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Hot gas components such as gas turbine blades are loaded with constant centrifugal stress at high temperatures and thus the formation of creep cavitation on grain boundaries is expedited. Conventionally cast polycrystalline Nickel-base superalloys used as blade materials are prone to creep cavitation as unfavourably orientated grain boundaries exist. This research presents a diffusion-based probabilistic creep model which describes the creep cavitation process on grain boundaries. It includes the three mechanisms: pore nucleation, growth, and coalescence. The calibration of the model has been carried out by analysing Alloy 247 as-received specimens and specimens with pre-strain. For the evaluation of the pore numbers and sizes, a deep learning model for pore detection was trained on light microscopic and scanning electron microscopy images. Electron backscatter diffraction images are analysed for further investigations regarding grain orientations and grain boundary angles to the loading direction. The calibrated model allows predictions of pore size distributions over time. [ABSTRACT FROM AUTHOR]

The study and modelling of material degradation processes, such as the initiation and growth of creep cavities in high-temperature applications, require a correlative and comprehensive knowledge of the microstructure. However, individual microscopy is limited to a small region and specific microstructural information of the specimen. This work demonstrates a novel correlative microscopy approach for characterising creep cavitation and establishing correlations with local microstructural parameters in a statistical manner. This approach combines datasets from stitched higher-resolution backscattered electron (BSE) images, XeF2 Focused Ion Beam (FIB) images, and backscattered electron diffraction (EBSD) maps with advanced image correlation techniques. Deep-learning image segmentation techniques and statistical analysis are applied to find relations between creep cavitation and local microstructural environment. This approach is demonstrated in a cyclic creep-tested 316H stainless steel specimen with extensive creep cavities. The results show that in this material, strain localization, grain boundary misorientation, and substantial precipitation dominate the nucleation of cavities, whereas other microstructural properties such as grain size and Schmid factor play smaller roles. This study presents the use of the correlative microscopy approach to provide new insights into creep cavitation behaviour and its implications for establishing creep cavitation damage models.

About 9 wt.% Cr steels are widely used in the design and fabrication of thick section components in combined cycle or coal-fired applications for working temperatures of 600 ~ 650°C. This family of materials possesses a martensitic microstructure stabilised by precipitates. The presence of nitrides, inclusions or evolution of second-phase particles may increase the metallurgical risk to creep. The chemical composition and microstructural evolution of 9 wt.% Cr steels contribute to thermal stability and long-term performance. In some specialist alloys, Ta is added to the composition which causes the formation of fine MX precipitates which are only present at the nanometre scale in tempered martensite, which hinders the recovery of dislocations and the migration of laths to extend creep life. However, the presence of large Ta-containing particles or inclusions in the 9 wt.% Cr steels may have a detrimental effect on its creep performance, as they may act as preferred sites for cavity nucleation. To fully appreciate the development of damage in these steels, it is necessary to link the pre- and post-test conditions, evaluate damage in the parent metal, develop procedures that provide consistency of results, and obtain statistically relevant data. The evolution of the Ta-containing phase has been tracked and quantified using a variety of correlative characterisation approaches. Utilising focused ion beam microscopy and two-dimensional electron-based microscopic characterisation, three-dimensional tomography has identified a strong relationship between creep cavities and Ta-containing phases from the early stages of creep. [ABSTRACT FROM AUTHOR]

The present work is performed on the main steam pipe system in Heleneholmsverket, a CHP in Sweden and consists of the following parts (i) numerical analysis of the in-service creep behaviour of the pipe system, (ii) creep testing of new and service exposed materials from welded components, (iii) characterisation of the creep damage distribution in creep tested welds of the actual piping. The entire system has been modelled for creep evaluation to make it possible to compare the simulated creep stress and strain distributions in selected welds with observed amounts of creep cavitation, which can be correlated to the accumulated creep strain. Creep data for the analyses were produced by creep testing of service exposed base and weld metals from a pipe weld and a T-piece branch weld from the system. In addition, the creep tested welds were studied metallographically to map the creep damage and make it possible to compare the damage development with the resulting creep stress and strain distributions in the weld. In the previous project also a T-piece branch weld was investigated in a similar way and those results were used for verification of the re-analyses in present project with the updated system model. The following results were achieved: The model of the entire steam pipe system was created in Abaqus and the strain distributions were verified in comparison to a corresponding elastic Caepipe model. The Norton creep law was used for the simulations. In addition, also primary creep was analysed. The effects of primary creep on the long-term creep behaviour was significant and the results shows the importance of including primary creep into the model. There is no effect of starts and stops on the stress and strain distributions in the system during creep. The system analysis results showed enhanced strains up to 2.1% at one bend and 0.5–1.0% in some parts of the system. Although replica testing had not been conducted directly at the bend the high strains indirectly agreed with the observations of small creep cracks had been observed in replica testing of in a weld at one of the ends of the actual bend. Furthermore, several components in the system have been exchanged due to creep crack formation. Moderate levels creep damage was observed in the pipe weld. The analysis of this pipe weld gave somewhat lower creep strains than expected. The stress and strain distributions matched with the maximum principal stress criterion but not with the von Mises stress that Abaqus uses for creep analyses by default. The analysis of the branch weld matched well with observed creep damage distributions whereas the maximum strain level of 0.4% appears to be rather low in comparison to the quite extensive creep damage. However, local constraint and multiaxiality in welds lead to significantly lower creep ductility compared to uniaxial creep and contribute to a reasonable agreement between the strain and the damage levels. The creep tests of service exposed material resulted in relatively high Norton creep law exponents and no shift to lower values at the lowest tested stresses. It is hardly possible to perform tests at even lower stresses and therefore the simulations at service conditions resulted in unreasonably low creep strains. [ABSTRACT FROM AUTHOR]

In the present study, the creep rupture behaviour and microstructural evolution of 2195-T84 Al-Li alloy are investigated at different tensile stresses. It is found that as the applied stress during the creep rupture process increases, the corresponding creep strain and creep rate significantly increase. Moreover, the evolution of microstructures, including precipitates, dislocation density and creep cavities at different stages is characterized using a transmission electron microscope (TEM), X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The main strengthening precipitates are identified as T1 (Al2CuLi) and θ′ (Al2Cu) phases. Obtained results show that as the applied stress increases, T1 and θ′ phases are gradually coarsened. This coarsening is more pronounced for θ′ phase. Furthermore, the creep cavities are mainly distributed at the interface between the insoluble second phase particles and the matrix, and their average sizes gradually increase as the applied stress increases. Meanwhile, the density and size of dimples on the fracture surface gradually decrease as the applied stress increases. Moreover, the main fracture mechanism changes from transgranular dimple fracture to quasi-cleavage fracture. Based on the microstructural evolution, a novel set of unified creep rupture constitutive model is proposed. The established model incorporates the evolution of microstructures, including the dislocation density, average length of T1 and θ′ precipitates and creep cavitation, and correlates microstructural variables with creep rate. The results calculated by the constitutive model are in good agreement with the experimental data, which validates the proposed model. [ABSTRACT FROM AUTHOR]

In this study, the creep deformation of ferritic/martensitic 9Cr-1Mo (Grade T91) steel boiler tubes that occurs during the operation of thermal power plants is studied using crystal elasto-viscoplasticity finite element analysis (CEV-FEA). The material properties and creep behaviour of Grade T91 were obtained through experimentation and computational analysis at different scales, e.g., molecular dynamics simulation, scanning electron microscopy, small punch (SP) tests. A triple junction of grains with various combinations of crystallographic orientations of polycrystalline Grade T91 was also simulated by CEV-FEA to investigate the stress concentration evolution during creep deformation. The stress concentration at the triple junction was then used as an input for the creep cavitation model, i.e., as the representative stress driving creep cavitation. The combination of CEV-FEA and SP test experiments can improve the evaluation of creep lifetime for as-received boiler tubes with minimum requirements for the amount of material and testing time. [ABSTRACT FROM AUTHOR]

Creep deformation of Super304H austenitic stainless steel boiler tubes in thermal power plants is evaluated using crystal elasto-viscoplastic finite element analysis (CEV-FEA). The material properties of Super304H at elevated temperatures are obtained from experiments and computations at different scales as follows: anisotropic elastic constants of Super304H at elevated temperatures (500–700°C) are obtained from molecular dynamics simulations; microstructures of virgin and service-exposed Super304H boiler tubes are observed using scanning electron microscopy; and macroscopic stress–strain and creep strain curves at 650°C are measured from tensile and miniature creep tests, respectively. Stress concentration evolution at a triple junction of grains featuring various combinations of crystallographic orientations in polycrystalline Super304H is evaluated using CEV-FEA during creep deformation. The remaining creep lifetime of polycrystalline Super304H austenitic stainless steel boiler tubes is estimated from the creep cavitation model, where the stress concentration at the triple junction is used as a representative stress driving creep cavitation. [ABSTRACT FROM AUTHOR]

A model has been developed to describe the nucleation and growth of creep pores in crystalline material under service conditions. The nucleation model is based on Becker-Döring (BD) nucleation theory using Helmholtz free energy, while for the growth of pores, a vacancy flux model towards nucleated or existing pores is utilized. The whole model is able to describe nucleation and growth rates of pores in the matrix (homogeneous nucleation), at grain boundaries, at triple and at quadruple grain boundary points as well as at particles/inclusions. Nucleation and growth rates of pores in creep process are considered to be a function of external and internal stress due to the residual stresses, working temperature, local microstructure (nucleation and growth of particles), nucleation sites, interfacial energy of grain boundaries and phase boundary energies, diffusion rates in different paths, and pore geometry. Interrupted creep tests are performed for 9Cr-1Mo martensitic (ASME Gr.91) steels under 66 MPa uniaxial creep loading at 650 °C to track the pore evolution after 0, 2189, 4009, 5272 and 8030 h. The model results are then compared to experimental findings in terms of mean pore size, and volume fraction. The model has good prediction and description power of the physical phenomena. [ABSTRACT FROM AUTHOR]

A systematic investigation has been carried out to asses the creep cavitation behaviour in austenitic stainless steel fusion zone by performing detailed microstructural and finite element analysis. Metallographic observations of the failed cross-weld joint specimens reveal that the cavities nucleated in the near-crown region and propagated towards the near-root region. Similar pattern of nucleation and growth of cavities was observed in weld joint tested at a different stress level. The preferential nucleation of creep cavities in the near-crown region of the weld joint can be attributed to both micro and macro inhomogeneities in the weld joint. The micro-inhomogeneity in the fusion zone, which could be characterized using electron backscatter diffraction studies, is a result of variations in morphology and formation of thermo-mechanically treated region across the weld pass interface. The considerable strength variation between the heat affected zone, base metal and fusion zone resulted in macro-inhomogeneity in the weld joint and introduced significant stress gradients which could be illustrated using finite element analysis. Both micro and macro-inhomogeneities have a synergistic role in determining the damage initiation location in multi-pass stainless steel weld joints subjected to creep. [ABSTRACT FROM AUTHOR]

In the present work, creep deformation and fracture behaviours of Sanicro 25 alloy were obtained based on long-term creep-strain tests. The multiscale precipitation behaviours were calculated thermodynamically and inspected by examination of the microstructure of the as-crept alloy. Creep cavitation nucleation near the M 23 C 6 precipitates and an increase in amount of Laves phase with increased creep time were observed. The nanoscale MX-type and Cu-rich phases were found distributed evenly within the as-crept alloy, which leads to the high creep resistance of Sanicro 25 at elevated temperatures. The dislocation substructure evolution was surveyed, and dislocation cells were discovered in abundance of the alloy during creep. The grain orientation transformation of the alloy before and after creep were analysed, and 〈111〉//RD and 〈001〉//RD were detected as the preferred orientations during creep. The number of low-misorientation angles grain boundaries increased in as-crept alloy due to the presence of grain rotation and dislocation cells. [ABSTRACT FROM AUTHOR]

Creep rupture can happen in two ways, brittle and ductile creep rupture. Brittle creep rupture of austenitic stainless steels proceeds with the nucleation, growth and coalescence of grain boundary cavities. A creep cavity nucleation model has been developed previously, which considers cavity nucleation at particles and sub-boundary corners due to grain boundary sliding. A modified constrained cavity growth model has been used to describe the cavity growth behavior with combination of the cavity nucleation models. In this paper, the brittle creep rupture has been analyzed by combining the cavity nucleation and growth models. The physically based models where no adjustable parameters were involved were used to predict the brittle creep rupture strength. On the other hand, previously developed basic models for ductile creep rupture based on exhaustion of the creep ductility have been used. The creep rupture strength of common austenitic stainless steels has been predicted quantitatively by taking both ductile and brittle rupture into account. The predicted rupture times for ductile rupture are longer than those for brittle rupture at high stresses and low temperatures with a reversed situation at low stresses and high temperatures. This reproduces the characteristic change in slope in the creep rupture curves. [ABSTRACT FROM AUTHOR]

Creep in metals and alloys has been observed and studied extensively over the past century. Most studies are based on constant load or less frequently on constant stress conditions. Under certain stress regimes during the period of service, such as creep-fatigue (cyclic), and under displacement-controlled loading the stress can relax. This paper uses novel millimetre length-scale beam bend geometry test specimens with constant displacement to simulate stress relaxation and explore cavity nucleation and early-stage growth/closure in a model polycrystalline material, namely oxygen-free high-conductivity copper. The role of changing grain size over the range 43 μm–2350 μm has been explored. Power-law creep theoretical modelling and finite element analyses have been adopted to predict creep relaxation and explain cavity nucleation and early-stage growth/closure for the test conditions. The results are compared with the experimental observations. The overall experimental and modelling outcomes are considered with respect to the underlying creep damage mechanism. [ABSTRACT FROM AUTHOR]

Diffusion controlled cavity growth models tend to exaggerate the growth rate. For this reason it is essential to take into account the restrictions caused by creep rate of the surrounding material, so called constrained growth. This has the consequence that the stress that the cavities are exposed to is reduced in comparison to the applied creep stress. Previous constrained growth models have been based on linear viscoplasticity. To avoid this limitation a new model for constrained growth has been formulated. Part of the work is based on a FEM study of expanding cavities in a creeping material. Compared with the previous constrained cavity growth models, the modified one gives lower reduced stresses and thereby lower cavity growth rates. By using recently developed cavity nucleation models, the modified creep cavity growth model can predict the cavity growth behaviour quantitatively for different types of austenitic stainless steels, such as 18Cr10Ni, 17Cr12NiNb and 17Cr12NiTi. [ABSTRACT FROM AUTHOR]

The creep crack growth (CCG) phenomena in the heat affected zone (HAZ) of Mod.9Cr–1Mo (P91) steel weldment is a lesser explored field than to that in the weld and the base material. In this study, constant load creep crack growth tests have been carried out at different applied loads at 898 K as per ASTM E1457 on the CT specimens prepared from ‘V-type’ and ‘half-K’ type of weld joints. The creep crack growth rate, as a function of C *, has been found to be higher in the HAZ as compared to the weld metal. Even the crack initiated in the weld has been found to deviate towards HAZ. These have been attributed to the combined effect of high constraint situation and high creep strain in the HAZ, leading to early crack initiation and its faster growth in this region. A detailed SEM based study of the crack initiation and growth mechanism in the HAZ has revealed that grain boundary cavitation, triple point cracking and subsequent sub-surface linking up of the cavities into microcracks have been responsible for extensive creep damage in the HAZ. [ABSTRACT FROM AUTHOR]

Even in properly processed Grade 91 steel, the long term performance and creep rupture strength of base metal is below that predicted from a simple extrapolation of short term data. One of the mechanisms responsible for this reduction in strength is the development of creep voids. Importantly, nucleation, growth and inter linkage of voids under long term creep conditions also results in a significant loss of creep ductility. Thus, elongations to rupture of around 5% in 100,000 h are now considered normal for creep tests on many tempered martensitic steels. Similarly, creep damage development in the heat affected zones of welds results in low ductility cracking at times below the minimum expected life of base metal. In all cases, the relatively brittle behavior is directly a consequence of creep void development. Indeed, the results of component root cause analysis have shown that crack development in Grade 91 steel in-service components is also a result of the formation of creep voids. The present paper examines background on the nucleation and development of creep voids in 9%Cr type martensitic steels, presents information regarding methods which allow proper characterization of the creep voids and discusses factors affecting creep fracture behavior in tempered martensitic steels. It is apparent that t he maximum zone of cavitation observed in Grade 91 steel welds occurred in a region in the heat affected zone which is ∼750 μm in width. This region corresponds to the band where the peak temperature during welding is in the range of ∼1150–920 °C.The cavity density in this band was over about 700 voids/mm 2 at an estimated creep life fraction of ∼99%. [ABSTRACT FROM AUTHOR]

• Industrial K-type thermocouples used in chemical processing plant were failing prematurely. • Visual inspection, fractography and metallography confirmed that the thermoelements had undergone creep cavitation damage. • Fractography and longitudinal micrographs confirmed the formation of r-type cavities predominantly along the transverse grain boundaries of the thermoelements. • First principle calculation estimated the stress endured by the thermoelements and was confirmed to be in the coble creep regime in the deformation mechanism map. • Selection of sheathing material (SS347) having significantly higher thermal expansion coefficient than that of the thermoelements (Chromel/Alumel) is adjudged as the root-cause for the in-service failure of the thermocouples. This work investigates premature failure of K-Type thermocouples that were used in a chemical processing plant. The work presents a detailed metallurgical failure analysis involving, site visit, visual inspection, metallography, optical microscopy, scanning electron microscopy, and fractography. The analysis reveals that the thermocouple wires endured creep cavitation damage during service and eventually failed by creep-rupture. Interestingly, the influence of creep on the degradation of industrial thermocouples is not detected and reported thus far. By presenting analytical stress calculations using temperature dependent materials properties, the current work reveals that the thermal expansion mismatch between various materials of thermocouple causes high tensile stresses in the thermocouple wires during elevated temperature services. The work concludes that high tensile stresses at elevated temperature operation caused premature creep failure of the K-type thermocouples. [ABSTRACT FROM AUTHOR]

Mesothermal chalcopyrite + pyrite + magnetite-bearing calcite-dominated vein deposits in the Repparfjord Tectonic Window, northern Norway, have been studied to constrain the mechanics of their emplacement and the timing of initial vein formation and reactivation. The veins cut across Paleoproterozoic tholeiitic metabasalts and present textural contrast between their hydrofractured, coarse-grained margins, and the fine-grained mylonitic cores. They formed under overall viscous conditions, although cyclically increased fluid pressures caused transient embrittlement. As mineral precipitation sealed the fractures, deformation was accommodated again viscously leading to mylonitization of the vein's core. Local brecciation of the calc-mylonite demonstrates the cyclicity of this process. Re–Os chalcopyrite–pyrite and K–Ar fault gouge dates define an almost continuous age range from ~ 2540 Ma to ~ 460 Ma. Regression of three Re–Os analyses yields a 2069 ± 14 Ma age ( 187 Os/ 188 Os = 0.18 ± 0.04), interpreted as the age of vein emplacement, sulfide precipitation, and initial frictional–viscous deformation. K–Ar ages are mixed ages that constrain a maximum age of faulting in association with the veins at approximately 460 Ma, hence indicating structural reactivation connected with Silurian Caledonian orogenic compression. The spread in Re–Os model ages reflects this reactivation, wherein renewed strain accommodation and circulating oxidizing fluids caused fracturing, dynamic recrystallization, and isotopic disturbance of the sulfides. The study provides evidence for fluid flow during viscous deformation and demonstrates that strain, and flow of oxidizing fluids, can have a significant yet localized control on the integrity of the Re–Os systematic in pyrite and chalcopyrite. [ABSTRACT FROM AUTHOR]

An ex-service Type 316H stainless steel which was subsequently aged at 500 °C for ~22×10 3 h was found to contain approximately 2% mixed ( δ and α ) ferrite distributed in localised regions of the microstructure. Preferred creep cavitation at boundaries was associated with these ferrite regions. Creep cavities associated with the austenite−austenite−ferrite boundary junctions, showed a lenticular morphology while austenite−austenite grain boundary creep cavities had a more spherical morphology. Details of the microstructure of these localised regions are described together with the creep cavity distribution. It has been established the extended heat treatment post-service did not result in any significant sintering of creep induced cavities. The results are discussed with respect to the contribution of the ferrite on the nucleation and growth of the creep cavities and the influence on the overall creep behaviour. An evaluation of the impact of low volume fractions of widely dispersed ferrite on the creep life-time of Type 316H stainless steel is discussed. [ABSTRACT FROM AUTHOR]

A sample of AISI type 316 stainless steel from a power station steam header, showing reheat cracking, was removed from service and has been examined by a combination of microscale X-ray computed tomography (CT), nanoscale serial section focused ion beam-scanning electron microscopy (FIB-SEM), energy dispersive X-ray (EDX) spectrum imaging and transmission electron microscopy (TEM). Multiscale three-dimensional analysis using correlative tomography allowed key regions to be found and analysed with high resolution techniques. The grain boundary analysed was decorated with micrometre sized, facetted cavities, M23C6 carbides, ferrite and G phase but no σ phase. Smaller intragranular M23C6 particles were also observed, close to the grain boundaries. This intimate coexistence suggests that the secondary phases will control the nucleation and growth of the cavities. Current models of cavitation, based on isolated idealised grain boundary cavities, are oversimplified. [ABSTRACT FROM AUTHOR]

The need for a new paradigm to estimate remaining creep life of service exposed steels is critically assessed. New approaches to residual life assessment are proposed, in the light of a decade's experience of the use of micro-tomography to characterise the three-dimensional (3D) nature of cavitation damage in structural materials. Imaging of conventional structural materials such as steels with high absorption to X-rays has been realised by synchrotron micro-tomography (SR-μCT), providing new insights into phenomena such as creep failure. The unique feature of SR-μCT studies is the direct imaging in 3D of cavities (hundreds of micrometres in size) present in the bulk, revealing the spatial characteristics and morphology of the creep voids. Quantitative analyses of the cavitation characteristics revealed by 3D datasets, when scaled with respect to time, stress and temperature, provide functional information suitable for developing constitutive equations for creep. The application of SR-μCT, a non-destructive technique providing high fidelity data, significantly reduces the ambiguity in developing functional relationships to predict creep failure. The explicit use of such constitutive equations to estimate the residual life of components in creep, and the consequent assessment of structural integrity, would prove invaluable. Micro-tomography studies related to creep in materials are reviewed, with special emphasis on a 10·86%Cr heat resistant steel, to demonstrate the type of data available for life assessment and design against creep failure. A brief discussion of current methods to estimate residual life in the light of recent 3D micro-tomography data follows. Finally, the possibility of new approaches, using micro-tomography data in conjunction with destructive 3D approaches such as serial sectioning, to formulate advanced residual life estimates, is briefly considered. [ABSTRACT FROM AUTHOR]

Abstract: Creep rupture behaviour of 316LN austenitic stainless steel weld joint fabricated by multi-pass shielded metal arc welding process has been studied at 923K over a stress range of 120–225MPa. The weld joint exhibited inferior creep rupture strength than the base metal and extensive creep cavitation in weld metal led to the premature failure. Weld metal microstructure was found highly inhomogeneous having different morphologies of delta (δ)-ferrite. The δ-ferrite transformed into intermetallic phases on creep exposure and the creep cavitation was associated with the intermetallic phases. Extensive creep cavitation in weld metal was confined to regions containing δ-ferrite with vermicular morphology. The region near the weld pass interface having globular δ-ferrite was less susceptible to creep cavitation. The globular δ-ferrite region possessed higher hardness than the vermicular δ-ferrite region. High dislocation density was observed in the globular δ-ferrite region which is a consequence of the microstructural modification by the heat input from the successive weld passes. The strength inhomogeneity between the globular and vermicular δ-ferrite regions together with the transformation of δ-ferrite into intermetallic phases produced pronounced creep cavitation in the vermicular δ-ferrite region which led to premature failure of the stainless steel weld joint. [Copyright &y& Elsevier]

Abstract: Creep tests on both plain and notched specimens having ratio of notch throat radius to notch root radius ranging from 1 to 20 keeping the specimen diameter to notch throat diameter ratio fixed at 1.67. The creep rupture life of the material was found to increase in presence of notch at all the applied stresses. The extent of increase in rupture life was more at higher applied stresses. Creep rupture life was found to increase with notch acuity and tend to saturate at larger notch acuity. SEM fractography investigation revealed the presence of both plasticity induced dimpled intragranular ductile failure and creep cavitation induced intergranular brittle failure. The relative proportion of plasticity induced ductile failure to creep cavitation induced brittle failure decreased with increase in notch acuity and decrease in applied stress. Finite element analysis was carried out to study the effect of notch acuity on the stress distribution across the notch during creep exposure and its effect on creep rupture life. The variation of stresses at the skeletal point with notch acuity was used to characterize the state of stress in the notched specimen. The reduction in von-Mises stress with notch acuity at the skeletal point has been considered for the increase in rupture life of the relatively ductile 2.25Cr-1Mo steel. [Copyright &y& Elsevier]

Abstract: Two three-dimensional (3-D) techniques, namely X-ray microtomography and serial sectioning, have been applied for characterization of creep cavitation behavior in tempered martensitic steel. For this purpose samples have been extracted from a series of specimens that were subjected to creep tests over the stress range of 120–180MPa at 600°C. The presence of creep voids in the series of samples was un-ambiguously detected in a non-destructive manner using synchrotron X-ray microtomography with a resolution of 1μm. The 3-D visualization of the datasets provided an assessment of the spatial distribution and morphology of the creep voids as a function of creep stress and high temperature exposure time. The quantitative analyses of the image datasets enabled the development of functional relationships between the macroscopic creep parameters (such as rupture ductility, applied stress, creep life) and cavitation characteristics (such as volume fraction, and number density). The quantitative analyses also provided an evaluation of manifestations of growth and coalescence processes in the respective datasets. A transition of cavitation behavior of the steel has been found to occur in the stress range of 120–150MPa at 600°C. The evolution in the pattern of cavitation and its relation to the prior-austenite boundary was explored by combining micro-tomography and serial sectioning techniques, which revealed a new possibility in the progress of cavitation in the long term creep exposed specimens of 9–12% Cr heat resistant steels. [Copyright &y& Elsevier]

The maximum principal stresses, von Mises effective stresses and principal facet stresses at the time of creep rupture were compared in uniaxial, biaxial, and triaxial stress states for AZ31 magnesium alloy. The creep rupture of this alloy was experimentally controlled by cavitation, which was the result of a low damage tolerance, λ. Creep deformation could be correlated with the von Mises effective stress parameter. The failure-mechanism control parameter governing the stress state coincided with the experimental results of the rupture of the materials under multiaxial stress states. Finally, the theoretical prediction based on constrained cavity growth and continuous nucleation agreed with the experimental rupture data to within a factor of three. [ABSTRACT FROM AUTHOR]

The number of fossil fueled power plants in electricity generation is still rising, making improvements to their efficiency essential. The development of new materials to withstand the higher service temperatures and pressures of newer, more efficient power plants is greatly aided by physics-based models, which can simulate the microstructural processes leading to their eventual failure. In this work, such a model is developed from classical nucleation theory and diffusion driven growth from vacancy condensation. This model predicts the shape and distribution of cavities which nucleate almost exclusively at grain boundaries during high temperature creep. Cavity radii, number density and phase fraction are validated quantitively against specimens of nickel-based alloys (617 and 625) tested at 700 °C and stresses between 160 and 185 MPa. The model's results agree well with the experimental results. However, they fail to represent the complex interlinking of cavities which occurs in tertiary creep. [ABSTRACT FROM AUTHOR]

The present paper addresses the effects of creep cavitation damage on other mechanical properties – chiefly those that affect behaviour outside the creep range. Such effects seem not to have been systematically studied, yet they are significant for the understanding and prediction of component integrity and life. The paper presents results obtained mainly as byproducts of research programmes on low alloy steels for both fossil and nuclear power plant and seeks to rationalise the findings to generate an overall picture of the effect. It is seen that a simple loss of effective section model is adequate to describe many of the phenomena observed, but that other factors may also need consideration. [ABSTRACT FROM AUTHOR]

Nickel based superalloys are critical to the safe operation of many energy conversion systems operating at high temperatures. Time dependent intergranular cracking of these alloys, under both sustained and cyclic loads, is dominated by environmental interactions at the crack tip. This review is concerned mainly with the interaction of oxygen in alloys used for combustion turbine discs, although interactions with other more aggressive species are considered. The phenomenology of this cracking is shown to be consistent with the same mechanism as that associated with oxygen embrittlement resulting from pre-exposure of uncracked material, and also with environmentally induced reduction in creep rupture life. Gas phase embrittlement (GPE), resulting from intergranular oxygen penetration, is shown to be responsible for all four streams of experimental observations. Three distinct processes of intergranular embrittlement involving oxidation reactions have been confirmed experimentally. One of these, the oxidation of intergranular sulphides, results in elemental sulphur embrittlement and subsequent local decohesion under stress. The other two, oxidation of carbon or carbides to form carbon dioxide gas bubbles and oxidation of strong oxide formers to form intergranular internal oxides, result in a reduction of the local ability to accommodate stress concentrations associated with sliding grain boundaries in an intermediate temperature range. This in turn leads to a temperature dependent minimum in ductility and maximum in crack propagation rate. Attempts to reduce the sensitivity to time dependent cracking based on chemistry (chromium level or trace element addition), microstructure control (using thermal–mechanical treatment or controlled cooling), or reversal of environmental embrittlement, are all considered. The conclusions form a basis for the development of life prediction methods for energy materials operating in diverse environments. [ABSTRACT FROM AUTHOR]

High-temperature rupture behavior of 5083-A1 alloy was tested for failure at 548K under multiaxial stress conditions : uniaxial tension using smooth bar specimens, biaxial shearing using double shear bar specimens, and triaxial tension using notched bar specimens. Rupture times were compared for uniaxial, biaxial, and triaxial stress conditions with respect to the maximum principal stress, the von Mises effective stress, and the principal facet stress. The results indicate that the von Mises effective and principal facet stresses give good correlation for the material investigated, and these parameters can predict creep life data under the multiaxial stress states with the rupture data obtained from specimens under the uniaxial stress. The results suggest that the creep rupture of this alloy under the testing condition is controlled by cavitation coupled with highly localized deformation process, such as grain boundary sliding. It is also conceivable that strain softening controls the highly localized deformation modes which result in cavitation damage in controlling rupture time of this alloy. [ABSTRACT FROM AUTHOR]

A specially designed compact tension (CT) specimen has been used to simulate reheat cracking observed in plant components operating above 500°C. The specimens were deformed in compression beyond yield and unloaded to produce high tensile residual stresses at the notch root. Test specimens were machined from an ex-service AISI Type 316H austenitic stainless steel steam header. After residual stressing, one specimen was crept for 4500h at 550°C. Residual strains along the ligament ofthe specimens were measured using synchrotron and neutron diffraction techniques before and after creep. A 3D finite element (FE) model was developed and used to predict the residual stress and strain fields in the two conditions. A good correlation has been found between measurements and FE predictions before and after creep. Application of a creep ductility-exhaustion model to the specimen predicts the onset of simulated reheat crack initiation during the test. The validity of this model will be assessed on destructive examination of the specimen. [ABSTRACT FROM AUTHOR]

A cavity nucleation model under creep-fatigue interaction, abbreviated as CH model, has been established. The key aspect of the CH model is accounting for the creep deformation through a self-consistent scheme. This allows the combined effect of grain boundary sliding (GBS) and crystal deformation on cavity nucleation to be assessed in Type 316 stainless steel over the temperature range of 500–600 °C. The CH model predicts that the local normal stress asymptotically approaches a non-zero saturation value over a long period of stress hold. This aligns with the continuous nucleation process under creep. The CH model has been used to calculate the cavity nucleation rates under various load-waveform scenarios, which include the stress and time applied during the transient and steady loadings. As a result, the load waveform characteristics that can maximise the cavity nucleation are identified. First, the pre-compressive hold time needs to be long, whereas the load reversal time needs to be short. Second, the tensile hold time can be shortened to some extent if time efficiency is the primary concern. Third, all the above-mentioned time-related parameters have their optimum values depending on the temperature and creep rate. Fourth, the unbalanced stress hold in favour of the tension enhances the nucleation rate, and the stress range is the controlling factor. In summary, the CH model provides an important guide to the design of creep-fatigue testing programme with the desire to promote creep cavitation damage. This mechanistic-based model provides underlying interpretations to the complexity of creep-fatigue interaction. [Display omitted] • A self-consistent scheme to model cavity nucleation under creep-fatigue. • Model the cavity nucleation rate under various creep-fatigue load waveforms. • Predict the deformation compatibility over the transient and steady loadings. • Establish the relationship between local normal stress and cavity nucleation rate. • Unravel the optimum time and stress condition to promote the cavity nucleation. [ABSTRACT FROM AUTHOR]

The microstructure in the heat affected zone of the multi-pass welds constructed using the 9% Cr tempered martensitic steels is complex and susceptible to premature creep failure. In the present research, a systematic investigation has been conducted after long-term creep exposure in the heat affected zone of a multi-pass weld on the 9% Cr Grade 92 steel to identify the sub-optimal microstructures with a high susceptibility to creep cavitation. The characterisation techniques employed include hardness mapping and a range of electron-based microscopy methods. The results provide quantitative data for microstructure and creep cavities. In this case, preferential creep cavitation has been confirmed in the regions within the heat affected zone that possesses a microstructure with an inhomogeneous distribution of the M 23 C 6 carbides and a refined martensitic grain structure. Creep cavities have been observed on the secondary phase particles including the Al 2 O 3 , the MnS and the BN phases. The observations from the current research confirm that creep cavitation preferentially occurs in the partially-transformed zones that were subjected to thermal cycles associated with peak temperatures between the Ac 1 and the Ac 3 transus temperatures during weld fabrication. Unlabelled Image • Creep damage in the heat affected zone of a multi-pass Grade 92 steel weld was evaluated after long-term creep exposure. • Creep damage was compared with the variation of microstructure in the heat affected zone. • Cavitation preferentially occurs in a refined microstructure with an inhomogeneous distribution of the M 23 C 6 carbides. • Creep cavities are preferentially formed on the Al 2 O 3 , MnS and BN particles. • The partially transformed zones are the most susceptible to creep cavitation. [ABSTRACT FROM AUTHOR]