Alloy 740H Research Articles

Full-field eigenstrain reconstruction for the investigation of residual stresses in finite length weldments

The reconstruction of welding residual stresses has been performed based on the assumption of continuous processing conditions, owing to the limitations of reconstruction methods. This assumption prevents the attainment of a comprehensive understanding of mechanical deformations in the entire weld zone. The voxel-based eigenstrain (inherent strain) reconstruction method emerges as an appropriate approach for the full-field reconstruction of residual stresses in finite length weldments without such assumption. The innovative reconstruction capability of this advanced method is presented in this study, aiming to comprehend all components of residual stresses and eigenstrains in finite length weldments with discontinuous processing properties using only a single component of experimental data belonging to a limited portion of the specimen. Numerical experiments were conducted to develop the tools. These tools were validated using a case study employing experimental data obtained through contact profilometry measurements from the surface of discharge machine cutting (minimally disturbing cutting) in a bead-on-plate weldment of Inconel alloy 740H designed for ultra-supercritical power generation. The reconstructed two components of normal residual stresses that do not align with the available experimental data were validated by independent neutron diffraction strain scanning quantifications. Furthermore, the reconstruction of residual stresses in the intact component prior to cutting was carried out using the eigenstrains associated with the post-cut state of the weldment.

Creep-fatigue life and damage evaluation under various strain waveforms for Ni-based Alloy 740H

This paper presents creep-fatigue life evaluation of a nickel-based Alloy 740H, for evaluating its performance as a material of advanced ultra-supercritical power plants. Strain controlled creep-fatigue tests were performed under six strain waveforms using solid bar specimens at 700 °C, and the effect of strain waveform on creep-fatigue life was discussed. Creep-fatigue lives were shorter than pure fatigue life depending on the strain waveform. The failure life in symmetrical waveform was reduced to about half as the strain rate decreased by one order of magnitude. The failure lives in the unsymmetrical waveforms were smaller than that in the symmetrical waveform and tensile slow straining was more detrimental to fatigue life than compressive slow straining. The creep-fatigue life decreased significantly due to strain hold, and tensile peak strain hold had more enhanced effect on fatigue life than compressive strain hold. Applicability of five existing approaches based on linear damage summation rule were discussed. In addition, a new approach based on modified ductility exhaustion model with several modifications was proposed in order to improve the life predictability of the classical approach. Time fraction rule with ASME failure envelope and new approach satisfactorily predicted the creep-fatigue lives almost within a scatter band of 2.

Relationship between Grain Size and Sample Thickness on the Creep-Rupture Performance of Thin Metallic Sheets of INCONEL Alloy 740H

A study was conducted on INCONEL® alloy 740H® to examine the role of grain size and sheet thickness on the alloy’s creep-rupture behavior. Three different starting sheet thicknesses were utilized and multiple heat-treatment conditions anticipated for compact heat exchanger (CHX) manufacturing were applied to produce a range of grain sizes. Creep-rupture testing was conducted at 750 °C for times up to about 6000 h and the results were compared to wrought databases. The data show that both creep strength and ductility were important factors in the overall creep performance of the sheets. Reductions in performance were observed due either to accelerated creep when grain size was fine or loss of rupture ductility when grain sizes approached the sheet thickness. Some combinations of heat-treatment and thickness were able to produce typical expected wrought creep properties. Historically a ‘rule-of-thumb’ requirement for creep testing suggests 3-5 grains per sample minimum dimension to ensure homogeneous behavior. This research shows that to ensure representative wrought creep performance (i.e. no effect of sample size), the sample minimum cross-section should be 10 times the average ASTM grain size. Statistical analysis of the microstructures suggests the population of larger grains as a controlling feature in creep failure.

Development and performance of INCONEL® alloy 740H® seam-welded piping

INCONEL® alloy 740H® is an age-hardenable nickel-based alloy approved for pressure vessels and piping within ASME Section I, VIII, and B31.1. Currently, the code applies a weld strength reduction factor (WSRF) of 0.7 to the allowable stresses for longitudinal seam welds in the time dependent creep regime. In this work, a full-scale seam weld was successfully produced using typical industrial practices. The component was solution heat-treated and aged after fabrication to improve the performance of the weldment. Tensile, bend, impact, and low-cycle fatigue tests showed the component met all the specification requirements and elevated temperature properties were within base metal expectations with failures predominately in the weld metal. Long-term creep tests, including large samples more representative of the entire weldment, were fabricated and tested to times in excess of 10,000 h at multiple temperatures. Analysis of the creep data supports a WSRF of 0.9 for the solution annealed + aged weldments in contrast to the WSRF of 0.7 currently applied to welded + aged weldments. Metallurgical analysis shows that the solution annealing causes recrystallization of the weld metal and reduces the chemical compositional and microstructural gradients in the weldments resulting in acceptable short-term performance. In long-term creep, evidence for coarsened zones in the weldments due to discontinuous coarsening reactions were identified as the mechanism leading to accelerated creep damage formation in the weld metal and sample failure.

Corrosion resistance of high nickel alloys in solar salt at 600 °C for up to 4000 h

This study focuses on the time dependent performance of the corrosion resistance of IN625, H230 and 740H alloys in the solar salt (60 wt% NaNO 3 , 40 wt% KNO 3 , at 600 °C) for up to 4000 h. Alloy IN625 showed the lowest mass change with parabolic oxidation kinetics that tends to stabilize with exposure time. Both 740H and H230 alloys deviated from the parabolic oxidation behavior beyond the 3000 h of exposure. Alloy 740H showed the highest mass loss due to the nonuniform surface oxidation and the high chromium dissolution rate. Internal oxidation was noticed with Alloy H230 due to the high tungsten concentrations. • Oxidation of Alloy IN625 was found to follow a parabolic behavior that tends to stabilize with time. • Oxidation of 740H and H230 alloys deviated from the parabolic behavior after 3000 h of exposure. • Alloy 740H showed a nonuniform oxide layer and so has the highest mass loss. • Alloy H230 suffered of internal oxidation due to the high tungsten concentration.

Long-term Creep Property and Structural Stability of Ni-Based Alloy 740H

Long-term Creep Property and Structural Stability of Ni-Based Alloy 740H

Effect of Ageing on Microstructure, Mechanical Properties and Creep Behavior of Alloy 740H

A study on the mechanical property changes and the creep behavior of a Ni-based alloy 740H on ageing at 800 °C for 10250 hours is conducted. Microstructural characterization reveals that the alloy initially contains fine γ′ phase and carbide precipitates within grains and at grain boundaries. Ageing does not affect the grain size but coarsens the γ′ phase and facilitates further precipitation of carbides at grain boundaries, which gradually form a continuous network. This significantly deteriorates the mechanical properties in ambient conditions with marked reductions in the impact energy and yield strength. The strength of the aged alloy further reduces at 710 °C. In contrast, ageing appears to have insignificant influence on the ductility, which is > 30 pct at both testing temperatures. Ageing also reduces the creep strength of the alloy and accelerates the creep rate. The creep stress exponent was found to increase from 6.7 to 11.2 after ageing, suggesting a transition in the deformation mechanism. The observed reductions in yield strength, impact toughness and creep strength are attributed to the coarsening of the γ′ precipitate, formation of network carbides at grain boundaries and formation of γ′ precipitate-free zone adjacent to grain boundary carbide network. This γ′ precipitate-free zone also weakens the grain boundary and facilitates grain boundary sliding at higher temperatures. However, under quasi-static conditions, γ′ precipitate-free zone undergoes plastic deformation and helps in preserving the ductility of the aged alloy. Implications of these changes are discussed in the context of high-temperature structural applications, such as in power plants.

An Alternative Casting Technique to Improve the Creep Resistance of Cast INCONEL Alloy 740H

The increasing performance requirements of power plant designs, such as advanced-ultra supercritical (A-USC), require the use of Ni-based superalloys to replace high-strength, ferritic-martensitic steels for components subjected to temperatures above 898 K (625 °C) and for austenitic stainless steels components at temperatures above 973 K (700 °C). To date, commercial Ni-based superalloy INCONEL 740H has been shown to be appropriate for use in A-USC power plants as boiler components in a wrought product. However, large complex components in boilers as well as other casings in the turbine and valve chest require castings of a thick-wall nature. Using the alloy in its cast form would be significantly valuable in terms of range of component size, geometry and complexity. Previous investigations revealed short creep lives from cast INCONEL alloy 740H. In this investigation, an alternative casting route that utilized a melt procedure resulting in a fine-grain casting, and in conjunction with a computationally optimized homogenization heat treatment, not only controlled the grain size and grain boundary structure but minimized chemistry variability and segregation. A primarily equiaxed and homogenous grain size distribution was obtained from this approach with better repartition of M23C6 carbides along the grain boundaries. Furthermore, better than 38 pct increase was obtained for this material in comparison to the creep life obtained from the best performing conventionally cast material. More importantly, the fine-grain homogenized (FGH) casting route resulted in the Larson–Miller plot for this material that coincided with that of wrought alloy 740. At low creep stresses (with a test still in progress), the FGH casting is resulting in higher values of the LMP than the wrought alloy.

Neutron strain scanning for experimental validation of the artificial intelligence based eigenstrain contour method

The demand for energy generation with low carbon emissions evoked the development of ultra-super critical technology that allows operating steam turbines at high temperature and pressure conditions. However, operating at extreme conditions necessitates careful consideration of structural integrity which is affected by residual stresses. Welding is used for joining of components of steam turbines, but this process causes the formation of residual stresses of complex form. Careful investigation is necessary to understand the distribution of potentially detrimental residual stress fields. Eigenstrain theory was previously used for the development of the artificial intelligence based eigenstrain (AI-eig) contour method that allowed advanced modelling of the behaviour of Inconel alloy 740H under thermo-mechanical loading conditions. Models created using this method are capable of evaluating the residual stress fields in the whole specimen or in the parts and slices created using electric discharge machining (EDM). In the previous applications of the AI-eig contour method, the determination of the distribution of eigenstrain in as-welded and heat-treated specimens was followed by the calculation of volumetric residual stresses. In this study, long- and short-transverse components of the residual strains determined by the AI-eig contour method applied to EDM-cut surfaces of the parts of as-welded and heat-treated specimens were validated using the neutron strain scanning method. The results demonstrate the effectiveness of the integrative modelling approach that enables the determination of eigenstrains in the whole specimen and the calculation of residual strains before and after the machining process.

On the analysis of post weld heat treatment residual stress relaxation in Inconel alloy 740H by combining the principles of artificial intelligence with the eigenstrain theory

Post weld heat treatment (PWHT) process has an important role on fabrication of advanced ultra-supercritical power plant turbines. This process relieves the residual stresses formed as a result of welding by converting elastic strains into creep strains. In order to analyse the residual stress relief mechanism during the PWHT process, a novel simulation approach based on experimental data was developed for the analysis of residual stress states from complex manufacturing processes which are welding and heat treatment. This model uses permanent plastic strains (eigenstrains) formed as a result of welding process to set the initial mechanical state of the sample. The distribution of eigenstrains in the whole body was determined using displacement data obtained from contour measurements. The use of eigenstrains to set the initial residual stress state of the creep model reduced the number of uncertainties. This allowed the use of the principles of artificial intelligence for the development of a new fuzzy finite element model (fFEM) that determines the eigenstrain-creep model parameters through an evolution process. Subsequent to the determination of the model parameters, conditions of the PWHT process are investigated to analyse residual stress relaxation in Inconel Alloy 740H weldments.

The phase decomposition with related element interaction and redistribution during long term aging of 740H superalloy

Abstracts Microstructure evolution of alloy 740H for advanced ultra-supercritical (AUSC) power plants are studied during long term aging with a temperature scope of 600–720 °C up to 10,000 h and 800 °C up to 2000 h. Field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM) and micro-chemical phase analysis are carried out for detailed investigation of phase evolution. Though phase morphology does not change dramatically during 720 °C aging, apparent element redistribution are detected after 200 h aging. Different from common knowledge, decomposition of MC carbide is detected during aging. Element redistribution relationship between γʹ phase, MC carbide and M23C6 carbide during 10,000 h aging is established based on quantitative analysis and morphology observation. Nb, Ti and C elements are released into matrix during decomposition of MC carbide. Nb and Ti elements diffuse into γ′ phase during growth of γ′ particles and Cr element is rejected out of the phase. The abundant C element released by MC carbide and Cr element rejected from γʹ phase are beneficial for growth of M23C6 carbide and weight fraction of M23C6 carbide doubles after 720 °C/10,000 h aging. This time-consuming and element diffusion related process can be modified into the reaction: MC+γ→(MC)t+γ′+(Cr)enrichment→M23C6+γ′. The evolutions of different phases are linked by redistribution of elements during long thermal exposure.

On the application of principles of artificial intelligence for eigenstrain reconstruction of volumetric residual stresses in non-uniform Inconel alloy 740H weldments

The eigenstrain theory provides a range of fruitful concepts for advanced modelling of the behaviour of materials and components obtained using sophisticated manufacturing routes, their response to thermal and mechanical loading, and deformation under fatigue and creep conditions. In recent years the method has been shown to be able to provide predictions of residual stresses for a limited range of processing and simulated service conditions for which experimental data is available. The authors recently presented advances in the use of eigenstrain-based analysis to include accurate determination of the domain and boundaries of eigenstrain fields in the weld zone. This approach allowed effective modelling of large-scale components and the determination of volumetric distributions of residual stresses through the use of additional model coefficients that need to be determined. Due to the non-linear dependence of the prediction on these parameters, the algorithm of the decision-making process has a profound influence on the cost of the simulation, and the reliability of its output. To address this challenge, the principles of Artificial Intelligence were adopted for use in the eigenstrain contour method to develop fuzzy Finite Element Model (fFEM) for the eigenstrain the reconstruction of residual stresses in large structures. The deterministic finite element eigenstrain model uses contour measurements for reconstruction process, and the developed fFEM behaves as an artificial agent to determine the coefficients of the deterministic finite element eigenstrain model. As an example application, as-welded and post-weld heat-treated specimens of non-uniform weldments of Inconel Alloy 740H were investigated using the proposed model. The results were verified using displacement measurements and residual stress calculations of the contour method. The determination of model coefficients by artificial agent allowed effective reconstruction of volumetric residual stresses in complex shaped components using limited data without the requirement of costly and destructive multi-cut experimental procedures.

Characterization of Discontinuous Coarsening Reaction Products in INCONEL® Alloy 740H® Fusion Welds

Characterization of γ′ coarsened zones (CZs) in alloy 740H fusion welds via a variety of electron microscopy techniques was conducted. The effects of solute partitioning during nonequilibrium solidification on the amount of strengthening precipitates along the grain boundaries were evaluated via electron-probe microanalysis and scanning electron microscopy. Electron backscatter diffraction was used to present evidence for the preferential growth of CZs toward regions of lower γ′ content, even if growth in that direction increases grain boundary area. Scanning electron microscopy and image analysis were used to quantify the propensity for CZs to develop along certain segments of the grain boundaries, as governed by the local variations in γ′ content. Scanning transmission electron microscopy with X-ray energy-dispersive spectrometry (XEDS) was used to assess the compositions of the matrix and precipitate phases within the CZs and to quantify the segregation of alloying components to the reaction front. Thermodynamic and kinetic modeling were used to compare calculated and experimental compositions. The work presented here provides new insight into the progression of the discontinuous coarsening (DC) reaction in a complex engineering alloy.

Modelling of creep and fracture properties of nickel based alloys

This paper reviews the differences between two nickel based alloys, Alloy 740 and Alloy 740H. Microstructural evolution models are used to forecast the changes in volume fraction and interparticle spacing of both grain boundary and intra-granular precipitates in the alloys. These data are then employed in continuum damage mechanics models to forecast creep curves, and in fracture mechanics models to forecast Charpy impact energies/energy. The results reveal the key microstructural features that control secondary and tertiary creep rate as well as the time dependence of Charpy impact energy after high temperature exposure.

Microstructural Evolution of INCONEL® Alloy 740H® Fusion Welds During Creep

Electron microscopy techniques have been used to investigate the cause of premature creep failure in the fusion zone of INCONEL® Alloy 740H® (INCONEL and 740H are registered trademarks of Special Metals Corporation) welds. The reduced creep rupture lives of all-weld-metal and cross-weld creep specimens (relative to base metal specimens) have been attributed to the presence of large grain boundary regions that were denuded in fine γ′ but contained coarse, elongated particles. Investigation of creep rupture specimens has revealed four factors that influence the formation of these coarsened zones, and the large particles found within them have been identified as γ′. Comparisons of the microstructural characteristics of these zones to the characteristics that are typical of denuded zones formed by a variety of mechanisms identified in the literature have been made. It is concluded that the mechanism of γ′-denuded zone formation in alloy 740H is discontinuous coarsening of the γ′ phase. The discontinuous reaction is catalyzed by the grain boundary migration and sliding which occur during creep and likely promoted by the inhomogeneous weld metal microstructure that results from solute segregation during solidification. The increased susceptibility to the formation of the observed γ′-denuded zones in the weld metal as compared to the base metal is discussed in the context of differences in the contributions to the driving force for the discontinuous coarsening reaction.

Homogenization and Dissolution Kinetics of Fusion Welds in INCONEL® Alloy 740H®

Thermodynamic and kinetic modeling were used to determine appropriate heat treatment schedules for homogenization and second phase dissolution in INCONEL® alloy 740H® (INCONEL and 740H are registered trademarks of Special Metals Corporation) fusion welds. Following these simulations, a two-step heat treatment process was applied to specimens from a single pass gas tungsten arc weld (GTAW). Scanning electron microscopy (SEM) has been used to assess the changes in the distribution of alloying elements as well as changes in the fraction of second phase particles within the fusion zone. Experimental results demonstrate that adequate homogenization of alloy 740H weld metal can be achieved by a 1373 K/4 h (1100 °C/4 h) treatment. Complete dissolution of second phase particles could not be completely achieved, even at exposure to temperatures near the alloy’s solidus temperature. These results are in good agreement with thermodynamic and kinetic predictions.

Hot working characteristics of nickel-base superalloy 740H during compression

The hot deformation behavior of nickel-based superalloy 740H has been investigated by hot compression experiments. The tests were carried out in the temperature range of 1050–1200°C with strain rates of 0.1–20s−1 to a total strain of 0.6. True stress–true strain curves and deformation microstructures were studied. The results show that dynamic recrystallization, which is the principal softening mechanism in the hot working of 740H, determines the shape of flow curves. The fraction of DRX grains at the strain of 0.6 is increased by deformation temperature while at a particular temperature, it is lower at strain rate of 1s−1 than at other strain rates. A hyperbolic-sine type equation has been established between peak stress and deformation conditions through Z parameter, giving activation energy value of 357.3kJ/mol. The hot working windows of alloy 740H was briefly discussed using the processing map. Temperature range of 1160–1200°C with strain rate larger than 10s−1 is the probable domain for hot extrusion of 740H.

Nickel Base Superalloys for Next Generation Coal Fired AUSC Power Plants

Utilities worldwide are facing increased demand for additional electricity, reduced plant emissions and greater efficiency. To meet this challenge will require increasing boiler temperature, pressure and coal ash corrosion resistance of the materials of boiler construction of future coal-fired boilers. This paper describes two new nickel-base alloys, INCONEL® alloys 740 and 740H™, aimed at meeting this challenge. The evolution of the early alloy 740 into alloy 740H is discussed. Since alloy 740's mechanical properties, coal-ash and steam corrosion resistance have already been extensively reported [1], [2], emphasis will be given to recent data on alloy 740H and particularly its commercial-scale manufacturability to steam pipe and its subsequent weldability and mechanical properties.