High Temperature Creep Tests Research Articles

Effect of Heat Treatment on Structure Evolution and Mechanical Property Strengthening of Low-Cobalt Nickel-Based Superalloy

In this paper, the microstructure of an alloy was regulated by means of strengthening solution aging, and microstructure observation and composition analysis were carried out by means of an optical microscope and X-ray diffractometer. Combined with the Vickers hardness tester, electronic universal testing machine and high-temperature persistent creep testing machine, the mechanical properties and high-temperature properties of the alloy were tested, and the strengthening mechanism of the alloy was explored. The results showed that the dendritic morphology and structure of the alloy decreased with an increase in temperature during the solution process, and the γ′ phase morphology also changed with the solution temperature: oval → cross → cubic. The γ′ phase after solid solution at 1295 °C was closest to the cubic form. Therefore, it is believed that the 1295 °C solution treatment had the best effect. In the aging process, the uniform cubic degree of γ′ phase distribution was the highest at 1090 °C. On the basis of fixed aging temperature (1090 °C), it was found that the volume fraction of the γ′ phase increased significantly after 8 h. The γ′ phase, which was closest to the cubic form, had the largest proportion of precipitation, and the volume fraction increased to 70.3%. The minimum carbide volume was 1.0%. The hardness of the alloy reached 435.8 HV; the yield strength increased to 280.1 MPa; and the durability of the alloy under the conditions of 1000 °C/230 MPa and 870 °C/655 MPa was 99.7 h and 42.7 h, respectively, which achieved the purpose of alloy design.

The high temperature creep and fracture behavior of Inconel 718 produced by additive manufacturing

High-temperature creep tests of additively manufactured (AM) and wrought nickel alloy 718 (IN718) were conducted at 650 °C and 704 °C at stresses ranging from 316 to 819 MPa. The AM alloy had a greater creep strength than the wrought alloy at nearly all testing conditions due to increased volume fractions of γ” in the matrix and carbides at grain boundaries; however, the creep rupture ductility of the AM material was significantly reduced at all testing conditions. Secondary ion mass spectrometry (SIMS) revealed that sulfur segregation, resulting from a lack of Mg in the AM alloy, is the most plausible explanation for the observed ductility loss in AM IN718. Overall, this work focuses on understanding and defining the mechanism of embrittlement in AM IN718.

Influences of Si-Ti combined alloying on microstructures and properties of laser welded socket joints of molybdenum

This study selected silicon (Si) and titanium (Ti) as alloying elements to explore influences of the Si:Ti mass ratio on performance of molybdenum bead on plate laser welded joints. When the Si:Ti ratio was 1:1, the silicides exhibited a uniform continuous distribution, and significantly increased room temperature tensile strength of molybdenum bead on plate from 48 MPa to 231.6 MPa. In addition, Si-Ti combined alloying was applied to Mo tube-end plug socket joints with the optimal Si:Ti ratio. Results showed that Si and Ti were uniformly distributed in the weld of laser welded socket joints. Addition of Si and Ti in the weld increased the fraction of low-angle grain boundaries (LAGBs), which was conducive to improving the bonding strength of grain boundaries. The second-phase particles Mo3Si and Ti5Si3 were observed in the weld after adding Si and Ti, which played a role in second-phase strengthening. The room-temperature tensile strength of laser welded socket joints of Mo with the Si:Ti mass ratio of 1:1 reached 301 MPa, and the joint showed mixed fractures. The fractures in the FZ exhibited morphologies of cleavage fractures, while those in the heat affected zone (HAZ) showed mixed morphologies of intergranular and cleavage fractures. Finally, the results of high-temperature nanoindentation creep testing showed that after applying a load of 8 mN for 600 s at 400℃, the creep displacement of the LW-socket joint was measured at 20.28 nm, whereas the LW-SiTi socket joint exhibited a creep displacement of only 6.63 nm. The research findings of this paper provide references for alloying methods in laser welding of molybdenum and molybdenum alloys.

High-Temperature Creep and Microstructure Evolution of Alloy 800H Weldments with Inconel 625 and Haynes 230 Filler Materials

Alloy 800H stands as one of the few code-qualified materials for fabricating in-core and out-of-core components operating in high-temperature reactors. Welding is a common practice for assembling these components; however, the selection of a suitable filler material is essential for enhancing the high-temperature creep resistance of Alloy 800H weldments in high-temperature applications. In this study, Inconel 625 and Haynes 230 filler materials were used to weld Alloy 800H plates by employing the gas tungsten arc welding technique. The high-temperature tensile and creep rupture properties, microstructural stability, and evolution of the weldments after high-temperature exposure were investigated and compared with those of Alloy 800H. The results show that both weldments exhibit enhanced tensile and creep behavior at 760 °C. The creep rupture times of the weldments with Inconel 625 filler and Haynes 230 filler materials were about two and three time longer, respectively, than those of Alloy 800H base metal when tested at 80 MPa and 760 °C. Carbides (MC and M23C6) were commonly observed in the microstructures of both the weld and base metals in the two weldments after high-temperature creep tests. However, the Inconel 625 filler weldment displayed detrimental δ and Laves phases in the fusion zone, and these precipitates could be potential sites for initiating cracks following prolonged high-temperature exposure. This study shows that the weldment with Haynes 230 filler material exhibit better phase stability and creep rupture properties than the one with Inconel 625, suggesting its potential for use as a candidate filler material for Alloy 800H for further investigation. This finding also emphasizes the critical consideration of microstructural evolutions and phase stability in evaluating high-temperature materials and their weldments in high-temperature reactor applications.

Creep and stress relaxation experiments of 1960-grade high-strength steel wire at elevated temperatures

With the increasing spacing of large-span structures, the application of ultra-high-strength steel wire with a strength approaching or exceeding 2000 MPa is rapidly expanding. However, the related research on its creep and stress relaxation behaviour is still limited. In this paper, a total of 30 high-temperature creep tests and 31 high-temperature stress relaxation tests were conducted on 1960-grade high-strength steel wire (1960-HSSW) with a diameter of 5.0 mm. The full process creep strain curves were obtained by reinserting and resetting the high-temperature extensometer. The maximum creep strain measured was 50.67 %. The creep and stress relaxation curves of different high-strength steel wires (steel strands) were compared, revealing significant variations in mechanical properties among different steel wires. The steady-state creep rate and high-temperature creep rupture critical stress ratio of the 1960-HSSW were given. Especially, the critical stress ratio became lower than the yield strength reduction coefficient at temperatures exceeding 400 °C and it indicated that the 1960-HSSW could undergo creep rupture before yield. The stress relaxation coefficient was introduced to consider the stress relaxation of 1960-HSSW. Stress relaxation curves for the 1960-HSSW were provided at temperatures ranging from 100 to 550 °C. Moreover, based on the microstructure observed under transmission electron microscopy, it was found that damage occurred in the 1960-HSSW after exposure to 300 °C. Therefore, a critical temperature of 300 °C was recommended for the fire protection of 1960-HSSW, which could be the theoretical support for its fire protection design. Additionally, a high-temperature creep prediction model, which includes expressions for the termination time, was established for the 1960-HSSW. As a result, a conservative prediction of the high-temperature creep for the 1960-HSSW was provided. Moreover, A stress relaxation coefficient model was also provided. The predicted results of the aforementioned models were in good agreement with experimental results.

Study on Creep Properties of Al-Zn-Mg-Cu Alloys

This article conducts high-temperature creep tests on an Al-6.5Zn-2.3Mg-2.5Cu-0.1Zr-0.2Sc alloy in a solid solution + aging state at 200 °C and 150–180 MPa. Characterization of the microstructure of the specimen after creep test fracture was performed using SEM and TEM. The results indicate that the steady-state creep rate range of the alloy was 10−9 to 10−8 s−1, which was positively correlated with applied stress, while the creep life was negatively correlated with applied stress. Through failure analysis, it was found that the main deformation mechanism of the alloy was the dislocation climbing mechanism. The fracture mode of the alloy was ductile fracture.

Validation of applicability of induction bending process to P91 piping of prototype Gen-IV sodium-cooled fast reactor (PGSFR)

The application of the induction bending process to pipe systems in various industrial fields is increasing. Recently, efforts have also been made to apply this bending process to nuclear power plants because it can innovatively reduce welded parts of the curved pipes, such as elbows. However, there have been no cases of the application of induction bending to the piping of nuclear power plants.In this study, the applicability of the P91 induction bending piping for the sodium-cooled fast reactor PGSFR was validated through high temperature low cycle fatigue tests and creep tests using P91 induction bending pipe specimens. The tests confirmed that the materials sufficiently satisfied the fatigue life and the creep rupture life requirements for P91 steel at 550 °C in the ASME B&PV Code, Sec. III, Div. 5. The results show that the effects of heating and bending by the induction bending process on the material properties were not significant and the induction bending process could be applicable to piping system of PGSFR well.

Creep Behavior Characterization of Nickel-Based Single-Crystal Superalloy DD6 Thin-Walled Specimens Based on a 3D-DIC Method.

The thickness debit effect of creep behavior has been a focal point of nickel-based single-crystal superalloy research, and there is a need for an advanced creep deformation measurement method. This study developed a novel high-temperature creep test system based on a single-camera stereo digital image correlation (DIC) method with four plane mirrors to conduct creep tests on thin-walled specimens of a nickel-based single-crystal alloy, DD6, with thicknesses of 0.6 mm and 1.2 mm under experimental conditions of 980 °C/250 MPa. The reliability of the single-camera stereo DIC method in measuring long-term deformation at a high temperature was experimentally verified. The experimental results show that the creep life of the thinner specimen was significantly shorter. It was found the lack of coordination in the creep deformation process of the edge and middle section of the thin-walled specimens may be an important factor in the thickness debit effect according to the full-field strain contour. By comparing the local strain curve at the rupture point with the average creep strain curve, it was found that the creep rate at the rupture point was less affected by the specimen thickness during the secondary creep stage, while the average creep rate in the working section significantly increased as the wall thickness decreased. The thicker specimen usually had a higher average rupture strain and higher damage tolerance, which prolonged the rupture time.

Fire resistance of low alloy Q420d high-strength steel column under high-temperature creep.

The high-temperature mechanical and creep characteristics of Q420D steel are investigated in this essay. To determine the steel's high-temperature yield strength, the high-temperature tensile test of Q420D steel was first performed. In the temperature range of 400°C-800°C, the high-temperature creep test under various pressures was conducted, and the creep strain curve over time was produced. Finite element analysis and comparison were done to examine the impact of creep strain on the Q420D steel column's bearing capacity under high-temperature conditions. The findings demonstrated that: Using Abaqus, a finite element fire resistance analysis of a Q420D steel column, was conducted while taking initial geometrical flaws, residual stress, and creep effect into account. As a result, the critical temperature of a Q420D steel column under various load ratios was determined. The largest deviation from the critical temperature in the standard GB51249-2017 was 2.9% when the creep effect was taken into account under the load ratio R = 0.3. The highest reduction in fire resistance limit time under low load ratio conditions, taking into account the creeping impact of Q420D steel columns, is 35%. The findings demonstrate that the high-temperature creep energy greatly lowers the steel column's fire resistance.

A Virtual Simulation Experiment of Mechanics: Material Deformation and Failure Based on Scanning Electron Microscopy.

This work presents a set of comprehensive virtual experiments to detect material deformation and failure. The most commonly used pieces of equipment in mechanics and material disciplines, such as a metallographic cutting machine and a high-temperature universal creep testing machine, are integrated into a web-based system to provide different experimental services to users in an immersive and interactive learning environment. The protocol in this work is divided into five subsections, namely, the preparation of the materials, molding the specimen, specimen characterization, specimen loading, nanoindenter installation, and SEM in situ experiments, and this protocol aims to provide an opportunity for users regarding the recognition of different equipment and the corresponding operations, as well as the enhancement of laboratory awareness, etc., using a virtual simulation approach. To provide clear guidance for the experiment, the system highlights the equipment/specimen to be used in the next step and marks the pathway that leads to the equipment with a conspicuous arrow. To mimic the hands-on experiment as closely as possible, we designed and developed a three-dimensional laboratory room, equipment, operations, and experimental procedures. Moreover, the virtual system also considers interactive exercises and registration before using chemicals during the experiment. Incorrect operations are also allowed, resulting in a warning message informing the user. The system can provide interactive and visualized experiments to users at different levels.

The role of yttrium micro-alloying on microstructure evolution and high-temperature mechanical properties of additively manufactured Inconel 718

The effects of yttrium (Y) addition on the microstructure and high-temperature mechanical properties of Inconel 718 have been investigated. Alloys containing a range of Y (0–0.58 wt%) were fabricated using selective laser melting, followed by solution and aging heat treatment. The mechanical properties were evaluated by high-temperature (650 °C) tensile and creep tests. The results showed that Y addition up to 0.07 wt% enhanced both tensile and creep ductility. Ductility was the highest in the 0.07 wt% Y-added specimen; further increases in Y content reduced both tensile and creep ductility. The ductility improvement by the small addition of Y was attributable to the grain boundary segregation of Y, which led to the morphological change of the δ phase precipitates and the stabilization of oxygen by forming Y2O3 at grain boundaries. However, the beneficial effect of Y on ductility was suppressed when Y content exceeded 0.07 wt%, owing to the precipitation of Y-rich Ni5Y and NbC phases at intergranular and interdendritic regions. On the other hand, the specimens with high Y contents were mechanically strengthened by the solid solution of Y and by the precipitation of Ni5Y and NbC phases.

A Comparative Study of High Temperature Tensile and Creep Testing Between Standard and Miniature Specimens: Applicability and Limits

This study concerns the quasi-static and time-dependent mechanical behavior obtained via the miniaturized electro-thermal mechanical testing (ETMT) approach for single crystal (SX) and conventional cast Mar-M-247 superalloy. The experimental outcome was benchmarked against standardized testing procedures. It is found that tensile yielding behavior can be captured accurately by the ETMT approach up to 1100 ^{circ }C, provided the appropriate type of thermocouple (T/C) is chosen. Furthermore, creep rupture behavior is underestimated by the miniaturized set-up. High repeatability of the rupture time was obtained for the SX case, whereas a significant scatter was observed for the conventional cast case. The discrepancies are assessed in detail; discussion centers around analytical and practical considerations, such as temperature uncertainty due to parasitic voltage and the choice of T/C, microstructural change as a result of the Joule heating, representative gauge volume, and strain rate non-linearity. Consequently, the applicability and limits of the miniaturized approach are examined critically, and improvements were suggested where appropriate.

Analysis on the fitness of heat-resistant steel as casing material in corrosive environment at high temperature

This study investigates the performance of heat-resistant steel in a corrosive environment at high temperatures, focusing on mechanical characteristics and corrosion behavior. Firstly, the performance of heat-resistant steel at high temperature was verified using mechanical property measurement, fracture failure analysis, and high-temperature creep test. The results indicate that the tensile strength and yield strength of heat-resistant steel at 350 °C are separately for 930 MPa and 776 MPa. The specimen has good creep resistance and no creep between 100 ∼ 350 °C. After establishing that the high-temperature performance of heat-resistant steel complies with the specifications, the weight loss and stress corrosion tests were carried out in a corrosive environment at high temperatures. The results reveal that the corrosion rate is 0.9504 mm/a at 350 °C, and the corrosion products mainly consist of FeSx, FeCO3 and Fe3O4. The four-point bending test specimen did not crack at all test temperatures. The results show that heat-resistant steel has excellent properties and can be used as the casing material of a complex deep well at high temperatures.

Experimental analysis of high-temperature creep in FV566 steel based on digital image correlation.

Digital image correlation (DIC) is an optical measurement method of material strain/displacement based on visible light illumination, which can be used for the measurement of long-term mechanical behavior. In this paper, an experimental method for analyzing high-temperature creep in FV566 steel material based on DIC was independently designed. Aiming at the problems of glass observation window medium refraction and thermal airflow disturbance in high-temperature testing, the corresponding correction methods were proposed to improve the measurement accuracy. Based on the above methods, high-temperature creep tests were carried out on three specimens with different shapes, and the strain concentration area at 600°C was calculated. Then, the influences of shape and other properties on material creep failure, stress distribution, and actual strain were investigated. Finally, the DIC calculation results were analyzed and compared with results of finite element analysis and the final fracture position of the specimen. The three results had a high degree of consistency, which verified that the proposed method can accurately measure and analyze the creep behavior of materials.

Optimizing Regenerant Content of Aged SBS Modified Asphalt Binder Based on High- and Low-Temperature Performance

The performance of the recycled asphalt pavement is directly influenced by the content of the used regenerant (i.e., rejuvenator and/or unaged asphalt binder). The goal of this study is to optimize regenerant content of aged styrene-butadiene-styrene (SBS) modified asphalt binder based on high-temperature creep and low-temperature relaxation performance tests. Initially, penetration, ductility, softening point, multiple stress creep recovery (MSCR), and bending beam rheometer (BBR) tests were carried out to investigate the rheological behavior of the aged SBS modified binder (AMB) regenerated with different contents of the rejuvenator and unaged SBS modified binder (UMB). A method is proposed, based on the rheological behavior of the regenerated AMB, to determine the optimum contents of the regenerants (i.e., rejuvenator and UMB). To reach a successful trade-off between low- and high-temperature properties of the regenerated AMB, the relaxation rate VR and nonrecoverable creep compliance J3.2 (at 3.2 kPa) were selected as the evaluation indices to settle the optimum contents of the rejuvenator and UMB. The results showed that the ductility of regenerated AMB is on par with the UMB when the contents of rejuvenator and UMB are 4% and 70%, respectively. The nonrecoverable creep compliance and relaxation rate of the regenerated AMB tended to increase with the increase in the content of the rejuvenator and UMB, whereas the recovery rate and relaxation modulus decreased. It indicated that the low-temperature performance increased while the high-temperature performance was adversely affected by the increase in the content of the regenerants. The VR and J3.2 of the regenerated AMB were comparable to those of unaged state, when the contents of the rejuvenator and UMB in the aged binder were 5% and 70%, respectively. It indicated that the high-temperature creep and low-temperature relaxation performance of the AMB reach a balance.

Thermomechanical processing of Type-4 composite cylinders under static load

Type-4 composite cylinders have recently gained recognition in global markets for storage of compressed gas as an alternative to the existing metallic cylinders. The developed cylinders are certified before commercial deployments, based on the safety requirements of the available standards for onboard compressed gas storage. However, various accidents relating to the sudden burst of these cylinders have been reported previously. The failure depends on a variety of reasons; failure due to thermal degradation has been addressed in this study. Finite element analysis is used to predict the ultimate hoop stress of composite rings; the model is experimentally validated using the NOL ring test as per the ASTM standards. An FEA model to virtually test the cylinder for hydrostatic pressure burst test, high temperature creep test and accelerated stress rupture test is also developed and experimentally validated. The composite cylinders are tested as per the testing criteria mentioned in ISO 11439 which is the safety standard for onboard compressed natural gas storage. The validated FEA models can be used to analyze the thermomechanical response of composite cylinders when subjected to a hydrostatic load under temporary thermal conditions or when operated at temperatures higher than the service temperature mentioned in the ISO standard.

Creep properties and relevant deformation mechanisms of two low-cost nickel-based single crystal superalloys at elevated temperatures

In this work, the creep properties and relevant deformation mechanisms of the two self-designed low-cost single crystal superalloys were studied under the conditions of 1038 °C/172 MPa and 982 °C/248 MPa. The experimental alloys both displayed superior creep properties than the typical second-generation single crystal superalloy René N5. During high temperature creep tests, the rafted γ′ phase became more and more twisted with the decrease of distance away from the fracture surface, which was caused by the existence of a stress gradient in the gauge length section of the fractured specimen. Under high temperature and low stress conditions, the Re-free alloy displayed denser dislocation networks under both test conditions. Based on the nodal reaction theory, the dislocation networks with different shapes formed by reactions of a/2<110> dislocations at the γ/γ′ interface. Furthermore, the morphology of dislocation networks could transform from initial rectangles to octagons and then to thick hexagons. Under the condition of 982 °C/248 MPa, some parallel dislocation arrays in γ′ particles caused by dislocations pile-up was found in the Re-low alloy, and the main obstacles to the motion of dislocations were K–W locks and a<010> superdislocations. Based on the calculated critical resolved shear stresses, the primary deformation mechanism of the two experimental alloys under elevated temperature and low stress conditions was identified as dislocations slipping in γ matrix and climbing over rafted γ′ phase.

Research on creep damage model of high alumina bricks

In order to understand the mechanism of lining failure caused by creep, it is significant to evaluate the three creep stages of lining materials. In this paper, the three stages creep curves of 3 MPa, 3.25 MPa and 3.5 MPa for high alumina bricks at 1350 °C were obtained by high temperature creep testing equipment. Then the K-R model, the Sinh model and the Liu-Murakami model combined with the Garofalo formula are used to fit the measured creep curves. The results show that all the three models are suitable for the description of the creep behavior and the creep life prediction of high alumina bricks, and the Sinh-Garofalo model has the best fitting effect. Therefore, the Sinh-Garofalo model is selected for finite element modeling. The reliability of the model is verified by comparing the simulated and analytical values. Finally, by comparing the model with/without considering the creep damage, it is found that the creep damage model can be used to characterize the creep damage evolution process and predict the life of structures, which can be further used in the failure study of furnace lining.

Quantitative models of high temperature creep microstructure-property correlation of a nickel-based single crystal superalloy with physical and statistical features

The microstructural evolution prediction and operating condition evaluation of nickel-based single crystal (SX) superalloys during creep are of great significance for damage assessment in service. In this paper, quantitative models of crept microstructural evolution of a nickel-based SX superalloy containing Re and Ru were constructed by using the machine learning method with physical and statistical features of microstructure. Firstly, a sequence of high temperature creep tests was conducted on high-throughput specimens with multiple conditions. The physical microstructural features, i.e., volume fraction (Vf), rafting degree (Ω), and rafts thickness (D) of γ′ precipitates of 8 different specimens were quantitated continuously. Secondly, the statistic features were introduced as supplementary to improve the specificity of microstructural features, using the two statistical methods of two-point correlation and principal component analysis (PCA). Then, two machine learning models were constructed through a neural network algorithm, to predict the microstructure under a certain creep condition and evaluate the creep condition with a certain microstructural feature. The validation creep test showed that these two models have good performance. The quantitative models constructed in this study have great significance in the alloy optimization and damage assessment of nickel-based SX superalloys, which can be extended to other SX superalloys.

Microstructure and mechanical properties of Mg–Zn–Y–Mn magnesium alloys with different Zn/Y atomic ratio

The effects of Zn/Y atomic ratio on the microstructure and mechanical properties of Mg–Zn–Y–Mn alloys were studied and analyzed using optical microscopy (OM), scanning electron microscopy (SEM), x-ray diffraction (XRD), differential scanning calorimetry (DSC), microhardness testing, high-temperature creep testing and an electronic universal testing machine. The results indicated that when Zn/Y ratio is 3:1, the alloy is mainly composed of α-Mg, I phase and W phase; when Zn/Y ratio is 1.5:1, the alloy is mainly composed of α-Mg and W phase; the alloys are all composed of α-Mg, W phase and LPSO phase when the Zn/Y ratio decreases to 1:1 or 0.8:1. With the decrease of Zn/Y ratio, the yield strength and tensile strength of alloy first increase and then decrease; the high-temperature creep resistance increases first and then decreases, and the elongation shows the trend of decreasing, increasing and then decreasing again. In total, the Mg94Zn2.5Y2.5Mn1 alloy exhibits the best mechanical properties and the best creep resistance, which is mainly related to the type and amount of the second phase.