Term Creep Research Articles

Creep deformation in a modified 9Cr-1Mo steel θ projection approach to prediction of creep properties

The θ projection concept has been developed by Evans and Wilshire in an attempt to use short term creep test data to predict long term behaviour. The analysis has been discussed as a three-stage procedure: (i) least squares curve fitting of the model equation to individual creep curves; (ii) mathematical description of the variations of the parameters in the model equation with test conditions; (iii) prediction of creep curves using the equations developed in stages (i) and (ii). When stages (i) and (ii) have been completed it should be possible to determine creep curves for any combination of test conditions if a suitable model equation is chosen and variation of the parameters in the model equation is sufficiently well characterised. Evans and Wilshire have used this methodology on a number of alloys and ceramics. In the present work the θ projection concept has been applied to a modified 9Cr-1Mo steel, whose creep properties have been presented elsewhere. It is found that the general form and shape of the creep curves can be represented by the normal θ projection equation, i.e. ε=θ1(1-exp(-θ2t)) + θ3(exp(θ4t)-1), but that the temperature and stress dependencies of many of the parameters in the equation, and the strain to failure, are difficult to describe accurately, using equations similar to those proposed by Evans and Wilshire. Hence, while the prediction of short term creep properties was good, long term predictions were less impressive.

Creep behavior of bone cement: a method for time extrapolation using time-temperature equivalence.

The clinical lifetime of poly(methyl methacrylate) (PMMA) bone cement is considerably longer than the time over which it is convenient to perform creep testing. Consequently, it is desirable to be able to predict the long term creep behavior of bone cement from the results of short term testing. A simple method is described for prediction of long term creep using the principle of time-temperature equivalence in polymers. The use of the method is illustrated using a commercial acrylic bone cement. A creep strain of approximately 0.6% is predicted after 400 days under a constant flexural stress of 2 MPa. The temperature range and stress levels over which it is appropriate to perform testing are described. Finally, the effects of physical aging on the accuracy of the method are discussed and creep data from aged cement are reported.

Deformation mechanisms in TiAl intermetallics—experiments and modeling

The mechanical properties of intermetallic γ-TiAl based materials depend strongly on the microstructure, which in turn is influenced by the alloy chemistry and the applied heat treatment. First, a computational study of the room temperature deformation behavior of γ-TiAl based two-phase alloys exhibiting a globular near-γ microstructure is presented. The micromechanical model is based on the unit-cell technique using the finite element method. In the applied crystal plasticity concept crystallographic slip and deformation twinning are taken into account as the dominant deformation mechanisms. The conclusions drawn from the simulations are discussed and compared to experimental results obtained from acoustic emission measurements and transmission electron microscopy investigations. Furthermore, the creep behavior of a designed fully lamellar (DFL) γ-TiAl microstructure is investigated. Differently spaced DFL microstructures were adjusted in order to investigate their influence on creep. The interface spacing was varied in the range of 1.2–0.14 μm by altering the cooling rates from 1 to 200 K/min, and short term creep tests were carried out in air under various temperature/load conditions. A first approach in modeling the steady state creep deformation of the fully lamellar material in question is presented. A power law description for diffusion controlled dislocation creep is proposed, and a structure factor is introduced which depends on the lamellar orientation with respect to the loading axis as well as on the mean lamellar interface spacing.

高Crフェライト系耐熱鋼における長時間クリープ強度に注目したCr含有量の最適化

高Crフェライト系耐熱鋼における長時間クリープ強度に注目したCr含有量の最適化

The effect of W and N addition on the mechanical properties of 10Cr steels

The effect of W and N on the creep properties and microstructural degradation in 10Cr steels was studied. Creep testing was performed to determine the creep rupture strength and minimum creep rate. Transmission electron microscopy was used to observe the microstructural degradation during the creep deformation. W and N which were added to the 10Cr steel increased the creep rupture strength and decreased the minimum creep rate. As W and N were added, the thermal stability of the subgrain and carbide was improved, thus the growth of the subgrain and carbide during creep deformation was restricted. In W added steel, the Laves phase played an important role in increasing creep rupture strength. But the impact toughness was rapidly degraded by the addition of W after aging at 600°C for 5000 hours. So one must evaluate more accurately the effect of the Laves phase on long term creep and impact properties. In N added steel, V(C, N) was precipitated in the lath boundary and within the lath. The size of the precipitates was 20–50 nm. The increase of creep rupture strength in N added steel may be due to the precipitate of the V(C, N). Future tests are required to clarify the effect of N on creep and impact properties.

Creep damage quantification of 2.25Cr–1Mo steel using scanning electron microscopy

The material 2.25Cr–1Mo alloy steel has been used extensively for high temperature applications in power generation plant for over five decades owing to its long term creep resistance. It has been recognised that the lifetime of a high temperature component containing pre-existing defects is dependent not only upon the material's crack propagation resistance but also upon an incubation period before crack growth during which a damage zone ahead of the defect tip develops. The extent of the damage occurring during this incubation period, before crack propagation, is dependent upon the stress intensity at the defect tip, the ductility of the material, and the microstructure in the damage zone. The present paper details a technique for quantifying the early stages of creep damage using image analysis in the scanning electron microscope, and compares the degree, distribution, and orientation of creep damage occurring in two microstructural variables of 2.25Cr–1Mo alloy steel. The paper describes the procedures necessary for generating consistent and reproducible quantitative analysis results, including specimen preparation, defect detection, and measurement criteria. The image analysis process, its accuracy, and application to the study of creep damage mechanisms occurring ahead of defects are discussed.

Quantification of precipitates in a 10%Cr steel using TEM and EFTEM

Modern (9–12)%Cr steels designated for power plants with higher steam parameters show a pronounced time dependent change in microstructure during purely thermal or creep stress exposure at temperatures around 600°C that determines their properties in service. In addition to other microstructural parameters, the state of the precipitates plays an important role for microstructural stability which is a prerequisite for long term creep strength. In order to support theoretical studies on precipitation growth and coarsening with more reliable experimental data, in this study a method is introduced for the quantification of the state of precipitates in (9–12)%Cr steels which is based on the application of different TEM methods. Therefore up to about 33 000 h aged specimens of the martensitic cast alloy G-X12CrMoWVNbN-10-1-1 were investigated by means of electron microscopy. The application of energy filtering transmission electron microscopy (EFTEM) allowed a reliable quantitative distinction between M23C6, VN, and Laves phases to establish the size distribution of these precipitates in different specimens conditions.

Effects of high temperature mechanical damage on physical properties of unidirectional Ti/SiC metal matrix composite

A miniaturised test system was used to investigate how the thermal and electrical properties of a unidirectionally reinforced titanium alloy (Ti–6Al–4V)/SiC (SM1140+)metal matrix composite change with mechanical damage at elevated temperature. Thermal conductivity and expansion measurements were obtained in the longitudinal and transverse direction both before and after short term strength and creep tests and at intervals during tests to assess changes in interface characteristics as functions of mechanical or thermal damage. The mechanical tests included monotonic stress–strain and ramp creep at temperatures between 500 and 650°C. The changes in thermal properties were compared with model predictions for the dependence of thermal properties on interface characteristics. The agreement was good for thermal expansion changes but not for thermal conductivity. This was ascribed to the nature of the damage at the interface that probably still allowed thermal transport but not mechanical load transfer.

Behaviour of heat resistant power plant steels undergoing variable long term loading conditions

High temperature components of power plants are in many cases subjected to variable loading conditions such as cyclic creep or strain cycling. Therefore the cyclic behaviour of important heat resistant steels has been investigated under stress control and to a smaller extent under strain control. The behaviour under cyclic creep rupture conditions was determined in long term experiments covering a wide range of loading conditions. This behaviour can be described with the modified life fraction rule and a factor concept of relative life for single stage cycles and additionally with a multistep hypothesis for multi-stage cycles. The description can be adapted to estimate the life consumption under service type random conditions. The strain cycling behaviour was investigated in regard to the conditions of the heated surface of large components. To simulate this type of loading and to generate long term creep fatigue data, a service type strain cycle was developed. Anisothermal and comparable isothermal cycles were carried out up to 8000 h and delivered similar results. Long term isothermal tests on a bainitic and a martensitic turbine rotor steel could economically be performed up to 70 000 h by means of package type tests combining short term strain cycling and long term creep cycling. Creep fatigue life under single stage and multi-stage conditions can be predicted on the basis of the generalized damage accumulation rule if an internal stress concept and preloading effects are considered. In the long term region governed by viscoelastic deformation, the creep fatigue behaviour can be approximated by the modified life fraction rule concept developed for cyclic creep. Cycle counter programs support the application of the life assessment concepts presented.

改良9Cr-1Mo鋼の長時間クリープ変形に伴う不均一回復とZ相の析出

改良9Cr-1Mo鋼の長時間クリープ変形に伴う不均一回復とZ相の析出

低合金フェライト鋼の長時間クリープ強度に及ぼす初期組織の影響

低合金フェライト鋼の長時間クリープ強度に及ぼす初期組織の影響

Designing with glass fiber reinforced vinyl composites. II: Long term creep considerations

Seven years ago we initiated a long-term study on the effect of time and temperature on the data sheet modulus values for glass fiber reinforced vinyl composites. Results after the first year showed that the measured creep modulii were higher than those predicted by time-temperature superposition of dynamic mechanical data. This paper presents five years of additional creep data and examines the effect of sample aging on the creep predictions.

Long Term Creep Behavior of Spent Fuel Cladding for Storage and Disposal

Long Term Creep Behavior of Spent Fuel Cladding for Storage and Disposal

Heterogeneous Changes in Microstructure and Degradation Behaviour of 9Cr-1Mo-V-Nb Steel During Long Term Creep

Heterogeneous Changes in Microstructure and Degradation Behaviour of 9Cr-1Mo-V-Nb Steel During Long Term Creep

Lifetime Prediction and Confirmation of Long Term Creep Resistance in Advanced Silicon Nitride Ceramics

Lifetime Prediction and Confirmation of Long Term Creep Resistance in Advanced Silicon Nitride Ceramics

Effect of Microstructure on Failure Behavior of Light Water Reactor Coolant Piping under Severe Accident Conditions

In a severe accident of light water reactors, the reactor coolant system (RCS) piping might be subjected to thermal loads caused by the decay heat of the deposited fission products and the heat transfer from the hot gases, with an internal pressure in some accident sequences. Tests on the RCS piping failure were performed along with high temperature tensile and creep rupture tests including metallography to investigate the failure behavior. The prediction of the 0.2% proof stress by Arrhenius equation is in good agreement with the measured stress above 800°C for served RCS piping materials. The modified Norton's Law for the short term creep rupture model agrees with the experimental values between 800 and 1,150°C for type 316 stainless steel. The microstructural change was discussed with the effect of the very rapid formation and resolution of the precipitation on the strength at high temperature. The result of the piping failure tests which simulated the severe accident conditions, i.e., in short-term at high-temperature, could support the plastic limit load prediction of the flow stress model using the 0.2% proof stress.

Stress relaxation in broken fibers in unidirectional composites: modeling and application to creep rupture analysis

This paper describes a model of stress relaxation in broken fibers in unidirectional metal matrix composites reinforced with long brittle fibers. A cylindrical cell with a broken fiber embedded in a power law creeping matrix is employed, and the broken fiber is assumed to have a bilinear distribution of axial stress. Then, on the basis of energy balance in the cell under constant overall strain, a relaxation equation of interfacial shear stress acting on stress recovery segments is derived in a simple form. The relaxation equation is approximated rationally and integrated to obtain an analytical solution, which is shown to agree fairly well with the numerical analysis of Du and McMeeking. (Du, Z.-Z., McMeeking, R.M., 1995. Creep models for metal matrix composites with long brittle fibres. J. Mech. Phys. Solids 43, 701–726.) Moreover, the relaxation equation is combined with Curtin's model (Curtin, W.A., 1991. Theory of mechanical properties of ceramic-matrix composites. J. Am. Ceram. Soc. 74, 2837–2845.), so that rupture time in long term creep is evaluated analytically and explicitly on the assumption of global load sharing. It is shown that the resulting relation represents well the dependence of creep rupture time on applied stress observed experimentally on a unidirectional metal matrix composite.

Prediction of atomic configurations in alloys

Improvements in the description of configurational thermodynamics have enabled appropriate evaluations of atomic configurations in the ordered and disordered phases. From the cluster variation method (CVM) calculations, the substitution behaviors of the third element in Ti 3Al and TiAl intermetallic compounds are in good agreement with those evaluated by ALCHEMI and X-ray diffraction. The equilibrium concentrations of solute elements and atomic pairs in the ferrite matrix were estimated by thermodynamic calculations with the sublattice model (Thermo-calc.) and CAM, respectively. A good correlation is observed between the long term creep strength of carbon steels at 773 K and 88 MPa and the concentrations of Mn–C and Mo–C atomic pairs, suggesting that the strength is controlled by these atomic pairs. In the case of Cr–Mo steels, however, the strengthening effect of M–C atomic pair is not observed at 823 K and 88 MPa, and the contribution of solute elements such as Cr, Mn and Mo in the ferrite matrix seems to be dominant.

Further analysis of the Monkman–Grant relationship for 2·25Cr–1Mo steel using creep data from the National Research Institute for Metals

The National Research Institute for Metals in Japan regularly publishes long term creep data which is not as readily available in the UK. Parametric extrapolation techniques have proved unsuccessful on such data sets and so interest has recently turned to the Monkman–Grant relationship. A number of shortcomings in past estimates of this relationship make them unsuitable for extrapolation. These include failure to make use of censored data and to identify the distribution for rupture lives, and neglect of the known dependency of the Monkman–Grant relationship on test conditions. This paper addresses these issues to derive an empirical relationship capable of extrapolation. Incorporating unfailed specimens into the analysis substantially altered the predictions obtained from the Monkman–Grant relationship. Uncertainty over the shape of the failure time distribution also increased the confidence limits associated with these predictions. The intercept of the Monkman–Grant relationship was shown to vary with absolute temperature and this was modelled to allow the relationship to be extrapolated to operating conditions for boiler tubes. A weak linear dependency of variability in rupture life on the minimum creep rate was also identified.

The Influence of Microstructural Aspects on the Service Behaviour of Advanced Power Plant Steels.

There is a world-wide substantial demand to increase the application temperature and design stresses of advanced creep resistant ferritic-martensitic 9-12 % Cr steels to increase the efficiency of thermal power plants. They show, compared to austenite grades, favourable physical properties-like good thermal conductivity and low coefficient of thermal expansion coupled with better service behaviour. In the frame work of the European research action COST 501 intensive investigations of these types of steels were performed. These alloys show during service conditions pronounced microstructural changes in the aged as well as in the stressed conditions. Therefore an extrapolation of short term creep tests to long times-i.e. 100 kh-using conventional methods is not successful because of changes in the acting creep mechanism governed by microstructural development. Therefore an intensive investigation and modelling of the microstructure of W- and Mo-alloyed advanced 9-12 % Cr steels, mainly G-X12 CrMoVWNbN 10-1-1, was applied to understand the mechanisms. Hardness tests, optical microscopy, SEM, TEM methods and thermodynamic equilibrium phase diagram calculations were performed. Very promising is the use of the EFTEM (Energy filtering TEM) method to quantify the precipitation sequence of the different types of precipitate: M23C6, MX, Laves Phase and Z-Phase as a function of time, temperature and straining. A new approach (Graz Model) was proposed for the improvement of the creep resistant alloys in terms of the α/γ-transformation and the Curie temperature respectively, which influence the microstructural stability. Both were found as important influencing factors. Experimentally it was found, that the alloys showing the highest creep resistance i.e. the Japanese grade NF616/P92 and the alloy B2, one of the COST 501 activity, show the highest degree of microstructural stability after creep exposure. Further investigations and modelling is necessary to develop an optimised alloy composition of advanced 9-12% Cr steels.