High Temperature Creep Characteristics Research Articles

Fabrication and high-temperature performance evaluation of silicon carbide – Hafnium carbide nanocomposite fiber

Silicon carbide fibers are used in silicon carbide fiber reinforced silicon carbide composite (SiCf/SiC) as reinforcement materials because of their outstanding mechanical properties, high-temperature durability and oxidation resistance. Owing to its excellent high-temperature performance, SiCf/SiC finds extensive application in materials exposed to extreme environments for long periods, such as nuclear fusion reactors, gas turbines, and rocket nozzles. Transition metal-based carbides, in particular hafnium carbide, have attracted considerable attention owing to their high melting point, high self-diffusion coefficient and chemical bonding energy. This study aims to improve the high-temperature creep characteristics of polycrystalline silicon carbide fibers by dispersing hafnium carbide nanocrystals in crystalline silicon carbide fibers and the changes in the microstructure, crystal phase and crystallite size were observed under various sintering conditions by different experiments. The results confirmed that hafnium carbide was uniformly dispersed, and that the crystal size grew to approximately 40 nm at a sintering temperature of 2000 °C. For high-temperature property evaluation, bend stress relaxation tests were conducted at temperatures in the range of 1000–1500 °C, and the effects of hafnium carbide were compared with those of other single silicon carbide fibers.

Assessment of High Temperature Creep Characteristics for P92 Steel Weldment in USC Boiler Header

Assessment of High Temperature Creep Characteristics for P92 Steel Weldment in USC Boiler Header

The Influence of Macrostructure of Nickelbased Superalloys IN713C and MAR 247 on the Characteristics of High-temperature Creep

Abstract The study consisted in assessing the influence of surface and volume modification on the characteristics of high-temperature creep of castings made of waste products of nickel-based superalloys IN 713C and the MAR-247. The results of high-temperature creep tests performed under conditions of two variants of research were analysed. The characteristics of creep according to variant I were obtained on the basis of earlier studies of these alloys with the parameters T=982°C, σ=150MPa [1]. Variant II included carrying out creep tests of alloy IN713C with the parameters T=760°C, σ =400MPa and alloy MAR247 with the parameters: T=982°C, σ=200MPa.Developed creep characteristics were compared with the results of these alloys with the parameters according to variant I of the study. It was observed that the conditions of experiments carried out depending upon the value of the creep test temperature and stress with the creep stability depends on the size of the macrograin (I variant of the studies) or such influence was not observed (II variant of the studies). Stability of samples with coarse structure in variant I of creep tests was significantly higher than the samples with fragmented grain. It was found that the observed stability conditions are dependent on the dominant deformation mechanisms under creep tests carried out - diffusion mechanism in variant I and a dislocation mechanism in variant II of the study. The conditions for the formation and growth of the cracks in the tested materials, including the morphological characteristics of their macro-and microstructure were tested

High-temperature creep data generation in elaboration of regulatory documents for metal-intensive power equipment

High-temperature creep characteristics of metals and alloys used in the elaboration of regulatory documents are determined by systems analysis of known representative experimental data on plastic deformation in static tension and creep as well as of long-term strength data. Their final refinement is performed with corresponding in-service monitoring results.

Effect of cyclic stress reduction on high temperature creep characteristics of solid solution Al−2·9 wt-%Mg alloy

Creep characteristics of solid solution Al−2·9 wt-%Mg alloy have been investigated under the effect of cyclic stress reduction of different amplitudes (12·47−26·18 MPa) and frequencies (0·20−0·44 Hz) at different working temperatures ranging from 533 to 593 K. It was found that increasing both amplitude and frequency of cyclic stress reduction resulted in an increase in both transient and steady state creep stages. The enhancement of creep rate was observed above either a threshold stress or a threshold frequency. The activation energy values of the mechanisms operating in both transient and steady state creep stages were found to be 130 and 171 kJ mol−1 respectively.

Characteristics of High Temperature Creep in Pure Aluminum Processed by Equal-Channel Angular Pressing

High purity aluminum was processed by equal-channel angular pressing (ECAP) to reduce the grain size to ~1.3 m. Tensile specimens were cut from the as-pressed billets and these specimens were tested under conditions of high temperature creep. The results show excellent creep properties with a well-defined region of steady-state flow. The flow behavior is analyzed by comparing the creep data with the predicted behavior for different fundamental creep mechanisms and by plotting a deformation mechanism map to provide a visual representation of the creep properties.

Development of New Alloys with Excellent High Temperature Creep Characteristics of 700°C or More

The carbon and nitrogen free new alloys which were composed of the supersaturated martensitic microstructure with high dislocation density before the creep test have been investigated systematically. These alloys were produced from the new approach which raised creep strength by the utilization of the reverse transformed austenite phase as a matrix and intermetallic compounds such as Laves and μ-phases as precipitates during creep test. It is important that these alloys are independent of any carbides and carbo-nitrides as strengthening factors. Creep behavior of the alloys is found to be different from that of the conventional high-Cr ferritic heat resistant steels. The minimum creep rates of the Fe-Ni alloys at 700°C are found to be much lower than that of the conventional steel, which is due to fine dispersion strengthening useful even at 700°C in these alloys. As a result carbon and nitrogen free alloys exhibited superior creep properties at temperatures more over 700°C, and steam oxidation resistance.

Effects of Ru on the high-temperature phase stability of Ni-base single-crystal superalloys

Microstructural instabilities associated with the precipitation of refractory-rich topologically-close-packed (TCP) phases within the microstructure of advanced Ni-base single-crystal superalloys were quantified in two nominally identical alloys with and without additions of Ru. Differences in the microstructural kinetics associated with the formation of TCP precipitates in these experimental single-crystal superalloys enabled the influence of Ru to be assessed. Detailed microstructural investigations were carried out on specimens subjected to prolonged isothermal exposures at elevated temperature. Even after 1000 hours at temperatures in excess of 1100 °C, the microstructure of the Ru-bearing alloy was highly resistant to the formation of TCP phases. Transmission electron micro-analysis (TEM) coupled with X-ray diffraction (XRD) was used to identify the characteristic crystal structures of the TCP precipitates in both alloys as being primarily the orthorhombic P and tetragonal σ phase. The sluggish precipitation kinetics of TCP phases in the Ru-bearing single-crystal Ni-base superalloy prevents the breakdown of the parent γ-γ′ microstructure and greatly enhances the high-temperature creep characteristics.

Creep Life Prediction of High Temperature Tube Materials for Power Plants

The high temperature creep behaviors of heat machine systems such as aircraft engines, boilers and turbines in power plants and nuclear reactor components have been considered as an important and needful fact. There are considerable research results available for the design of high temperature tube materials in power plants, based on uni-axial tension creep tests. However, few studies on the Initial Strain Method (ISM) capable of securing repair, maintenance, cost loss and life loss have been made. In this method, a long time prediction of high temperature creep characteristics can be dramatically reduced through a short time experiment. The purpose of present study is to investigate the high temperature creep life of 1Cr-0.5Mo steel using the Initial Strain Method. The creep test was performed at 500°C, 550°C and 600°C under a pure loading. In the prediction of creep life for 1Cr-0.5Mo steel, the equation of ISM was superior to those of LMP. Especially, the long time prediction of creep life was identified to improve the reliability.

High Temperature Creep Characteristics Evaluation for Degraded Heat Resistance Steel of Power Plant by Mini-Specimen

In this study the new creep test using miniaturized specimen(10100.5 ㎣) was performed to evaluate the creep characteristics for degraded materials of 2.25Cr-1Mo steel. For this creep test, the artificially aged materials for 330 hrs and 1820hrs at were used. The test temperatures applied for the creep deformation of miniaturized specimens was X and the applied loads were between 45 kg∼80 kg. After creep test, macro- and microscopic observation were conducted by the scanning electron microscope(SEM). The creep curves depended definitely on applied load and microstructure and showed the three stages of creep behavior like uniaxial tensile creep curves. The load exponents of virgin, 330 hrs and 1820 hrs materials based on creep rate showed 14.8, 9.5 and 8.3 at respectively, The 1820 hrs material showed the lowest load exponent and this behavior was also observed in the case of load exponent based on creep rupture time. In contrast to virgin material which exhibited fined dimple fractography, a lot of carbides like net structure and voids were observed on the fractography of degraded materials.

Microstructure and mechanical properties of directionally solidified γ/γ-α eutectic composites

The microstructure and mechanical properties of four directionally solidified γ/γ-α eutectic composites with chemical composition Ni-20.1Mo-13.6Al, Ni-20.2Mo-14.1Al-3.5Cr, Ni-20.4Mo-14.7Al-1.8V and Ni-18.4Mo-13.9Al-2.0V-0.5Re (at.%) are characterised. The crystallographic relationships between the coexisting phases, the effect of the growth rate of eutectic composite on the microstructural parameters and experimentally determined critical values for the stability of planar solid-liquid interface are outlined and discussed. An analytical model for prediction of the tensile offset yield stress based on the concept of Orowan stress is proposed. The fracture micromechanisms identified during tensile in-situ straining in SEM and fracture mode are described. High-temperature creep characteristics including stress exponents and activation energies for creep are calculated and compared to those of other intermetallic Ni3Al-based alloys. An analytical model for steady-state creep behaviour is presented. Creep fracture mode and effect of coexisting phases on initiation and crack propagation are discussed.

초기 연신율법을 이용한 크리프 수명예측 평가

The high temperature creep behavior of heat machine systems such as aircraft engines, boilers and turbines in power plants and nuclear reactor components have been considered as an important and needful fact. There are considerable research results available for the design of high temperature tube materials in power plants. However, few studies on the Initial Strain Method (ISM) capable of securing repair. maintenance. cost loss and life loss have been made. In this method, a long time prediction of high temperature creep characteristics can be dramatically induced through a short time experiment. The purpose of present study is to investigate the high temperature creep life of Udimet 720, SCM 440-STD6l and ICr-0.5Mo steel using the ISM. The creep test was performed at 400℃ to 700℃ under a pure loading. In the prediction of creep life for each materials, the equation of ISM was superior of Larson-Miller Parameter (LMP). Especially, the long time prediction of creep life was identified to improve the reliability

Indentation test for the investigation of high-temperature plasticity of materials

High-temperature behaviour of materials has been investigated by indentation creep methods. Materials investigated belong to three typical groups of materials: glasses, pure metals and alloys. Indentation tests were performed with flat-ended cylindrical and hemispherical punches. It is shown that the hemispherical punch is also proper for the determination of the high-temperature creep characteristics of the materials investigated. It is also proved that there is a unique correspondence between the results obtained by the two indentation methods.

A numerical analysis of fracture and high temperature creep characteristics of brittle matrix composites with discontinuous ductile reinforcements

The role of the material parameters and interface characteristics in the fracture and creep behavior of discontinuous ductile fiber-reinforced brittle matrix composite systems was investigated numerically. To simulate the fracture behavior, the ductile fibers were modelled using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. The matrix was assumed to be elastic and fails according to requirements of a stress criterion. The debonding behavior at the fiber interfaces was simulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. Results indicate that for rigid interfaces between the ductile reinforcing phase and the matrix the contribution of the ductile reinforcement to the work-of-fracture value (toughness) of the composite increases with less exhaustion of its work-hardening capacity before the onset of matrix failure. Therefore the failure strength and elastic modulus values of the matrix become important material parameters. In the case of interfacial debonding the load transfer to the discontinuous reinforcements after matrix failure should be somehow maintained for utilization to full capacity of the reinforcement and interfacial behavior. In the creep regime, for rigidly bonded interfaces the creep rate of the composite is not significantly influenced by the material properties and geometric parameters of the ductile reinforcing phase owing to the development of triaxial stress state and constrained deformation in the reinforcement. For debonding interfaces the geometric parameters of the reinforcing phase become important; however, even with very weak interfacial behavior, low composite creep rates can be achieved by suitable selection of the geometric parameters of the ductile reinforcing phase. Significant increases in room temperature fracture toughness can be achieved without extensively sacrificing the creep strength by ductile discontinuous reinforcements.

A Numerical Analysis of Fracture and High Temperature Creep Characteristics of Composites with Discontinuous Ductile Reinforcements

ABSTRACTThe role of the material parameters on the fracture and creep behavior of discontinuous ductile fiber reinforced brittle matrix composite system was numerically investigated. For simulation of fracture behavior, the ductile fibers were modeled using a constitutive relationship that accounts for strength degradation resulting from nucleation and growth of the voids. The matrix is assumed to be elastic and fails according to requirements of a stress criterion. Results indicate that the contribution of ductile reinforcement to the work of fracture value (toughness) of the composite increases with less exhaustion of its work hardening capacity before the onset of matrix failure. At creep regime, for rigidly bonded interfaces, the creep rate of the composite is not significantly influenced by the material properties of the ductile reinforcing phase due to development of large hydrostatic stress and constrained deformation in the reinforcement. Significant increases in room temperature fracture toughness can be achieved without extensively sacrificing the creep strength by ductile discontinuous reinforcements.

Microfracture of metals during high-temperature creep

1. A change from creep at moderate temperatures to high-temperature creep is accompanied by equalization of the defect density Δρ/ρ in the volume and in surface layers of polycrystalline metals resulting from microdiscontinuities. In this case Δρ/ρ varies with the applied stress, while the character of accumulation of defects in the process of creep changes. 2. The characteristics of high-temperature creep of polycrystalline materials are evidently due to a considerable extent to the activation of healing of microcracks in presurface layers at elevated temperatures.

Creep of dispersion-strengthened alloys

The importance of dispersed second-phase particles in providing strengthening of alloys both at room temperature and at high temperatures has long been recognized and the effects of dispersoids have been the subject of many research studies. These studies have led to a considerable understanding of the role of particles but inconsistencies still remain both in the experimental observations and in the interpretation of these observations. The purpose of this review is to examine the characteristics of high temperature creep of dispersion-strengthened alloys from the viewpoint of the possible mechanism involved. Three mechanisms of plastic deformation are distinguished and discussed. Specifically, these are examined in relation to oxide-dispersion-strengthened nickel-based alloys as these have potentially good high temperature corrosion resistance and useful creep resistance to high fractions of their melting points. These attractive properties at temperatures in excess of about 1000°C, where the useful creep strength of creep-resisting steels and superalloys rapidly diminishes, offer what at present appears to be the most technologically feasible development of materials for engineering applications at temperatures of 1100°C and above.

Choice of a getter for the high-temperature mechanical testing of niobium and its alloys

The high-temperature creep properties of Nb were studied. It was found that in determining the high-temperature creep characteristics of niobium in testing machines with oil-vapor vacuum pumps, porous titanium getter provides effective protection, if at the same time the working part of the specimen is screened with niobium foil. (JRD)

Transition from class I to class II creep behaviors at low stresses in Pb-9Sn

Abstract The high-temperature creep characteristics were studied in Pb-9%Sn alloy in a stress range of 10−5-10−3 G at various temperatures. The experimental results indicated a transition from micro creep to climb creep mechanisms at a stress level of about 7 × 10−5 G. These results were compared with the earlier findings in Al-3 Mg solid solution alloy and were analysed in terms of the glide-climb model for high-temperature creep.

High temperature creep characteristics of Al-6.3at%Mg alloys containing small additional elements

The effects of small amounts of an additional element (Ag, Si or Zn) on the creep characteristics of Al-6.3at% Mg alloys were investigated for the specimens which had been subjected to two kinds of heat-treatments.The heat-treatment (A) was an attempt to obtain dispersed coarse particles. The specimens were solution heat treated at 400°C for 1hr. and furnace-cooled to 250°C at a cooling rate of 25°C/hr. Then, they were maintained at that temperature for 6 days and water quenched.The heat-treatment (B) was to produce dispersed fine particles by much nucleation. The specimens were solution heat treated at 400°C for 1hr. and water quenched. Then, they were maintained at 150°C for 2 days, at 200°C for 2 days, and at 250°C for 6 days, and then again water quenched. The heat treatments had remarkable effects on the creep characteristics such as activation energy (Q) and stress exponent (n) expressed in the following state equation of steady creep;es=Aσnexp(-Q/RT)The activation energy (Q) and the stress exponent (n) for the creep at 250°C of the specimens heat-treated by (A) method were about 37kcal/mol and about 3, respectively. These values were approximately equal to those of Al-4.4at%Mg solid solution alloys. On the other hand, the activation energy and the stress exponent for the specimens heat-treated by (B) method were 2731kcal/mol and about 4 respectively, under the same testing conditions. The rate controlling step of the former treatment (A) would be the dragging of solute atmospheres around the jogs in screw dislocations and that of the latter treatment (B) would be due to the cross slip of dislocations. The difference between the effects of both treatments would be attributed to the change in transition temperature range from the cross-slip mechanism to the vacancy controlling mechanism.The three kinds of additional elements had little effects on the high temperature creep characteristics of Al-6.3at%Mg alloys. It would be attributed to the fact that the additional elements had been dissolved in β phases of Al-Mg alloys during the heat treatments before the creep.