Creep Rupture Time Research Articles

Influence of thermal-varying material properties on stress calculation for creep life assessment of seamless medium-carbon steel boiler tubes

Seamless medium-carbon steel boiler tubes from a thermoelectric plant are studied to predict their remanent life. An alternative for stresses calculation in creep life-assessment method is presented. The proposed stress calculation incorporates not only material dimensions and working pressure but also thermal-varying properties of the material and the radial temperature gradient in the tube wall. The methodology for creep life prediction established by ISO / TR 7468-1981 is used. It is compared with the API std 530 to show that the predicted creep rupture times can result longer when thermal-varying properties and the radial temperature gradient in the tube wall are considered. This enhanced creep failure prediction method may favour a cost reduction avoiding premature maintenance and unnecessary plant shutdowns.

Study on creep behavior and microstructure of a Fe–Cr–Al–Nb–W ferrtic heat-resistant steel based on C, N and Al element control

In this work, the effects of C, N, and Al on the microstructures and creep properties of Fe–Cr–Al–Nb–W ferritic alloys were investigated through scanning electron microscopy, x-ray diffractometry, as well as uniaxial creep testing and hardness testing. The results demonstrated that the matrix of the Fe–Cr–Al–Nb–W heat-resistant steel was ferrite, while the precipitation phases were Laves phases, M23C6 carbides and MX nitride phases. M23C6 and MX precipitated at grain interior, Laves phases precipitated at grain interior, grain boundary and around MX phases. C, N and Al affected microstructure and creep of heat-resistant steel. As the Al content increased or as N content decreased along with the C increase, the average diameter of the Laves phases, along with M23C6 and MX phase grain interior decrease. Moreover, the unit density increased and the phase spacing decreased, which led to the minimum creep rate decrease and creep rupture time increase. Compared to M23C6, Laves and MX phases mainly affected the alloy creep strength. The decrease of Laves phase continuity coefficient (ratio of Laves phase particle spacing and size) on the grain boundary was conducive to the plasticity improvement of heat-resistant steel.

Creep rupture time prediction methods of 2.25Cr-1Mo steel under multiaxial stress gradient condition

Creep rupture time prediction methods of 2.25Cr-1Mo steel under multiaxial stress gradient condition

Creep Rupture Assessment of New Heat-Resistant Sanicro 25 Steel Using Different Life Prediction Approaches

The paper deals with creep life and creep strength estimations of Sanicro 25 steel through various continuum damage models and modified extrapolation methods. Four different continuum damage models, i.e., modified K-R model, Dyson model, W-T model and modified W-T model considering the stress-dependent failure strain, were incorporated into numerical simulations using user subroutines so as to analyze the creep strain behavior and the creep rupture time. In the case of 700 °C, the predicted creep rupture lifetimes determined from modified K-R model, Dyson model, W-T model and modified W-T model all matched with the experimental data, where the modified W-T model provided the best prediction. However, at 750 °C, only the predictions using W-T model and modified K-R model matched with the experimental data. The Dyson model and modified W-T model underestimated the creep rupture lifetime. Besides the creep damage approaches, a modified extrapolation method developed by Wilshire (based on the stress-dependent creep activation energy) was also tested and verified to make predictions of creep lifetime under various stress conditions more easily than the creep damage models. A general comparison of the analyses made with different life prediction methods in Sanicro 25 steel was presented in this paper.

Fatigue–oxidation–creep damage model under axial-torsional thermo-mechanical loading

A fatigue–oxidation–creep damage model that can take into account the effect of multiaxial cyclic feature on the damage mechanism is proposed under axial-torsional thermo-mechanical fatigue loading. In the proposed model, the effects of non-proportional additional hardening on fatigue, oxidation, and creep damages are considered, and the variation of oxidation damage under different high temperature loading conditions is also described. Moreover, the intergranular creep damage needs to be equivalent to the transgranular damage before accumulating with the fatigue and oxidation damages. The fatigue, oxidation, and creep damages can be expressed as the fractions of fatigue life, critical crack length, and creep rupture time, respectively, which allows the linear accumulation of different types of damages on the basis of life fraction rule. In addition, the proposed model is validated by various fatigue experimental results, including uniaxial thermo-mechanical fatigue, axial-torsional thermo-mechanical fatigue, and isothermal axial-torsional fatigue under proportional and non-proportional loadings. The results showed that the errors are within a factor of 2.

Prediction of long-term creep life of 9Cr–1Mo–V–Nb steel using artificial neural network

In this work, the method of artificial neural network was employed to predict the long-term creep rupture time of 9Cr–1Mo–V–Nb steel using the NIMS datasheet. In order to verify the performance of this method, the long-term creep rupture times of 23 000–41000 h were predicted using the data lower than 17 000 h. Meanwhile, the detailed analyses were carried out by comparison with the traditional time-temperature parametric (TTP) methods, such as Larson-Miller, Manson-Harferd, and Orr-Sherby-Dorn method. The results showed that by the artificial neural network method, the predicted creep rupture times above had an average relative error of 17%, which was significantly lower than those of TTP methods. It further demonstrated that the artificial neural network offers a convenient tool to predict the accurate creep rupture time of 9Cr–1Mo–V–Nb steel due to its robust ability in law learning and extrapolation generalization.

Effect of Deformation on the Mechanical Properties of Sn-5wt%Sb Alloy

Sn–5wt%Sb is one of the materials considered for replacing Pb-bearing alloys in electronic packaging. The mechanical response of Sn–5wt% Sb solder alloy has been tested under different strain rates and three deformation temperatures. The behavior of true strain–time of Sn–5wt% Sb solder alloy has been investigated over strain rates of and deformation temperatures of 313, 333, and 353 K. Three-load creep tests were carried out at each temperature for of the wire samples to alloys. The deformation behavior and grain growth mechanism were investigated by strain-time curve analysis and microstructure observations. The results obtained show that the general characteristics of strain-time curve and microstructure of Sn-5wt% Sb alloy sensitively depend on the deformation temperature and strain rate. New free grains have been nucleated in microstructures in the process of dynamic recrystallization. These grains grow during deformation, forming coarser structure and elongation. The dynamic recrystallization and grain growth increase with increasing deformation temperature and decreasing strain rate. From the steady state creep rate the stress exponent is described in terms of the heat treatment temperatures. The stress exponent (n) were determined to clarify the deformation mechanism. Based on the n values, it is suggested that the rate controlling creep-deformation mechanism is dislocation climb. This study revealed that the solder alloy Sn–5wt%Sb have potential to give a good combination of higher creep resistance and rupture time.

Application of ductility exhaustion based damage model to predict creep rupture time of grade 92 steel

To accurately predict the creep rupture time of notched bar becomes a challenge to academia and structural engineer due to complex stress-strain distribution around the notch throat. This paper presents a Finite Element (FE) simulation employing ductility exhaustion based damage model to predict creep rupture time of multiaxial notched bar Grade 92 steel. Three different notch acuity, η = 2.5, 5.0 and 12.0 were simulated and the FE predicted rupture time was compared to the available experimental rupture data. The reduction of creep ductility due to geometrical constraint is considered during the simulation by employing the void growth model. Further reduction in ductility in long term period arises from internal microstructural changes or damage is also accounted. Furthermore, empirical-type exponential prediction model coupled with skeletal stresses is applied to provide upper/lower bounds for short and long term rupture data. It is found that the FE prediction agreed well with the experimental data. At short-term, notched bar ruptured is controlled by the von-Mises stress while at long-term the rupture is controlled by the maximum principal stress.

Effect of precipitate evolution on creep damage of reduced activation ferritic/martensitic steel friction stir welded joint

This paper reports on the precipitate evolution and its influence on the high temperature creep resistance of 9% Cr martensite ferritic steel FSW joint. The creep rupture time of the FSW joints welded by 400 rpm and 200 rpm are 859.29 h and 1173.8 h respectively, while that of BM is 3398 h. The creep fracture occurs in ICHAZ where a pronounced increase of coarse Laves phase is detected. Most of the Laves phase particles are large in size and mainly nucleate and grow rapidly at the micrograin boundaries adjoining the M23C6 particles where W, and other elements segregate. The large Laves phase particles will deteriorate the pinning effect of M23C6, weaken the solid solution strengthening effect and attribute to the increased cavity nucleation. In addition the size of Laves phase in ICHAZ is bigger than that in stir zone (SZ). These reasons explain why creep fracture occurs in the ICHAZ and the time to rupture will be shorter than base metal.

Thermo-Mechanical Fatigue of SOFC Glass-Ceramic Sealant/Steel Interconnect Joint in a Reducing Atmosphere

Thermo-mechanical fatigue (TMF) behavior of a joint between a solid oxide fuel cell glass-ceramic sealant and an interconnect steel is investigated in a reducing atmosphere (H2–7 vol% H2O) under cyclic shear and tensile loadings combined with cyclic temperature change between 40°C and 800°C. Experimental results indicate TMF life of shear specimen is increased with a decrease in the end stress applied at 800°C which dominates the shear TMF life. The accumulated duration of loading at peak temperature range (795–800°C) in TMF test is comparable with the estimated creep rupture time at 800°C for shear specimens. TMF fracture of the shear specimens mainly occurs at the interface between the glass-ceramic layer and chromia layer. For tensile specimens, TMF life is controlled by the end stresses applied at 800°C and 40°C. For tensile TMF specimens, fracture mostly takes place within the glass-ceramic layer and at the chromia/glass-ceramic interface.

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.

A multi-scale failure-probability-based fatigue or creep rupture life model for estimating component co-reliability

The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Committee has recently developed a new Section XI (Nuclear Components Inspection) Division 2 Code named “Reliability and Integrity Management (RIM)." RIM incorporates a new concept known as “System-Based Code (SBC)" originally due to Asada and his colleagues (2001–2004), where an integrated approach from design to service inspection is introduced using an overall statistical quantity known as “reliability index,” or, “co-reliability target.” In this paper, we develop a new theory of fatigue and creep rupture life modeling for metal alloys at room and elevated temperatures such that the co-reliability target can be estimated from fatigue and creep rupture test data. To illustrate an application of this new modeling approach, we include two numerical examples using (a) the fatigue failure data of six AISI 4340 steel specimens at room temperature (Dowling, N. E., 1973) and (b) the creep rupture time data of 37 specimens of 1.3Mn-0.5Mo-0.5Ni steel plates at 500 C (NRIM, 1987). Significance and limitations of this new fatigue and creep rupture life modeling approach are presented and discussed.

Modeling and Optimization of Creep Behavior of Ni-Based Superalloys GTD-111 and IN-738LC Using Central Composite Design

In this study, the effects of temperature, stress, and type of materials and their interactions on the creep rate and rupture time were investigated by using central composite design (CCD). An experimental plan for CCD with two numerical factors, one categorical factor, and two levels was used to optimize the required number of experiments. Temperatures of 800 and 900 °C and stresses of 250 and 450 MPa were selected as factors for GTD-111 and IN-738LC superalloys, respectively. Experimental and numerical results showed that the main effects of factors and their interactions are significant on the creep rate and rupture time. Among all factors, the effects of temperature and stress dominated other factors. Moreover, it was indicated that the combination between temperature and stress is much more effective on creep rate response than on rupture time. The high creep rate and the low rupture time values were obtained at the highest stress and temperature for IN-738LC. With the same experimental condition, creep rate values were the most and rupture time values were the least for IN-738LC in comparison with GTD-111.

A Study of Microstructure Evolution During Creep of 9Cr-1Mo Steel Using Ultrasonic and Hardness Measurements

In this work, microstructural changes during creep of 9Cr-1Mo steel were characterized using ultrasonic and hardness measurements. The creep testing of this steel was firstly performed at 873 K under the initial stresses of 160 and 175 MPa, respectively. Then, eight specimens under each stress level were prepared by the interrupted tests to simulate various creep states. Subsequently, two parameters of ultrasonic attenuation and velocity were introduced by the ultrasonic testing. Experiment results showed that regardless of the stresses, the ultrasonic attenuation experiences a peak at about 20% of the creep rupture time and a minimum value at about 50%, showing a strong correlation between ultrasonic attenuation and the creep life fraction. In addition to a slight increase during the primary creep, the ultrasonic velocity exhibits a significant change during the tertiary creep. Based on the analysis of microstructure changes, it indicated that the ultrasonic attenuation is significantly sensitive to the dynamic changes of the dislocation. In contrast, the velocity is mainly related to the evolution of precipitates. The Vickers hardness decreases monotonously during the creep process and presents a linear relation with the inverse of average diameter of M23C6 carbides during the secondary and tertiary creep. Accordingly, it suggested that the present ultrasonic and hardness measurements provided a convenient tool to evaluate the microstructure evolution during creep degradation of heat-resistant steels.

Investigation on Creep Properties of Welded Joint of Modified 9Cr-1Mo Martensitic Heat-Resistant Steel

The creep rupture behavior of weld joint of modified 9Cr-1Mo martensitic heat-resistant steel is investigated by conducting creep tests. The creep strain-time curves of 838K under different stress levels are obtained. The effects of different stress conditions on the creep rupture lifetime are analyzed. Based on the creep test data, the relationship between minimum creep rate and stress, creep rupture time is analyzed. The test results show that the creep rupture occurred in the fine-grained heat-affected zone. The microstructure evolution during creep process is studied by metallographic analysis and fracture analysis.

Creep life prediction for Sn-5Sb lead-free solder alloy: Model and experiment

Extensive application of the newly developed lead-free solder alloys demands thorough acquaintance and deep understanding of their in-service behavior and performance. A Sn-5Sb alloy is one of the widely-used soldering materials well-thought-out for substituting lead-based ones to be used in electronic packaging applications. In this research work, the θ projection approach was used to model the creep characteristics of the Sn-5Sb solder at 500 K taking the advantage of nonlinear least square fitting (NLSF). The appropriate cut-off strain for defining the precise four θ parameters was determined considering statistical criteria used for curve-fitting and then, the acceptable linear dependency of four θ-parameters over imposed stress was investigated. As a significant design parameter, the predicted minimum creep rate was predicted using the mentioned mathematical formulations and a good agreement with the experimental data was obtained. This proposed model that offers major practical advantages get the potential to model the creep curves, creep rate and the time to reach a limiting strain in a wide range of stresses unlike traditional techniques which only relies on quantifying few creep factors like minimum creep rate and rupture time.

Tensile and creep behavior of microvascular based self-healing composites: Experimental study

A 3D microvascular network which was generated by 3D printing method was within the composite specimens. Healing efficiency of the composites was investigated using mechanical tensile and creep tests. The experimental results revealed that despite mechanical properties degradation in the specimens due to the presence of the embedded network, significant healing efficiency can be obtained at both room and elevated temperatures by the presented healing method. The maximum healing efficiency of 89% was obtained in tensile strength for the specimens, which were healed for 7 days. Moreover, creep behavior of the self-healing specimens showed promising results and the healing efficiency was 83% in creep rupture time of the composites at 90 °C.

Prediction of Creep Rupture Time Using Constitutive Laws and Damage Rules in 9Cr–1Mo–V–Nb Steel Welds

Long-term creep tests for the parent metal, a simulated heat-affected zone (HAZ) and a weld joint were conducted based on 9Cr–1Mo–V–Nb steel. The creep rupture time, tr, for the weld joint was predicted by computationally simulating creep damage based on Norton’s law combined with the time exhaustion rule (TER), where the equivalent stress (σeq), maximum principal stress (σ1), or Huddleston stress (σhud) was used to evaluate the rupture time. Creep damage analysis was also conducted based on the Hayhurst-type damage mechanics rule (HDR), in which creep rupture time was evaluated in relation to the rupture stress, σr. The computed creep rupture times and damage distributions were compared with the experimentally obtained rupture time and void distribution of the actual weld joint, respectively. Furthermore, the effect of the bevel angle of the HAZ was examined. The key findings from this study were as follows: (1) cautious predictions could be obtained for a loading stress of 80 MPa and higher; (2) the computed fracture initiation positions were on the HAZ boundaries, consistent with the type IV fracture observed at the stress of 80 MPa; (3) the magnitudes of the predicted rupture times were tr (σ1) < tr (σhud) ≈ tr (σr) < tr (σeq); (4) the bevel angle dependence previously reported was reasonably reproduced with the TER models that used σeq and σhud and with the HDR model.

泥质盐岩单轴蠕变寿命研究

The creep life is an important problem in the research of rock rheology. For the lack of long-term creep experiment data, the creep rupture time is difficult to estimate. The complete stress-strain uniaxial compression experiment of the muddy salt rock was conducted, and the uniaxial creep test with the Chen method was done. The creep curve cluster under different stress levels was gained through processing of the creep curves, thereby the isochronal stress-strain curve cluster was obtained. Based on the fitting analysis, the variation law for the secant modulus of isochronal stress-strain curves with time and the mathematical model for isochronal stress-strain curves were established. The relationship between isochronal stress-strain curves and the complete stress-strain curve was considered to get the mathematical expressions of the creep failure strength and failure strain with the creep lifetime respectively. The research results can be used to estimate the creep lifetime, the long-term strength, the long-term elastic modulus, the creep failure line and the termination line of muddy salt rock, also make a reference for the estimation of the creep lifetime of similar rocks.

Clarification of creep damage condition and rupture time prediction method for round notch bar specimens on Modified 9Cr-1Mo Steel under creep loading

Clarification of creep damage condition and rupture time prediction method for round notch bar specimens on Modified 9Cr-1Mo Steel under creep loading