Creep Rupture Mechanism Research Articles

Creep rupture properties of dissimilar welded joint of ferritic/martensitic P92 steel and Inconel 617 alloy

The dissimilar welded joint (DWJ) comprising P92 steel and Inconel 617 (IN617) is frequently utilized in advanced ultra-supercritical (AUSC) boilers. In present work the creep rupture behaviour, rupture mechanism and microstructure evolution of DWJ of P92 steel and IN617 with Ni-based ERNiCrCoMo-1 filler, were examined at 650 °C under the stress range of 120–200 MPa. The results of the creep test indicated that the location of creep rupture varied across different testing conditions, encompassing areas such as P92 base metal and the fine-grained heat-affected zone (FGHAZ). The creep tested specimens within the stress range of 180–200 MPa exhibited failure originating from P92 base metal, predominantly influenced by plastic deformation. The failure manifested in a transgranular mode, driven by the formation, growth, and eventually coalescence of microvoids. The creep tested specimens within the stress range of 120–150 MPa displayed the characteristic ofType IV failure, primarily attributed to matrix softening and the absence of adequate precipitates to pin the PAGBs. Microstructural characterization unveiled the presence of microvoids along the PAGBs, facilitating microvoid nucleation primarily owing to the existence of the coarse brittle carbide phase. The growth of these microvoids over a period of time during tertiary stage of creep which lead to their coalescence and the formation of microcracks, ultimately resulting in premature intergranular failure. The specimen creep exposed at 650 °C under the lower stress of 120 MPa exhibited higher creep rupture life for 432 h. The SEM/EDS study of the crept sample of 120 MPa is also confirmed the presence of intermetallic laves phases in regions near the fracture tip and in the heat-affected zones (HAZs). Although the creep tested specimens at 150 MPa and 120 MPa exhibited failure in the FGHAZ and their elemental SEM/EDS analysis confirmed the presence of an oxide layer and notch formation near the interface of P92 base metal and ErNiCrCoMo-1 filler weld. The FESEM study revealed the growth of the oxide layer in the notch region, and it also showed the presence of multiple cracks and microvoids in the oxide layer. The formation and propagation of the oxide notch mainly led to the interfacial failure. The crept specimens subjected to 180 MPa and 200 MPa did not exhibit any cracks or microvoids in HAZs. However, specimens that failed under applied stresses of 120 MPa and 150 MPa revealed the presence of a high density of microvoids in the FGHAZ, inter-critical heat-affected zone (ICHAZ), and in the region of the coarse-grained heat-affected zone (CGHAZ) adjacent to the interface attached with an oxide layer.

Investigation of Multiaxial Creep Rupture Mechanisms and Life Prediction in High-Temperature Alloys Under Complex Environments

Investigation of Multiaxial Creep Rupture Mechanisms and Life Prediction in High-Temperature Alloys Under Complex Environments

A unified creep life prediction method and fracture mechanism of high-temperature alloys under multiaxial stress

In this paper, a unified creep rupture life prediction method based on continuum damage mechanics under multiaxial stress was proposed. By optimizing the traditional skeletal point stress method, the number of models that need to be computed during the simulation was reduced by 5/6, and the generality of the method for different notch sizes was significantly improved. Creep tests on Ni-based superalloy round bar specimens with different notch sizes, which verify the accuracy of the method, were conducted. Furthermore, creep rupture mechanism analyses were conducted for different notch sizes by combining finite element simulations with experiments, stress and damage were analyzed, and void growth was explained based on triaxial stress states.

Influence of Replacing Molybdenum with Tungsten on the Creep Fracture Property of Waspaloy Nickel-Based Alloy

Alloys meeting the requirements of “700 °C and above” advanced ultra-super-critical technology, with higher thermal efficiency, have been developed in recent years. Here, a new wrought Ni-based superalloy with excellent high-temperature creep strength based on Waspaloy has been developed and is proposed as a candidate material for application in 700 °C class advanced ultra-super-critical steam turbine blades. In this new alloy, the Molybdenum (Mo) in Waspaloy is partially replaced by Tungsten (W). Creep tests have shown that this new Ni-based alloy has a 70 MPa higher creep-rupture strength than that of Waspaloy at 700 °C by extrapolating the experimental data. Detailed creep-rupture mechanisms have been analyzed by means of scanning electron microscopy, transmission electron microscopy, and chemical phase analysis with a view to devising potential approaches for performance improvements. The results showed that the partial replacement of Mo by W had negligible effect on the composition of carbides precipitated in the alloy. Instead, the amount of the γ′ phase was significantly increased, and mismatch between the γ and γ′ phases was reduced. In this way, the stability of the γ′ phase was increased, its coarsening rate was reduced, and its critical shear stress was increased. As a result, the high-temperature creep-fracture strength of the new alloy was increased.

Evolution of creep rupture mechanism in advanced powder metallurgy superalloys with tantalum addition

To further understand the effect of Ta additive on the creep damage for polycrystalline Ni-base superalloys, creep rupture tests on powder metallurgy (PM) superalloys with various Ta were carried out under different conditions in this study. The results indicated that the multiple crack propagation and damage mechanisms could occur at the identical creep condition. These different fracture mode areas had distinct boundaries on the fracture surface, its size could be altered by Ta addition. However, the effects of Ta were radically changed by the creep conditions. The creep rupture mechanism transformed significantly with temperatures and applied stresses, and even subversive laws appeared in the creep life for high-Ta alloys. The substructure observation after creep rupture demonstrated that the annealing coherent twin boundaries gradually lose coherence with the matrix, which could become sites for cavities nucleation or crack propagation that followed. Furthermore, the creep-induced microtwins were critical for the creep damage and rupture. The electron back-scattered diffraction (EBSD), high-resolution transmission electron microscope (TEM) imaging, and quantitative elemental analysis assisted in comprehending these alterable creep rupture mechanisms.

Indentation Creep Response and Rupture Mechanisms in GLARE: Experimental and Statistical Evaluation

Abstract Fiber metal laminates (FMLs) are structural materials that play a vital role in the aviation sector, in which they are exposed to cyclic long-term loading. This article investigates creep response and rupture mechanisms of glass laminate aluminum reinforced epoxy (GLARE) FMLs based on microindentation creep testing. Specimens were manufactured by compression molding, and experiments were conducted under different test parameters, namely, aluminum surface treatment (in terms of surface roughness), creep chamber temperature, indentation load, and run time. In addition, rupture mechanisms were analyzed using scanning electron microscopy. The individual interaction of control parameters on creep strain and mass loss response was studied using Taguchi method. Among the studied factors, specimen condition and creep chamber temperature predominantly control creep rupture, whereas load and time were less important, as confirmed by statistical predictions using linear regression.

Creep rupture mechanisms and life prediction of IN617 for VHTR applications

The creep rupture mechanisms of IN617 were systematically investigated by establishing the relationship between microstructural characteristics and creep performance, including rupture time, percentage elongation, microhardness, and minimum creep rate. The Wilshire equation was employed to extrapolate long-term creep life by normalizing the stress, σ , using the ultimate tensile strength, σ TS . The results indicated that at temperatures ≤950 °C, when σ / σ YS values were >0.2 ( σ YS = yield strength), the creep rupture mechanism involved the sliding and migration of recrystallized grain boundaries, leading to matrix softening and necking fractures. When applied stresses ranged from 0.12 σ TS to 0.2 σ YS , creep voids along the grain boundaries became a pronounced feature, implying the rupture mechanism of the weakening of the grain boundaries without sufficient secondary-phase pinning. At a low stress level ( σ / σ TS < 0.12), creep ruptures were governed by a composite mechanism of microstructural degradation and internal oxidization, which was reflected in the brittle fractures caused by the connection of cracks along carbide networks and internal oxidized voids. A temperature–stress–rupture mechanism map was constructed by combining a comparison and summary from the literature. The Wilshire extrapolation curve described the creep rupture times well with temperatures and stresses, and the turning point of the curve suggested that microstructural degradation and internal oxidization increased the minimum creep rates and accelerated creep rupture.

Influence of thermal ageing on creep rupture mechanism and creep life of P92 ferritic steel

In order to analyze the effect of thermal ageing on creep rupture mechanism and establish a method for predicting the creep rupture life of P92 heat-resistant steel under thermal ageing, creep tests were performed on P92 steel specimens, aged at 650 °C for different times under different applied stresses. Optical microscopy and scanning electron microscopy were utilized to observe the microstructure after thermal ageing and creep rupture. Voids and cracks were distributed in the grains of P92 steel before thermal ageing, whereas the voids were clearly distributed around M23C6 precipitates and Laves phase along sub-grain boundaries after thermal ageing. The creep damage tolerance factor of the unaged and short-time aged samples ranged from 2.5 to 5, and the creep rupture was caused by dislocation movement. After high-temperature thermal ageing for 3000 h, the damage tolerance factor increased to > 5 and creep rupture was caused by precipitate coarsening. A theoretical method was established to predict the creep rupture life of heat-resistant P92 steel under thermal ageing, providing consistent results with the American Society of Mechanical Engineers (ASME) long-term test and trend.

Creep rupture mechanism and microstructure evolution around film-cooling holes in nickel-based single crystal superalloy specimen

The fracture mechanism and microstructure evolution around film-cooling holes in a nickel-based single crystal superalloy specimen under tensile creep at high temperatures were investigated using thin-walled plate specimens with close-packed film-cooling holes. The results demonstrated that the film-cooling hole structure has a weakening effect on the specimen for the creep life and creep ductility. In the specimen with the film-cooling holes, a large number of small cracks formed from initial cracks along the direction of the maximum resolved shear stress during the high-temperature creep process, and the cracks propagated along the direction perpendicular to the loading stress direction. Further, the microstructure evolution of the γ′ phase around the edge of the film-cooling holes was closely related to the local stress. While N-type rafting occurred in the high-stress zone, no significant change of microstructure was observed in the low-stress zone. Further, a modified damage model of the crystal plasticity theory was used to simulate the deformation of the specimen with cooling holes under multi-axial stress state, and the results of finite element analysis (FEA) agreed with both the test results and fracture morphology.

Anisotropic creep fracture mechanism and microstructural evolution in nickel-based single crystal specimen with a center film hole

The influence of orientation on the creep rupture properties of nickel-based single crystal superalloy with a center film hole was carried out at 980 °C and 300 MPa. The mechanisms of creep rupture and microstructural evolution of the specimens were studied through macro- and micro-morphological observations and finite element analysis. The central film hole exhibited the creep-strengthening effect on the nickel-based single crystal plate specimens. The degree of strengthening was related to crystal orientations, and the most prominent effect was observed in the [011] orientation. The steady-state creep rate of the specimens with central film holes decreased in the order of [001] > [011] > [111], which was completely opposite of the specimens without central holes. The creep rupture mechanism occurred due to the growth and aggregation of micro-cracks and voids. The microstructural evolution in the strengthening phase γ’ of the specimens with central film holes was closely related to the local stress state. In the high-stress zone where fractures originated, the rafting directions of the [011] and [111] orientations formed a 50° angle with the loading axis, and that of the [001] orientation was perpendicular to the loading axis. The modified crystal plastic-damage model was employed to perform the analysis of stress and damage around the center hole. The simulation results concorded well with the experimentally observed fracture behaviors.

Creep Characteristics and Fracture Mechanisms of a Ni‐Based Superalloy with δ Phases at Intermediate Temperatures

The creep and fracture characteristics of a Ni‐based superalloy with preprecipitated δ phases are researched at the intermediate temperatures (630–670 °C). The influences of external stress and temperature on the creep properties and rupture mechanisms are analyzed. It is observed that the creep rupture time decreases, whereas the minimum creep rate increases, as the creep temperature or external stress is raised. With raising the external stress, the failure pattern changes from the mixed intergranular/transgranular mode to the intergranular fracture. A Monkman–Grant (MG) model is established to characterize the relationship between the residual creep life and experimental parameters (external stress and temperature). The small error between the forecasted and measured residual creep life verifies that the established MG model can be applied to evaluate the creep life of the studied superalloy.

Creep behavior and cavitation evolution of 15CrMoG steel at high temperatures

15CrMoG steel is a type of heat-resistant steel frequently used in boiler and piping systems. Creep properties of the 15CrMoG steel at service temperatures are not much documented due to the difficulties in obtaining long-term creep data. Herein, the creep behavior and the cavity evolution of 15CrMoG steel were investigated based on 20,000 h of creep tests at varied temperatures. Creep curves were analyzed to elucidate the creep behavior and creep rupture mechanism of the 15CrMoG steel. A continuum damage model was adopted to fit the rupture stress versus creep time data, and the results showed the reliability of this model in describing the creep behavior and predicting the creep life. The creep rupture stress at 20,000 h decreased significantly with the increase in the temperature in the tested temperature range. The cavitation in the 15CrMoG steel samples occurring during the creep tests was also examined by microscopic analysis, the results of which confirmed that the cavitation evolution is responsible for the reduced mechanical performance and finally creep rupture of the steel. This work provides valuable high-temperature creep data of the 15CrMoG heat-resistant steel and insights into evaluation and prediction of long-term creep behavior at high temperatures.

Creep rupture properties of dissimilar metal weld between Inconel 617B and modified 9%Cr martensitic steel

In this study, creep rupture behaviors and rupture mechanism of dissimilar metal weld (DMW) between Inconel 617B and modified 9%Cr martensitic heat-resistant steel (hereinafter referred to 9Cr steel) with Inconel 617 filler metal were investigated. Creep tests were carried out at 600 °C and 620 °C in the stress range 130–240 MPa. Optical microscopy (OM), scanning electron microscopy (SEM), Electro-Probe Microanalyzer (EPMA) and micro-hardness test were used to examine the creep rupture behaviors and microstructure characteristics of the DMW. The results indicated that under different creep conditions, the rupture locations varied, including 9Cr base metal (BM), the intercritical heat affected zone (ICHAZ) of 9Cr steel, and the interface between weld metal (WM) and 9Cr steel. The rupture behavior occurring in the 9Cr BM was mainly controlled by plastic deformation, and the growth and coalescence of dimples eventually led to transgranular fracture. The rupture behavior occurring in the ICHAZ was caused by type IV crack due to matrix softening and lack of sufficient precipitates pinning the grain boundaries (GBs). The rupture behavior occurring at the interface was mainly related to the oxide notch forming at the interface. The formation and propagation of the oxide notch resulted in the interfacial failure.

Estimating the Influences of Prior Residual Stress on the Creep Rupture Mechanism for P92 Steel

Creep damage is one of the main failure mechanisms of high Cr heat-resistant steel in power plants. Due to the complex changes of stress, strain, and damage at the tip of a creep crack with time, it is difficult to accurately evaluate the effects of residual stress on the creep rupture mechanism. In this study, two levels of residual stress were introduced in P92 high Cr alloy specimens using the local out-of-plane compression approach. The specimens were then subjected to thermal exposure at the temperature of 650 °C for accelerated creep tests. The chemical composition of P92 specimens was obtained using an FLS980-stm Edinburgh fluorescence spectrometer. Then, the constitutive coupling relation between the temperature and material intrinsic flow stress was established based on the Gibbs free energy principle. The effects of prior residual stress on the creep rupture mechanism were investigated by the finite element method (FEM) and experimental method. A comparison of the experimental and simulated results demonstrates that the effect of prior residual stress on the propagation of micro-cracks and the creep rupture time is significant. In sum, the transgranular fracture and the intergranular fracture can be observed in micrographs when the value of prior residual stress exceeds and is less than the material intrinsic flow stress, respectively.

Creep-induced microstructural evolution in a nickel-based superalloy designed for advanced ultra-supercritical boilers

Long-term microstructural stability is a major concern for candidate materials used in advanced ultra-supercritical (A-USC) power plants. In the present work, precipitation behavior of a nickel based superalloy designed for A-USC boilers, with a base composition of Ni-25Cr-20Co, was investigated during long-term thermal exposure under different stresses at 1043 K. The results showed that the coarsening of γ′ precipitates in the grain interior had occurred during thermal exposure in the presence and absence of stress. The microstructure around grain boundary was altered remarkably under stress. The precipitate-free zones (PFZs) commonly existed near the grain boundary in the gauge sections of specimens after long-term creep. Coarsened and elongated particles in the PFZs were identified as γ′-Ni3 (Ti, Al) precipitates by chemical composition analysis and diffraction pattern. Furthermore, the morphology and distribution of MC-type carbides have no significant changes observed due to their good stability, but discontinuous M23C6 appeared under stress, due to formation of PFZs resulting in dissolution of the adjacent M23C6. Besides, the creep-rupture mechanism was discussed to explore a potential approach for improving creep properties.

Creep behavior of Grade 91 steel under uniaxial and multiaxial state of stress

Creep rupture behavior of Grade 91 heat-resistant steel used for steam cooler under uniaxial and multiaxial state of stress was investigated. Creep tests were conducted at the temperature of 923K under the stress 125MPa. The notch root radii (r) of doubled circumferentially U-notched specimens were 0.6 and 6 mm. The creep rupture life of Grade 91 steel was found to increase with the increasing of notch acuity ratio. The creep rupture mechanism was investigated based on the SEM fractography analysis.

Creep deformation and rupture mechanism of an advanced wrought Ni[sbnd]Fe-based superalloy for 700 °C class A-USC steam turbine rotor application

The 700 °C class Advanced Ultra-Super-Critical (A-USC) technology with higher thermal efficiency is being developed for next-generation coal-fired power plant in recent years to reduce environmental impacts of energy consumption. A new wrought NiFe-based superalloy with excellent high creep strength and low cost has been developed recently and evaluated as a candidate material for the 700 °C class A-USC steam turbine rotor application. Creep tests were conducted on the new NiFe-based alloy at 700 °C/200 MPa, 700 °C/250 MPa and 725 °C/150 MPa using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The creep-rupture testing results show the new NiFe-based alloy has better creep-rupture strength than the existing commercially available NiFe-based and even some Ni-based alloys. The focus of this study is on understanding the relative important causes of better creep resistance and reduced rupture life in grain interiors and grain boundaries. Detailed creep deformation behaviors and related creep-rupture mechanisms were analyzed and discussed to explore the potential approaches for performance improvements. The results presented are helpful in providing a new approach to design and develop novel high performance alloys for A-USC steam turbine rotor application.

Creep damage mechanism and γ′-phase morphology of a V-notched round bar in Ni-based single crystal superalloys

The circumferential V-type notched specimen has been designed to investigate the effect of multiaxial stress state on creep rupture behavior of Ni-based single crystal superalloys at 980°C. The stress on the minimum net-section is chosen to be 400MPa. Tests on smooth specimens were also carried out under 400MPa for comparison. Time to creep fracture was found to be higher in the notched specimens. Scanning Electrical Microscope (SEM) analysis indicated that rupture initiates at the notch root in the notched specimen and at the inner in the smooth specimen. Finite element analysis was carried out to understand the stress distribution and the creep damage evolution under multiaxial stress state. The morphology the evolution of γ′-phase proved that directional coarsening was strongly dependent on the local principal stress field. These findings can be helpful to understand the effect of notch on the creep rupture behavior and mechanism, especially on the γ′-phase morphology evolution.

Cinétique de ruptures de fibres dans un matériau composite UD soumis à la traction avec une matrice viscoélastique

Structural applications of fiber reinforced polymers are strongly dependent on long-term durability studies. Subject to sustained loading, the material undergoes several phenomena reducing its mechanical performances and potentially leading to its failure. To understand the creep rupture mechanisms and identify relevant mechanical parameters that will need to be adjusted during the composition of the material, modeling at the fibers scale seems adequate. In this article a shear-lag model will be presented. This 2D model integrates a viscoelastic behavior of the matrix and a stochastic distribution of fibers strength. Results showing the main phenomena occurring in the stress redistribution in the composite material will also be presented. © 2013 Lavoisier.

Microstructural analysis on creep properties of dissimilar materials joints between T92 martensitic and HR3C austenitic steels

This paper reports the creep rupture behaviors of T92/HR3C dissimilar materials joints. HR3C austenitic steel is welded with T92 martensitic steel to obtain T92/HR3C dissimilar materials joints. After welding, creep test is carried out at 625 °C in the stress range 110–180 MPa. The rupture locations of creep specimens are examined by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the creep rupture mechanism of the joint is dependent on stress. When stresses ≥140 MPa, rupture location is at the T92 base material part and the connection of adjacent dimples results in transcrystalline fracture. For stresses <140 MPa, rupture location is at the T92 coarse-grained HAZ (CGHAZ) and M 23C 6 particles grown up as well as Laves phase ((Fe, Cr) 2 (W, Mo)) precipitation on grain boundaries leads to the creep cavities and intergranular fracture.