Creep Rupture Lifetime Research Articles

X-ray Diffraction Analysis of Creep-deformed Nitrogen-added 316LN Stainless Steels

X-ray diffraction detailed analysis of nitrogen-added austenitic 316LN stainless steels with different nitrogen concentrations (0.07, 0.11, 0.14, 0.22 wt.%) was carried out in creep-tested (at 923 K, 200 MPa) and solution-annealed conditions. The effect of nitrogen on lattice parameter was studied before and after creep testing. Size and strain were estimated through Williamson–Hall analysis. High nitrogen-added 316LNSS showed lower dislocation density. Increase in creep rupture lifetime with nitrogen content correlated well with the lattice parameter, crystallite size, and strain.

Crystallographic texture and multiscale boundaries mediated creep anisotropy in additively manufactured Ni-based Hastelloy C276 superalloy

The microstructure and high-temperature creep mechanisms of Ni-based Hastelloy C276 superalloy fabricated using wire and arc-based directed energy deposition were investigated systematically and innovatively. The microstructural investigation revealed that the as-fabricated samples comprise γ-Ni matrix and topologically close-packed (TCP) P phase precipitates. The γ matrix subgrains and grains are spread over multiple highly textured columnar dendrites, with a majority of γ <001> crystallographic orientations closely aligned along the deposition direction. Moreover, the interdendritic regions exhibit severe Mo segregation, P phase particles, and dislocation bands. Creep tests were conducted on miniature samples under various temperature and stress conditions, loaded either in the deposition direction (DD) or travel direction (TD). DD samples exhibit lower minimum strain rates, greater strains-to-failure, and longer creep rupture lifetimes than TD samples, indicating significant creep anisotropy. Dislocation creep was identified as the primary creep mechanism for both DD and TD conditions. During creep, dynamic precipitation of TCP phases occurred in the interdendritic regions, resulting in varying creep resistance between interdendritic and dendritic core regions. Isostress and isostrain models, considering both crystallographic texture and precipitation strengthening, reasonably predicted the observed creep anisotropy during the secondary creep stage. Additionally, variations in the Schmid factor led to significant deformation incompatibility among dendrites in TD samples. Dislocation accumulation in TD sample interdendritic regions promoted new grain nucleation, triggering dynamic recrystallisation, facilitating grain boundary sliding, and accelerating tertiary creep. Furthermore, TCP phase particles in the interdendritic regions contributed to microcrack development, further accelerating creep fracture, especially in the TD condition.

Predicting the creep-rupture lifetime of a cast austenitic stainless steel using Larson-Miller and Wilshire parametric approaches

An experimental dataset of just over 100 creep tests of a cast austenitic stainless steel, CF8C-Plus, was analyzed by two temperature-compensated parametric models (Larson-Miller, Wilshire et al.) to predict long-term lifetimes as functions of temperature and stress. The dataset and associated regression analyses showed greater scatter than typically found in recent similar studies of wrought Ni-based alloys by the same two models and was attributed to the microstructural inhomogeneity of the cast stainless steel. Qualitatively, the Larson-Miller formalism showed greater lifetime prediction accuracy than the Wilshire approach, with the latter model's predictive ability being particularly degraded by the presence of two very significant outlier results. This observation suggests that the Larson-Miller approach is more robust when treating rupture-time datasets that show particularly wide experimental scatter. Despite the differences in the overall predictive ability, both models yielded similar predictions of the applied stress at which CF8C-Plus would have a creep-limited lifetime of 100,000 h when loaded below the yield point.

The Effect of Forging and Heat Treatment Variables on Microstructure and Mechanical Properties of a Re-Bearing Powder-Metallurgy Nickel Base Superalloy

In our previous works, the effects of forging and heat treatment variables on microstructure evolution and mechanical properties have been studied for an ingot-metallurgy Re-bearing nickel base superalloy. To overcome the issues associated with the production of large-scale ingots and fine-grained workpieces, in the present work, the effect of hot forging and heat treatment variables was studied in a Re-bearing nickel base superalloy prepared via powder metallurgy. The purpose of the study was to reach the properly balanced mechanical properties for the potential use of the superalloy as a disc material. The initial as-HIPed workpieces were subjected to different hot forging and post-forging heat treatment or only to heat treatment (no forging). For the processed workpieces, the recrystallization behavior, size, morphology and volume fraction of γ′ precipitates were evaluated by scanning electron microscopy followed by a study of mechanical properties. The most properly balanced mechanical properties (strength, ductility, creep resistance and creep rupture lifetime) were reached for the γ grain size of dγ ≈ 13.6 µm. A finer and coarser γ grain size (down to dγ ≈ 2.6 and up to dγ ≈ 37.5 µm) even when superimposed with a higher volume fraction of dispersed secondary γ′ precipitates (in the case of dγ = 27–37.5 µm) was associated with worse mechanical properties.

Creep rupture lifetime evaluation and creep deformation analysis using θ-projection and Blackburn-type methods for forged Ti43Al5V4Nb

ABSTRACT The high-temperature creep properties of Ti43Al5V4Nb were investigated experimentally and analyzed numerically. Creep testing of forged Ti43Al5V4Nb at 650°C and 760°C was conducted to investigate the creep rupture lifetime and high-temperature properties. Steady-state and accelerated creep regions were dominant in the high-temperature environment, indicating rupture ductility. The creep rupture lifetime in the range of 600°C−800°C was evaluated uniformly using the Larson – Miller and Orr – Sherby–Dorn parameters. The θ-projection method can be used to draw creep curves for experimental data up to a lifetime ratio of 0.9, and it was possible to represent the strain immediately before rupture.. The creep rupture lifetime could be predicted within a factor of two using the predicted minimum strain rate and the Monkman – Grant law. A Blackburn-type method was found to represents the transition and steady-state creep ranges accurately. Thesemethods were effective in predicting the 1% strain occurrence time

Statistical Comparison of Creep-Rupture Lifetime Predictions of Single and Two-Step Aged Wrought Haynes 282 Alloy

AbstractThe paired and Welch's t-tests were used to determine if statistically significant differences exist between the predicted creep rupture life of single- and two-step aged wrought Haynes 282 nickel superalloy. Both tests indicated that there is no difference in the temperature range of 700–800 °C. These results provide a quantitative perspective to support the conclusions reached in previous publications that the creep rupture strengths of wrought single- and two-step aged Haynes 282 are very similar or the same. The lack of creep rupture data above 800 °C for two-step aged H282 precluded extending the statistical analysis above that temperature.

Effect of Heat-to-Heat Variability on Long-Term Creep Rupture Lifetime Predictions of Single Step Aged Wrought Haynes 282 Alloy

Abstract The impact of a weaker heat of the wrought form of Haynes 282 nickel superalloy on predicted 100,000 h creep rupture strength (CRS) and hence maximum allowable working stress (MAWS) was determined by correlating the creep rupture data of the two stronger heats of material that were used (along with the third weaker heat) to develop the ASME Code Case for this alloy. In comparison with the established MAWS values, estimated MAWS values were 3–7% higher in the temperature range of 700–800 °C and up to 30% higher above 800 °C when the weaker heat was removed from the data analysis. These results show the importance of minimizing heat to heat variability of properties that affect high temperature creep strength, including grain size, which can be affected by parameters that might not be included in the alloy specification. For nickel superalloys, such as Haynes 282, design optimization and manufacturing control of grain size, in addition to chemical composition and gamma prime content and size, can provide measurable improvements in MAWS values established by ASME in the time-dependent region, where these alloys are usually employed.

Verification of creep properties and rupture lifetime evaluation methods using small diameter Sn-3.0Ag-0.5Cu/Sn-58Bi/Sn-5Sb specimens

In this study, creep testing methods for three solder specimens with a diameter of 3 mm were investigated. The creep rupture lifetime of the 3 mm diameter Sn-3.0Ag-0.5Cu specimen was evaluated in comparison with reference data acquired from a 10 mm diameter sample, and it was determined that there was no significant effect of specimen size on the creep rupture lifetime. The creep curve for Sn-3.0Ag-0.5Cu revealed a significant increase in the strain ratio during the later stage of its lifetime; whereas, Sn-58Bi and Sn-5Sb showed substantial increases in the strain ratio during the early stages of their lifetimes. Strong correlations were observed between the minimum strain rate and applied stress for all solders, confirming the validity of the Norton rule. The activation energy of each solder was calculated from the creep tests performed on the small diameter specimens. The activation energies for Sn-3.0Ag-0.5Cu and Sn-58Bi were approximately 94 kJ/mol and 65 kJ/mol, respectively, whereas the activation energy for Sn-5Sb varied significantly from 91.0 to 169.9 kJ/mol.

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.

Creep behavior of nuclear grade 316LN austenitic stainless steel at 873 K and 923 K

Creep deformation and rupture behavior of nitrogen-alloyed (0.14 wt.%) nuclear grade 316LN austenitic stainless steel were investigated for the varying stress levels at 873 K and 923 K. The power-law dependency of creep properties such as steady-state creep rate and rupture life on applied stress was observed. For a given applied stress condition, a systematic increase in strain to failure was noticed with increasing temperature from 873 K to 923 K. Irrespective of test temperatures, creep rupture elongation of the steel increased with the increase in rupture lifetime ( $t_{r}$ ) for $t_{r} > 1000$ h. Analysis indicated that the interdependency between creep properties could be well described by the modified Monkman-Grant relationship. The predominance of inter-granular fracture arising from the triple point cracks and/or coalescence of cavities was observed at all the tested conditions for the steel. The enhanced tendency for wedge cracking was noticed for high stress levels at 873 K and 923 K. The evaluated damage tolerance factor $(\lambda ) < 5$ and the calculated ratio between time to reach the Monkman-Grant strain and creep rupture lifetime in the range of 0.69 to 0.80 indicated the accumulation of Monkman-Grant strain for the major fraction of lifetime during creep deformation of 316LN steel.

Composition rules of Ni-base single crystal superalloys and its influence on creep properties via a cluster formula approach

The present work investigated the composition evolution of the TMS series of Ni-base single crystal (SC) superalloys in light of the cluster formula approach systematically. The cluster formula of SC superalloys could be expressed with {[}overline{{{text{Al}}}} {-} overline{{{text{Ni}}}} 12{](}overline{{{text{Al}}}} {, }overline{{{text{Cr}}}} {)}m, in which all the alloying elements were classified into three groups, Al series (overline{{{text{Al}}}}), Cr series (overline{{{text{Cr}}}}), and Ni series (overline{{{text{Ni}}}}). It was found that the total atom number (Z) of the cluster formula units for TMS series of superalloys varies from Z ~ 17 to Z ~ 15.5, and then to Z ~ 16 with the alloy development from the 1st to the 6th generation, in which the superalloys with prominent creep resistance possess an ideal cluster formula of {[}overline{{{text{Al}}}} {-} overline{{{text{Ni}}}} 12{](}overline{{{text{Al}}}} 1.5overline{{{text{Cr}}}} 1.5{)} with Z = 16. Similar tendency of composition evolution also appears in the PWA and CMSX series of SC superalloys. Typical TMS series of superalloys with prominent creep properties generally exhibit a moderate lattice misfit of γ/γ′ which could render alloys with appropriate particle size of cuboidal γ′ precipitates to acquire a maximum strength increment by precipitation strengthening mechanism. More importantly, the relationship between the lattice misfit (δ) of γ/γ′ and the creep rupture lifetime (tr) of superalloys was then established, showing a linear correlation in the form of lgtr–lg|δ|3/2 at both conditions of 900 °C/392 MPa and 1100 °C/137 MPa. Combined with the lattice misfit, the cluster formula approach would provide a new way to modify or optimize the compositions of Ni-base superalloys for further improvement of creep property.

Correlation of creep fracture lifetime with microstructure evolution and cavity behaviors in G115 martensitic heat-resistant steel

In order to deepen the understanding towards the creep rupture lifetime of G115 martensitic heat-resistant steel, a series of creep tests using 130–200MPa loading stresses at 923K were carried out to investigate the as-crept microstructural characteristics and cavity behaviors. The significant microstructure deformation and a dramatic increase in the areal density of high-angle grain boundaries occur when the stress loading is over 160MPa. Apart from the strain concentration at elongated grain boundaries and refined grains, the elevated stress also leads to the appearance of a strong α-fiber texture with {001}<110>//RD. The nucleation and growth of cavities can be effectively accelerated by the stress elevation, and ductile fracture is determined as the dominate mechanism within the test stresses. However, a transition towards brittle intergranular fracture is observed due to the rapid coarsening of Laves phase. Furthermore, a good prediction for creep lifetimes of G115 steel has been obtained based on the constrained diffusional cavity growth model.

Data Assessment Method to Support the Development of Creep-Resistant Alloys

This work introduces a methodology to assess data quality for the tensile, creep/stress relaxation, and fatigue properties of alloys (as well as metadata associated with manufacture) as a part of a project to develop new materials for extreme environments. The extreme environments in question deal with those found in the power generation sector. Data quality assessment is needed to ensure the reliability of data used in analytics to develop new materials for the power generation sector and to predict the performance of established materials in current use. As data quality metrics have not been standardized for material properties data, quality rating guidelines are developed here for the aspects of data completeness, accuracy, usability, and standardization. The specific design requirements for heat-resistant alloy development were considered in creating each metric. Establishing the quality of a dataset in these areas will enable robust analysis. High-quality data can be set aside to develop predictive models. Lower-quality data need not be discarded but can be used for experimental design. Determining the quality of a materials dataset will also provide additional metadata with the data resource and will promote data reusability. A sample high-quality dataset is presented to indicate the typical data attributes collected from relevant mechanical property testing results, which were considered when generating the data quality metrics. A data template of these attributes was created as a tool for data generators and collectors to promote uniformity and reusability of alloy data. The sparsity of the sample dataset was calculated in order to highlight the areas where data gaps pose a challenge for reliable prediction of creep rupture lifetime.

Dependence of Creep-Rupture Lifetime for Rods under Tension in an Aggressive Environment on the Shape of a Single-Cell Cross-Section

The long-term strength of rods under tension in an aggressive environment is investigated. We consider rods of various shapes of a single-cell cross-section (circle, square, and rectangles with different aspect ratios), provided that the areas of these cross-sections are equal. To determine the level of inclusions from aggressive environment into the rod at different time moments, approximate diffusion equations based on the motion of diffusion fronts from the rod surface are used. The high accuracy of the obtained approximation is shown. To assess the influence of an aggressive environment on long-term strength, the Yu. N. Rabotnov kinetic theory with two structural parameters (material damage and concentration of environment elements in the rod material) is used. The dependence of the time to rupture for the rods with various cross-section shapes under the same level of tensile stress is obtained. It is shown that the time to rupture for a rod with square cross-section under the indicated conditions exceeds the time to rupture for a rod with circular cross-section. In the case of rods with the same cross-section shape, the relation of the cross-section perimeter to time to rupture for the rod is considered. The minimum time to rupture for the considered rods is observed for rods of rectangular cross-section with the smallest thickness

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.

Type IV failure in weldment of creep resistant ferritic alloys: II. Creep fracture and lifetime prediction

The creep rupture behavior (Type IV failure) for weldments of creep strength enhanced ferritic steel is numerically analyzed, using an integrated microstructure- and micromechanics-based finite element model. To account for the large microstructure gradients across weldments, a two-dimensional digital microstructure is constructed based on the actual observed microstructure of ferritic steel weldment by using the Voronoi-tessellation method. According to the fracture mechanism studies and literature experimental observations, the Type IV failure is identified as an intergranular creep fracture in the fine-grained or intercritical heated affected zone (FGHAZ or ICHAZ). In the present study, the following micromechanics model is employed to determine the micromechanical and microstructural origins for the failure process above, accounting for the underlying physical fracture mechanisms at different length scales, including nucleation of grain boundary cavities, their growth by competition of grain boundary diffusion and grain interior creep, viscous grain boundary sliding, and the emergence of microcracks by coalescence and their evolution to the ultimate failure. The methodology demonstrates the capabilities in modeling the Type IV failure and providing quantitative creep rupture lifetime prediction which shows an excellent agreement with long-term creep experimental data for creep strength enhanced ferritic steels and their weldments. In particular, the drop-off in time to rupture at high temperatures and low stress levels in the creep rupture curves is quantitatively predicted, and the transition of failure mechanisms from creep-controlled to diffusion-controlled creep fracture mechanism is illustrated.

Screening of heritage data for improving toughness of creep-resistant martensitic steels

Data science techniques were used to quantify the effect of alloying additions on the tensile behavior of martensitic steels. The effort was undertaken to exploit the heritage data to establish the next experimental design space for the class of 9–12 wt% Cr steels for the application of turbine rotors with an operating temperature of 650 ∘C and above. Linear, lasso, and multivariate multiple regression models were utilized to identify which alloying elements contribute towards strength and ductility. Visualization techniques such as t-distributed stochastic neighbor embedding and pair-wire element specific comparisons were utilized to explore information gaps that exist within the data. The study found that tantalum, recently added to improve the creep rupture lifetime, does not show any effect on tensile properties. All combined, the results suggest that the low tempering temperature has compensated for the low alloying additions in the past, therefore, new experiments are needed to isolate the effects of tempering temperature from those of individual elements.

Creep Void Formation and Rupture Lifetime in Multiaxial Stress States

This paper discusses creep void formation and rupture lifetimes in multiaxial stress states for a SUS 304 stainless steel at elevated temperatures. Biaxial and triaxial tension creep tests were performed using a cruciform and a cubic specimen, respectively. These two types of the specimens were designed to achieve uniform equi-biaxial and equi-triaxial stress distributions by a finite element analysis in the gage parts. Void formation at grain boundaries was observed by intermitting biaxial creep tests and by interrupting triaxial creep tests. Creep rupture lifetimes were also obtained in biaxial and triaxial creep tests. Biaxial stresses increase the void formation but give a little influence on a creep rupture lifetime in the correlation with von Mises equivalent stress. Triaxial stresses also increase the void formation and drastically reduce a creep rupture lifetime in the correlation with von Mises equivalent stress. Evident void formation in an equi-triaxial stress condition demonstrates that von Mises equivalent stress is not a suitable measure to evaluate creep damage development in multiaxial stress states. A new equivalent stress is proposed to evaluate creep rupture lifetimes in biaxial and triaxial stress states.

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 Damage Evaluation using Uniaxial Miniature Specimens for Multiaxially Damaged Components

This study discusses a method of evaluating multiaxial creep damage for high temperature components subjected to multiaxial creep damage utilizing a miniature creep testing. A new model of the damage evaluation is proposed based on a linear creep damage accumulation and von Mises equivalent stress being a measure of multiaxial creep damage. The model clearly indicates that conventional creep damage evaluation methods utilizing a unidirectional miniature creep testing give an unconservative estimate in some cases for multiaxially creep damaged components. To verify the appropriateness of the proposed model, multiaxial creep tests were performed using cruciform specimens of a Mod.9Cr-1Mo steel. Miniature specimens in two directions were machined from a pre-damaged cruciform specimen and the uniaxial creep rupture lifetimes of the miniature specimens demonstrate the validity of the proposed model.