High Temperature Creep Rupture Research Articles

Tensile and creep resistance improvement of GH4975 alloy for turbine discs via electron beam smelting layer solidification technology

A difficult-to-deform superalloy for turbine disks, which can be used at 850 °C for a long time, was prepared by electron beam melting layer solidification (EBSL) technology with high purity and fine structure. Subsequently, the ingot was processed by hot extrusion technology instead of conventional hot forging, and then the tensile and creep tests were performed after standard heat treatment. The results show that the EBSL alloy exhibits excellent high-temperature strength and creep rupture life compared to the conventional preparation method. Among them, the yield strengths at 20 °C and 850 °C are 8.1 % and 5.2 % higher than those in the literature, and the creep rupture life at 850 °C and 412 MPa is 1.53 times higher than that in the literature. The dislocation-precipitate interaction reveals that dislocation shearing γ′ phase is the major mechanism at 20 °C and 850 °C tensile, and Orowan looping and stacking fault shearing are the decisive mechanism during creep at 850 °C and 412 MPa. MC carbides remaining during solidification are the main source of cracks during deformation by fracture analysis. The reasons for refining the microstructure and improving the purity by electron beam melting are analyzed and explained. We believe this work can provide new ideas and methods for preparing superalloys.

High-Temperature Creep and Microstructure Evolution of Alloy 800H Weldments with Inconel 625 and Haynes 230 Filler Materials

Alloy 800H stands as one of the few code-qualified materials for fabricating in-core and out-of-core components operating in high-temperature reactors. Welding is a common practice for assembling these components; however, the selection of a suitable filler material is essential for enhancing the high-temperature creep resistance of Alloy 800H weldments in high-temperature applications. In this study, Inconel 625 and Haynes 230 filler materials were used to weld Alloy 800H plates by employing the gas tungsten arc welding technique. The high-temperature tensile and creep rupture properties, microstructural stability, and evolution of the weldments after high-temperature exposure were investigated and compared with those of Alloy 800H. The results show that both weldments exhibit enhanced tensile and creep behavior at 760 °C. The creep rupture times of the weldments with Inconel 625 filler and Haynes 230 filler materials were about two and three time longer, respectively, than those of Alloy 800H base metal when tested at 80 MPa and 760 °C. Carbides (MC and M23C6) were commonly observed in the microstructures of both the weld and base metals in the two weldments after high-temperature creep tests. However, the Inconel 625 filler weldment displayed detrimental δ and Laves phases in the fusion zone, and these precipitates could be potential sites for initiating cracks following prolonged high-temperature exposure. This study shows that the weldment with Haynes 230 filler material exhibit better phase stability and creep rupture properties than the one with Inconel 625, suggesting its potential for use as a candidate filler material for Alloy 800H for further investigation. This finding also emphasizes the critical consideration of microstructural evolutions and phase stability in evaluating high-temperature materials and their weldments in high-temperature reactor applications.

Influence of Ru on high-temperature creep rupture properties and oxidation behavior of Ni-based single-crystal superalloys

The present study investigates the microstructure, high-temperature creep rupture properties and oxidation resistance of three fourth-generation single-crystal Ni-based superalloys containing 0, 3.5 and 7 wt% of Ru. The addition of Ru changes the partitioning behavior of the alloying elements, decreases γ' size and increases the γ/γ' lattice misfit. The creep resistance keeps increasing as Ru content rises up 7 wt% under 1100 °C/140 MPa. However, it was discovered that 7 wt% Ru alloy has inferior creep resistance compared with the 3.5 wt% Ru one under 1150 °C/140 MPa, although no TCP precipitation was observed in this 7 wt% Ru-containing alloy. It was revealed that 7 wt% Ru-addition caused significant γ' dissolution and induced the generation of creep cavities, which may account for the anomaly. Furthermore, the increase in Ru content up to 7 wt% keeps degrading the oxidation resistance of the superalloys by facilitating the formation of pores and cracks within the oxide scales, which also accelerate scale spallation. This work suggests that excess Ru addition may adversely affect both the mechanical properties and the oxidation resistance of the fourth-generation Ni-based superalloys.

Creep and stress relaxation experiments of 1960-grade high-strength steel wire at elevated temperatures

With the increasing spacing of large-span structures, the application of ultra-high-strength steel wire with a strength approaching or exceeding 2000 MPa is rapidly expanding. However, the related research on its creep and stress relaxation behaviour is still limited. In this paper, a total of 30 high-temperature creep tests and 31 high-temperature stress relaxation tests were conducted on 1960-grade high-strength steel wire (1960-HSSW) with a diameter of 5.0 mm. The full process creep strain curves were obtained by reinserting and resetting the high-temperature extensometer. The maximum creep strain measured was 50.67 %. The creep and stress relaxation curves of different high-strength steel wires (steel strands) were compared, revealing significant variations in mechanical properties among different steel wires. The steady-state creep rate and high-temperature creep rupture critical stress ratio of the 1960-HSSW were given. Especially, the critical stress ratio became lower than the yield strength reduction coefficient at temperatures exceeding 400 °C and it indicated that the 1960-HSSW could undergo creep rupture before yield. The stress relaxation coefficient was introduced to consider the stress relaxation of 1960-HSSW. Stress relaxation curves for the 1960-HSSW were provided at temperatures ranging from 100 to 550 °C. Moreover, based on the microstructure observed under transmission electron microscopy, it was found that damage occurred in the 1960-HSSW after exposure to 300 °C. Therefore, a critical temperature of 300 °C was recommended for the fire protection of 1960-HSSW, which could be the theoretical support for its fire protection design. Additionally, a high-temperature creep prediction model, which includes expressions for the termination time, was established for the 1960-HSSW. As a result, a conservative prediction of the high-temperature creep for the 1960-HSSW was provided. Moreover, A stress relaxation coefficient model was also provided. The predicted results of the aforementioned models were in good agreement with experimental results.

Influence of laser power on the high-temperature creep rupture life of 304L stainless steel manufactured by selective laser melting

To study the influence of laser power on the high-temperature durability performance of 304L stainless steel, its effect on the porosity, hardness, relative density, and creep rupture life have been studied using a single-factor experiment. The relative optimal process parameters were obtained, and the microstructure and mechanical properties of the as-built samples were characterized. The results showed that the creep rupture life reached a peak at 55H for the following process parameters: scanning speed = 900 mm/s, layer thickness = 0.03 mm, hatch space = 0.1 mm, laser power = 280 W, room temperature hardness = 91.5 HRB, and relative density = 99.4%. Furthermore, the microcellular structure was uniform and dense with few pores, the orientation of the fine grains at the boundary of the molten pool was random, and the orientation of the larger columnar crystals in the molten pool had a specific preference along the <001> direction. After 55.1 h (maximum) duration at a temperature of 700 °C, the cellular substructure, that had been formed by a large number of dislocation clusters, had dissipated significantly. The dislocation lines were more evident, and a large number of dislocations had interacted with spherical nano-precipitates (enriched with O–Si–Mn nanoparticles).

Design and application of double stove wires high-temperature furnace in high-temperature creep rupture test

This paper presented a high-temperature furnace with double electric stove wires for the test of high-temperature creep rupture, adopted the 5 mm electric stove wire in diameter, and the wires are made up of the working stove wire and the alternate stove wire. When the tester system detects the working stove wire is fractured, it will automatically and quickly switch the power supply circuit from the working stove wire to the alternate stove wire. Then the alternate stove wire starts to heat to keep the high-temperature furnace working, reducing the loss caused by the interruption of the test on account of the fractured stove wire. Experimental result indicates that the high-temperature furnace with double electric stove wires can solve the problem that when the stove wire is fractured, the test is difficult to proceed for a long time and high-temperature creep rupture test with the classical high-temperature furnace. Besides, the 5 mm in diameter electric stove wire can also meet the temperature control on the ultra-high-temperature (such as 1200 °C) Creep test.

Enhancing mechanical properties and creep performance of 304H and inconel 617 superalloy dissimilar welds for Advanced Ultra Super Critical power plants

This investigation is conducted to find improvements in the mechanical properties of the dissimilar welds between austenitic stainless steel and Nickel superalloy used in the manufacturing of Advanced Ultra Super Critical (AUSC) power plants boiler components. Nickel superalloys (IN617) have excellent creep rupture properties, so they are employed in higher temperature regions (above 700 °C) of the boiler. In contrast, austenitic stainless steel (304H) is used in moderate temperature regions (up to 650 °C) part of the boiler, also striking off the economic balance need. The welding is carried out with manual multipass Gas Tungsten Arc Welding using an IN617 filler to compare 304H and IN617 weld without and with a buttering layer. Multiple attenuated buttering material layers are deposited at the 304H side, further machining the butter weld layer to the weld groove and closure weld to the IN617 part. The buttering technique improves mechanical properties and high-temperature creep rupture life through microstructure transformation. The dissimilar weld characterization shows a supplementary dendritic growth layer for the buttering deposition. The tensile strength is found to be 728.214 MPa with buttering higher than without buttering weld (620.25 MPa). The V-notch Charpy impact toughness value of the buttered weld sample is 66 J, greater than the non-buttered sample, i.e., 63.25 J. The creep rupture duration significantly increased with buttering. Strain hardening at higher applied stress helps to resist creep failure even at higher temperatures.

Design of heat-resistant Al–Mg –Zn–Cu–Ni quinary alloy: Controlling intermetallic phases and mechanical performance at elevated temperature

Owing to good formability, wrought Al alloys are used in a wide range of industries, including turbochargers in automobile engines. The development of Al alloys with increased high-temperature strength is necessary to improve the fuel efficiency of engines. In this study, a modified heat-resistant Al–Mg–Zn–Cu–Ni quinary alloy with superior high-temperature strength and creep rupture life at 200 °C (potential temperature in service for compressor radial impellers) was designed. The thermodynamic assessments provided a modified alloy composition of Al–5Mg–3.5Zn–2Cu–2Ni (at %) exhibiting α-Al phase in equilibrium with T-Al6Mg11Zn11 and Al3(Ni, Cu)2 phases at elevated temperatures. The experimental alloy showed granular T and Al3(Ni, Cu)2 phases often located on grain boundaries in the α-Al matrix, although the undissolved Al3Ni phase (formed in solidification) locally remained. Apart from the T phase, the η-Zn2Mg phase containing Cu element preferentially precipitated on grain boundaries. A large number of fine precipitates were dispersed homogeneously in the grain interior after the aging at 200 °C or 300 °C. Compared to the base alloy with a ternary composition of Al–5Mg–3.5Zn (at %) and conventional Al alloys, the designed quinary alloy (pre-aged at 200 °C for 10 h) showed a high strength at elevated temperatures above 200 °C. The creep-rupture life at 200 °C/105 MPa was remarkably extended to 665 h by the addition of Cu and Ni elements into the base ternary alloy. The rupture life was more than ten times longer than one (57 h) of the ternary alloys. The alloy modification based on the thermodynamic calculations provides an effective approach for designing lightweight Al alloys with superior high-temperature strength and creep rupture life.

High-temperature creep rupture behavior of dissimilar welded joints in martensitic heat resistant steels

The creep rupture behavior and microstructure changes of dissimilar welded joints of two types of martensitic heat resistant steel F92 and Co3W2, under a creep testing of 115–200 MPa and 873–893 K, were investigated. Fracture position and mode change from transgranular ductile fracture in parent F92 to intergranular brittle fracture in the fine grain heat affected zone (type IV cracking) near F92, with the drop of applied stress. Microstructural degradation resulted from creep with respect to dislocation density decrease, precipitates coarsening, dislocation cells, microvoids, macroscopic secondary cracks, and fracture surface morphology are researched based on detailed experimental results. The creep rupture data are analyzed employing Norton’s power law, Monkman–Grant relation, creep damage tolerance, and Larson–Miller parameter. Main creep damage micromechanisms are analyzed with the aid of calculated and experimental data. The extrapolating creep rupture stress for a creep life of 100,000 h are obtained and the influence of stress change and temperature change on creep life are compared in terms of calculated normalized creep life growth rate.

Microstructures and high-temperature failure analyses of 25Cr-35Ni-Nb + MA/32Cr-20Ni-Nb dissimilar welded joint with different filler wires

To address the issue of lacking scientific guidance for welding 25Cr-35Ni-Nb + MA/32Cr-20Ni-Nb joint in petrochemical plant applications, the joint microstructures were studied by adopting four types of filler wires. The high-temperature tensile tests and creep rupture tests were performed to investigate joint failure mechanisms. Cellular/columnar dendritic grains were formed at the fusion line region with similar orientations to adjacent un-melted grains. The fine-sized dendritic grains generated at the central weld seam presented a larger misorientation owing to a lowered cooling rate. The lesser alloying element in filler wire led to a larger area with cellular/columnar dendritic grains. The carbide precipitates mostly located at grain boundaries either in the form of particles or necklace structures depending upon the filler wire compositions. The heterogeneous microstructure was thought to be the main reason for the failure at weld seam and fusion line region with large hardness fluctuations when joints welded by the filler wire with a more diverse alloying element. The filler wire with the least introduction of alloying elements, i.e., ERNiCrCoMo-1, gave rise to a more uniform microstructure and hence higher joint property without failing at the weld seam, which was therefore considered as the most suitable filler wire candidate.

Creep rupture life prediction of nickel-based superalloys based on data fusion

Creep rupture life is a key performance parameter of nickel-based superalloys, which directly affects the engineering service behavior of components. In this study, a machine learning method based on data fusion was proposed to predict the properties of new alloys with the properties of existing alloys, or to predict other related properties of the same alloy, so as to solve the problem of accurate prediction of creep rupture life caused by the lack of accumulated data. Using the existing nickel-based superalloy creep rupture life data accumulated in the NIMS database, a creep rupture life prediction model was established using key alloy factor screening and feature-based transfer learning strategy (FS-SVR). The performance of GH4169D alloy was successfully predicted by the properties of GH4169 alloy, and the high temperature creep rupture life was predicted by the low temperature creep rupture life of GH4169 and GH4169D alloys, and the prediction accuracy reached more than 90%. The research results not only provide a fast method for predicting the creep properties of novel nickel-based superalloys, but also provide a reference case for data fusion to assist the research and development of new materials.

Experimental investigation on high-temperature creep behavior of carbon fiber reinforced polymer cable

Both steady state-constant load and temperature (ST-CLT), and transient state-constant load and temperature (TR-CLT) tests were conducted on carbon fiber reinforced polymer (CFRP) cable in order to investigate the corresponding high-temperature creep behavior. The failure modes, creep strains and rupture times of CFRP cables exposed to high temperature were obtained in ST-CLT and TR-CLT tests, while the residual tensile strength and elastic modulus of specimens without rupture after high-temperature creep tests were determined by conducting high-temperature tensile tests. The variation law of high-temperature creep strains and rupture times of CFRP cable obtained from ST-CLT tests were compared to these of TR-CLT tests to identify the effect of temperature-load path. A practical model for characterizing the high-temperature creep behavior of CFRP cable was developed. Finally, the sustained stress levels and rupture times of specimens exposed to high temperature were analyzed to predict the high-temperature creep rupture stress level of CFRP cable. The obtained results indicated that the rupture morphology of high-temperature creep rupture specimens was the completely soft fiber bundles with “fluffy” pattern. The high-temperature creep strains of CFRP cables in ST-CLT and TR-CLT tests were highly depended on temperature, sustained stress level and time. The presence of pre-tension load before heating accelerated the damage of strength and stiffness for CFRP cable at high temperature, and the damage degree increased with the increasing sustained stress level and high-temperature. The developed practical models for CFRP cables under ST-CLT and TR-CLT conditions can accurately predict the high-temperature creep strains. In the fire resistant design of pre-stressed structures with CFRP cables, the effect of high-temperature creep rupture stress of CFRP cable was not negligible.

Microstructural evolution of Super304H stainless steel during high-temperature creep rupture

This study is aimed to explore the microstructural evolution of Super304H stainless steel during long-term creep. Creep-rupture specimens of this steel are analyzed via microhardness measurement, metallographic structure observation, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The results show that the matrix structure of the steel was stable during high-temperature stress-rupture test without grain growth. Twin boundaries in austenite gradually disappeared with the prolongation of time, which process was accelerated at higher temperatures. The steel hardness sharply increased in the initial stage of high-temperature stress-rupture test due to the second-phase precipitation, and then gradually dropped with time, due to the coarsening of precipitates. The Cu-rich phase and secondary Nb(C,N) with the size less than 50 nm, and M23C6 were the main precipitated phases in the high-temperature stress-rupture test. This allowed the steel under study to retain a high stress-rupture limit due to the precipitation-strengthening effect. The microstructure aging resulted in the formation of M23C6 chain at the grain boundary and coarsening of M23C6 and primary Nb(C,N) in grains. With the coarsening of precipitated phases, hardness and stress-rupture limit of Super304H stainless steel decreased.

Stress Rupture Properties of Welded Joints of Austenitic Heat-resistant Super304H Steel at 650 °C

With the help of metalloscope, scanning electron microscope and its energy dispersive spectroscopy, this paper investigates the stress rupture properties at high temperature and microstructure changes of the welded joints of austenitic heat-resistant Super304H steel at 650 °C as well as the effects of microstructure changes on stress rupture properties by high temperature creep rupture testing. The results show that at 650 °C, the welded joint of Super304H steel has high breaking strength at elevated temperature, and the fracture position is on the base material away from the fusion line, and the fracture type is mainly a transcrystalline rupture. The microscopic process of creep rupture of Super304H steel welded joints is the nucleation and growth of the void and crack, while the large bulk Nb(C,N) is the main nucleation site of void. As the careep time increases, M23C6 on the grain boundary is also the main nucleation site of the void.

Creep rupture strength of F12 steel welded joint after long-term service

Creep rupture strength of F12 steel welded joint which was used as the main steam pipe and serviced 165000 hours was investigated by using high temperature creep rupture testing. Microstructure analysis results show that the microstructure of F12 steel welded joint became embrittlement significantly after long-term service. The precipitates mainly consist of M23C6 carbide and Laves phase, and the amount of M23C6 phase is more, which precipitated along the grain boundaries in bulk or chainlike with larger size. At the same time, there are larger inclusions with directional distribution, in which these inclusions have poor bonding with substrate and easily become crack source during the process of high temperature creep. The high temperature enduring strength of welded joint of F12 steel decreased significantly, it is necessary to enhance the examination of the microstructure and life assessment of pipeline in the next overhaul period.

Effect of Ru on the concentration distribution of elements and creep properties of nickel-based superalloy

The effects of Ru on creep properties, microstructure and elemental concentration distribution of nickel..based single crystal superalloy containing without Ru and 2% Ru are studied by means of creep tests under high temperature, morphology observation after heat treatment and determination of elemental concentration distribution by atomic probe. The results show that Ru can inhibit the precipitation of TCP phase and improve the solid solubility of W element in the alloy and the creep life of the alloy at high temperature. At the same time, the addition of element Ru can make the alloy to produce “reverse distribution” effect, improve the alloying degree of γ’S phase and the effect of solution strength, which final significantly improve the high temperature creep rupture properties of the alloy containing Ru.

A new approach to predict creep rupture of Grade 92 steel under multiaxial stress states

The extension of the available stress-based predictive models from uniaxial to multiaxial feature is uncertain due to the difficulty to justify the model transferability to the cases which involves complex deformation and fracture mechanisms. This paper presents a new approach to predict the creep rupture life of Grade 92 steel under multiaxial stress state as the case of notched bar samples using Kachanov's Continuum Damage Mechanics (CDM) model. Combining the CDM model with the concept of Hayhurst's representative stress and accounting two extreme conditions; fully dislocation-controlled and diffusion-controlled creep mechanisms provide the bounds for short- and long-term rupture data of notched bar up to 10,000 h. Although the CDM is scientifically relevant and fundamental in their approach, it is generally complex and contains too many variables. Therefore, it needs proper measurements or numerically intensive to make the model acceptable for industrial applications. Alternatively, the recent developed strain-based exponential-type predictive model which links globally uniform failure strain with a multiaxial constraint factor was employed with the intention to reduce the large number of CDM parameters. The approach is relatively simple yet reliable to be used for high temperature creep rupture assessment.

Research on Remaining Life Evaluation Method of T92 Steel for Superheater Tube Based on Oxide Layer Growth

T92 steel is widely used in high-temperature superheater of supercritical power station boiler, and the researches on its remaining life have always been the hot spot of scholars around the world. In this paper, based on the analysis of high-temperature creep rupture strength data, the optimum C value of Larson–Miller parameter formula for T92 steel is obtained by using isothermal extrapolation method. On this basis, combining the relationship between the thickness of the oxide layer and the operating time, the relationship between the thickness of the oxide layer and the temperature of the tube wall and the relationship between the thickness of the oxide layer and the stress of the tube wall, a new remaining life evaluation formula is deduced. Finally, based on the cumulative creep damage life evaluation method, the service life of superheater tube is divided into three stages, which provides a new reference for the service life evaluation of superheater tube of ultra-supercritical boiler.

Creep rupture behavior and microstructural evolution of modified 9Cr–1Mo heat-resistant steel

High-temperature creep rupture behavior of modified 9Cr–1Mo steel used for steam cooler was investigated at temperature of 838 and 923 K and stress ranging from 100 to 250 MPa. Based on the analysis of creep rate–time curves, it is found that the creep rupture life decreases with the increase in the applied stress and temperature. The creep damage tolerance factor has been identified as a value of 8. In the normalized and tempered condition, the studied steel shows typical martensitic microstructure with Cr-rich M23C6 and Nb- or V-rich MX precipitates. Moreover, the Laves phase has been found along the grain boundaries. The fracture morphology characterized by field emission scanning electron microscope is adopted to reveal the creep failure mechanisms. The investigated results indicate the occurrence of the transgranular fracture under all the creep test conditions.

Effect of Base Metal Misorientation on Single-Crystal Superalloy Transient Liquid-Phase Bonded Joints

In this paper, experiments and theoretical analysis were conducted to elucidate the internal relation between base metal misorientation and the characteristics of DD5 single-crystal superalloy TLP joints. The alloy was bonded at 1513 K for 10 h in vacuum environment. High-temperature creep rupture tests were carried out to determine the joint’s quality. Results revealed that base metal misorientation exerted a remarkable influence on DD5 alloy TLP joints. The joints of base metals with [001] orientation were used in this research. When the misorientation was smaller than 9°-10°, the single crystallization of joints can be realized and joints can achieve excellent mechanical properties. When the misorientation was more than 10°, there would be second-phase particles distributed in grain boundaries, thus resulting in poor mechanical property of joint; moreover, the characteristics of joint did not change with base metal misorientation in this case.