Creep Rupture Time Research Articles

Development of Ultra-Miniature Creep Testing Method

A uniaxial creep test method using an ultra-miniature creep (Ultra-MC) specimen with a size equivalent to that of a small punch creep (SPC) specimen was developed in this study. It was confirmed that the devised Ultra-MC specimen had the self-alignment function and geometry optimized by FEM analysis. In addition, by comparing the creep test results of the Ultra-MC specimen with the standard specimen (ϕ6), the applicability of Ultra-MC testing method was confirmed. The creep rupture time of the Ultra-MC specimen was found to be good agreement with that of the standard specimen, and the deviation of the creep rupture time was small. On the other hand, the creep deformation of the Ultra-MC specimen was larger than that of the standard specimen. By FEM analysis, this was found to be due to the creep deformation in the shoulder portion of the Ultra-MC specimen. So, a modification procedure for adjusting the creep property of the Ultra-MC specimen to that of the standard specimen was proposed.

Effect of prior austenitisation temperature on creep rupture in Grade 22 steel

ABSTRACT The uniaxial tensile creep resistance at 600°C for Grade 22 bainitic steel was measured as function of prior normalisation heat treatment which was carried out in the range 900–1100°C. A decrease in creep rate is favoured by larger prior austenite grain size, but creep ductility is substantially reduced. Of particular interest is the significant difference in creep performance for the 900°C and 1000°C heat treatment conditions. Although the prior austenite grain size is only marginally different for the two conditions, the creep rupture time is substantially longer for the 1000°C condition. Dilatometry tests, which simulate the normalisation thermal cycles, indicate differences in phase dissolution during normalisation and hence it is proposed that variations in solute distribution account for the differences in creep behaviour.

Analysis of Monkman-Grant relationships and damage tolerance factor for creep of 25Cr35NiNb micro-alloyed steel

Creep behavior of high Cr austenitic stainless steel was investigated at temperatures ranging from 650 to 1050 °C and stress levels in the range 47–120 MPa. Primary, secondary and tertiary creep strains were analyzed in terms of minimum creep rate, constant true stress and temperature. Minimum creep rates, strain to rupture and rupture time were analyzed in terms of Monkman-Grant and modified Monkman-Grant relation. Good correlation between minimum creep rate and rupture time using both Monkman-Grant relationships is observed for the creep data on the steel. Validity of the relationship between time to reach Monkman-Grant ductility tMGD and rupture life tr in terms of damage tolerance factor λ was established for the steel. Effect of temperature on microstructure and carbide content of the steel is presented in this paper. Microstructural studies reveal failure mechanisms by nucleation and growth of cavities around carbide particles at grain boundary regions. Based on the critical continuum damage mechanics approach, the ratio of tMGD to tr express by constant fCDM separated safe and unsafe regimes of creep behavior of the steel.

Formation of Cells and Subgrains and Its Influence on Properties

During plastic deformation, cells and subgrains are created in most alloys. This is collectively referred as the formation of a substructure. There is extensive qualitative information about substructures in the literature, but quantitative modeling has only appeared recently. In this paper, basic models for the formation of substructure during creep and deformation at constant strain rate are presented. It is demonstrated that the models can give at least an approximate description of available experimental data. The presence of substructure can have a dramatic impact on properties. It is well-known that prior cold work can significantly increase the creep strength. Cold work of copper can raise the creep rupture time by up to six orders of magnitude. During plastic deformation dislocations with opposite Burgers vectors move in different directions creating polarized or unbalanced dislocations. Since the unbalanced dislocations are not exposed to static recovery, they form a stable dislocation structure. Taking the role of the unbalanced dislocations into account, the full increase of the creep strength after cold work can quantitatively be explained (without the use of adjustable parameters). Additionally, the shape of the creep curves that varies with the amount of cold work can be modeled. The substructure is also of importance for the modeling of creep curves for material without cold work. In power-law breakdown, the stress exponent can be 50 or more. This should imply that there would be a huge increase in the creep rate with increasing strain, but that is not observed. The reason is that the unbalanced dislocations form a back stress that acts against the increase in the true stress. Taking the back stress into account, it has been possible to model creep curves for copper at near ambient temperatures. This effect must be taken into account in stress analysis to avoid overestimating the creep rate by many orders of magnitude.

A deep learning-aided prediction approach for creep rupture time of Fe–Cr–Ni heat-resistant alloys by integrating textual and visual features

Prediction of creep properties is crucial but still very challenging for the heat-resistant alloys, for which data-driven materials genome technologies provide a promising approach. To this end, we have explored the application of deep learning to quantitatively predict the creep rupture time of Fe–Cr–Ni heat-resistant alloys using textual (chemical composition and creep testing conditions) and visual (as-cast microstructure) data. The multi-layer perceptron (MLP) and the convolutional neural network (CNN) are adopted to analyze the textual and visual data, respectively. Then the multi-source heterogeneous data are integrated by the fusion deep learning model, which achieves a significant enhancement in prediction accuracy compared with the single MLP and CNN models. The results demonstrate that the fusion model is efficient in leveraging information from diverse types of data and learning the relation between various features and creep properties in a high-throughput, statistically robust, and physically meaningful manner. The developed model can be regarded as a new workflow to predict creep properties and give insights into the inverse design of alloys. Our work is equally applicable to other classes of materials and properties as well, where the accumulated data can be reconsidered to guide discoveries.

Effects of applied stress and grain size on creep-rupture lifetime prediction for Haynes 282 alloy

A multiple-heat set of tensile and creep-rupture data generated from a single laboratory was used to establish and evaluate predictive equations for the average creep-limited lifetime of an aged wrought Haynes 282 alloy using two different time- and temperature-compensated parametric models (those of Larson and Miller and Wilshire et al.). The range of temperatures and stresses used to produce the creep-rupture data used in the analyses revealed that two distinct regimes, above and below the proportional limit, were necessary for more accurate predictions of creep lifetimes over the range of conditions studied. Furthermore, a 2x difference in grain size amongst the three heats revealed a breakdown in the relationship between ultimate tensile strength and creep-rupture time, thus impacting the predictive ability of the Wilshire model. Lifetime predictions based on the formulations from the respective models were compared to other available Haynes 282 datasets and found to reasonably replicate the experimental trends established in other studies.

Transmission electron microscopy of precipitation in fine-grained heat-affected zone of Grade91 steel weld during creep exposure

Precipitation during creep exposure at 550 °C in the fine grain heat-affected zone of a Grade91 steel weld produced by gas tungsten arc welding with no filler and post-weld heat treatment at 760 °C for 30 min. has been investigated. The applied tension load during creep testing was 165 MPa, the creep rupture time was 1138 hrs. (100% creep aging), and the creep exposure times were 911, 569, 341, 223 and 114 hrs. for 80%, 50%, 30%, 20% and 10% creep aging, respectively. Light microscopy and scanning electron microscopy were used to examine the heat-affected zone in the steel weld. Grain size in the fine grain heat-affected zone was measured by a circular intercept method on secondary electron images from scanning electron microscopy. Precipitates were detached from the fine grain heat-affected zone by a carbon-film extraction replica technique and investigated by transmission electron microscopy. During creep, a continual increase in grain size was observed in the fine grain heat-affected zone, starting from about 1.70 µm after post-weld heat treatment to about 2.30 µm at 100% creep aging. Precipitates found during creep were M23C6, M7C3, M6C, MX, M2X and high-Si Laves phase. M23C6 was found in all creep conditions until rupture, mainly at prior austenite grain boundaries and sub-boundaries. M7C3, M6C and high-Si Laves phase were observed during creep aging, mainly nearby grain boundaries and sub-boundaries. Relatively fine MX and M2X were found after 20% creep aging until rupture, mainly within grain interiors. Detailed criteria for identification of these precipitate phases, based mainly on energy dispersive X-ray spectrometry and selected area electron diffraction using transmission electron microscopy, is given and a major cause of premature loss of creep strength in Grade91 steel weld is discussed. This is important in understanding creep behavior of steel parts used for boiler tubing, heat exchangers, and steam piping systems in fossil-fuel power plants.

In Situ Creep Rupture Life Prediction Methods for 316FR Austenitic Stainless-Steel Weldment

The present paper proposes new creep life prediction methods that can be used in situ as an in-service inspection tool for the condition-based monitoring of high-temperature structure components of 316FR stainless steel that has been developed in Japan for fast breeder reactors. The methods were based on two types of the modified theta methods and the combination of the modified theta and modified omega methods and were applied to creep deformation data during creep tests made at different applied stress levels at 823K and 923 K in air. Creep lives were predicted by the proposed three methods at arbitrary time in the continuous measurement of deformation during the creep tests. The results show that the combination of the modified and modified omega methods gives the most accurate prediction of creep rupture time at the two levels of test temperature for different applied stress levels.

Creep Remaining-Life Assessment of 2.25Cr-1Mo Steel Hot Reheat Steam Piping by Small Punch Test

In order to investigate the adaptability of small punch (SP) creep testing technique to the remaining-life assessment, the SP creep test was carried out with 2.25Cr-1Mo steel hot rehear steam piping, which had been actually used in a fossil power plant for long periods of time. The SP load (F) was converted to the stress (σ) by three different equations, which were derived based on the displacement to maximum load in the SP test (um) and the deflection to minimum deflection rate in the SP creep test (umin) for correlating the SP creep rupture data with the uniaxial ones. The experimental results showed that the SP creep rupture time of specimen removed at around outer surface of piping tended to be slightly shorter than those taken at around inner surface and center. It was also found that the SP creep rupture data were relatively in good agreement with the uniaxial ones by converting F to σ with the equations, and the creep remaining-life was well predicted by the extrapolation of short-term SP creep rupture data. The highest prediction accuracy was obtained by using the equation derived from the SP test result, that is, um. Consequently, it was confirmed that the SP testing technique could be a strong tool for creep remaining-life assessment of boiler piping.

Development of a creep testing machine with the maximum temperature 1000℃ for utilizing miniature specimens and evaluation of creep strength for IN713C-MIM

This paper presents the development of a creep testing machine with a maximum temperature 1000°C for utilizing miniature specimens and evaluation of creep strength for IN713C-MIM. The remaining life assessment of actual devices using the miniature specimens are expected as the better method because it can decrease the damage to them by sampling a specimen, so the test dates about miniature specimens are required. This study shows the developed testing machine and verification test results as well as some experimental results using IN713C-MIM. However, the MIM-materials creep strength is lower than the casting materials, although the experimental result are very few. This research aims at meeting the above needs and creating the superior heat-resistant alloy. The creep rupture time of IN713C-MIM and improvement of strength have been done by authors, but more detailed evaluating of the strength is also necessary. For this study, a suitable testing was required to evaluate creep strength more precisely.

Creep deformation and rupture behavior of 10Cr-3Co-2W heat-resistant steel weldments in ultra supercritical power units

Creep deformation and rupture behavior of a martensitic heat resistant steel (10Cr-3Co-2W) welded joints prepared by vacuum electron beam welding for supercritical power generation unit are investigated employing traditional uniaxial tensile creep, 898 K (625 ℃) to 948 K (675 ℃) and 105 MPa to 250 MPa, and room temperature nanoindentation on different welding micro-zones. The rupture location shifts from base metal (ductile rupture) for the shortest creep time of 74 h, to heat affected zone (brittle Type IV rupture) for the rest crept specimens. Accordingly, a softening phenomenon is observed in fine grain heat affected zone in crept specimens for longer creep times, where voids, precipitates and reduced dislocation density are found and further discussed. The extrapolating creep rupture time under various creep temperature and stress combinations and the 100,000 h creep rupture strength for different temperatures are predicted employing the Larson-Miller parameter method, which satisfies the industry service condition. Compared with tensile creep the nanoindentation creep yields faster creep rates and higher stress exponents; and the largest strain rate sensitivity value (m) in weld metal implies that weld metal has the best creep resistance while heat affected zone and base metal are more likely to appear creep rupture.

Creep rupture in carbon nanotube-based viscoplastic nanocomposites

While the linear viscoelastic characteristics of polymer composites have been extensively studied, the nonlinear creep with progressive damage leading to the ultimate creep rupture in CNT-based viscoplastic nanocomposites still remains a challenging issue. In this paper, a unified approach is presented to study the whole lifetime of creep process including deformation, damage, and creep rupture. First, based on the concept of secant viscosity, a linear viscoelastic comparison composite is introduced to mimic the nonlinear viscoplastic nanocomposite, with the linear part further formulated in the Laplace transformed domain. Then, both progressive degradation of the interphase and creep damage of the polymer matrix are presented. A joint field-fluctuation method and work-rate equivalence is subsequently utilized to calculate the effective Mises stress and hydrostatic stress of the viscoplastic matrix. The principle of irreversible thermodynamics is then invoked to derive the thermodynamic driving force to characterize the evolution of creep damage and rupture process. After the theory is developed, it is demonstrated that the predicted effective creep strains of multi-walled CNT/polypropylene nanocomposites are calibrated by the experiments spanning over the whole three stages of primary creep, secondary creep, and tertiary creep. Both creep rupture time and rupture strain of the overall nanocomposite are found to increase with CNT volume concentration. It is concluded that population of CNT nanofillers can remarkably enhance the creep rupture resistance of CNT/polymer viscoplastic nanocomposites.

Finite Element Modelling of Creep Rupture on Grade 91 Steel using Monkman-Grant Ductility based Damage Model

Failure strain is a main parameter used in the ductility exhaustion based damage model in which the accuracy of the prediction is dependent on its input value. The experimental measured has indicated that the value of strain at fracture is extensively scattered, therefore may affect the prediction. This paper presents the result of creep rupture time using a modified creep damage model incorporating Monkman-Grant (MG) failure strain as an alternative to strain at fracture. Both strains at fracture and MG failure strain are separately employed in the damage model to predict the failure time of uniaxial smooth specimen and notched bar with different acuity ratios of 3.0 and 20. The FE model of the specimen is loaded under different stress values and the multiaxial failure strain at each stress level is estimated using Cock and Ashby void growth model. The predicted creep rupture time that is compared to the experimental data (in a range of 40-1000 hours) showing a good agreement within the scatter band of +/-factor of 2. Both approaches using strain at fracture and MG failure strain can be used in predicting the creep failure under uniaxial and multiaxial features. The advantage of using MG strain is that the laboratory creep testing can be interrupted prior to specimen fractured or once the secondary creep deformation occurs. Meanwhile, the determination of strain at fracture needs longer test duration where the test can bestopped only when the specimen broken.

Microstructure and high-temperature mechanical properties of RAFM steel processed by friction stir processing combined with tempering heat treatment

In this study, the microstructure and high-temperature mechanical properties of a 9Cr-1.5W reduced activation ferritic/martensitic (RAFM) steel processed by friction stir processing (FSP) combined with a tempering treatment were investigated. During the FSP, lath martensite microstructures containing needle-like M3C and nano-sized MX precipitates were formed in the steel owing to the severe plastic deformation and high cooling rate. The steel subjected to FSP and the tempering treatment at 700 °C for 60 min (FSP-T60) had a fine-grained microstructure consisting of ferrite and martensite, and a large number of M23C6 precipitates formed at the grain boundaries and within the grains. The FSP-T60 steel showed an increase of ∼100 MPa in the tensile strength at 600 °C and a high elongation of 26% as compared to the steel prepared via traditional hot rolling, normalizing, and tempering. The FSP-T60 steel also exhibited improved creep resistance with a creep rupture time of 4523 h at 600 °C/210 MPa. The increase in the strength and ductility of the steel can be attributed to its refined grain structure and the high density and dispersion of the M23C6 precipitates. Meanwhile, the homogeneous distribution and refinement of M23C6 carbides could reduce the growth rate of the Laves phase during creep, thus extending the formation time of the creep cavities and cracks and further increasing the creep rupture time of the steel.

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.

EXTRAPOLATION OF SHORT TERM CREEP DATA INTO LONG TERM RUPTURE TIME PREDICTIONS VIA CAVITY NUCLEATION MEASUREMENTS

ABSTRACT Due to the demand, especially by the power generation and the aerospace industries, for materials withstanding prolonged high temperatures – and relatively high stresses – it has become imperative to make long-term (>100,000 hrs) creep rupture time predictions using short-term/practical experiments. The current standard – based on numerical curve-fitting of data rather than natural law, and thus having limited success – is to start with accelerated experiments, i.e. tests under modified stress-temperature conditions so that rupture can occur in realistic experimental time scales, and then extrapolate the results ‘indefinitely’. In the present work, the phenomenon of creep cavity nucleation is exploited in order to propose a novel methodology for creep rupture time predictions via interrupted/short-term experiments – measuring cavity nucleation rates – conducted under true operating conditions; utilising the interrelation between creep cavity nucleation rates and minimum strain rates, and the coupling of the latter with rupture times via the Monkman-Grant ductility.

High temperature damage development in Ni-alloy/steel dissimilar metal welds

ABSTRACT The development of high-temperature damage in Ni-alloy/steel dissimilar metal welds and its influence on creep-rupture (CR) times and creep-fatigue (CF) endurances is examined. The importance of a systematic and detailed post-test examination of the damage generated in x-weld microstructures is emphasised, and available evidence for Alloy-617/1%Cr-steel welds at 550°C and Alloy-617/10%Cr-steel welds at 575°C is characterised. CR and CF weld strength factors for the two DMWs are determined by the respective kinetics of microstructural developments adjacent to the fusion line and in the FG/ICHAZ of the steel. While the influence of near-weld microstructural evolution in the steel part of Alloy-617/1%Cr-steel DMWs is similar for both CR and CF damage development at 550°C, the same cannot be concluded for Alloy-617/10%Cr-steel DMWs at 575°C. Creep-fatigue damage in Alloy-617/10%Cr-steel DMWs initially develops in the over-tempered parent material with an increasing influence of FG/ICHAZ CF damage generation with reduced strain range.

Progress on microstructure features and creep properties of prospective Tin-Silver-Zinc alloy using a magnetic field

In the current work, applying a rotating magnetic field (RMF) is an innovative approach to improve the microstructure features and creep resistance of Sn-2.0Ag-2.0Zn (SAZ) alloy. The results revealed that RMF does not change intermetallic compounds (IMCs) constituents furthermore SAZ alloy with applying a magnetic field (SAZ-B) exhibited microstructure refinement and homogeneous distribution of IMCs. Moreover, SAZ-B displayed more creep resistance (∼366%) and greater creep rupture time (∼56.4%) than those of SAZ alloy. These results have great implications in improving the alloy’s performance for industrial applications.

Hot corrosion-creep interaction in IN718 under simulated marine environment: Introducing strain-associated-time (SAT) plots for comprehensive understanding

Creep-rupture behaviour of IN718 with deposit of 87.5 wt.%Na2SO4+5 wt.%NaCl+7.5 wt.%NaVO3 (3SM) in the stress range of 550–850 MPa at 650 °C is investigated. 3SM shortens the creep-rupture time by ≈70–90 %. Comprehensive microstructural analysis suggests that oxygen-induced-dynamic- embrittlement, sulphidation followed by oxidation, and vanadic-hot corrosion operate during creep. Creep-hot corrosion interaction influence on rupture strain is explained by introducing strain-associated-time (SAT) and Δtε plots, for the first time. These plots accompanied by detailed microstructural examination demonstrate that deleterious influence of 3SM increases with decreasing stress. Further, SAT plots and EPMA analysis confirm that oxygen-induced-dynamic-embrittlement commences from ∼9% strain under 3SM at 550 MPa.

Damage Model Determination for Predicting Creep Rupture Time of 2 1/4Cr–1Mo Steel Weld Joints

In a creep damage analysis for a weld joint (WJ) using a damage model, it is important to select a damage model that properly reflects complex conditions of stress generated in the interface between the base metal and the heat-affected zone (HAZ). For 2 1/4Cr–1Mo steel, we examined what type of scalar stress is appropriate to properly reflects the conditions of stress generated around the HAZ for accurately predicting creep rupture time, in the framework of the time exhaustion rule, which evaluates damage as a function of stress. For a specimen of a round rod WJ through which an HAZ penetrates obliquely, we compared actual tests and finite element method (FEM) analysis, in temperature-stress ranges from 120 to 160 MPa at 823 K, and from 80 to 100 MPa at 873 K. Using FEM calculations, we evaluated the rupture time based on the time exhaustion rule, considering three types of scalar stress: the maximum principal stress, the equivalent stress, and the Huddleston stress. Comparing with the tests, we found that the scalar stress that gives an appropriate rupture time is the Huddleston stress. In this stress, the damage accumulation due to the creep deformation in the base metal (BM)-HAZ interface and the cavity formation due to the hydrostatic pressure are taken into account. Analyses of the stress distribution inside and outside of the HAZ and in the BM-HAZ interface indicated that, for the 2 1/4Cr–1Mo steel, the cavity formation due to the tensile hydrostatic pressure is also important when we evaluate damage. We optimized a parameter b, which regulates the balance in the Huddleston stress, so that the observed results are reproduced with the determination coefficient of R2 = 0.948, and we obtained the value of b as b = 0.34.