Miniature Creep Research Articles

Creep damage evaluation via uniaxial miniature testing for multiaxially stressed Mod.9Cr–1Mo steel components

In this study, creep damage in a biaxially creep-damaged Mod.9Cr–1Mo component has been examined via uniaxial miniature creep testing. Using cruciform specimens, biaxial tension creep tests were conducted at principal stress ratios (λ) of λ=0.0 and 1.0 at 923 K. The principal stress ratio is the ratio of the y-directional principal stress (σy) to the x-directional principal stress (σx) in a cruciform specimen (λ=σy/σx). The equi-biaxial creep test was interrupted at a half lifetime, and two miniature specimens were machined from the damaged cruciform specimen in the x- and y-direction. The applicability of the linear cumulative creep damage rule was assessed using the results of the miniature lifetimes. The linear cumulative creep damage rule gave an unconservative estimate to the remaining life of a biaxially damaged cruciform specimen. Using the hyperbolic constitutive relationship, the unconservative estimate according to the damage accumulation was explained.

The Effect of Specimen Size and Test Procedure on the Creep Behavior of ME21 Magnesium Alloy

Lightweight constructions and materials offer the opportunity to reduce CO2 emissions in the transport sector. As components in vehicles are often exposed to higher temperatures above 40% of the melting temperature, there is a risk of creep. The creep behavior usually is investigated based on standard procedures. However, lightweight constructions frequently have dimensions not adequately represented by standardized specimen geometries. Therefore, comparative creep experiments on non-standardized miniature and standardized specimens are performed. Due to a modified test procedure specified by a miniature creep device, only the very first primary creep stage shows a minor influence, but subsequently, no effect on the creep process is detected. The creep behavior of hot extruded and heat treated ME21 magnesium alloy is investigated. It is observed that the creep parameters determined by the miniature and standard creep tests are different. As the deviations are systematic, qualitatively, evidence of the creep behavior is achieved. The creep parameters obtained, and particularly the creep strain and the strain rate, show a higher creep resistance of the miniature specimen. An initial higher number of twinned grains and possible multiaxiality in the gauge volume of the miniature specimen can be responsible.

Creep deformation and rupture behaviour of service-exposed Super304H steel boiler tubes

ABSTRACT 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.

On the influence of crystallography on creep of circular notched single crystal superalloy specimens

The present work contributes to a better understanding of the effect of stress multiaxiality on the creep behavior of single crystal Ni-base superalloys. For this purpose we studied the creep deformation and rupture behavior of double notched miniature creep tensile specimens loaded in three crystallographic directions [100], [110] and [111] (creep conditions: 950 °C and 400 MPa net section stress). Crystal plasticity finite element method (CPFEM) was used to analyze the creep stress and strain distributions during creep. Double notched specimens have the advantage that when one notch fails, the other is still intact and allows to study a material state which is close to rupture. No notch root cracking was observed, while microstructural damage (pores and micro cracks) were frequently observed in the center of the notch root region. This is in agreement with the FEM results (high axial stress and high hydrostatic stress in the center of the notched specimen). Twinning was observed in the notch regions of [110] and [111] specimens, and <112> {111} twins were detected and analyzed using orientation imaging scanning electron microscopy. The present work shows that high lattice rotations can be detected in SXs after creep fracture, but they are associated with the high strains accumulated in the final rupture event.

Creep lifetime prediction of virgin and service-exposed Super304H austenitic stainless steel boiler tubes based on hierarchical multiscale analysis and creep cavitation model

ABSTRACTCreep 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.

ICME Framework for Damage Assessment and Remaining Creep Life Prediction of In-Service Turbine Blades Manufactured with Ni-Based Superalloys

Accurate creep life prediction is necessary for the evaluation of the remaining creep lives of in-service turbine blades and for the design of new turbine blades in aircraft engines. In this study, an integrated computational material engineering methodology for predicting the remaining creep life of in-service turbine blades was developed by taking a microstructural criterion and creep strain criterion into consideration, and combining artificial neural networks with a modified θ projection model to assess the service temperature, stress, degradation time, and existing creep strain. To explore the application of the method and verify its accuracy, the microstructural degradations at different locations of two directionally solidified superalloy DZ125 turbine blades, which were in-service for 300 h and 980 h in different engines, were characterized and quantified. Using these results, the remaining creep life of the microstructures at different locations of the blade was predicted. Finally, these creep life prediction results were experimentally verified using miniature creep test specimens. The development of this new method provides a reference for the design and service evaluation of turbine blades made of directional solidified and single-crystal Ni-based superalloys.

Creep properties of single crystal Ni-base superalloys (SX): A comparison between conventionally cast and additive manufactured CMSX-4 materials

The present work compares the microstructures and the creep properties of two types of single crystal Ni-base superalloy CMSX-4 materials (SXs). One was produced by conventional directional solidification Bridgman processing. The other was manufactured by selective electron beam melting (SEBM). The microstructures of the two types of materials are compared with emphasis placed on the large (dendritic/interdendritic regions) and small scale (γ-matrix/γ′-precipitates) microstructural heterogeneities, which characterize SX microstructures and their evolution during processing, heat treatment and creep. It is shown that heat treated SEBM materials have creep properties, which match or even outperform those of conventionally processed SX materials. Creep properties were assessed using a miniature creep test technique where [001] miniature tensile creep specimens were tested in the high temperature/low stress (1050 °C, 160 MPa) and in the low temperature/high stress (850 °C, 600 MPa) creep regimes. The creep behavior is interpreted based on microstructural results, which were obtained using analytical scanning and transmission electron microscopy (SEM and TEM).

On the rejuvenation of crept Ni-Base single crystal superalloys (SX) by hot isostatic pressing (HIP)

In the present work, we study the effect of HIP rejuvenation treatments on the creep behavior and residual life of a pre-crept single crystal Ni-base superalloy of type CMSX-4. The present work combines miniature creep experiments of precisely oriented [001] tensile creep specimens with HIP treatments and quantitative analysis of scanning and transmission electron micrographs. A HIP-rejuvenation treatment after 5.0% creep pre-strain significantly improves creep strength. However, the microstructural damage induced by the creep pre-deformation could not be fully removed. In a series of sequential creep/HIP/creep-experiments, increasing levels of pre-deformation result in increasing levels of creep rates even after identical HIP-rejuvenation treatments. The memory effect, which causes this phenomenon, is related to an accumulation of permanent microstructural damage, which is not associated with rafting or cavitation. The mechanical results obtained in the present work are interpreted based on microstructural results on the γ/γʼ-microstructure (γ-channel widths and γʼ-size), on the pore population (number density of pores, pore size distributions and pore area fractions) and dislocation substructures which have formed during creep. The results are discussed in the light of previous results reported in the literature.

Creep Damage Evaluation using Uniaxial Miniature Specimens for Multiaxially Damaged Components

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

Miniature creep tests using plate specimens bonded by electron beam welding

Many aged thermal power plants restarted to supply sufficient electricity for Japanese industries and daily activities after the seismic earthquake. Reasonable guidelines for assessment of their integrity and estimation of remaining life have been required to operate them with a safety margin. Miniature Creep (MC) test is recognized as a semi-destructive technique to examine the degradation level of aged high temperature components. The MC thin plate specimen of 2.25Cr-1Mo steel and the similar one welded by Electron Beam (EB) for gauge length were employed to evaluate the validity of the EB welded MC thin plate specimen. A series of creep tests using the MC thin plate specimens were conducted in vacuum at 600°C. Then, following conclusions were obtained: (1) Creep rupture lives of the MC thin plate specimens welded by the EB were nearly the same as those of standard ones. (2) Vickers’ hardness in the welded zone decreased in a short time after the test started and reached almost the same as or below that in the base metal. (3) Rupture lives of the EB welded MC thin plate specimen at higher stress levels were longer than those of the standard MC thin plate specimen.

Correlation and capability of using site inspection data and small specimen creep testing for a service-exposed CrMoV pipe section

ABSTRACTThis paper presents for the first time an investigation on the creep damage evolution of an ex-service CrMoV pipe section through impression creep test (ICT) and metallurgical inspection data. The study emphasises the importance of correlating the operating conditions (temperature and stress) of power plant components with the results from metallurgical examinations and small specimen creep tests. The paper seeks for a correlation among micro- and macro-hardness measurements, surface replicas data and minimum creep strain rates (obtained by ICTs) of the parent material of the pipe section. Also, optical and scanning electron microscope (SEM) micrographs have been used to assess possible metallurgical differences through the thickness of the pipe section. This investigation shows how miniature creep test specimen data could be practically used in a holistic approach for the evaluation of life consumption of power plant components and concludes that the studied parent material could have been retired from service too early.

Creep deformation and rupture behaviour of service exposed P91 weld and base steel measured by miniature tensile creep testing

Early cracking had occurred at a weld joint of a service-exposed P91 steel hot reheat pipe branch. In this study, a series of miniature tensile creep tests were conducted for both the base and weld materials extracted from the retired P91 pipe weld. Tests were conducted at various stress levels at 569 °C to measure the creep properties and investigate the failure mechanisms. Microstructural analyses were also carried out for post-test specimens. The measured creep strengths were represented in terms of the Larson-Miller parameter, and compared to literature data. Most of the data fell within the 95% error band of grade 91 steel. However, the weld metal exhibited higher creep rupture strengths. It showed creep brittle fracture, and base metal showed creep ductile behaviour. The weld metal sometimes had non-uniform microstructures, yielding a seriously tempered zones with minor tempered martensites, thereby causing early creep ductile failure of the weld.

Determination of Creep Damage Properties from a Miniature Three Point Bending Specimen Using an Inverse Approach

In this work, a novel method for determining the creep damage properties which can be used to represent the full life until failure from a miniature bending creep test specimen is developed based on a mathematical analysis and an inverse approach. Using the Kachanov-Rabotnov creep damage model, a mathematical expression for the deflection of a simply supported, rectangular miniature thin beam creep test specimen, under three-point bending (TPB), is derived. The outputs of these equations are iterated numerically using a MATLAB program. The time-dependent deflection curves are computed as the virtue TPB tests at various loads. The accuracy of the mathematical solutions is evaluated by the corresponding results obtained from finite element analysis. On this basis, an inverse method is then developed to obtain the creep and damage constants using a MATLAB optimisation scheme, where the primary creep is neglected. The results obtained for a power plant Grade 91 Cr steel is used for demonstration. The inverse approach developed has potential applications for assessing the high temperature material strength as part of a NDT procedure and for deriving the full life creep damage constitutive properties from a small volume of material, in particular, for various microstructure regions within a heat-affected zone of weldments, e.g. of power plant pipelines and aero-engine components.

MC試験による6万時間および23万時間経年ボイラー水冷管材の損傷評価

MC試験による6万時間および23万時間経年ボイラー水冷管材の損傷評価

直径1mm丸棒引張型ミニチュア試験片を用いたCrMoV鍛鋼のクリープ損傷評価

Creep damage assessment of high temperature components in aged thermal power plants is necessary to maintain reliable operation. Although an accurate creep damage assessment result is obtained by a destructive assessment method in which creep tests are performed, it is difficult to cut a standard size specimen from actual components. Therefore, development of testing and damage assessment methods by using a miniature specimen are expected. In this study, creep tests of a CrMoV forging steel were conducted by using a solid bar tensile type miniature creep specimen with 1mm diameter at 600℃. It was demonstrated that equivalent creep deformation and rupture properties to those obtained from the standard size specimen are obtained by the miniature specimen under stress level below 200MPa. Creep damage materials were produced by interrupting creep tests performed by using a standard sized smooth bar specimen and two types of round notch bar specimen with radiuses at notch root of 0.5mm(R0.5) and 2.0mm(R2.0). Uniaxial tensile creep tests were performed by using the miniature creep specimens taken from the plain specimens and the notch roots of the notch specimens. Although accurate creep damage assessment results were obtained from the creep tests with the miniature creep specimens taken from the plain specimens, it should be noted that the results may give under estimation for higher creep damaged condition. Creep damage assessment results based on the creep tests with miniature specimens also gave under estimation for the creep damaged material of R0.5 due to lower axial creep strain accumulation under multiaxial stress states before interrupting the creep tests. On the other hand, accurate creep damage assessment results were obtained for the creep damage materials of R2.0 indicating that creep tests with the miniature specimens may be applicable to creep damage assessment of components under certain range of stress multiaxiality.

Double minimum creep of single crystal Ni-base superalloys

Low temperature (750 °C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the 〈001〉 direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work.

GS0410 JIS規格外の微小試験片によるクリープ破断特性

Some destructive techniques are required to evaluate the degradation of components for thermal power plants. However, conventional uniaxial creep specimens were sometimes too large to collect a sample from local area of target components. Miniature Creep (MC) test that employs smaller size of the specimen can be collected from the local site is well known as a better semi-destructive testing technique. Although the MC specimen of round bar needed a sample of 4 mm in thickness, that of thin plate shape can be collected under 2 mm in thickness. In this study, MC tests using thin plate and round bar specimens were performed in Argon gas and in vacuum to evaluate their creep properties. Test results were compared with uniaxial ones employing 2.25Cr-1Mo steel. It was shown that creep rupture lives of the MC plate specimens in vacuum were a similar trend to those of uniaxial creep, and that the MC test using the thin plate is able to substitute for the conventional uniaxial creep test.

9CrMAG鋼溶接継手のクリープ強度評価

Mod.9Cr steels are widely used as boiler materials in ultra-super critical power plants. In this study, creep tests were conducted using miniature specimens with 1mm diameter machined from a base metal, weld metal and a heat affected zone (HAZ) in a Mod.9Cr-1Mo welded joint. Creep deformation and rupture behaviors of the miniature base metal specimens were almost identical with those obtained from standard size specimens. Material constants of the Norton law were determined from obtained datum by the miniature creep specimens. Creep rupture times of the miniature welded joint specimens were much lower than those of the standard size specimens. The finite element creep analysis results indicated that the triaxiality factor in the HAZ of the miniature welded joint is smaller than that of the standard size welded joint resulting in faster creep strain rate in the miniature specimen under the same applied stress.

ミニチュアクリープ試験によるボイラ水冷管の健全性評価

ミニチュアクリープ試験によるボイラ水冷管の健全性評価

10805 微小丸棒および微小平板試験片によるクリープ破断特性

Some destructive techniques are required to evaluate the degradation of components under elevated temperatures. However, conventional uniaxial creep specimens were sometimes too large to collect them from local area of target components. Miniature Creep (MC) test has been recognized as a semi-destructive technique to examine the damage condition of components. They employ small size specimens that can reduce the conventional uniaxial creep specimen in similarity. In this study, MC tests using round bar and thin plate specimens were performed in Argon gas to measure creep properties, and compared with uniaxial creep test results employing 2.25Cr-1Mo steel. It was shown that MC rupture lives were a similar trend those of uniaxial creep, and the MC test is able to substitute for conventional uniaxial creep test.