Long-term creep strength predictions from short-term creep test data for high Cr creep-resistant steels and microstructural evolution origin of over-predictions

A new tensile creep model that integrates the tensile strength at creep temperature is investigated for its generic applicability in predicting the long-term creep strengths from short-term creep test data for high Cr creep-resistant steels using creep and tensile strength data measured for a grade of 11Cr steel. The results show that, when the long-term creep strengths are specified by stresses producing the required minimum creep rate, they can be accurately predicted using short-term creep test data. However, when they are specified by stresses giving the required creep rupture time, using only short-term creep test data will lead to over-predictions. The microstructure evolution origin of such over-predictions is traced to the Z-phase precipitation during creep in creep-resistant steels with more than 9 wt.% Cr. The conventional concept on the relationship between creep test stress and creep mechanisms is also re-evaluated in light of the new results.     

Effect of NiCr and NiCrAl coatings on the creep resistance of a Ni alloy

The effect of plasma sprayed NiCr and NiCrAl coatings on the creep resistance of Nickel alloy 690 at temperature of 1033 K was investigated. Experimental results showed that the coatings had a beneficial effect on the improvement of the creep resistance of substrate. However, there was almost no difference in the creep lives between the NiCr and NiCrAl coated specimens at a given stress level. The relation between the applied stress and time to rupture of the coated specimens can be estimated by using Larson-Miller equation. For the coated specimens tested at low applied stress levels, the product of the minimum creep rate and the time to rupture was a constant value. The θ projection method can be used to accurately characterize the creep behavior of the coated specimens. The variation of the creep strain along with time predicted by using θ projection method agreed well with the experimental results.     

Mismatch effect in creep properties on creep crack growth behavior in welded joints

The finite element method based on ductility exhaustion model was used to systematically investigate the mismatch effect in creep properties on creep crack growth (CCG) behavior in welded joints. The crack-tip damage, stress states, CCG paths, CCG rate and rupture life were calculated for different configurations of creep properties between weldment constituents under the same load level, and the creep life assessment and design for welded joints were discussed. The results show that when the zone containing the crack is softer than at least one of the other two surrounding materials or both, the creep crack propagates straight along the initial crack plane. Otherwise, it will form a second crack in the soft material near interface. These simulation results were confirmed by the experimental observations in the literature, and the mechanism was analyzed. The harder surrounding materials can lead to higher CCG rate and shorter rupture life due to the higher constraint given from them. The early initiation and propagation of the second cracks increase CCG rate and reduce rupture life, and the incubation time of the second cracks in soft materials near interfaces should be accurately determined in the creep life assessment and design for the welded joints. A proper mismatch design with harder material containing crack and softer surrounding material can improve CCG properties of welded joints (decreasing CCG rate and prolong rupture life).     

Relationship between lifetime under creep and constant stress rate for polymer-matrix composites

The present article reviews two existing theoretical approaches for creep failure criteria of viscoelastic materials. One criterion is based on the continuum damage mechanics (CDM) and the other is based on the fracture mechanics extended to viscoelastic materials. Although both theoretical frameworks are based on different physical concepts, the deduced lifetime expressions turn out to be equivalent even though its parameters have different physical interpretation. It is proved that both theoretical frameworks, when extended to variable stress loading cases, imply the linear cumulative damage (LCD) law. Additionally the relationship obtained between the creep–rupture and constant stress rate until failure is very simple. Moreover this simple relationship is obtained independently by two different cumulative damage laws, which do not obey the LCD law, and by experimental evidence using published data for two different polymer-matrix composites (PMC). Finally a micromechanical model, used for creep–rupture of unidirectional composites, is extended for constant stress rate until failure to corroborate the simple relationship obtained between the creep–rupture and constant stress rate until failure.     

Coupled modeling of steady-state creep rate and creep rupture strength for metals

New methods of coupled mathematical modeling of steady-state creep rate and creep rupture strength of metals in tension have been devised. Two nonlinear fractional power functions with four material constants are used as basic dependences of the steady-state creep and creep rupture life on stress. Computation of the functions is based on optimally solving two nonlinear and inconsistent — in the conventional meaning — equations sets by the method of minimization of quadratic residuals. The authors outline the methods for calculating material constants, which were used to derive analytical expressions that optimally approximate the test results for 10Kh15N27T3MR steel at 600°C under various stresses. A method of piecewise-linear approximation of creep rupture strength test results, which involves the use of a two-segment broken line, is put forward. It implies that the locations of kinks as well as other numerical characteristics of the broken line are determined from the condition of the line’s optimal arrangement relative to the experimental data points. The method takes a more comprehensive account of various damage accumulation mechanisms in steel under various stress levels. __________ Translated from Problemy Prochnosti, No. 4, pp. 25–35, July–August, 2008.     

Comparison of creep strength and intrinsic ductility for serviced and reheat treated T91 steel based on stress relaxation testing

Abstract

High precision stress relaxation tests (SRT) at four temperatures were conducted on T91 (9%Cr) steel after extended boiler service, and also after re-heat treatment. Relative differences in creep strength, measured over five decades in strain rate were dependent on test temperature. Using an established correlation between strain rate sensitivity and elongation at failure, intrinsic ductility values as a function of stress and test temperature were determined. The general trend of a minimum in ductility in terms of stress or strain rate was consistent with long term creep rupture data on T91, and with literature data on alloy steels. However, the precision and repeatability of the SRT analysis contrasted with the appreciable scatter and heat to heat variation in traditional testing. It is argued that the current creep strength evaluation based on the nearly constant state measurement from the SRT test is superior to the measurement of stress dependence of minimum creep rate in traditional creep rupture testing. The complexity of primary creep in laboratory testing, which may not be significant at operating stresses where loading strains may be fully recoverable (anelastic), does not apply to the SRT. Since very low strain rates are achieved in a one day test, the procedures for setting of design allowables and design analysis based on the SRT data should not be significantly different from current practice. This technique offers accelerated alloy development and optimisation for creep strength and also ductility, and hence resistance to notch sensitivity.     

Inclusion of primary creep in the estimation of the C t parameter

The problem of time-dependent fracture under transient creep conditions is investigated via finite element analyses of fracture specimens with stationary cracks. The constitutive models consist of linear elasticity with combinations of power-law secondary creep and two primary creep laws. Two proposed parameters are studied. One is a contour integral, C(t), which characterizes the crack tip singularity strength. The other one, C _t, is evaluated based on the load line deflection rate and has been used successfully in correlating experimental creep crack growth data.It is evident that accurate constitutive modeling is essential to good agreement with experimental data. The inclusion of primary creep resolves earlier discrepancies between the experimental and analytical load line deflection rates which are used to calculate the respective values of C _t. The loading boundary condition is also an important factor that has been addressed. A more general formulation of C _twhich includes primary creep is presented. In small scale and transition creep, the C _tparameter does not characterize the crack tip stress singularity but rather is related to the crack tip creep zone growth rate. At times past transition time, C _tand C(t) both approach a path-independent integral, C ^*(t), which characterizes the stationary crack tip stress field. The relationship between C _tand C(t) is discussed. The interpretation and estimation of the C _tparameter are given based on the numerical results and analytical manipulations.     

Analysis of creep rupture in a notched tensile bar

For circumferentially notched, round tensile bars the creep rupture behaviour is analysed, based on constitutive relations that account for the nucleation and growth of grain boundary cavities in polycrystalline metals at high temperatures. Both diffusive cavity growth and growth by dislocation creep of the surrounding grains is incorporated in the model, and in some cases free grain boundary sliding is assumed. Failure by cavity coalescence is predicted at small overall strains in the range where cavity growth is constrained by the rate of dislocation creep of the grains, whereas outside this range large occur prior to failure.In the analyses for notched specimens, where the stress fields are strongly non-uniform, first failure occurs at the notch tip, and subsequently a macroscopic crack grows into the material. Various combinations of material parameters are considered, and in most cases the crack is found to grow in the plane of the notch. The results are related to earlier experimental and computational investigations of creep rupture in notched bars.     

Experimental study of remaining creep life of SA-304L stainless steel using small punch creep test

In this paper, the remaining creep life of SA-304L stainless steel at elevated temperatures is studied. The small punch creep tests (SPCT) were performed on SA-304L virgin and aged materials at constant loads. Times to fracture and the minimum deflection rate in SPCTs were recorded, and the time-temperature parametric analysis was performed, based on experimental results. The constants of the Larson–Miller parameter and the Monkman–Grant relationship were obtained for different consumed creep life ratios, and eventually the equations were established to explain the variation of constants of the Larson–Miller parameter and the Monkman-Grant relationship with respect to the consumed creep life ratio.     

A three-stage constitutive model for the creep behaviours of high-Cr steels at elevated temperatures

In this paper, a three-dimensional constitutive model is proposed to simulate the creep behaviours of high-Cr steels at elevated temperatures. In the model, the minimum creep rate and the average creep rupture time at different temperatures and stress levels are predicted by adopting two Larson–Miller parameters. The decrease of the creep rate during the primary creep stage is captured by introducing an internal variable representing the strain hardening effect. The material parameters of the model can be identified by using the conventional experimental results. Both the strain- and stress-driven algorithms are designed to solve the constitutive evolution equations. The response of high-Cr steels during the whole creep procedure can be predicted at a quantitative level by the current model. Further implementing the model into a finite element software, the global creep behaviours of high-Cr components under realistic loading conditions can be simulated.     

The multiaxial creep ductility of austenitic stainless steels

Calculations of creep damage under conditions of strain control are often carried out using either a time fraction approach or a ductility exhaustion approach. In practice, calculations of creep damage are further complicated by the presence of multiaxial states of stress. In the case of the time fraction approach, there are a number of models that can be used to predict the effect of state of stress on creep rupture strength. In particular, Huddleston developed a model from data on stainless steels. The R5 procedure uses a ductility exhaustion approach to calculate creep damage and includes a model for use under triaxial states of stress. The aim of this paper is to describe the development of this model, which is based on considerations of cavity nucleation and growth and was developed from multiaxial creep data on Type 304 and 316 steels.     

Tensile creep of whisker-reinforced silicon nitride

This paper presents a study of the creep and creep rupture behaviour of hot-pressed silicon nitride reinforced with 30 vol% SiC whiskers. The material was tested in both tension and compression at temperatures ranging from 1100 to 1250°C for periods as long as 1000 h. A comparison was made between the creep behaviour of whisker-reinforced and whisker-free silicon nitride. Principal findings were: (i) transient creep due to devitrification of the intergranular phase dominates high-temperature creep behaviour; (ii) at high temperatures and stresses, cavitation at the whisker-silicon nitride interface enhances the creep rate and reduces the lifetime of the silicon nitride composite; (iii) resistance to creep deformation is greater in compression than in tension; (iv) the time to rupture is a power function of the creep rate, so that the temperature and stress dependence of the failure time is determined solely by the temperature and stress dependence of the creep rate; (v) as a consequence of differences in grain morphology and glass composition between whisker-free and whisker-reinforced material, little effect of whisker additions on the creep rate was observed.     

Examining the mechanisms of sand creep using DEM simulations

In this study, DEM simulations of triaxial creep tests on dense and loose sand samples were carried out to examine the micromechanics involved during creep. The simulated creep responses reproduce qualitatively the published experimental results. During the primary creep, the creep stress is gradually borne by the contact normal forces instead of contact tangential forces so that the columnar particle structures can be formed. This process also leads to a continuous decrease in the creep rate. The columnar structures eventually are completely formed and the creep rate reaches a minimum. However, the structures become meta-stable and susceptible to buckling. This explains why a sand packing does not show an extended period of secondary creep in the experiment. Buckling of the columnar structures also gives rise to maximum dilatancy and a sharp transition of the major fabric orientation of weak forces from horizontal to vertical. The continuous buckling process of columnar structures increases the creep rate and sliding ratios of contacts during the tertiary creep. In addition, the trend of contact tangential forces decreasing and contact normal forces increasing is reversed. Finally creep rupture occurs as the creep stress–strain line intersects the complete stress–strain curve. All the creep samples follow their original volume-change tendency to continue their dilation or contraction response during creep.     

Isochronous creep rupture loci on deviatoric plane and its application to P92 steel creep behaviour under multiaxial stress state

The transformation relationship of the coordinate variables between principal stress space and deviatoric stress plane has been deduced and the isochronous creep rupture loci of disparate criteria have been described on deviatoric stress plane so as to analyze the creep behaviour under multiaxial stress state. The creep experiments of P92 steel smooth and notched specimens subjected to various stresses at 650 °C have been conducted. A modified constitutive model for the creep of P92 steel has been proposed and used to simulate the creep of P92 steel notched specimens with FEA software. The FEA results were consistent with the experimental data and the fracture morphology observation. It was found that the Hayhurst criterion had the best correlation with the experimental results of P92 steel under multiaxial stress state than other criteria through the comparison of the isochronous creep rupture loci on deviatoric plane.     

The effect of constraint on creep fracture assessments

This paper describes a preliminary examination of the effect of in-plane constraint on creep crack growth under widespread creep conditions using the Q stress. Plane strain is assumed. Damage models for fracture of the process zone based on both ductility exhaustion and stress rupture are shown to predict a variation of the crack growth rate with Q. Lower levels of constraint lead to lower crack growth rates for a given C*. The results are used to outline a high temperature failure assessment diagram approach to constraint-dependent creep crack growth. This revised version was published online in July 2006 with corrections to the Cover Date.     

Assessment of power law creep constants of Gr91 steel using small punch creep tests

A method for determining the power law creep constants using the small punch (SP) creep test is studied. We performed elastic-plastic-secondary creep finite-element (FE) analysis of Gr91 (ASTM A387 GR91 CL2) steel using the properties at 565 °C to investigate the evolution of stress and strain rate at the weakest location of the SP creep specimen, i.e. at the annular region located at about 0.7 mm from the centre of the specimen. Empirical relations that correlate the applied load to the equivalent stress and the punch displacement rate to the equivalent creep strain rate are suggested on the basis of the finite-element stress analysis results. These simple relations enable us to achieve the constitutive relation of equivalent stress and equivalent creep strain rate under small punch creep test condition. To validate this approach, SP creep tests were conducted and creep constants were evaluated by using the proposed relations. These evaluated creep constants were then compared with those measured from standard uniaxial creep test. It is shown that creep constants evaluated from the SP creep test and the proposed method are in a good agreement with those from the uniaxial creep test.     

Creep crack growth rate predictions in 316H steel using stress dependent creep ductility

Abstract

Short and long term trends in creep crack growth (CCG) rate data over test times of 500–30?000 h are available for Austenitic Type 316H stainless steel at 550°C using compact tension, C(T), specimens. The relationship between CCG rate and its dependence on creep ductility, strain rate and plastic strain levels has been examined. Uniaxial creep data from a number of batches of 316H stainless steel, over the temperature range 550–750°C, have been collected and analysed. Power-law correlations have been determined between the creep ductility, creep rupture times and average creep strain rate data with stress σ normalised by flow stress σ_0·2 over the range 0·2<σ/σ_0·2<3 for uniaxial creep tests times between 100 and 100?000 h. Creep ductility exhibits upper shelf and lower shelf values which are joined by a stress dependent transition region. The creep strain rate and creep rupture exponents have been correlated with stress using a two-stage power-law fit over the stress range 0·2<σ/σ_0·2<3 for temperatures between 550 and 750°C, where it is known that power-law creep dominates. For temperature and stress ranges where no data are currently available, the data trend lines have been extrapolated to provide predictions over the full stress range. A stress dependent creep ductility and strain rate model has been implemented in a ductility exhaustion constraint based damage model using finite element (FE) analysis to predict CCG rates in 316H stainless steel at 550°C. The predicted CCG results are compared to analytical constant creep ductility CCG models (termed NSW models), assuming both plane stress and plane strain conditions, and validated against long and short term CCG test data at 550°C. Good agreement has been found between the FE predicted CCG trends and the available experimental data over a wide stress range although it has been shown that upper-bound NSW plane strain predictions for long term tests are overly conservative.     

析出相对Ti60钛合金蠕变和持久性能的影响

研究了固溶态的Si、硅化物以及α2相对Ti60高温钛合金蠕变和持久性能的影响.结果表明,α片层之间析出的硅化物能提高Ti60钛合金的600℃蠕变抗力,且当α片层内部有α2相析出时蠕变抗力提高更明显,但是硅化物的大量析出和大颗粒硅化物的存在却降低了Ti60钛合金的600℃持久性能;α2相的析出同时提高材料的蠕变抗力和持久性能;减少硅化物的析出以提高固溶态的Si对低应力下蠕变抗力的作用不显著,但是能改善高应力下的持久性能.在蠕变和持久实验条件下固溶态的硅和硅化物的不同作用,可通过不同外加应力水平下材料变形机制的差异加以解释.     

A micromechanics model of creep deformation in dispersion-strengthened metals

A micromechanics model, in which work-hardening caused by second-phase particles and a recovery process by diffusion of atoms were taken into account, has been proposed for explaining the creep deformation of dispersion-strengthened metals in high-temperature creep. A constitutive equation of the projection was employed to describe the whole creep curves from the onset of loading to rupture. The results of the calculations based on the present model have been compared with those of experiments on the carbon steels containing spherical cementite particles. There was a correlation between the experimental creep curves and the calculated ones. The changes in the calculated creep strain and creep rate with time have also been compared with the experimental results on carbon steels. The micromechanics model was found to be applicable to any kind of two-phase material, if the constitutive equation was appropriately chosen.     

Research on representative stress and fracture ductility of P92 steel under multiaxial creep

Creep experiments on both plain and notched specimens were conducted at 650 °C over a stress range of 120–185 MPa. The notch strengthening effect was found to exhibit in notched specimens. By using stress components at the skeletal point, several expressions of representative stress were compared to validate their effectiveness in predicting creep rupture lives of P92 steel under multiaxial stress states. The results showed that Hayhurst representative stress was more suitable for life predictions of P92 steel. In the mean time, the relationship between the fracture ductility and multiaxiality was presented to investigate the influence of the multiaxial stress states on creep rupture behavior of P92 steel. A more reasonable prediction model was proposed, and the validity of the model was verified by experimental data.