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In this paper, the creep‐fatigue responses of the nickel‐based superalloy Inconel 718 have been investigated by conducting hybrid stress–strain‐controlled creep‐fatigue interaction (HCFI) loading experiments at 650°C with different dwell stresses and dwell periods. Deformation responses demonstrate that stress dwell leads to complex cyclic responses, such as an increase in softening rate and changes in inelastic strain rate. Then, a novel creep equivalent stress determination method is proposed based on stress relaxed characteristics. After modifying the continuum damage mechanics (CDM) model with this method, accurate prediction results of the creep‐fatigue cyclic life under strain‐controlled creep‐fatigue loading and HCFI loading can be obtained. Finally, the damage evolutions of Inconel 718 under HCFI loading are analyzed by CDM model. The results of damage evolution indicate that the trend of damage accumulation under different test conditions is consistent with the trend of inelastic strain in the material. Highlights: The creep‐fatigue responses of Inconel 718 under HCFI loading are investigated.A creep equivalent stress determination method based on stress relaxation is proposed.Accurate life prediction results obtained under various loadings at high temperature. [ABSTRACT FROM AUTHOR]

Creep behaviour of P91, 12Cr1MoV steels and the P91/12Cr1MoV dissimilar joint were investigated at 823 K. Results show that the creep strength of P91 is much higher than 12Cr1MoV and than the dissimilar joint. The stress dependence of minimum creep rate and rupture life for both steels and the dissimilar joint obeyed Norton’s power law. The values of stress exponent are similar for 12Cr1MoV steel and the dissimilar joint in high stress region, indicating similar creep mechanism. However, the creep behaviour at 140 MPa for the dissimilar joint showed deviation from the other higher stresses, indicating different creep mechanism as the stress is decreasing. Microstructure showed that creep ruptured on the 12Cr1MoV side of the dissimilar joint as conducted in the high stress region, whereas ruptured on the carbon decarburized zone as conducted in the low stress region. Fracture location changed from 12Cr1MoV base metal to inter-critical heat affected zone as the creep time going. A modified theta equation was proposed to predict the creep behaviour, and the random errors from fitting to experimental creep data were smaller than obtained from the traditional theta method. The predicted creep behaviour showed good agreement with experimental ones. [ABSTRACT FROM PUBLISHER]

• The creep-fatigue responses of Inconel 718 affected by overloading are investigated. • Fracture appearance and microstructure are analyzed after overloading. • An equivalent damage to quantify the creep fatigue damage affected by overloading. • Accurate residual life prediction results after overloading obtained. Inconel 718 is a material for manufacturing turbine disks, which is subjected to complex loads of creep-fatigue during service. This paper examines the effect of constant stress overloading at various lifespan stages on subsequent cyclic deformation, fracture behavior, and residual lifetime in hybrid stress and strain-controlled creep-fatigue interaction (HCFI) experiments conducted at 650℃. Overloading of 800 MPa constant stress with 180 min dwell time is applied at the 10 %, 25 %, 50 %, 65 %, and 70 % lifespan of HCFI tests. Results show that overloading reduces the residual life of the material obviously and makes a significant impact on the subsequent creep-fatigue deformation behavior, and additional damage is induced when overloading is applied towards the end of the lifespan. Moreover, more severe grain boundary damage leading to the final intergranular fracture is identified through microstructural analysis following overloading. Finally, equivalent damage is proposed to quantify the creep-fatigue damage affected by overloading, resulting in accurate life prediction outcomes. [ABSTRACT FROM AUTHOR]

The high-temperature superheater outlet header (Outlet Header) in ultra-supercritical (USC) thermal power plants is subjected to high temperatures and pressures, which increases the risk of creep failure. To assess the structural reliability of the Outlet Header, it is necessary to consider the impact of uncertainty factors. Furthermore, the diverse operating conditions make reliability assessment inconvenient. This study evaluates the creep life reliability of the Outlet Header based on material uncertainty and simplifies the assessment process using machine learning methods. Considering the scatter of creep rupture data, the uncertainty of material constants in the Larson-Miller (LM) model is quantified by randomly sampling a specific number of creep rupture life data. Based on the results of uncertainty quantification and finite element analysis, the distribution of the Outlet Header's creep life is obtained to calculate its reliability under design life. Machine learning is employed to assist in the reliability assessment of creep life under different operating conditions of Outlet Header. The results indicate that Artificial Neural Network (ANN) demonstrates good performance in this study, and an assessment diagram based on the ANN has been constructed. This approach provides a practical solution for assessing the reliability of high-temperature components in engineering. • Establishes a P92 steel creep rupture dataset and quantifies the uncertainty of Larson-Miller material constants. • Conducts reliability assessment of the creep life of the high-temperature superheater outlet header. • Utilizes ANN to construct reliability assessment diagram under different operating conditions. [ABSTRACT FROM AUTHOR]

In the petrochemical industry, the centrifugally cast austenitic 20Cr32Ni1Nb stainless steel used in steam reformer furnace's hot outlet manifold experiences creep failure and microstructural degradation, impacting its structural integrity. Understanding the causes of these issues is crucial for addressing the associated challenges. This study examines the creep deformation behaviour, damage mechanisms, and microstructural evolution of 20Cr32Ni1Nb steel under stresses of 22–50 MPa at temperatures of 1163 K (890 °C) and 1223 K (950 °C). Analysis of creep and creep strain rate curves revealed the presence of both primary and tertiary creep stages, with the power law relationship governing the stress-dependent minimum creep rate and rupture life. The variance of creep damage tolerance factor was also analysed, with results showing that in high-stress regimes, loss of section and necking along with cavity growth may initiate tertiary creep, while in low-stress regimes, microstructural degradation, specifically G-phase formation, may initiate tertiary creep. As the duration of creep exposure increases, microstructures were observed to degrade, with the growth and coarseness of NbC and M 23 C 6 carbides. The structure of the space group for the Ni–Nb–Si enriched G-phase in 20Cr32Ni1Nb steel was found to be Fm 3 ‾ m , with a lattice parameter of about 1.14 nm. • The distinct values of creep damage tolerance factor were obtained in low and high stress regimes. • The structure of space group for the Ni–Nb–Si enriched G-phase in 20Cr32Ni1Nb steel is identified. • The growth and coarsening of NbC and M 23 C 6 carbides, and G-phase formation lead to creep strength degradation of 20Cr32Ni1Nb steel. [ABSTRACT FROM AUTHOR]

This work is devoted to the study of the influence of prior low cycle fatigue (LCF) on microstructure evolution and subsequent creep behavior of P92 steel. Different strain amplitudes prior LCF tests ranging from 0.25% to 0.6% were performed at 650 °C with constant strain rate of 1.0 × 10 −3  s −1 and thereafter subsequent creep tests were conducted at 130 MPa in the same temperature. Optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) were utilized to clarify prior LCF damages of various strain amplitudes on subsequent creep behaviors. Results reveal that the increase of lifetime fraction and the increase of strain amplitude lead to the nonlinear degradation of subsequent creep properties. However, the degradation behavior shows a tendency to be saturated after 70% lifetime fraction or after 0.4% strain amplitude, respectively. The degradation is lifetime fraction and strain amplitude of prior LCF dependent. Further analysis of microstructure indicates that the degradation behavior is attributed to the different evolution behavior of martensite laths driven by cyclic deformation during prior LCF. Additionally, fractographs of creep failed specimens were analyzed in detail to correlate the different subsequent creep behaviors. [ABSTRACT FROM AUTHOR]

We developed an analytical model to predict the equivalent creep strain rate of a lattice truss panel structure. The model, which takes into account the effects of the bonded node and the intersection node of the trusses, is well validated by finite element analysis. Compared with Hodge and Dunand model, this model obtains a more accurate prediction result. The creep deformation of the panel structure is controlled by the creep of vertical trusses parallel to the applied load. The equivalent creep strain rate is determined by five key parameters including punching angle, cutting angle, truss thickness, width and length. A slight change of truss dimension can lead to a big variation of the creep rate by orders of magnitude. With the increase of punching angle and cutting angle, the relative density decreases and the stresses in the trusses increase, leading to an increase of creep rate. With the increase of truss thickness and width, the creep rate decreases because the relative density increases and the stresses in the truss decrease. As the truss length increases, the creep rate increases due to the decrease of relative density and the increase of stresses in the truss. [ABSTRACT FROM AUTHOR]

• Simulations of hybrid-controlled creep-fatigue interaction (HCFI) were performed. • Simulated results of cyclic deformation matched well with the experimental data. • Increase in computational time of the extended model was limited. A numerical process based on crystal plasticity finite element method was implemented to describe the cyclic stress–strain response and especially the creep deformation of 9 %Cr steel under hybrid stress–strain controlled creep-fatigue interaction loading at 650 °C. An extended model based on Armstrong and Frederick (A-F) hardening law was used to capture the creep strain. Compared with experimental results, the creep strain performed by A-F model was small while extended model predicted well. The cyclic softening and decline tendency of relaxation have been simulated. Simulations of other loading modes with extended model also show a good agreement with experimental results. [ABSTRACT FROM AUTHOR]

The deformation and damage mechanisms of welded joints have gained high interest for their widespread use in power plants. The inter-critical heat-affected zone (ICHAZ) is generally identified as the weakest region in the whole welded joint. However, the micro-regions' nanoindentation and creep results in the present work reveal that the weakest position moves to base material after fatigue loading. It is found from electron backscatter diffraction (EBSD) analysis that the early saturated grain growth in the ICHAZ contribute to the movement of the weakest region. It is important to note that compared with the creep strength in the as-received condition, prior fatigue loading enhances the creep deformation. It is also observed from three-dimensional X-ray tests that large fatigue cycles and long hold time of fatigue loading accelerate the nucleation of creep voids. Finally, a fatigue damage parameter based on the nanoindentation microhardness is proposed, which is validated to be capable of capturing the effect of various loadings on the subsequent creep life and creep fracture location. These results benefit the understanding of the creep behaviour of welded joints after fatigue damage and provide a guide for the design and damage evaluation of welded components. [Display omitted] • The weakest region is quantitatively proved to change with the fatigue loading. • The nucleation of creep voids is accelerated by large fatigue cycles and long hold time of fatigue loading. • The correlation between the fatigue damage and remaining creep fracture behaviour is revealed. [ABSTRACT FROM AUTHOR]

• The hybrid-controlled creep-fatigue interaction (HCFI) tests have been performed. • All of the conducted HCFI tests can be divided into two regions. • The failure mechanisms of the HCFI tests in different regions have been clarified. • A new life prediction method based on the steady growth rate has been proposed. The hybrid stress–strain controlled creep-fatigue interaction (HCFI) loadings at various dwell conditions were conducted on 9%Cr steel at 625 °C to investigate the deformation and damage mechanisms. Results reveal that the HCFI tests with conservative dwell conditions show complicated cyclic responses and low creep strain. The failure specimens demonstrate strong creep-fatigue interaction damage (CFID) with many branched surface cracks. With the increase in dwell stress and time, the HCFI tests show continuous softening and remarkable creep deformation. The creep-dominant damage (CDD) featured by internal creep cavities and blunt surface cracks leads to the final failure. [ABSTRACT FROM AUTHOR]

• A novel hybrid-controlled creep fatigue interaction (HCFI) loading was developed. • The complicated cyclic responses of P92 steel under HCFI loadings were found. • The fatigue life decreases with dwell stress and dwell period under HCFI loadings. • The proposed model can predict the life of both HCFI and strain-controlled tests. In contrast to conventional strain-controlled creep-fatigue interaction (CCFI) loadings, a novel hybrid stress- and strain-controlled creep-fatigue interaction (HCFI) loadings were developed on P92 steel. Dwell stresses ranging from 140 MPa to 170 MPa, and dwell periods of 300 s, 600 s and 1800 s were employed at 625 °C. The test responses demonstrate that cyclic softening and hardening effects lead to complicated cyclic responses. In addition, the failure life under HCFI loading is observed to be reduced with the dwell stress and dwell period. Finally, a viscosity-based model is proposed to predict the lifetime of both HCFI and CCFI tests. [ABSTRACT FROM AUTHOR]

Strain cycling prior to a constant load exposure is expected to influence the creep properties. The present work aimed to evaluate the effect of prior low cycle fatigue (LCF) damage on the remaining creep properties of P92 steel welded joints. Strain amplitudes of 0.25%, 0.4% and 0.6% and fatigue cycles ranging from 10% to 80% of the number of cycles to failure (N f) were introduced in creep specimens to represent fatigue damage. A critical analysis of the creep test responses revealed that a high strain amplitude and high fatigue cycles induced a clear reduction in the remaining creep life; conversely, prior LCF also led to a significant increase in the creep rupture strain. It was found that prior LCF loading produced its saturated effect after fatigue cycles at 70% N f or a strain amplitude of 0.4%. It is important to note that the degradation of the remaining creep life of a welded joint was more severe than that found in homogeneous base metal. It was also observed that the failure location and development of creep voids were dependent on both the number of fatigue cycles and strain amplitude. Additionally, the prior fatigue damage was evaluated using a defined damage parameter that revealed two nonlinear stages of fatigue damage. • The remaining creep properties of welded joint are sensitive to prior LCF loading. • Fracture location and creep voids are both fatigue cycles and strain amplitude dependent. • Two nonlinear stages behaviour of the fatigue damage on creep life was revealed. [ABSTRACT FROM AUTHOR]

• Kinematic damage equations that represent the recovery of martensite lath and decline of dislocation density are proposed. • The proposed model is capable of capturing effects of various prior fatigue loadings. • Satisfactory prediction of various creep fracture behaviors is achieved. Predicting the remnant creep fracture life precisely is crucial for ensuring safety of high temperature components. This study presents a microstructural damage mechanics-based model for creep fracture of 9%Cr steel under prior fatigue loading. Microstructure observation reveals that the decrease of dislocation density and the growth of martensite lath width occurred during prior fatigue process contribute to the degradation of creep strength. Particularly, coarsening of martensite lath width plays the dominated role. To take into account the effect of the prior fatigue loading, kinematic damage equations that represent the evolution of dislocation density and martensite lath are proposed in the developed model. With the proposed model, creep fracture life and creep failure strain at various lifetime factions, strain amplitudes and hold times of prior fatigue loading can be satisfactorily predicted, which manifests that the proposed model is robust in capturing the effects of various prior fatigue loadings. The proposed model is also shown to be able to accurately predict prolonged creep deformation of other similar steel after different prior fatigue loadings. [ABSTRACT FROM AUTHOR]