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The UK High Temperature Power Plant Forum have completed a long term cross-weld creep test programme on a Grade 92 steel pipe. This paper describes an analysis of the creep test data. The weld strength factor (WSF), i.e. the ratio of cross-weld strength to base material strength for a specified rupture time and temperature, is shown to be a unique function of a dimensionless parameter S, the creep test stress divided by ultimate tensile strength at the test temperature. The key practical finding is that, since a single universal cross-weld creep life parametric is applicable to all temperatures, long-term behaviour at service temperatures should reliably be predictable from short-term tests at accelerated temperatures. [ABSTRACT FROM AUTHOR]

In this paper, a new high-temperature methodology for determining the 'effective' corrosion rates of internal pressure-tested cladding tubes, based on the reduced creep failure times under exposure, is presented. The methodology has been developed to estimate the combined damage accumulation of corrosion and creep and to establish temperature limits for the use of 1515Ti claddings in high-temperature Lead-Bismuth-Eutectic (LBE). The results are compared to static corrosion rates determined from earlier test campaigns in stagnant LBE. All tests are conducted on 24% cold worked 1515Ti cladding tubes that are the SCK CEN reference material for the MYRRHA design efforts. The virgin material creep model that is the basis for the corrosion rate estimation, has been fitted on an extended creep failure database and a life-fraction rule is applied to accommodate for the life reduction caused by the sought 'constant' effective corrosion rate. [ABSTRACT FROM AUTHOR]

Regarding the application of the Monkman-Grant relationship to creep strength enhanced ferritic steels such as modified 9Cr-1Mo and 12Cr steels, there have been many reports showing that results tend to be an overestimation of the creep life in the long-term region. To address this issue, an author conducted a study to improve the estimation accuracy in the long-term region by the Monkman Grant relationship. Analysing the published creep test data for modified 9Cr-1Mo steel, the author found that the product of the minimum creep strain rate and rupture time (ƐMG) is not constant but depends on the loading conditions, which are test temperature and stress. Then, the author proposed an extended Monkman-Grant relationship considering the dependence of ƐMG on the loading conditions. When this model was applied to the data for modified 9Cr-1Mo and 12Cr steels, the creep life was predicted with good accuracy from the short-term to long-term regions. [ABSTRACT FROM AUTHOR]

P92 (9Cr-1.8W–0.5Mo-NbV) martensite steel is widely used in high-temperature pipes of ultra-supercritical boilers in China. During high-temperature operation, microstructural evolution of the heat affected zone (HAZ) across welded joint leads to type IV cracking failure. In this article, microstructural evolution and type IV cracking of P92 steel welded joint in an actual 605℃ supercritical boiler were analysed. The results show the cracking appeared in the fine-grained heat affected zone (FGHAZ). Laves phase and M23C6 phase clustered around the cavity. Their precipitation and coarsening promote the nucleation and growth of cavities. The Z phase was also observed in the heated affected zone. Compared with other zones, FGHAZ has lower dislocation density and more degeneration of tempered martensite lath, the coarsening of Laves phase and M23C6 phase is more severer in FGHAZ. These factors led to the deterioration of the creep properties and type IV cracking. [ABSTRACT FROM AUTHOR]

The cast coarse-grained polycrystalline Alloy 247 is investigated, which is typically used in the rear turbine blades of land-based gas turbines. High rotational speeds up to 10,000 rpm in combination with high temperatures induce creep deformation. Additionally, gas turbine blades are subjected to high-frequency cyclic stresses. This complex loading state requires the investigation of the interaction of creep damage and high-cycle fatigue (HCF). Therefore, HCF tests with a stress ratio of Rσ = −1 were carried out on as-received and pre-crept specimens. In comparison, an HCF test series with high mean stresses (Rσ = 0.5) was conducted to consider the superposition of creep and fatigue. Since nickel-base alloys exhibit a pronounced elastic anisotropy, the grain orientations were considered in the damage analysis. The results show that creep-induced grain boundary damage is a dominant structural parameter for the HCF behaviour of Alloy 247 in both, sequential and superpositioned creep-fatigue loading. [ABSTRACT FROM AUTHOR]

Alleima 3R60TM is an AISI 316/316 L type of stainless steel. This alloy shows extra-long creep lives. At 700°C with an applied stress of 45 MPa, the specimen broke first after 240 131 hours, a 140% longer creep life than the predicted. The reasons have been studied using SEM/EDS, EBSD, ECCI and TEM. Two unexpected phenomena have been observed. One is the presence of homogeneously dispersed small Mo2Fe4CrSi(Ni0.5)f063type of precipitates in the matrix. According to the thermodynamic calculation, this phase should not form in this alloy at 700°C. The other is the fine grains formed in the matrix during the creep-testing at 700°C for such an extra-long time. It is believed that both small precipitates and fine grain size contribute to this extra-long creep life of the steel. With the microstructural study and thermodynamic simulation, the mechanisms to form precipitates and fine grains have been discussed. [ABSTRACT FROM AUTHOR]

Trends in life with respect to stress can vary significantly based on the form of the chosen Arrhenius equation, and this in turn is fundamentally influenced by the nature of the rate controlling microstructural obstacle. Recognising this crucial aspect is important since shortcomings in creep models are in many cases attributed to myriad microstructural changes, while in fact, it is the basic creep mechanism which needs to be properly accounted for. It is proposed that nano-scale vanadium carbonitrides and dislocation nodes are the rate controlling obstacles in ferritic steels and superalloys, respectively, and this defines the basic rate equation to be used for these materials. With these definitions in place, simple constitutive equations, similar in spirit to the Larson-Miller parameter are derived. The ability of these equations to predict long-term behaviour and remaining life is demonstrated using data on P91 boiler tubes and IN738 superalloy. [ABSTRACT FROM AUTHOR]

A study of the mechanical properties of Ti64 alloy through Small Punch Test is presented in this paper. The method and the validity of the correlation coefficients developed for nuclear steels are first verified on wrought alloy and then applied to additive manufactured material. The yield strength values obtained agree with the literature and domains of negative strain rate sensitivity could be highlighted for the microstructure resulting from additive manufacturing. The creep tests carried out showed good agreement between the present results and those reported in the literature on usual creep representations. This allows to validate the models used and to extend the small punch methods to the case of titanium alloys. The results are used for comparing the mechanical behaviour of additive manufactured Ti64 alloy to that of wrought alloys. [ABSTRACT FROM AUTHOR]

Solid solution strengthened (SSS) Ni-based superalloys, such as A617, show superior creep resistance at 700°C. Established for many years in land- and aero-based gas turbines, these materials are increasingly being considered for use in high-temperature thermal power plants. Apart from SSS, the creep strength in A617 stems from γ' and carbide precipitates. In this work, a microstructurally based creep model for A617 is presented. Mobile dislocations in the model interact with fine grain-interior precipitates, and grain boundaries act as dislocation sources/sinks. The model is capable of simulating creep curves and time-to-rupture (TTR) diagrams based on the evolution of mobile dislocations. At lower stresses, the accuracy of modelled TTR can be improved by adding diffusion creep to dislocation creep. The simulated evolution of dislocation densities is realistic compared to the literature data. The reduction of area of ruptured samples was included in a damage factor, enabling the consideration of creep ductility. [ABSTRACT FROM AUTHOR]

This work deals with a physically based creep model of the martensitic 9% Cr-steel P91. In particular, we quantitatively study the effect of precipitates located in the subgrain interior on the creep curve and the lifetime of the material. Our creep model is capable of simulating the microstructural evolution parallel to creep curves and has been further developed towards predicting time-to-rupture (TTR) diagrams. We demonstrate how the number density, size and shape of the precipitates in the subgrain interior affect the creep curve and thus the lifetime of the material. Three according parameter studies on the material P91 compare microstructures with/without precipitates and/or assuming specific shapes by introducing aspect ratios. These simulations can be used as a basis for further development of materials with regard to the precipitation phases. [ABSTRACT FROM AUTHOR]

We demonstrate a dislocation density-based crystal plasticity (CP) model approach for simulating mesoscale deformation and damage. The existing CP framework is extended to be compatible with the oxygen-free phosphorous copper microstructure that is the focus of this study. The key aim is to introduce relevant plastic deformation mechanisms and to develop a failure model capable of depicting creep damage in the material. The effect of local variations in material is evaluated, and the model response is compared with experiments and characterisation. The basis of this work is CP material modelling, including grain orientation and size, obtained using electron backscatter diffraction and experimental test data of real relaxation test specimens. This will yield a realistic description of texture and grain shape and, ultimately, accurate stress–strain response at the microstructural level for further evaluation of performance with respect to material creep(−fatigue) damage. [ABSTRACT FROM AUTHOR]

The life-limiting fitness for service of high-temperature components is of interest in design, fabrication and later assessments of remaining creep life. Of the associated indicators, strain reflects creep in design and in service, while the discontinuities like creep cavities are targeted in the in-service inspections. Microstructure and hardness can provide supporting information on the material condition. Here, we assess such indicators for the creep-associated damage, particularly at the early stages. Improvements appear possible, e.g. in microscopy to support metallographic inspections and in utilising the widening inspection experience on newer materials. The present work successfully integrated the Wilshire/LCSP creep strain and rupture models with FE analysis for predicting creep strain evolution. Since the model applies for the whole creep curve, it can be used for early stages of creep down to the limit of negligible creep and to the lower limit of the window where creep cavitation damage can be observed. [ABSTRACT FROM AUTHOR]

The design and operation of conventional power plant components is affected by load changes, with the balancing of renewable energy generation leading to an increase in warm and hot starts. Essential parts of these power plant components are flanges e.g. connecting pipes and turbine housings. While being structural rather simple, many influences affect the functionality of these flanges, like the high-temperature behaviour of the bolt material or the temperature distribution in the components. This paper presents parts of a recently finished research project on different influences on the relaxation behaviour of flanges. To investigate the influence of the bolt material, tests were carried out on a model of an IP turbine flange using martensitic X12CrMoWVNbN10-1-1 and nickel-based Ni80A bolts. Each tests included 2000 h of steady state and 3000 h of transient load with a retightening of the bolts after 1500 h. Exemplary relaxation tests on the X12 material provide additional information towards the observed behaviour in the flange tests. [ABSTRACT FROM AUTHOR]

For the service life calculation of high-temperature components the knowledge of the creep behaviour of the materials used remain essential. Over decades, many methods have been developed for extrapolating creep rupture strengths. The challenge with these Creep Rupture Data Assessments (CRDAs), however, always remains evaluating the predictive accuracy of creep life. New computer-aided calculation methods allow the use of extensive data on the casts and other experimental data, as well as the application of probabilistic methods. Within the ECCC, software tools are being developed that both leverage the capabilities of new powerful computer-aided computational methods and allow for simultaneous assessment with post-assessment testing in accordance with ECCC recommendations. The authors would like to point out that despite all available tools and guidelines, the expertise and experience of the assessor is an indispensable guarantor for a reliable evaluation. [ABSTRACT FROM AUTHOR]

For heat resistant alloys, long-term structural stability at high temperatures is a critical issue for alloy design and applications. In this paper, the long-term creep behaviours and structural stabilities of six heat resistant high Ni alloys and austenitic stainless steels have been studied. The longest creep rupture life is up to 359 283 hours. High Ni and Cr alloys show a good combination of high creep and oxidation resistances. Precipitation of nano MX particles with a very low growth rate improves long-term creep resistance at high temperatures. Long-term stable multiple nanoprecipitates of MX, Cu-rich, Laves and M23C6 phases can greatly contribute to the creep strength. Low Ni austenitic stainless steels show comparatively low oxidation and creep resistances. It was first found that at 800°C, Cr2N could form in the low Ni steel with a long-term crept by the absorption of nitrogen from the air into the matrix. [ABSTRACT FROM AUTHOR]

A uniaxial creep model that describes creep over a wide stress range was developed for P91 steel using an artificial neural network (ANN). The training dataset was based on measurements from uniaxial creep tests and information derived from a combination of the logistic creep strain prediction and the Wilshire models. The ANN model reproduces the training dataset with high accuracy (R2 = 0.975; RMSE (Root Mean Square Error) = 0.19). The model can be easily implemented in finite element analysis (FEA) codes since it provides an analytical expression of the true creep rate as a function of temperature, true stress and true creep strain. In FEA simulations under the same conditions as the training dataset, the model provides times to rupture and minimum creep rates very close to those in the training dataset. The model can be adapted for heats with different properties from the average behaviour of the training dataset by means of a stress-scaling factor. [ABSTRACT FROM AUTHOR]

The aim of the paper is to investigate experimentally the fatigue and creep-fatigue material behaviour of improved materials and welded joints for the application in flexible future power plants. These materials promise a reduction in manufacturing costs as well as an increase in flexibility by providing enhanced creep strength thorugh a wall thickness reduction. At the temperature range between 500°C −550°C, the investigation focusses on the creep and low-cycle fatigue behaviour of dissimilar welded joints from conventional materials (bainitic and martensitic materials T24 and T92) to nickel-based alloys (A617B and HR6W) fabricated as tubes. At the temperature range between 700°C and 750°C, it focusses on the creep, low-cycle fatigue and creep-fatigue behaviour of similar and dissimilar welded joints from nickel-based alloys (A740H, A617B and A263) fabricated as tubes and as pipes. Metallographic investigations after testing provide support for understanding the influence of temperature, strain amplitude and dwell time on the microstructure change and the fatigue strength. [ABSTRACT FROM AUTHOR]

The microstructural anisotropy caused by the highly oriented solidification of the metal Laser Powder Bed Fusion (PBF-LB/M) process affects mechanical short- and long-term properties. Component build orientation influences grain morphology and orientation, and thus, mechanical properties. While the creep behaviour of samples manufactured parallel and perpendicular to the build direction are studied intensively, the 45° build orientation remains uncharacterised. In this study, IN738LC creep samples are manufactured via PBF-LB/M in three build orientations (0°, 45° and 90°). While the results of 90° and 0° are as expected, where 90° achieves the longest time to rupture and largest rupture strain, the 45° specimen shows the least fracture time. Differences in microstructure and twinning behaviour are identified as one of the root causes for this unexpected behaviour. This study discusses the correlation between microstructure, twinning and build orientation and their effect on creep behaviour, with special focus on the 45° build orientation. [ABSTRACT FROM AUTHOR]

This work explores the influence of various post-build heat treatment procedures on the microstructural characteristics and creep response of laser powder bed fusion (L-PBF) Mar M509 superalloy. Besides MC-type carbides formed primarily during the L-PBF process, the obtained results indicate the influence of heat treatment on the nucleation of M6C and M7C3 carbides due to elemental segregation. Short-term creep-rupture tests have been conducted at 900°C/110MPa on eight specimens, each subjected to a specifically individual heat treatment procedure. Rupture times are found to be sensitive to the heat treatment applied and vary from ~178 to ~641 hours. Detailed microscopic characterisation of ruptured specimens highlights the synergistic influence of oxidation and M7C3/M6C precipitate-free zone formation on macroscopic intergranular failure facilitated by grain boundary sliding along favourably oriented small grains. Besides notable grain growth, the presence of a serrated grain boundary structure with relatively less MC-type carbide coarsening is found to offer the best creep resistance among the eight L-PBF specimens. Further improvement of the elevated temperature performance of the alloy is seen to require a focus on the L-PBF process conditions to optimise the morphology of MC carbides, as they form. [ABSTRACT FROM AUTHOR]

In this paper several aspects of single line minimum creep rate and time to rupture models are examined that have not been discussed previously in the literature – the origin and boundary condition behaviour of some models; and the existence and relevance (for industrial applications) of temperature turnback. It is found that some published time to rupture models will predict very finite times to rupture at zero stress and temperatures of industrial use of the alloys, and that temperature turnback is mathematically possible even with physics-based models, but unlike stress turnback, the conditions at which temperature turnback would occur are usually of no physical or industrial relevance. [ABSTRACT FROM AUTHOR]

The effects of Re/Re on the element distribution and creep performance of nickel-based single crystal alloy by using atom probe tomography (APT) and creeping property testing has been studied. Results show that the elements Re, Ru, W, Mo, Cr, and Co are the former of γ phase; the elements are distributed in γ and γ' phases of two alloys according to various ratios. Some of the Ru atoms in Re-containing alloy make more Al, Ta atoms are dissolved in γ matrix and make more Mo, W, and Re atoms dissolving in γ' phase, which increases the alloying degree of γ", γ phases to enhance the strength and creep resistance of alloy. Particularly, the Re and W atoms dissolved in γ" phase are excluded, during creep, for the enrichmrnt in γ phase near the interface to form their peak content; the lattice distortion coming from the enriched Re and W atoms restrains dislocations gliding to delay the γ' phase from being sheared. The deformed mechanisms of alloy in the later stage of creep are the dislocations gliding in γ phase and shearing γ' phase. Wherein, the dislocations of shearing γ' phase are both glided on {111} planes and cross-glided from {111} to {001} planes to form the KW locks, the ones that restrain the gliding and cross-gliding of dislocations for improving the creep resistance of alloy. While the interaction of the Ru with Re and W atoms make some Reand W atoms reserved in the γ′ phase to delay the diffusion of other elements, which prevents the KW locks from being released, to keep the good resistance of alloy. [ABSTRACT FROM AUTHOR]

The γ/γ' microstructure is a key factor for the mechanical performance of nickel-based single crystal superalloys. Experimental results have shown that the size and shape of the γ' precipitates are not uniform and follow certain probability distributions. In order to study the effect of the microstructure on the mechanical behavior of the superalloys, a method is proposed to reproduce its initial microstructure with the distribution function of its γ′ particle size. The effect of different microstructures on the creep behavior at different locations of a real blade is predicted to demonstrate the application of the method. It can be found from the simulation results that the mechanical behavior of the blade is related to not only the thickness of the blade wall but also the position in the blade. [ABSTRACT FROM AUTHOR]

The fatigue and creep–fatigue properties of 316SS formed by selective laser melting (SLM) were investigated, considering the effects of different peak stresses, stress rates and peak-holding times on the ratcheting and creep-ratcheting behaviour of SLM 316 SS specimens at different sampling angles (0°, 90°) and high temperatures (550°C, 650°C, 750°C). Results show that the 90° specimens exhibited better fatigue life than that of the 0° specimens. Moreover, the ratcheting evolution can be divided into three stages, but the first stage is very short and the main ratcheting behaviour takes place in the second and third stages. Interestingly, it was found that at stress rates above 10 MPa/s, the ratcheting evolution curves are highly overlapping. Furthermore, the creep–fatigue interaction promotes the evolution of material damage. Besides, creep increases the total strain of 316SS even with a short dwell time (0.5 min). [ABSTRACT FROM AUTHOR]

To help determine remaining lifetime of pressure vessels suffering creep, the authors have previously developed a method and presented promising results using a combination of AC and DC electrical potential drop (EPD) on-line monitoring, detecting both final cracking as well as incipient creep damage. The latter was tentatively ascribed to the development of cavitation damage, but recent modelling and separate off-line measurements have shown that cavitation is unlikely to provide enough of a change in electrical properties to explain all of the variations previously observed. Here we gather the results obtained to date, and review their likely relationships in an attempt to obtain a greater insight into the mechanisms at play. Whilst changes in both on-line and off-line EPD are largely in accord, the belief now is that the changes seen cannot be fully explained by cavitation development and that EPD is responding to other creep induced phenomena as well. [ABSTRACT FROM AUTHOR]

The paper presents a critical analysis of the procedures for defect propagation assessment recommended by British standards and their application to rotating components. The way of applying these procedures was shown by an example of a steam turbine rotor with particular use of a characteristic strain model of high-temperature creep. Calculations of the flaws located at the rotor axis have shown that their growth in the design life is small. The performed analyses have also shown that crack development due to low-cycle fatigue is comparable to that due to creep and cannot be ignored under typical service condition. It was also shown that there is no risk of brittle fracture in the rotor under typical start-up conditions. The presented results prove the usefulness of the methods included in the British standards and guidelines for crack growth assessment in components subject to centrifugal load. [ABSTRACT FROM AUTHOR]

The creep response of AISI 316 and AISI 316 L was analysed to provide a coherent picture of the material behaviour, valid for both conventional wrought and additively manufactured steels. Literature evidences were considered. The analysis demonstrated that the presence of a fine distribution of particles, precipitated during creep exposure in both AISI 316 and 316 L(N), should be taken into account. A recent model, expressly developed for particle-strengthened alloys, was successfully used to describe the minimum creep rate dependence on applied stress for AISI 316 and 316 L(N). The same set of constitutive equations, in combination with the composite model, was then used for describing creep data obtained by testing the AISI 316 L produced by additive manufacturing. The main difference with wrought materials was that the microstructure of the additively manufactured alloy was composed by cells (soft zones) and thick walls heavily decorated with ultrafine oxide particles (hard zones). [ABSTRACT FROM AUTHOR]

The effects of the chemical composition and heat treatment on the creep properties of Grade 91 steel base metal and welded joint were investigated by conducting creep tests on materials whose Al, Cr, and Ni contents were systematically varied or the normalising and tempering temperatures were varied. The effect of the chemical composition on the creep strength was similar for the base metal and the welded joint. That is, the creep strength decreased with increasing Al content, whereas the Ni content had little effect on the creep strength. Regarding the effect of heat treatment, the creep strength increased with increasing normalising temperature and decreasing tempering temperature for the base metal and the welded joint. The comparison of the test results of the welded joints with those of the base metals revealed a positive correlation between the creep strength of the base metals and that of the welded joints. [ABSTRACT FROM AUTHOR]

Microstructural characteristics of abnormal structure in long-term used Gr.91 steel weldments and its effect on creep strength of welded joint are presented. The abnormal structure is defined as a structure in which the hardness is clearly lower than that in the normal region, and the grain is coarsened. Creep strength of the abnormal structure is significantly lower than that of normal weld metal. There is little effect of abnormal structure formed in the HAZ on creep life of standard welded joint specimensr. In case of abnormal structure formed in the weld metal, times to creep rupture of standard specimens are shorter than those of specimens without abnormal structure. Full-thickness welded joint specimen consisting more than half abnormal structure in the weld metal ruptured at the weld metal before the HAZ. Time to creep rupture decreased with increasing the abnormal structure ratio in thickness direction. [ABSTRACT FROM AUTHOR]

TP347H (18Cr-10Ni-Nb) austenite steel has high creep strength and has been used as the material of tubes in 600°C USC power plants in China. The TP347H materials used in actual power plants are useful for estimating the changes of material properties caused by long-term ageing and damage at low stress conditions. An understanding of the long-term microstructural evolution under actual use conditions is a key for the improvement of these heat resistant steels. In this article, microstructural evolution of TP347H steel under different service conditions in power plants was studied using optical microscope (OM), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) techniques. The results show that M23C6, MX and σ phases were found to precipitate. A quantitative characterisation of microstructure evolution was evaluated during long-term exposure, focusing on the content and the size of precipitates in the TP347H steel after long-term service. [ABSTRACT FROM AUTHOR]

The design of grade 92 steel seamless pipes has progressed significantly, but very little has been published on heavy wall forged pipes. In this work, microstructure and creep properties of 180 mm thick F92 pipes both normalised and tempered (N&T) and quenched and tempered (Q&T) are studied. Creep behaviour was assessed using both uniaxial tests and small punch creep test data analysis. Quenching treatment was able to develop cooling rates greater than 0.6 °C/s, limiting the presence of ferrite to less than 1%, while in case of normalising, the ferrite percentage was 3–4.5% (average cooling rate < 0.12 °C/s). Consequently, the Q&T steel exhibited creep strength of 20–25% higher than that of the N&T forged pipe of the same thickness. These results were exploited for the setup of a new proprietary heat treatment able to develop on heavy wall forged pipes' creep strength levels similar to those of N&T thin seamless pipes. [ABSTRACT FROM AUTHOR]

Additive manufacturing of alloys has gained attention by industry as it provides economical production of complex-configuration engine parts with fewer joining steps and greater geometric freedom. The creep behaviour of additively manufactured (AM) Inconel 625 and 718 at 650 and 800 °C was compared with that of the wrought alloy after 24 hour and long-term tests up to about one year. Inconel 718 is precipitation strengthened while 625 is solid-solution strengthened. It was discovered that the creep strength of the 625 and 718 produced by additively manufacturing is essentially identical than that of the wrought alloy. Complications occur, however, with a marked loss of ductility in the AM alloys. Fatigue and impact toughness were adversely affected as well. The basis for these degradations was particularly investigated. [ABSTRACT FROM AUTHOR]

Additive manufactured components are increasingly being applied in operational equipment. If these components are going to be used at higher temperatures, their creep behaviour needs to be verified. In the ECCC workgroup 3C an exploratory study of the short-term creep behaviour of the HASTELLOY® X alloy produced by Selective Laser Melting was carried out, with the aim of characterising the material behaviour and collecting input for the heat treatment optimisation, if necessary. Hastelloy X is a nickel-chromium-iron-molybdenum alloy for applications where a combination of oxidation resistance and high-temperature strength is required together with good fabricability. Creep test results up to about 3000 h were compared with creep results of conventionally produced Hastelloy X. Two orientations were tested: parallel and perpendicular to the building direction. In addition, the microstructure of the ruptured creep specimens was investigated to determine the fracture location/mechanism and the main microstructural features. The obtained results can be considered promising and give useful indications in the perspective of LPBF material application at high temperatures. [ABSTRACT FROM AUTHOR]

ASME BPVC Section VIII Division 2 ('ASME VIII(2)') and ASME B31.1 are being used not only for low-temperature design but also for high-temperature design in the creep range in supercritical thermal power plants. However, creep effects in ASME VIII(2) and B31.1 are taken into account implicitly, and there could be significant technical issues in terms of conservatism in the case of long-term operation in the creep range because their design evaluation results do not change across plant operation time. The conservatisms of ASME VIII(2) and B31.1 have been quantified based on comparisons with nuclear-grade design rules of ASME BPVC Section III Division 5 and RCC-MRx which take creep hold time into account explicitly. It was shown that ASME VIII(2) and B31.1 results could be non-conservative if operation time exceeds a certain time limit, which raises concerns around non-conservatism in the case of long-term operation within the creep range. [ABSTRACT FROM AUTHOR]

For modern creep-resistant steels, ductility is primarily controlled by cavitation, since brittle rupture is dominating during long-term service. In recent years, the present authors have formulated fundamental models for nucleation and growth of cavities that have been verified to be able to describe experiments for austenitic stainless steels. These equations, together with models for dislocation creep, are used in this paper to present basic modelling results for the creep ductility of austenitic stainless steels for the first time. New results are also presented for ductile rupture, where the elongation values are mainly governed by plastic instability and necking. The Hart criterion is used to identify the strain where the instability forms. Modelling shows that the instability grows very slowly and that its size is not significant until close to rupture. These facts are used to demonstrate that ductility during ductile rupture can be predicted from necking behaviour. [ABSTRACT FROM AUTHOR]

Microstructural characterisation was performed on the gauge section of two creep rupture samples manufactured from Super 304H; an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The samples had been exposed to uniaxial creep tests (Sample A: 700°C and 75Mpa, Sample B: 650°C and 120Mpa) where both exhibited low creep ductility. Six phases were identified: MX (Nb-rich), sigma, Cu-rich, M23C6, modified Z phase and Fe (BCC). Quantification of Nb-rich and Cu-rich particles revealed ~10% and ~16% more Nb-rich and Cu-rich particles respectively in Sample A versus Sample B. The amount of sigma and the average particle size for the Nb and Cu-rich particles was similar for both samples. There appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation which could have contributed to the low creep ductility exhibited by these two creep samples. [ABSTRACT FROM AUTHOR]

Modified hyperbolic sine minimum creep rate equations were used to correlate uniaxial minimum creep strain rate data of 1/2Cr-1/2Mo-1/4 V and Grade 22 steel alloys and to correlate creep life data of Grade 22 steel when coupled with the Monkman-Grant equation. Hyperbolic sine equations were selected because of their physical basis in dislocation creep and numerical stability. Both equations provided very good fits to minimum strain rate data over a wide range of temperatures and stresses. Reasonable correlations of a large Grade 22 creep life data set were obtained with goodness of fit nearly equivalent to those obtained with the Larson-Miller and Wilshire equations, and with more conservative predictions of long-term creep strength. However, all these equations provided a poor fit to the entire data set when only short-term data (tr < 5,000 h) were correlated, and they predicted lower long-term creep rupture strengths vs. the full data set correlations. [ABSTRACT FROM AUTHOR]

Microstructure evolution and damage mechanisms during the creep deformation of a directionally solidified Ni-based superalloy were evaluated. The creep deformation behaviour was studied along the longitudinal direction (i.e. along the solidification direction), at a moderate temperature of 870 °C, and applied stresses ranging from 310 to 450 MPa concerning the service conditions in industrial gas turbine blades. It was shown that at the lower stress levels of about 310 MPa, the average creep strain rate decreased by two orders of magnitude, while tertiary creep appeared as the dominant stage. Progressive coarsening of γ' precipitates was also observed, meantime γ' precipitate shearing was confirmed to occur over the evaluated range of stresses. [ABSTRACT FROM AUTHOR]

Metallic structures operating at high temperatures can have creep as a life-limiting mechanism. The limit can arise by nucleation and growth of creep cavities that eventually merge becoming the leading damage of the fracture mechanisms. Understanding and modelling for creep cavitation can therefore help in prediction of structural safety and condition of such structures. The present paper will introduce a new model to predict creep cavitation damage so that the associated creep strain is also modelled out of the predictive expression without losing accuracy in the time to critical damage and corresponding residual life. The model was verified for OFHC copper, low-alloy 0.5CMV steel and higher-alloy X20 steel, to cover a wide range of polycrystalline metals and their microstructures. The new model performed as well as or better than the classical approach which requires access to measured strain. The model was applied to predict creep cavitation damage from time, stress and temperature as predictive variables. [ABSTRACT FROM AUTHOR]

The nanoindentation creep behaviour of crept HR3C austenitic steels was investigated to study the microstructural evolution of the steel after long-term creep at 650 °C. The corresponding microstructure evolution was characterised by optical microscopy, scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Results show that the nanoindentation hardness and creep displacement decreased with increasing creep time. The dominated nanoindentation creep mechanism was most likely changed from dislocation slip to grain boundary (GB) sliding induced by the decrease of dislocations density inside the grain and coarsening of carbides at the GB. The coarsening and growth of the precipitates distributed along the GB weakened the pinning effect of the precipitates and accelerated the migration of the grain and twin boundaries. With prolonged the creep time, large numbers of cavities were formed at the grain boundaries, which might lead to terrible intergranular corrosion and brittle cracking under the action of stress and high temperature. [ABSTRACT FROM AUTHOR]

Proverbially 'To travel hopefully is better than to arrive.' Many roads have been travelled in search of a short-term mechanical test that would reliably predict long-term component creep life. The motive is clear; whatever progress is made in inspection and monitoring, direct measurement of the relevant properties of the subject material – archival or ex-service – will always provide the most reliable indicator of its state and performance capability. The difficulties are equally clear: mechanism changes over the stress, temperature and time ranges involved; issues of specimen size and geometry – and their relation to stress state; adequate emulation of in-service variation in operating condition; and, for post-exposure tests, translating information between the sample position and the critical location. This paper focuses on: issues of mechanism change, both in simple extrapolation and across boundaries of stress state and test type identification of parameters that allow true comparison between forward creep, stress relaxation and constant strain rate testing Data obtained on low-alloy ferritic steels are examined and analysed, largely using the framework of continuum damage mechanics. Particular attention is given to the nature of the equilibria represented by tensile instability and the minimum creep rate. It is concluded that the 'instant test' is an unreachable ideal. However, the knowledge and understanding gained through the search have benefitted the larger objective and – as in every legendary quest – have made the journey worthwhile. [ABSTRACT FROM AUTHOR]

Electrical potential drop (EPD) is a powerful technique to gauge crack depth in many contexts, including fracture, creep and fatigue testing, and in-field NDE, however it has only seen limited use for monitoring pre-crack creep cavitation damage. The authors have previously reported promising results using a combination of AC and DC EPD on large pressure vessel creep tests, even detecting incipient damage. However, that study lacked linkage to the underlying microstructural mechanisms. Here we present the results of a more fundamental creep study using EPD on P91 pressure vessel steel specimens taken from weldments, suffering HAZ-related Type IV cracking. This work confirmed that EPD could detect incipient damage, with ACPD being particularly sensitive to life fraction, but allowed linkage to changes in material properties, such as permeability and resistivity. Discussion of the implications for future off-line (in-field) NDE methods and on-line continuous monitoring of high-temperature plant components is also made. [ABSTRACT FROM AUTHOR]

A Re-containing 10% Cr-3% Co-3% W steel with low N and high B contents subjected to normalising at 1050°C and tempering at 770°C was crept at 650°C under the applied stresses ranging from 200 to 100 MPa with a step of 20 MPa. During creep both the temperature and the strain affect the tempered martensite lath structure that causes the thermal and strain-induced recoveries, respectively, and degradation of structure. Both these recoveries lead to premature failure. The microstructure in the gauge and grip portions of the crept samples were investigated separately to estimate the contributions of temperature and strain during creep to the evolution of tempered martensite lath structure of the Re-containing 10% Cr-3% Co-3% W steel. The W depletion from the solid solution was accelerated by creep strain during short-term creep. The lath width and particle sizes were stable during long-term ageing at temperature of 650°C in the grip portions of the crept samples. The width of martensitic laths was 350 nm after 13,600 h of long-term ageing. The creep strain facilitates the recovery of the structure causing the subgrain formation with a mean size of subgrains of 1.1 μm and particle coarsening in the gauge and neck sections. The precipitation of W-rich Laves phase particles was observed in both grip and gauge portions. The constant coarsening rate of M23C6 carbides was significantly lower during long-term ageing than that during creep, whereas the constant coarsening rates of Laves phase particles were comparable in both grip and gauge sections of the crept samples. [ABSTRACT FROM AUTHOR]

Damage equivalent to that seen over very long times in Grade 91 steel welded components was successfully formed in laboratory, feature sized samples tested under controlled conditions. This paper compares the creep behaviour of welds made using a typical preparation with results for novel step weld geometries. When well-designed the step geometry offsets the heat affected zone (HAZ) in the through thickness of the weld which would be equivalent to the pressure boundary in a pipe. The step in the weld profile thus provides benefit to in-service performance because it prevents the rapid link up of aligned creep cavitation that forms in the HAZ of welds made using typical preparations. The results demonstrate that a well-designed step weld geometry increases the creep rupture life and improves damage tolerance. In turn, the increased duration of stable crack growth promotes the opportunity for detection of creep damage using nondestructive testing methods. [ABSTRACT FROM AUTHOR]

By performing solution treatment at various temperatures, creep property measurements, and microstructural observations, the influence of the solution temperature on the microstructure uniformity and high-temperature creep properties of the DZ125 nickel-based alloy were studied. The results showed that after the solution and aging treatment of the alloy under a relatively low temperature of 1230°C, microstructure inhomogeneities in the dendrite/inter-dendrite regions appeared, and the coarse g¢ phase and eutectic structure in the casted alloy were not removed. The processing and aging treatment of the alloy under a higher solution temperature of 1260°C caused the coarse g¢ phase and eutectic structure within the inter-dendritic area of the alloy to be eliminated, thus enabling the uniform distribution of the fine cubic g¢ phase with a high-volume fraction between the dendrite/inter-dendrite regions and the enhancement of the creep performance of the alloy. [ABSTRACT FROM AUTHOR]

The present work deals with the microstructure evolution during creep of P91 steel. In particular, the changes of prior austenite grain (PAG) and low-angle grain boundary (LAGB) during creep were studied using the electron backscatter diffraction (EBSD). Firstly, a series of crept specimens of P91 steel were prepared at 893 K under a constant stress of 145 MPa. Then, the EBSD measurements of these specimens were carried out. Subsequently, the PAGs were reconstructed successfully with the measured EBSD data, and the resultant distribution of LAGBs was calculated. Consequently, the obtained results suggested that the size of PAGs has not changed significantly during the whole creep. For the distribution of LAGBs, however, the peak shifted obviously to high angles during creep. In addition, it demonstrated that the EBSD method offers an effective and quantitative tool to characterise the microstructure evolution during creep of P91 steel. [ABSTRACT FROM AUTHOR]

Brittle rupture during creep of materials is due to the formation and evolution of inter-granular cavities. Creep cavity nucleation rates – contrary to growth rates – are unpredictable, thus hindering forecasts about the lifespan of any material/specimen under creep. Via rigorous theoretical calculation, it is shown that creep cavity nucleation rates are linked to the Monkman–Grant ductility and that they can be quantitatively estimated through this link. Hence, a relevant phenomenological formula is verified, and creep-induced material-porosity estimations become achievable which in turn lead to improved predictions of rupture times or strains or other possible creep-porosity-related material properties like elastic moduli, thermal and electrical conductivities. [ABSTRACT FROM AUTHOR]

Effects of microalloying the grade 91 steel with boron along with control on nitrogen content (P91BN) on its microstructure, tensile and creep properties have been investigated and compared with those of the standard grade 91 steel (P91). The P91BN steel possesses finer martensite lath structure and M23C6 precipitates in comparison with the P91 steel. Tensile strengths of the P91BN steel were comparable with the P91 steel up to 823 K. The P91BN steel has possessed higher tensile strengths than the P91 steel at higher temperatures (above 823 K). Microalloying the steel with boron decreases the creep deformation rate with significant increase in creep rupture life. Finer lath width, higher number density of fine M23C6 precipitates in pre-creep condition along with sluggish coarsening of M23C6 precipitates resulting in better stabilisation of martensite structural features in the P91 steel on boron micro-alloying, leads to the significant improvement in high temperature tensile and creep strengths. [ABSTRACT FROM AUTHOR]

The determination of creep properties of the components operating at high temperatures in thermal power plants is an enormous concern for safe life assessment. In this study, a series of miniature creep and tension tests were carried out with service-exposed (54,750 h and 68,550 h at 650 °C) and virgin Super304H steel materials. The power law creep behaviour, creep rupture life, and tensile properties were compared among the virgin and service-exposed materials. The creep strength of the 54,750-h-serviced material was comparable to that of the virgin material, while it decreased significantly for the 68,550-h-serviced material. The degradation of the properties of the service-exposed material was discussed based on metallurgical analyses and precipitation mechanisms. [ABSTRACT FROM AUTHOR]

Creep crack growth is known to be the dominant failure mechanism in high-temperature components. Particularly in welded structures operating at elevated temperatures, cracks are often found to initiate and propagate in the vicinity of the weld region which can eventually penetrate into the base material after a long period of operation. In this study, creep crack growth tests have been performed on specimens extracted from an ex-service 316 H welded component to examine the crack initiation and growth behaviour in near-weld regions. The results show that the cracking behaviour of the base metal in near-weld specimens is similar to the as-received 316 H data set, suggesting that the material inhomogeneity would not influence the crack propagation behaviour in service-exposed components. Moreover, the test results show that the crack initiation and growth behaviour of the HAZ specimens can be estimated in much shorter time scales by performing tests on pre-compressed material. [ABSTRACT FROM AUTHOR]

This paper studies high-temperature creep deformation of Mg retort tubes considering the decarburisation of steel. A material model was developed for finite element modelling that relates the activation energy of steel in the steady-state creep rate equation (Q) to its carbon content. Carbon content was described as a function of radial position along the retort thickness (r) and service time (t). The time-dependent activation energy, Q(C(r,t)), was incorporated in the steady-state creep equation. The FEM results of the decarburising model were compared to those of the non-decarburising model and an actually failed retort for verification. The newly developed model presents more accurate results compared to the constant material model (non-decarburising). The FE model was used to predict the effects of various process parameters (temperature, internal pressure, and furnace carbon activity) and geometric parameters (retort diameter and thickness) on the creep deformation of retorts. [ABSTRACT FROM AUTHOR]