Rupture Life Research Articles

Stress Rupture Life Prediction Method for Notched Specimens Based on Minimum Average Von Mises Equivalent Stress

Creep tests were carried out on notched plate specimens of nickel-based superalloy GH4169 with different stress concentration coefficients. It was found that the duration of the first stage of the creep curve increases with the increase of stress concentration coefficient, while the fracture ductility decreases with the increase of stress concentration coefficient. To predict the life of notched plate specimens, four constitutive models were used to analyze the stress and strain of the notches. It was found that the average Von Mises equivalent stress (AVES) on the minimum notch section first decreases and then increases with the creep time, resulting in a minimum value. The minimum average Von Mises equivalent stress (MAVES) is considered as the characteristic stress of notched specimens in this paper. The creep life equation is fitted according to the results of creep tests of smooth specimens, and then the predicted life of notched specimens is obtained by substituting the minimum average Von Mises equivalent stress of notched specimens into the creep equation. The prediction results of the four constitutive models are within 2 times the dispersion band, and the three-stage model is within the 1.5 times dispersion band.

Role of stabilization heat treatment inducing γ′-γ″ co-precipitates and grain boundary η phase on tensile and creep behaviors of Inconel 706

The effects of the stabilization conditions on the microstructure evolution in Ni-Fe-based superalloy Inconel 706 were investigated. In addition, the relationship between the microstructure and tensile and creep properties of the alloy was investigated. Stabilization at 800–840 °C resulted in the precipitation of the η phase at the grain boundaries. With an increase in the stabilization temperature, the η phase fraction increased. Non-compact γ′-γ″ co-precipitates were also formed at 800–840 °C, and their size was the largest when the stabilization was carried out at 840 °C. After double aging, the sample stabilized at 840 °C showed a cuboidal γ′ phase covered entirely with γ″ films, i.e. compact γ′-γ″ co-precipitates were observed along with a coarsened lamellae η phase. The double-aged sample stabilized at 800 °C (S80) exhibited the highest yield strength because of the presence of fine γ′ and γ″ precipitates. The double-aged samples stabilized at 810 and 840 °C (S81 and S84, respectively), exhibited similar yield strengths. Isolated stacking faults were generated in the γ′ particles of the S81 and S84 samples during the tensile deformation, indicating the high mobility of perfect dislocations in the γ-matrix. After creep at 650 °C/650 MPa, S81 and S84 exhibited higher creep strain at a given time than S80. This higher creep strain can be attributed to the synergistic effect of the increased dislocation mobility and η phase fraction, which resulted in strain accumulation at the grain boundaries. The anti-phase boundary shearing observed only in the S84 sample contributed to the strengthening of its γ-matrix and increased its creep rupture life.

Short-Term Creep-Rupture Behaviour of AISI 310S Austenitic Stainless Steel Sheets Manufactured in Salem Steel Plant, a Special Steels Unit of Steel Authority of India Limited, Ministry of Steel, Government of India

The creep behavior of AISI 310S stainless steel taken from SAIL’s Salem stainless steel plant has been investigated by constant load tensile creep test at the temperatures of 973, 1023, and 1073 K and loads of 66.6, 74.8, 86.6, and 94.8 MPa. It exhibits steadystate creep behavior in most test conditions. The double logarithm plot of rupture life and applied stress yielded straight lines at all the three test temperatures indicating that power-law creep due to dislocation climb is the operating mechanism of creep deformation. Linear relationship was obtained for plots of logarithm of rupture life against inverse temperature obeying Arrhenius type of temperature dependence with activation energy of 340 kJ/mol. The stress-rupture data yielded a master curve of Larson-Miller parameter. The plot of Monkman-Grant relationship is typical indicating that rupture is controlled by growth of grain boundary cavities. The metallographic examination of crept samples revealed formation of grain boundary voids and cracks leading to intergranular creep fracture. Deformation twins and carbide precipitates were also observed. Creep-rupture properties are compared with that of AISI 600 ironbased superalloy to analyze quantitatively its behavior

Experimental and numerical study on creep behaviors of 2D twill woven quartz fiber/silica matrix composites

This study investigates the creep deformation, damage, and rupture behaviors of 2D woven SiO2/SiO2 composites via experimental and numerical methods. In situ monotonic tensile tests and creep tests were conducted at 900 °C using a self-designed experimental system and digital image correlation. The tested specimens were characterized by X-ray computed tomography and scanning electron microscopy to conduct quantitative analyses and fracture observations. The obtained creep strain–time curves consist of primary and secondary stages, similar to the creep strain–time curves of most ceramic matrix composites. The matrix at the intersection of fiber bundles cracked under tensile loading. During subsequent creep loading, the propagation of matrix cracks, interfacial debonding, and fiber breakage in longitudinal fiber bundles were observed. At the mesoscale, the creep rupture entails a mechanism analogous to that observed in the monotonic tensile tests. Overall, the SiO2/SiO2 composites employed in this study exhibit excellent potential for long-term operation under mechanical loads at high temperatures. Next, a micromechanics-based creep model was proposed to simulate the creep behavior of the composites. In this model, the primary creep law and rule of mixtures were combined to describe the stress redistribution of various constituents and predict the deformation of the composites. In addition, the rupture life was predicted based on the global load-sharing model, two-parameter Weibull model, and shear lag model. The degradation of the matrix modulus and fiber strength was also considered to improve the accuracy of the simulation. The predicted results were in good agreement with the experimental data.

Effect of withdrawal rate on the microstructure and mechanical properties of a novel monocrystalline CoNi-based superalloy

Withdrawal rate is a critical parameter that affects the microstructure during single-crystal growth. In the present work, the effect of the withdrawal rate (3, 4.5, 6, and 9 mm/min) on the microstructure and mechanical properties of a new CoNi-based single-crystal alloy in the as-cast and heat-treated states was investigated in detail. As the solidification distance decreased with increasing withdrawal rate, the dendritic structures and γ′ phase in as-cast microstructures were significantly refined, the element (e.g. W, Al, Ta, Ti, etc.) segregation aggravated, and the volume fraction of the intercrystalline phase increased from 11.9% to 25.2%, which led to a decrease in the creep rupture life (from 90.4 h to 57.8 h) of the as-cast alloy at 1000 °C/150 MPa. By contrast, the microstructure and mechanical properties of the heat-treated alloys did not appear to be a strong function of the withdrawal rate, which was probably due to the fully heat treatment eliminated the effects of the withdrawal rate and homogenized microstructure. This investigation provides theoretical support and a practical basis for the growth of large-scale single crystals from CoNi-based superalloys.

An interpretation of creep rupture properties of grade 91 steel service- exposed for 100,000 hours at 600 °C

ABSTRACT Creep rupture data of grade 91 steel used as hot reheat pipes have been reported recently. The data are investigated by comparison with the creep rupture database of the steel in normalised and tempered (NT) states. Four regions H, M, L and G with different values of stress exponent appear in creep of grade 91 steel. The pipes were used in long-term region G (around 600 °C for 100,000 hours) in fossil-fired power plants. Medium- and low-stress regions M and L are confirmed to exist in the service-exposed states. Stress exponents in the two regions are kept unchanged in all the pipes, but the service exposure reduces their remaining creep rupture lives. The reduction is significant in the pipes made by cold bending. The service exposure retards the appearance of long-term region G. Causes of these findings are discussed in this paper.

Creep Behavior of Compact γ′-γ″ Coprecipitation Strengthened IN718-Variant Superalloy

The development of high-temperature heavy-duty turbine disk materials is critical for improving the overall efficiency of combined cycle power plants. An alloy development strategy to this end involves superalloys strengthened by ‘compact’ γ′-γ″ coprecipitates. Compact morphology of coprecipitates consists of a cuboidal γ′ precipitate such that γ″ discs coat its six {001} faces. The present work is an attempt to investigate the microstructure and creep behavior of a fully aged alloy exhibiting compact coprecipitates. We conducted heat treatments, detailed microstructural characterization, and creep testing at 1200 °F (649 °C) on an IN718-variant alloy. Our results indicate that aged IN718-27 samples exhibit a relatively uniform distribution of compact coprecipitates, irrespective of the cooling rate. However, the alloy ruptured at low strains during creep tests at 1200 °F (649 °C). At 100 ksi (689 MPa) load, the alloy fails around 0.1% strain, and 75 ksi (517 MPa) loading causes rupture at 0.3% strain. We also report extensive intergranular failure in all the tested samples, which is attributed to cracking along grain boundary precipitates. The results suggest that while the compact coprecipitates are indeed thermally stable during thermomechanical processing, the microstructure of the alloy needs to be optimized for better creep strength and rupture life.

Accurate Effective Stress Measures: Predicting Creep Life for 3D Stresses Using 2D and 1D Creep Rupture Simulations and Data

Operating structural components experience complex loading conditions resulting in 3D stress states. Current design practice estimates multiaxial creep rupture life by mapping a general state of stress to a uniaxial creep rupture correlation using effective stress measures. The data supporting the development of effective stress measures are nearly always only uniaxial and biaxial, as 3D creep rupture tests are not widely available. This limitation means current effective stress measures must extrapolate from 2D to 3D stress states, potentially introducing extrapolation error. In this work, we use a physics-based, crystal plasticity finite element model to simulate uniaxial, biaxial, and triaxial creep rupture. We use the virtual dataset to assess the accuracy of current and novel effective stress measures in extrapolating from 2D to 3D stresses and also explore how the predictive accuracy of the effective stress measures might change if experimental 3D rupture data was available. We confirm these conclusions, based on simulation data, against multiaxial creep rupture experimental data for several materials, drawn from the literature. The results of the virtual experiments show that calibrating effective stress measures using triaxial test data would significantly improve accuracy and that some effective stress measures are more accurate than others, particularly for highly triaxial stress states. Results obtained using experimental data confirm the numerical findings and suggest that a unified effective stress measure should include an explicit dependence on the first stress invariant, the maximum tensile principal stress, and the von Mises stress.

Experimental Study of Multiaxial Stress State on Creep Rupture Life and Ductility of P91 Steel

Experimental Study of Multiaxial Stress State on Creep Rupture Life and Ductility of P91 Steel

Precipitation behavior of δ phase and its effect on stress rupture properties of selective laser-melted Inconel 718 superalloy

The selective laser-melted (SLM) Inconel 718(IN718) specimens were heat-treated with different solution regimes after the hot isostatic pressing (HIP). The results show that δ phase forms at grain boundary and presents needle-like pattern, and the size and volume fraction of δ phase increase with the increase of holding time and the decrease of solution temperature. The equilibrium volume fraction of needle-like δ phase in SLM-built IN718 is higher than that of short rod-like δ phase in forged-IN718 under the same heat treatment. The SLM-built IN718 specimen without δ phase at grain boundary has the best stress rupture properties, while the specimen with the largest volume fraction of δ phase at grain boundary exhibits the worst stress rupture properties. This is attributed to that the needle-like δ phase at grain boundary promotes the nucleation of void-formed cracks and accelerates the deformation damage. The analysis of the responses to the coordinated deformation capacity of grain orientation and grain size indicates that there exist the heterogeneous deformation and stress/strain concentration at grain boundary. The existence of δ phase influences the coordinated deformation capacity and promotes the stress/strain concentration around grain boundary region. Especially, the needle-like δ phase at grain boundary leads to the more detrimental effect on the rupture life and ductility of SLM-built IN718 alloy on compared with the forged-IN718 alloy.

CREEP RUPTURE LIFE OF PRE-STRAINED SUPERALLOY N07080

The creep of the pre-strained superalloy N07080 is described in this work. The pre-strain was achieved by warm rolling at 1050 oC.-The warm rolling was performed due to additional strengthening, i.e increasing of the superalloy hardness.-The pre-strain drastically reduces the creep rupture life of the superalloy compared to the creep rupture life of the standard heat treated superalloy.-The drastic reductionof the creep rupture life is result of rapid creep cavity nucleation on stress concentration sites along primary grain boundaries of the pre-strained superalloy.-Recrystallization eliminates potential sites for rapid cavity nucleation and prolongates the creep rupture life.

The thin-wall debit of a typical cast polycrystalline M951 alloy

The effect of specimen thickness on the rupture properties of cast Ni-based polycrystal superalloy M951 was investigated in this paper. The results of the rupture tests in air under 980 ​°C/90 ​MPa revealed that the rupture life and elongation obviously decreased with decreasing specimen thickness. The thin-wall effect for polycrystal superalloy M951 is associated with several factors. First, the surface oxidation and internal nitridation induce the reduction in the load-bearing section, which accelerates the rupture fracture of thin specimens. Second, the combined effect of oxidation and stress at the surface grain boundary facilitates the premature initiation and propagation of the surface cracks in thin specimen. Third, the through-grain boundaries introduce by specimens machining are detrimental for rupture property. A direct comparison of rupture properties of thin-wall samples with different grain morphologies indicate that thin-wall effect can be diminished by avoiding through-grain boundary introduction.

Influence of Ta on the intermediate temperature creep behavior of a single crystal superalloy

The influence of Ta on the intermediate temperature creep behavior and deformation mechanism in a Ni-based single crystal superalloy has been investigated at 760 oC and 750 MPa. The results indicate that there is a significant increasing of creep rupture life after the addition of Ta. Lattice misfit between γ and γ′ phase at 760 oC increases in the Ta-containing alloy. Microstructure showed that the formation of stacking faults in the γ matrix and denser dislocation network at the γ/γ′ interfaces caused by Ta addition are found during the primary creep stage. And a large amount of stacking fault locks which act as an important pattern in the strengthening generate in the γ′ precipitates, resulting in a lower steady state creep rate and longer steady state creep life in Ta-containing alloy. And possible formation mechanism of the stacking fault locks is also discussed.

Creep Deformation Property and Creep Life Evaluation of Super304H

Abstract Creep deformation behavior, creep strength property, and microstructural evolution during creep exposure were investigated on Super 304H steel for boiler tube. In the high stress and lower temperature regime, creep rupture strength of Super 304H steel is higher than that of SUS304H steel. The slope of stress versus time to rupture curve of Super 304H steel, however, becomes steeper with increase in creep exposure time and temperature, and the creep rupture strength of Super 304H steel becomes closer to that of SUS304H steel after the tens of thousands of hours at 700 °C and above. In the short-term, at 600 °C, creep rupture ductility increases with increase in creep rupture life. However, it tends to decrease after showing the maximum value and the creep rupture ductility decreases with increase in temperature. The complex shape of creep rate versus time curves, with two minima in creep rate, was observed at 600 °C. Several type precipitates of niobium carbonitride (Nb(C,N)), Z phase (NbCrN), and copper were observed in Super 304H steel, as well as M23C6 carbide and sigma phase observed in SUS304H steel. The change in slope of stress versus time to rupture curve is caused by disappearance of precipitation strengthening effect during creep exposure. Accuracy of creep rupture life evaluation was improved by stress range splitting method which takes into accounts of the change in slope of stress versus time to rupture curves was demonstrated.

Creep crack growth modelling of Grade 91 vessel weldments using a modified ductility based damage model

This paper reports creep crack growth modelling of Grade 91 vessel weldments at high temperature using a modified ductility-based continuum damage mechanics (CDM) model, which is based on the Kachanov-Rabotnov CDM and utilizing the concept of ductility exhaustion. The proposed model holds a key advantage over existing models in that it requires fewer material constants to be identified and calibrated. The new modified model was implemented into a user-defined subroutine in ABAQUS and then used to predict the creep crack growth of Grade 91 vessel weldments. Creep crack initiation and growth in the vessels were predicted to occur in the heat-affected zones of the weldments associated with the end closures, which is in good agreement with the corresponding vessel tests. Further, the predicted creep rupture lives of the notched bars and the vessels using the proposed model correlated reasonably well against the experimental results. This suggests that the modified ductility-based damage model can be used for creep crack growth and life prediction for high temperature structures with confidence.

Numerical Prediction of Creep Rupture Life of Ex-Service and As-Received Grade 91 Steel at 873 K

Infallible creep rupture life prediction of high temperature steel needs long hours of robust testing over a domain of stress and temperature. A substantial amount of effort has been made to develop alternative methods to reduce the time and cost of testing. This study presents a finite element analysis coupled with a ductility based damage model to predict creep rupture time under the influence of multiaxial stress state of ex-service and as-received Grade 91 steel at 873 K. Three notched bar samples with different acuity ratios of 2.28, 3.0 and 4.56 are modelled in commercial Finite Element (FE) software, ABAQUS v6.14 in order to induce different stress state levels at notch throat area and investigate its effect on rupture time. The strain-based ductility exhaustion damage approach is employed to quantify the damage state. The multiaxial ductility of the material that is required to determine the damage state is estimated using triaxiality-ductility Cock and Ashby relation. Further reduction of the ductility due to the different creep mechanisms over a short and long time is also accounted for in the prediction. To simulate the different material conditions: ex-service and as-received material, different creep coefficients (A) have been assigned in the numerical modelling. In the case of ex-service material, using mean best fit data of minimum creep strain rate gives a good life prediction, while for new material, the lower bound creep coefficient should be employed to yield a comparable result with experimental data. It is also notable that ex-service material deforms faster than as-received material at the same stress level. Moreover, higher acuity provokes damage to concentrate on the small area around the notch, which initiates higher rupture life expectancy. It also found out that, the stress triaxiality and the equivalent creep strain influence the location of damage initiation around the notch area.

Creep damage and interaction behavior of neighboring notches in components at elevated temperature

Previous studies have focused on creep damage evolution behavior of components with one isolated notch, but two or more notches may be placed at the components due to the function requirements, the limitation of the structural size, or may be induced by chemical/physical damage processes. In this work, numerical analyses and creep tests on creep damage and interaction behaviors of neighboring notches in components under creep conditions are investigated. Effects of structural configurations on creep damage and interaction behavior are discussed. Results indicate that the notch still indicates a strengthening effect on rupture life of components despite the interaction behavior of the two adjacent notches. A negative interaction effect on rupture life of components with two notches is found. Creep damage and interaction behaviors of the components with two neighboring notches of various distance values are related to structural configurations of the components, including notch acuity ratio, notch open angle and notch depth.

Effect of nickel-based filler metal types on creep properties of dissimilar metal welds between Inconel 617B and 10% Cr martensitic steel

Creep rupture behaviors of dissimilar metal welds (DMWs) between Inconel 617B and 10%Cr martensitic steel with two different filler metals Inconel 617 and 82 were comparatively investigated at 600 °C to study the effect of nickel-based filler metal types on creep properties. Both the crept DMWs ruptured on the steel side prematurely compared to steel base metal (BM). The failure mode of DMW shifts from interfacial failure at weld metal (WM)/steel interface to Type Ⅳ crack in the intercritical heat-affected zone (ICHAZ) of steel with longer rupture life at the same creep condition by replacing Inconel 617 filler metal with Inconel 82. Type Ⅳ crack was related to matrix softening and fine prior austenite grain boundaries (PAGBs) scarcely decorated by precipitates in the ICHAZ. Interfacial failure was controlled by creep voids nucleation, growth and coalescence at the interface, while oxide notch only triggered crack initiation. The failure mode shift is because Inconel 82 filler metal compared to Inconel 617 can achieve slighter creep deformation concentration in the HAZ closely adjacent to interface and avoid interfacial failure of the DMW due to the lower creep strength gradient across the interface it induced. Based on this study, creep strength compatibility should be considered when selecting nickel-based filler metals for similar DMWs to obtain prolonged creep life.

Weakening modes for the heat affected zone in Inconel 625 superalloy welded joints under different high temperature rupture conditions

Inconel 625 superalloy (IN625) was generally welded as the base metal to fabricate the rotors in steam turbine with temperature over 700 °C. In this case the weakening modes for the heat affected zone (HAZ) in IN625 welded joints during the stress rupture test are systematically investigated in the present study. The stress rupture test results show a dependence of rupture time on stress lg (t) = −0.017 σ + 5.85, which can be achieved to predict the rupture life with different stress levels. Besides, all the samples are deemed as intergranular fracture with columnar grain morphology appeared on the fracture surface. The microhardness distribution reflects that all the specimens ruptured in the HAZ. It is deemed that the chain-like particles can weaken the binding force of the continuous grains and therefore cause the resultant fracture in the region with comparative low microhardness of HAZ. With the decrease of the applied stress, the formation of acicular δ phase (Ni3Nb) can reduce the deformation capacity of the HAZ by consuming the precipitate strengthened element Nb. Hence, the micro-voids tend to initiate along the grain boundaries. The coarse Laves phase particles with hard and brittle features become the dominated factor with the further increased rupture time. The micro-voids initiate and converge around the Laves phase under the stress to form micro-cracks, eventually cause the fracture of the ruptured specimens.

Study on creep properties of nickel-based superalloy blades based on microstructure characteristics

Creep experiments were performed on miniature K480 nickel-based superalloy specimens at 835 °C and 400 MPa. These specimens were cut from turbine blades processed with different casting processes and pouring temperatures. The microstructure analysis, fracture morphology analysis, and grain size analysis were carried out by scanning electron microscopy (SEM), indicating that the influence of the moulding process and pouring temperature on the microstructure of the alloy is mainly reflected in the difference in the size of crystal grains, grain boundary precipitates and casting defects, which in turn affects the high temperature creep properties of the alloy. Based on crystal plasticity theory, a polycrystalline alloy viscoplastic constitutive equation considering the moulding process and pouring temperature parameters is established, meanwhile, the geometric models of the K480 alloy were established based on the observations of its microscopic structure under SEM. The study revealed the relationship between the casting process of the blade, the microstructure of the material, and the creep property of the specimen. The established plastic constitutive model can predict blade stress rupture life under different casting processes.