMatPro software. The amount of Ti and Nb should be controlled by 1.50wt.%, and meanwhile, Al should be 1.0-2.0wt.%. If Mo and W were added the amount of Mo should be in the range of 1.0-2.0wt. %, and W should be about 1.0wt.%. Based on these results, three types of new alloys were designed, which contain Ni-Cr-Co-Mo type (1#), Ni-Cr-Co-W type (2#) and Ni-Cr-Co-Mo-W type (3#). Compared with the Ni-Cr-Co type Inconel 740H alloy, the room temperature strength, high temperature strength and high temperature durable performance of the three new alloys improved, which can provide the evidence and reference to optimize the chemical composition of Inconel 740H alloy, i.e., adding 1.50wt.% Mo and 1.0wt.% W individually or together.

Oxidation Behaviors of Inconel 740H in Air and Dynamic Steam

Abstract Inconel 740H alloy is a candidate material for 700°C advanced ultra-supercritical (A-USC) coal-fired power plants application as superheater/reheater tube. In this work, oxidation behavior of Inconel 740H alloy was studied in static air at 750°C and 850°C, and in dynamic pure steam at 750°C, respectively. The alloy was oxidized approximately following a parabolic law in three test environment. In the static air, the oxidation rate at 850°C was about 50 times of that at the 750°C. More NiCrMn spinal and TiO2 were detected after oxidation at 850°C. Cr2O3, however, was the main oxidation product at 750°C. In the pure steam, Cr2O3 was still the main oxidation product. The oxidation rate was about 2.6 times of that in static air, but the surface roughness was much smaller and edges of oxide particles were more blurred. There was no evidence of cracks or spallation in three test environments.

Oxidation Behavior and Mechanism of Inconel 740H Alloy for Advanced Ultra-supercritical Power Plants Between 1050 and 1170 °C

The oxidation behavior and mechanism of as-cast Inconel 740H superalloy, newly developed for advanced ultra-supercritical power plants superheater and reheater pipes, were investigated in the temperature range of 1050–1170 °C in air. Isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used in this study. The results showed that the oxidation of Inconel 740H was a diffusion-controlled process and controlled by the outward diffusion of metallic elements and inward penetration of oxygen. Owing to the complicated chemical composition of the alloy, different oxides tended to form in the oxidation process. Cross-sectional characterization investigation indicated that the oxide-scale layer consisted of three parts when the temperature was high or the oxidation time was long: an internal oxidation zone composed of Al2O3 and TiO2, a continuous and dense middle layer of Cr2O3 containing precipitates of TiO2 and an outmost layer of NiCr2O4 spinel. In the formation process of NiCr2O4, NiO was found in the early transient stage. With further oxidation, TiO2 finally could not be detected in the internal oxidation zone as a result of outwards diffusion of Ti and its limited concentration in the matrix.

Heat Treatment of a Candidate Material for 700 °C A-USC Power Plants

The preliminary chemical composition optimization and heat treatment parameters of Inconel 740H were investigated systematically. Six heats of experimental alloys (designated as heat 401 to heat 406) with varied chemical compositions were subjected to standard heat treatment at four different solution temperatures. Due to its superior combination of strength, ductility, and toughness together with its minor mechanical property changes at different solution temperatures, heat 405 was chosen as an optimized one to conduct further heat treatment investigations. The evolution of grain size with solution temperature for heat 405 was studied and the optimal solution temperature was determined. After solution treatment at the optimal temperature, four different cooling rates were applied to investigate the effect of cooling rate on the size distribution modal and mean radius of the γ′ precipitates. No bi-modal size distribution of γ′ precipitates was found for all cooling rates and water cooling was recommended for Inconel 740H. In addition, the effect of pre-aging at 800 °C for 16 h on the microstructure of Inconel 740H aged for a long time was studied. It is found that pre-aging at 800 °C would result in bi-modal size distribution of γ′ precipitates after long time aging at 750 °C compared with mono-modal size distribution in samples without pre-aging, which might be caused by the difference in initial size distribution modal before Ostwald ripening of γ′ precipitates in samples with and without pre-aging at 800 °C.

Microstructure evolution and mechanical properties of Inconel 740H during aging at 750 °C

The microstructure evolution of Inconel 740H during aging at 750°C for up to 3000h was investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), three-dimensional atom probe (3DAP) analysis and micro-phase analysis. The mechanical properties of samples after aging were also studied. The grain size increased substantially during the whole aging period. Two different types of grain boundary carbides were observed; block-shaped and needle-shaped. Both were identified to be M23C6 by selected area diffraction measurements. The grain boundary carbides did not coarsen significantly during aging, with the weight fraction increasing only from 0.20% to 0.28%. In contrast, a much higher coarsening rate of γ′ precipitates was observed, as evidenced from both TEM observations and SAXS analysis. 3DAP was used to study the elemental partitioning behavior between γ′ precipitates and γ matrix as well as the evolution in width of the γ/γ′ interface. A large increase in the width of γ/γ′ interfaces was seen between 1000h and 3000h aging. In addition, for the sample aged at 750°C for 3000h, Cr enrichment on the γ matrix side of the γ/γ′ interface was found. Tensile tests at 750°C of the aged samples showed a gradual decrease in elevated-temperature yield strength after 500h, when this alloy was over-aged. The critical precipitate size for the transition from precipitate cutting by weakly coupled dislocations to strongly coupled dislocations for Inconel 740H was calculated to be approximately 50nm, which agrees well with the experiment measurements of elevated-temperature yield strength. The room temperature impact toughness of all samples decreased during aging as the grain size kept growing.