Creep Damage Evaluation Research Articles

Development of Creep Damage Evaluation Method for Cr-Mo-V Steel Forgings. 1st Report. Separation of Strain Component Due to Grain-Boundary Sliding from Creep and Creep-Fatigue Deforluation.

Strain component due to grain-boundary sliding was attempted to separate from creep and creep- fatigue deformation in order to evaluate creep damage as the function of the accumulation of grain-boundary sliding. It was supposed that inelastic strain is composed of 4 components i. e.εgb, εpl, εcr andεdp, where εgb is the strain due to grain-boundary sliding, εpl is plastic strain, εcr is creep strain and εdp is some inelastic strain categorized between εpl and εcr. Considering the inequality of each strain component and the inner stresses due to inharmonious values of components, the equations were devised by trial and error to simulate the creep and creep-fatigue behavior. The result of simulation with the evaluation of creep damage by εgb shows good agreement with the result of experiment in creep deformation, creep-fatigue deformation and creep rupture behavior, but the agreement with creep-fatigue failure behavior of experiment is not enough. This may suggest that creep damage is not only the function of sliding of grain-boundary but also the function of normal stress.

電気化学的手法によるＷ強化型９％Ｃｒフェライト系耐熱鋼のクリープ損傷評価

In order to develop a methodology for creep damage evaluation of W alloyed 9%Cr ferritic steel KA-STBA29/KA-STPA29 (ASME T92/P92), which is the candidate material for an ultra-super-critical fossil power plant, changes in electrochemical property of the steel caused by creep have been investigated. Experimental results on electrochemical polarization measurements revealed that the peak current density “Ip”, which appeared at a specific potential during potentiodynamic polarization measurement in 1N-KOH solution, increased with thermal aging and creep damage and reflected not only the thermal effect but also the stress effect on creep damage. The Ip value corresponded to the selective dissolution volume of precipitates (M23C6 and Laves phase) and the increase in the Ip due to creep damage reflected the increase in amount of chromium precipitated as Laves phase. It was also found that the difference in the Ip value between the as-tempered material and the creep damaged materials (ΔIp) were uniquely correlated with a newly proposed parameter “t·F(T)·exp(-Q(σ)/RT)” irrespective of test conditions, which is the parameter based on Laves phase precipitation kinetics. On the basis of such correlation between ΔIp and t·F(T)·exp(-Q(σ)/RT), the test temperature or the applied stress of the creep damaged materials can be nondestructively estimated by electrochemical polarization measurements.

A Creep Damage Evaluation Method for Welded Joints of Mod. 9Cr-1Mo Steel. The Application of Gurson Model.

This paper proposes a new numerical method of a creep damage evaluation for modified 9Cr-1Mo steel welded joints. The creep damage mechanisms of this steel are mainly the nucleation of creep voids, their growth and coalescence in a softening region of its heat affected zone. Creep constitutive equations have been formulated using the Gurson Model which was originally developed for the ductile fracture analysis of porous materials. The damage parameter of this model is the void volume fraction that is easy to compare with measured damage values. To include the coupling effects between damage and deformation, the locally coupled technique has been employed, because the damaged area in a welded joint of this steel is small in comparison with the wall thickness. The accuracy of this proposed method has been examined by simulating uniaxial creep tests on round bar specimens of welded joints. Their lives have been predicted within a factor of two.

918 SUS304ステンレス鋼の3軸引張クリープ試験(GS-3 熱変形・高温疲労・熱疲労)

This paper describes the new triaxial creep testing and creep damage evaluation in a triaxial stress state. Equi-triaxial tension creep tests were carried out using SUS304 stainless steel at 923K. Triaxial creep damage was evaluated by utilizing a miniature creep testing for triaxially damaged specimens. Maximum principal, Mises and Huddleston stresses were applied to the test results and the suitability of the stresses was discussed for multiaxial creep damage evaluation.

516 内圧クリープ試験による改良 9Cr-1Mo 鋼溶接継手のクリープ損傷評価

516 内圧クリープ試験による改良 9Cr-1Mo 鋼溶接継手のクリープ損傷評価

A Proposal of "Maximum Fraction of Voids on the Grain Boundary Line Parameter Method". A Method of Evaluating Creep Damage in Coarse Grain Areas of Heat Affected Zones in Welded Low-Alloy Steel Used for Boilers.

Through investigations of the deterioration of serviced boilers and through time series investigations involving serviced-size creep rupture tests, it has been verified that damage tends to progress locally in the coarse-grain areas of heat affected zones of welded low-alloy boiler steel ; in other words, void development and interconnection progresses noticeably starting from specific grain boundaries. Focusing on this aspect, a "Maximum Fraction of Voids on the Grain boundary Line Parameter (M Parameter) Method" is proposed here as a new method of evaluating creep damage. Using this method, the authors quantified void-induced damage and summarized its relationship with the Creep Life Fraction(t/tr). The results showed that a distribution was found along a gentle convex curve. Sensitivity and accuracy comparisons made with the conventional void area ratio method verified that this new method reacts with good sensitivity and produces accurate results pertaining to the consumed service life. In short, this new evaluation method is easier to use than the conventional method and is highly reliable.

Electromagnetic Acoustic Resonance to Assess Creep Damage in 2.25 Cr-1 Mo Steels.

This paper describes an ultrasonic technique for evaluating creep damage in 2.25 Cr-1 Mo steel exposed to the temperature of 923 K at various stresses. The technique is based on the electromagnetic resonance (EMAR), which is a combination of the resonant technique and a noncontacting electromagnetic acoustic transducer (EMAT). We obtain the ultrasonic velocity from the resonant frequency and the attenuation coefficient from the ringdown curve at a resonance. Two types of EMATs are used. One is the bulk wave EMAT for measuring the frequency dependence of shear wave attenuation in plate samples. The other is the axial-shear-wave EMAT for detecting the attenuation change of shear waves traveling in the circumferential direction of cylindrical samples. The attenuation showed much larger sensitivity to the damage accumulation than the resonant frequency. Approaching the rapture, it becomes ten time as large as the initial value. The frequency dependence of the attenuation also showed a remarkable change with the damage. The EMAR technicue has been proven to possess a large potential for the practical nondestructive/noncontact damage monitoring ; the EMAR is capable of sensing the attenuation evolution and indicating the damage advance to predict the creep life of the metals.

LIFE PREDICTION FOR ADVANCED FERRITIC STEELS SUBJECT TO THERMAL FATIGUE

Abstract— Thermal fatigue is a well recognised source of damage in headers and steam piping of thermoelectric power plant. This topic has been extensively examined in the past for low alloy ferritic steels typically used in such applications. Experimental evidence obtained in low cycle fatigue testing with tensile hold times on Modified 9Cr1Mo and E911 steels suggests that the Linear Damage Summation rule conventionally used in engineering codes for high temperature damage analysis may not be particulary appropriate for the advanced 9Cr steel family. For this reason two alternatives have been examined: (a) a strain based creep damage evaluation using the R5 ductility exhaustion approach and (b) a creep‐fatigue continuum damage mechanics method. The potential advantages and disadvantages of both are discussed. In addition, results from low cycle fatigue and thermomechanical fatigue tests on crossweld specimens machined from welded joints in the Mod.9Cr1Mo alloy are evaluated. Even if the usual cyclic life reduction factor of 2 with respect to base material behaviour appears adequate to account for the mean trend of cross‐weld results, the large variability observed risks making the use of such a factor non‐conservative for accurate life prediction.

Nondestructive evaluation of creep damage in power-plant steam generators and piping by magnetic measurements

Magnetic hysteresis measurements have been used to evaluate creep damage in power plant weldments. This method relies on the sensitivity of the magnetic properties of steels, such as coercivity, remanence and hysteresis loss, to microstructural changes occurring during creep. During high temperature creep there is a significant change in microstructure such as the formation of voids, dislocation networks and grain boundary cavities. The evolution of these defects during creep affects the magnetic properties by changing the impedance to magnetic domain wall motion and also by introducing internal demagnetizing fields associated with cavities. The present paper discusses results obtained from on-site inspection of creep damaged Cr-Mo steel welds at two thermal power plants. One of the objectives of this research was to establish whether there were any trends in the magnetic properties as a result of creep damage which could be used later as part of a more comprehensive screening procedure for monitoring the progress of creep damage.

Comparison between calculations and experimental results in lifetime evaluations

Summary A comparison is made between methods of creep damage evaluation, such as non‐destructive testing, metallographic examination by means of replicas and theoretical analysis. This comparison is achieved by the analysis of two case studies which were considered particularly significant due to the contradictory results obtained according to the different methods used. The case studies were drawn from the experience of the Italian Institute of Welding which has the facilities to examine the problem of life extension.

２．２５Ｃｒ１Ｍｏ鋼ボイラ伝熱管の実機使用環境下の損傷シミュレーション法の開発と寿命評価

The heating tubes, which operate under most adverse conditions of all boiler components, deserve exceptional care about their remaining service lifetime, with particular regard to accumulation of damage due to creep and progress of corrosion due to oxidation. The current practice of such evaluation is to perform appropriate destructive tests on the tubings sampled out of a bundle concerned, but the creep test does not return accurate enough estimates because the damage that has been accumulated in service is yet too small, giving rise to large scattering of data. In addition, there is a matter of rise of metal temperature due to internal formation of steam-induced oxide film, hence degradation of thermal conduction: since the metal temperature can exceed the initially set design operation temperature as the service is prolonged, this increment will have to be incorporated in the creep damage evaluation. Also, rise of effective stress arising from loss of wall thickness due to external formation of scale has to be considered. This report introduces a comprehensive damage assessment simulation program that takes these changes into account and discusses its authenticity on comparison with damage evaluation conducted by TEM.

Proposal of Creep-Fatigue Life Evaluation Method for Ferritic Alloy Steels.

Establishment of creep-fatigue life evaluation methods for high-temperature components in thermal power plants is important for assessment of remaining life. In this study, creep-fatigue tests were conducted by a high-temperature fatigue testing machine with two rotor materials and one boiler material. In order to study the creep damage mechanism during the stress relaxation process, a creep mechanism map was established based on the creep defomation theory. From the detailed comparison between the mechanism map and observed failure results, it was suggested that creep damage during relaxation should be divided into matrix creep damage and grain boundary creep damage. Considering these two types of damage for evaluation of creep damage during relaxation, a nonlinear damage accumulation model for creep-fatigue damage evaluation was newly proposed. Creep-fatigue life for all materials could be predicted well by a life prediction equation derived from the model.

Ultrasonic evaluation of creep damage in steel

Creep processes are the main processes responsible for the failure of structural materials in prolonged service at high temperatures and pressures. Failure starts with the formation of cavities in the material. This paper deals with an ultrasonic method to evaluate creep damage. Measurements of both ultrasonic attenuation and velocity, together with improved theoretical models, allow the porosity of the material and the size and concentration of cavities to be determined. Good agreement between the results and data obtained from a metallographic study indicates the usefulness of the method for in-service evaluation of creep damage.

Study on Highly Accurate Evaluation of Creep Void Area Fraction Rate by Use of Image Processing Technique.

For life extension of fossil power plants, one of the most important subjects is to evaluate creep damage in high-temperature components such as boilers and turbines. A creep void area fraction rate method, by which damage is evaluated based on visible information from a material texture image, is a useful method with applicability to damage detection in actual plants. In this study, an evaluation system for creep void area fraction rate assisted by an image-processing technique was newly developed. It was ascertained that the developed system enabled efficient and objective evaluation with the same accuracy as the conventional manual method. Statistical properties of creep void distribution were studied using data acquired with the system. The relation between measurement conditions and evaluation accuracy was examined, and reasonable measurement conditions were proposed.

高温材料損傷の非破壊検出に関する研究 （第７報） 蒸気タービン高温ボルトの余寿命評価技術

In order to improve the accuracy of remaining life evaluation for high-temperature bolts of 12Cr-Mo-W-V steel in long-time use, creep tests on its notched specimens were carried out and non-destructive evaluation of creep damage for the bolt threads were investigated. Bending creep rupture tests were also made for the bolt threads used for a long-time. The results obtained are as follows:(1) A bending creep rupture test equipment was developed to obtain directly the creep life expenditure of bolts. Specimens were taken from a bolt used over a long period and bending creep tests were carried out using this equipment. The results showed that the creep life expenditure of this bolt was small.(2) The creep void observation, hardness, eddy current and X-ray diffraction methods were applied to creep damage evaluation. In these methods except for the eddy current method, a good correlation was confirmed between the detected values and creep life expenditure. The creep void observation is considered as the most suitable method from the viewpoint of evaluation accuracy and applicability to the bolt threads.

Ｃｒ‐Ｍｏ‐Ｖ鍛鋼の損傷力学に基づくクリープ損傷評価

In order to investigate the applicability of a creep damage evaluation method based on damage mechanics, creep tests on the specimens from five kinds of Cr-Mo-V steel differing in impurity content were carried out at 550°C-21kgf/mm2 and 575°C-16kgf/mm2. The results obtained are as follows:(1) When calculation was carried out on the basis of the generalized damage equations derived from the Kachanov-Rabotonov equations, the calculated results corresponded relatively well to the experimental results for both the creep curve and creep damage parameters.(2) The generalized damage equations in (1) are widely applicable to the damage evaluation of actual components under intergranular fracture conditions.(3) Among the creep damage parameters, the A parameter value corresponded best to the calculated results of ω in comparison with the other two parameters, i.e. the cavity number and fraction of area. Therefore, the A parameter value is the suitable quantitative parameter for cavities in arranging them with t/tr.(4) Consequently the equation to estimate the creep life fraction from the A parameter value is given by:t/tr=1-(1-A)η+1 and: η+1=nλ/(λ-1) where n and λ are material constants and A is the A parameter value.Furthermore, in consideration of dependence on material composition, λ is denoted by:λ=10(1-Y)where Y is the creep ductility parameter, defined as the Y-factor.On the basis of the above equations, a creep damage evaluation method to estimate the creep life fraction from the A parameter value is proposed.

Appearance and Growth of Creep Voids, and their Relationship with Stage of Creep Curves.

The aim of this work is to show that creep voids or cavities are observed at the secondary stage of the creep curve. Using commercially pure aluminum, creep damage tests were carried out at the temperatures of 200°C and 250°C. Creep tests were interrupted at several points in the creep curve, and the number of creep voids, creep void area per a unit area, and change in density in specimens were measured to evaluate creep damage. It was found that creep voids were formed in the middle of secondary creep when the fracture mode was intergraunlar, and that they developed rapidly with increasing creep strain. The creep damage parameterω introduced by Kachanov and Rabatonov agreed with the dimensionless value of change in density. There was no significant difference between secondary and tertiary creep in view of the creep damage measured.

Evaluation of Tertiary Creep Damage on Pure Aluminium with both Methods of Change in Density and the Ratio of Creep Voids in an Area.

The purpose of this paper is to evaluate creep damage along creep curves quantitatively. The present design code for the structures in high-temperature service strictly prohibits use of materials in tertiary creep. However, the creep properties of newly developed heat-resistant materials are characterized by the larger proportion of tertiary creep to creep curves. Using commercially pure alminium, creep damage was measured as a function of change in density (-ΔD/D), and as that of the ratio of creep void area to total area (Avoid/A). Each value is expressed as a power law of creep strain ec in the range of the latter secondary creep section to tertiary creep. Although they were dependent on the magnitude of initial stresses, the relationship between dimensionless values of change in density and creep strain against their values at rupture can be reduced to a curve despite change in stress.

Evaluation of creep damage in some major components of power generator.

Life extension of installed machinery and equipment has become a matter of strategic importance with significant economic implications especially in power plants.The paper presents a complex of methods aimed at assessment of the residual service lives of major components of power plant machinery (steam turbines, steam line systems) under creep conditions. The methods are based on a mathematical model of properties of creep-resistant steels based on the dislocation mechanism for which stochastic behaviour of material in the course of the strain mechanism has been assumed. A method is proposed to evaluate the equivalent service load in actual service conditions by means of a computerized data acquisition. The method is applied to monitor creep damage of steam turbine pipelines. Creep damage of turbine rotors and casings and plastic deformation in their critical points were estimated numerically.The actual time span of reliable operation of a component is assessed by these method and compared with results obtained by diagnostic methods prepared in cooperation with manufactures of power generating machinery.

ボイラ伝熱管のクリープ損傷評価

Nondestructive and destructive residual life evaluation techniques for high temperature component materials have been studied by many researchers, and recently some of these techniques have been applied to actual power plants. But in the case of creep damage evaluation of boiler tubes, these techniques have been applied only at typical locations because of a huge number of tubes to be inspected and the unreliability of metal temperature assessments.For developing creep damage evaluation techniques for boiler tubes, 3 tubes (JIS STB52D) were taken from an actual power plant after a prolonged service and then examined. These tubes were metallographically observed, and several kinds of creep tests were carried out to evaluate the metal temperature in service and the extent of creep damage. The relationships of the area fraction of cavities and the diametral strain to the creep damage were also discussed. The results obtained are summarised as follows:1) The metal temperature in service and the creep damage of each tube could be evaluated properly by using the results obtained from creep rupture tests.2) The area fraction of cavities observed near the tube surface increased with creep damage, and this behaviour was similar to that of SUS321HTB.3) The diametral strain ed showed a good correlation with the creep damage, and ed increased rapidly at the later stage of the creep life. The diametral strain can be an effective parameter to evaluate the creep damage of tubes.