Creep Rupture Characteristics Research Articles

Evaluation of the Creep Rupture Behavior of Alloy 690 Steam Generator Tubes Considering the Pressure Ramp Rate

Abstract Steam generator tubes are among the main components that form the pressure boundary of a nuclear power plant and studies of ruptures of steam generator (SG) tubes are important because they can ensure the safety of a nuclear power plant in case of a severe accident. The materials used previously for SG tubes around the world have been replaced and will be replaced by Alloy 690 given its improved corrosion resistance relative to that of Alloy 600. However, studies of the high-temperature creep and creep-rupture characteristics of SG tubes made of Alloy 690 are insufficient compared to those focusing on Alloy 600. In this study, several creep tests were conducted using half-tube shape specimens of the Alloy 690 material at temperatures ranging from 650 °C to 850 °C and stresses in the range of 30–350 MPa, with failure times to creep rupture ranging from 3 h to 870 h. Based on the creep test results, creep life predictions were then made using the well-known Larson–Miller parameter (LMP) method. Steam generator tube rupture tests were also conducted under the conditions of a constant temperature and pressure ramp using SG tube specimens. As the pressure ramp rate increases, the failure behavior changes due to the rapid change in strain near the crack tip. The rupture test equipment was designed and manufactured to simulate the transient state (rapid temperature and pressure changes) in the event of a severe accident condition. After the rupture test, the damage to the SG tubes was predicted using a creep rupture model and a flow stress model. A modified creep rupture model for Alloy 690 SG tube material is proposed based on the experimental results. A correction factor of 1.7 in the modified creep rupture model was derived for the Alloy 690 material. The predicted failure pressure was in good agreement with the experimental failure pressure.

Creep Rupture Characteristics of Cladding Tubes of FC92B and FC92N: Candidate Alloys of SFR Fuel Cladding Tube Materials

Creep Rupture Characteristics of Cladding Tubes of FC92B and FC92N: Candidate Alloys of SFR Fuel Cladding Tube Materials

Modern Powder Metallurgy: Chemical Composition Design for Improved Heat Resistant Alloys

The modern approach to the design of heat-resistant metal alloys (HRAs) is analyzed, according to which the creep rupture characteristics of an alloy are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of a matrix phase. The main attention is paid to the concept of “low alloying additions” to polycrystalline alloys with transition metals, because of which the cohesive strength of the GBs and the cohesion energy of the alloy matrix are increased. This approach is especially important in relation to alloys obtained by powder metallurgy, which, in the compacted state, are fine-grained polycrystals. The methodology for calculating the key parameters of the theory (the energy of impurity segregation to the grain boundaries Egb and to the free surface Efs, as well as the values of the partial molar energy of the cohesion of the alloys) from the first principles is given. The results of applying the theory to the study of Ni-, Cr- and Ti-based alloys and the development of new HRAs based on them are presented. Typical defects in the microstructures of objects obtained using additive technologies (AT) and the application efficiency of standard methods of processing powder alloys (Hot Isostatic Pressing (HIP), heat treatment (HT)) to improve the microstructure and increase the mechanical properties are considered.

Plant-specific assessment of the natural circulation-induced creep rupture characteristics in the RCS pressure boundary during the SBO accident

In nuclear power plants (NPPs), one of the important yet uncertain phenomena during a station blackout (SBO) accident is the possibility for a counter-current natural circulation flow in the reactor coolant system (RCS) pressure boundary, which could divert the core melt progression and/or the in-vessel fission product (FP) distribution and, in turn, lead to a heatup and eventual creep rupture. From the risk and accident management points of view, one of key concerns is to predict more explicitly whether such a creep rupture can occur in the RCS pressure boundary, and if it indeed can, also predict the most probable time and the most likely location. This paper presents plant-specific analysis results on the potential characteristics of natural circulation-induced creep rupture of the RCS pressure boundary that could potentially take place during an SBO accident of a reference pressurized water reactor (APR1400). MELCOR-CFD (Computational Fluid Dynamics) coupled analysis was performed to determine more precisely the relevant physical parameter that could greatly influence on the behaviour of natural circulation and the ensuing creep rupture. Multiple sensitivity analyses were also carried out to investigate the influence that particular MELCOR modeling schemes and plant conditions have in estimating the RCS creep rupture characteristics. The analysis results and relevant insights were summarized in terms of particular points of interest.

A Grain Size-Dependent Equation for Creep Rupture Life of Grade 91 Steel Verified Up To 233,000 Hours

Abstract Grade 91 steel is widely used as steam pipes in ultrasupercritical (USC) steam boilers. In residual creep life assessment of the pipes by calculation, one needs creep rupture life of the steel as a function of stress and temperature in a time range longer than 105 h. Four regions with different creep rupture characteristics appear in a stress versus creep rupture life diagram of the steel. Main steam pipes made of the steel are used in a long-term region with low values of stress exponent and activation energy for creep rupture life (referred to as region G in this paper). Creep rupture lives of the steel in this region vary from heat to heat depending on their prior austenite grain size. This paper proposes a grain size-dependent equation representing creep rupture life of the steel in region G. The equation is verified with creep rupture data up to 232,833 h at 600 °C. Region G is absent in some heats with a large grain size. The equation can rationalize the absence in the heats. In a stress versus creep rupture life diagram of grade 92 steel, there is the same long-term region G. In the region, a creep rupture life of each heat is dependent on its grain size as is the case in grade 91 steel. The proposed equation accords well with the creep rupture lives of the grade 92 steel in region G.

First-principles-aided design of a new Ni-base superalloy: Influence of transition metal alloying elements on grain boundary and bulk cohesion

A new approach to the design of Ni-base polycrystalline superalloys is proposed. In this approach, we assume that the creep–rupture characteristics of a superalloy are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of γ matrix. The ideal work of separation, Wsep, of a GB is used as a fundamental thermodynamic quantity that controls the mechanical strength of an interface, whereas the partial cohesive energy, χ, of an alloy component serves to characterize its contribution into the strength of the bulk. Using the Σ5 (210)[100] symmetric tilt GB as a representative high-angle GB in Ni, we calculate Wsep,χ, and GB segregation energies, Eseg, for the complete set of 4d and 5d transition metal impurities, to which we add B (a typical microalloying addition), S and Bi (notoriously known as harmful impurities in Ni-base superalloys). The purpose of the analysis is to identify the elements that demonstrate a high tendency to segregate to GBs, have positive (preferably high) partial cohesive energies in the bulk, and have positive impact on Wsep of GBs. We refer to these elements as low-alloying additions. Our study reveals Zr, Hf, Nb, Ta and B as the most promising low-alloying additions. Our next step is to introduce the elements found in the first step into a new powder metallurgy (P/M) Ni-base superalloy. The results of the subsequent testing confirm that the newly created P/M superalloy indeed demonstrates superior mechanical properties at high temperatures compared to the existing Russian P/M alloy EP741NP.

A nonlinear creep damage model for brittle rocks based on time-dependent damage

Many experiments conducted under the three-dimensional stress condition reveal that damage of brittle rocks shows obvious deformational and time-dependent effect. Based on triaxial rheological test results, the creep behaviours and creep rupture characteristics of metamorphic volcanic fine conglomerate were investigated. It is found that the test results agree well with numerical curve, and the relationship between creep damage variable and loading time can be fitted by an exponential function. By analysing the tertiary creep, a new nonlinear viscoplasticity model on the basis of creep damage was proposed, and then, the three-dimensional nonlinear creep damage constitutive equations were deduced and validated by tests. Comparisons between numerical predication and test data show that this nonlinear creep damage model is valid and reasonable. Therefore, it can better describe the complete creep phases of the metamorphic volcanic fine conglomerate, which include primary creep, secondary creep and tertiary creep.

Review of Creep Cavitation and Rupture of Low Cr Alloy and its Weldment

This paper presents a review of creep cavitation and rupture of low Cr alloy and its weldment, particular in the heat-affected zone (HAZ). Creep damage is one of the serious problems for the high temperature industry. One of the computational approaches is continuum damage mechanics which has been developed and applied complementary to the experimental approach and assists in the safe operation. However, the existing creep damage constitutive equations are not developed specifically for low stress. Therefore, in order to form the physical bases for the development of creep damage constitutive equation, it is necessary to critically review the creep cavitation and rupture characteristics of low Cr alloy and its weldment.

Bending creep of beams in aggressive media

The effect of an aggressive medium on high-temperature creep and creep-rupture characteristics of beams in pure bending is considered. Creep modeling is based on the Rabotnov kinetic theory that involves two structural parameters, i.e., damage and the concentration of the environmental chemical elements in a beam. The problem of beam bending allows for the difference in the material creep processes in tension and compression. The novelty of the problem under consideration is the use of the governing and kinetic equations in the form of singular fractional power dependencies of the creep rate and damage accumulation rate on the stress; instantaneous elastic deformations are neglected because they are low compared to creep deformations. The singularity makes it possible to allow for nonlinear viscosity together with instantaneous fracture characteristics.

Creep–rupture characteristics of polycrystalline oxide ceramic to 1600°C

Creep in polycrystalline oxide ceramics at temperatures 1400 – 1600°C occurs by diffusion. The creep curves are very sensitive to the presence of nonequilibrium defects and stress concentrators which were “annealed” during cooling after high-temperature kilning of the materials. Apparently, rupture is due to vacancies forming in stretch-zone sections tied to crystal boundaries followed by their coagulation on the boundaries. Cracks form and develop differently in different materials, depending on the crystal-chemical properties of the materials.

Long-term creep and creep rupture characteristics of TiAl-base intermetallics

The present study investigates creep in three TiAl-base intermetallic alloys, Ti–48Al–2Cr–2Nb–1B, Ti–46Al–7Nb–0.6Cr–0.2Ni–0.1Si and Ti–46Al–2W–0.5Si–0.7B (all at.%), in the range of rupture times up to 25 000 h. Creep behaviour of the individual alloys at 1023 K is assessed and compared and reasons for creep strength variations are discussed. Results suggest that dislocation processes contribute to the creep strain accumulation even at creep rates as low as 7 × 10 −10 s −1. Finally the applicability of Monkman–Grant-type relationships is critically analyzed. It is shown that these relationships stem from a general functional identity that is valid for any continuous creep curve. The role of the creep curve shape factor S is highlighted.

New generation of Ni-based superalloys designed on the basis of first-principles calculations

A new approach to the design of Ni-based single crystal superalloys is proposed. It is based on a concept that under given structural conditions, the creep-rupture characteristics of superalloys are mainly determined by interatomic bonding given by the cohesive energy. In order to characterize the individual contribution of each alloying element to the strength properties at high temperature, we introduce a parameter, χ, which is the partial molar cohesive energy of an alloy component. This parameter is then obtained in the total energy first-principles calculations for a usual set of alloying elements. We demonstrate that creep-rupture characteristics of alloys indeed correlate with the total gain partial molar cohesive energy due to alloying and find that W, Ta, and Re have the highest values of χ, and should therefore play the major role in providing high-temperature strength of superalloys. Based on this finding, we design three new superalloys with a high content of W and show that they have superior creep-rupture properties compared not only with their counterparts with the lower content of W, but also with the best Ru-bearing Ni-based superalloys.

Assessment of creep rupture life of weldments of martensitic steels

Abstract Martensitic steels are of major importance for modern high efficient power plants. Their long-term creep rupture characteristics are essential for the economic and reliable design of high temperature components. However, the weldments in such components have to be considered as a potential failure area under long-term creep loading as the heat affected zone of ferritic creep resistant materials (for example new martensitic steels) is exposed to significant microstructural changes due to the heat input during welding. Furthermore, the stress situation and multiaxiality of the stress state in the weld region changes permanently due to creep and relaxation. Evaluation therefore has to be based on experimental investigation that means uniaxial creep tests of crossweld creep specimens. The microstructrural characterisation of the heat affected zone and the comparison with the situation in the optimised base material has also to be taken into account. Heat affected zone simulation of specimens and the relevant experimental investigation deliver an important basis for the modelling of the stress – strain situation. All these aspects have to be combined in the inelastic computation in order to consider all relevant effects: the individual creep strength in the different regions of the heat affected zone as well as the influence of the subsequent state of stress multiaxiality. The paper highlights recent and on-going research activities in Germany in this domain. It also demonstrates the importance of inelastic calculation for the design of welded pipes.

Creep rupture curve for simultaneous creep deformation and degradation of geosynthetic reinforcement

The viscous property of polymer geosynthetic reinforcement, which causes creep deformation, is first summarised. The creep deformation and associated creep rupture characteristics, when subjected to long-term sustained loading under the following three conditions, are numerically simulated based on a non-linear three-component rheology model: (a) the load–strain behaviour is always free from material degradation; (b) the load–strain behaviour exhibits simultaneous viscous effects and material degradation as in usual actual field cases; and (c) the sustained loading starts after full material degradation has taken place. Case (c) is the one assumed in the currently most popular design method, in which the long-term tensile design strength is obtained by separately applying reduction factors for creep rupture and material degradation. This method underestimates the true creep rupture strength to an extent that depends on the viscous and material degradation properties of the geosynthetic reinforcement. In this paper a new method to obtain the design tensile strength is proposed, taking into account the new creep–rupture curve for simultaneous creep deformation and degradation.

Microstructure and creep rupture characteristics of an ultra-low carbon ferritic/martensitic heat-resistant steel

An ultra-low carbon ferritic/martensitic heat-resistant steel with 9.37Cr, 3.2W, 4.0Co, 0.22V, 0.09Nb, 0.068N and 0.007Ti (in mass%) was prepared. Nano-sized MX precipitates were found to distribute densely and homogeneously in the matrix within martensitic lath after normalizing-and-tempering heat treatment. During creep, Fe 2 W Laves phase was formed in the lath and grain boundaries. The oxide inclusions and coarse nitride particles were the crack initiation sites and their interfaces with matrix were the main propagation path of cracks.

Creep and Creep Rupture Properties of Sn-8Zn-3Bi Solder

This paper studies creep and creep rupture properties of Sn-8Zn-3Bi lead free solder. Static tensile creep and creep rupture tests were carried out using the solder at 313K, 353K and 398K. Equations for predicting the rupture time and creep strain of the solder were proposed by analyzing the experimental data. The equations predicted the experimental rupture times within a factor of two and the experimental creep strains within a factor of 1.25. The creep rupture and creep deformation characteristics of Sn-8Zn-3Bi were compared with those of Sn-37Pb, Sn-95Pb, and Sn-3.5Ag solders. Sn-8Zn-3Bi solder had a superior creep resitance to the two Sn-Pb lead and Sn-Ag lead free solders. The relationship between the creep strain rate and relaxation behavior in creep-fatigue test was discussed for the three solders.

On Creep Rupture Characteristics in STS304 Stainless Steels

In this study, the creep rupture tests of STS304 stainless steels were investigated at three different elevated temperatures of 600, 650 and 700 under the constant creep stresses. Creep rupture characteristics such as creep stress, creep rupture time, steady state creep rate and so on were evaluated. The behaviors of creep rate curve and initial strain are compared at three different elevated temperatures. The stress exponent (n) at 600, 650 and 700 based on steady state creep rate showed 22.5, 20.6 and 11.4 respectively. By increasing the temperature, the stress exponent is decreased. At the temperature of 700, the lowest stress exponents are shown and this behavior is also observed in the case of stress exponent based on rupture time. The creep life prediction by LMP method is presented and the equation of this result is as follows: T(logtr+20)=-0.005152-14.56+24126.

Creep rupture strength of heat affected zone for 9Cr ferritic heat resisting steels

This paper describes creep rupture characteristics of weld heat affected zone, HAZ for 9Cr ferritic steels that are promising materials for nuclear energy uses. In general, creep rupture strength in the heat affected zone of peak temperature between 900 and 1000°C is lower than that in the base metal for ferritic steels. Grain refinement and coagulation of carbides for 9Cr–1Mo steels cause decrease in creep rupture strength of the HAZ. The hardness in the simulated HAZ heated to around 1000°C decreases during creep. This seems to be related to weakening of the HAZ at 1000°C. However, substitution of W for Mo is very effective in enhancement of creep rupture strength of the HAZ due to higher stability of carbides and increase in quantity of precipitated carbides during creep rupture test.

An analysis of grain boundary sliding and grain boundary cavitation in discontinuously reinforced composites

In this study, the creep cavitation and rupture characteristics of polycrystalline matrix material and discontinuously reinforced composites are investigated including grain boundary sliding behavior, reinforcement aspect ratio and interfacial behavior between the reinforcement and surrounding matrix grains. Free sliding of the grain boundaries, a continuous nucleation of the grain boundary cavities, their diffusional growth and coalescence to form grain boundary facet cracks are fully accounted for in the analyses. The results indicate that, with sliding grain boundaries, the stress enhancement factor for the composites is much higher than the one observed for the matrix material and its value increases with increasing reinforcement aspect ratio, reduction in the matrix grain size and sliding interfacial behavior between the reinforcement and the matrix. For the composites, the influence of grain boundary sliding on the creep life is reduced by the stress concentration effect that is seen at the end of the reinforcements. In contrast with the behavior of polycrystalline matrix material in composites after the formation of the first facet crack, resulting from the coalescence of the cavities, a significant time is required for the formation of the other grain boundary facet cracks across the ligament to cause final rupture. The results also show that experimentally observed higher creep exponents or stress dependent creep exponent values in discontinuously reinforced composites can occur as a result of creep damage evolution behavior.

A finite element method analysis of the role of interface behavior in the creep rupture characteristics of a discontinuously reinforced composite with sliding grain boundaries

In this study, the role of interfacial behavior in the creep cavitation and rupture characteristics of a discontinuously reinforced composite is investigated by using the finite element method and the results are compared with the results obtained for an unreinforced polycrystalline matrix material having identical grain morphology. Free sliding of the grain boundaries, a continuous nucleation of the grain boundary cavities, their diffusional growth and coalescence to form grain boundary facet cracks are fully accounted for in the analyses. Although the composites having failure at the interface exhibited significantly lower minimum creep rates, their rupture time could be as short as or even shorter than that seen for the unreinforced polycrystalline matrix material. In contrast with the behavior of polycrystalline matrix material, in composites after the formation of the first facet crack, resulting from the coalescence of cavities, a significant time is required for the formation of the other grain boundary facet cracks across the ligament to cause final rupture. Therefore, the life predictions based on the models of cavity coalescence on a single grain boundary facet or interfacial debonding will significantly underestimate the composite creep life.