Evaluation of long-term creep damage in high Cr ferritic steel welds

Abstract

The creep strength of high Cr ferritic heat-resistant steel welds decreases due to the formation of Type IV creep damage in the heat affected zone (HAZ) during long-term service at high temperatures. In order to elucidate the processes of Type IV creep damage, creep rupture and creep interruption tests using ASME Gr.91 and Gr.122 steel welds were conducted. It was found that creep voids formed at an early stage (0.2 of life) and coalesced to form a macro crack at 0.8 of life for the Gr.91 steel weld. On the other hand, for the Gr.122 steel weld, a small number of Type IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life and rapid crack growth occurred after that. Differences of creep damage behaviour between the Gr.91 and Gr.122 steel welds are discussed. The Type IV creep damage distributions obtained were compared with analytical results using the finite element method and damage mechanics.     

Modelling of damage development and failure in notched-bar multiaxial creep tests

Finite‐element predictions of creep rupture in notched specimens are presented in this work. A damage model linked to the creep strain rate and stress triaxiality has been used to predict creep life under multiaxial stress conditions and the predictions have been compared with experimental data for a C–Mn steel. Finite‐element analyses have been conducted using primary–secondary (PS) and primary–secondary–tertiary (PST) creep laws. As expected a PST analysis gives a shorter predicted rupture life than a PS analysis. An additional term was included in the model to allow for an increase in hydrostatic strain due to creep damage. The incorporation of this term improved the agreement between the experimental data and the finite‐element predictions. A further enhancement to the model was to model the initiation and growth of a sharp crack in the vicinity of the notch, through the use of a nodal release technique linked to the damage evolution. It was found that the predictions obtained using the nodal release technique were very similar to those from the PST creep model incorporating the hydrostatic damage term. The effect of mesh size has also been examined and the finite‐element predictions were seen to be quite mesh sensitive with a finer mesh generally giving a shorter predicted life.     

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.     

Creep behaviour of a unidirectional SM1140+/Ti-6242 composite

The creep behaviour of a unidirectionally reinforced SM1140+/Ti-6242 composite at 500 °C in a vacuum atmosphere is investigated. Results are presented for creep tests performed in longitudinal and transverse direction, respectively. Microstructural observations show that the main damage mechanisms are filament fracture in the longitudinal direction and interface debonding in the transverse direction. In both cases, creep life can be estimated with the help of models taking into account the damage mechanisms.     

THERMAL FATIGUE OF MAR-M509 SUPERALLOY—II. EVALUATION OF LIFE PREDICTION MODELS

Abstract— Thermal fatigue data on MAR-M509 reported in a companion paper were used to evaluate four life prediction models. The temperature-stress-strain history of the critical element at the thin edge of wedge specimens was computed for this purpose. The analysis method uses a finite element computation of the temperature field and a uniaxial calculation of the stress-strain cycle using a cyclic viscoplastic constitutive equation. The influence of specimen geometry and of maximum temperature on the thermal fatigue life to initiate a macroscopic crack was accounted for by variations in stress and mechanical strain ranges at the thin edge. The accumulated and cyclic creep damage models were found to overestimate thermal fatigue life in all the cases. Models which describe oxidation fatigue crack growth interactions, and which are borne out by metallographic observations on wedge specimens, were found to give reliable predictions of thermal fatigue life.     

Cumulative damage model based on equivalent fatigue under multiaxial thermomechanical random loading

A new creep–fatigue damage cumulative model is proposed under multiaxial thermomechanical random loading, in which the damage at high temperature can be divided into the pure fatigue damage and the equivalent fatigue damage from creep. During the damage accumulation process, the elementary percentage of the equivalent fatigue damage increment is proportional to that of the creep damage increment, and the creep damage is converted to the equivalent fatigue damage. Moreover, combined with a multiaxial cyclic counting method, a life prediction method is developed based on the proposed creep–fatigue damage cumulative model. In the developed life prediction method, the effects of nonproportional hardening on the fatigue and creep damages are considered, and the influence of mean stress on damage is also taken into account. The thermomechanical fatigue experimental data for thin‐walled tubular specimen of superalloy GH4169 under multiaxial constant amplitude and variable amplitude loadings were used to verify the proposed model. The results showed that the proposed method can obtain satisfactory life prediction results.     

A generalized frequency separation–strain energy damage function model for low cycle fatigue–creep life prediction

Fatigue–creep interaction is a key factor for the failures of many engineering components and structures under high temperature and cyclic loading. These fatigue–creep life prediction issues are significant in selection, design and safety assessments of those components. Based on the frequency‐modified Manson–Coffin equation and Ostergren's model, a new model for high temperature low cycle fatigue (HTLCF), a generalized frequency separation–strain energy damage function model is developed. The approach used in this model to reflect the effects of time‐dependent damaging mechanisms on HTLCF life is different from those used in all the earlier models. A new strain energy damage function is used to reduce the difference between the approximate strain energy and real strain energy absorbed during the damage process. This proposed model can describe the effects of different time‐dependent damaging mechanisms on HTLCF life more accurately than others. Comparing traditional frequency separation technique (FS) and strain energy frequency‐modified approach (SEFS), the proposed model is widely applicable and more precise in predicting the life of fatigue–creep interaction. Experimental data from existing literature are used to demonstrate the feasibility and applicability of the proposed model. A good agreement is found between the predicted results and experimental data.     

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.     

Effect of Fatigue Damage Parameter on the Cumulative Life of a Turbine Rotor Under Multiaxial Loading

The effect of the fatigue damage parameter on the cumulative life of a high-speed turbine rotor has been estimated through finite element approach. Two most commonly used multiaxial fatigue damage models based on critical plane approach-Fatemi Socie (FS) model, and Kandil Brown and Miller model (KBM) have been used to estimate the fatigue life. Structural integrity test was carried out in spin test facility to validate the simulation results. KBM model for fatigue life estimation and LMP model for creep damage predicted a cumulative life within a factor of 1.5 scatter band of the experimental value. The combination of FS model for fatigue life estimation and LMP model could predict cumulative life only within a scatter band of 2. Some of the shortcomings attributed to LDS method can be obviated using a suitable fatigue damage parameter. The study provides invaluable input and confidence for the life prediction of high-speed gas turbine rotors.     

Damage mechanics based predictions of creep crack growth in 316 stainless steel

This paper describes a novel modelling process for creep crack growth prediction of a 316 stainless steel using continuum damage mechanics, in conjunction with finite element (FE) analysis. A damage material behaviour model, proposed by Liu and Murakami [1], was used which is believed to have advantages in modelling components with cracks. The methods used to obtain the material properties in the multiaxial form of the creep damage and creep strain equations are described, based on uniaxial creep and creep crack growth test data obtained at 600 °C. Most of the material constants were obtained from uniaxial creep test data. However, a novel procedure was developed to determine the tri-axial stress state parameter in the damage model by use of creep crack growth data obtained from testing of compact tension (CT) specimens. The full set of material properties derived were then used to model the creep crack growth for a set of thumbnail crack specimen creep tests which were also tested at 600 °C. Excellent predictions have been achieved when comparing the predicted surface profiles to those obtained from experiments. The results obtained clearly show the validity and capability of the continuum damage modelling approach, which has been established, in modelling the creep crack growth for components with complex initial crack shapes.     

Estimating the remanent creep life of power plant components

This paper examines the current status of remanent creep life assessment methods for power plant components. Consideration is given mainly to predictive techniques based on post-service examination and testing with application to low alloy ferritic components in fossil plant. The requirements for producing methodologies, namely the development of mechanistic and parametric models for creep damage and failure, are discussed together with aspects on the measurement of the relevant creep damage feature or property. Techniques considered include physical and mechanical property measurement, metallographic examination, strain measurement, and accelerated creep and rupture testing. Methods based on accelerated testing are discussed in detail; extrapolative techniques and application of the life fraction rule are considered both from an experimental and mechanistic viewpoint. Finally, attention is given to the choice of representative stress to apply to uniaxial data. The influence of material parameters on the representative stress is emphasized and upper and lower bounds appropriate to creep brittle and creep ductile material states are considered.     

Creep behavior and life assessment of anisotropic bicrystals with a void and without void in different kinds of grain boundaries

Based on crystallographic theory, a creep constitutive relationship and a life predictive model have been presented. The crystallographic creep constitutive relationship has been implemented as a user subroutine ’CRPLAW' to MACR. Bicrystal models containing a void in the grain boundary and bicrystal model without void have been studied by the finite element method. Different loading direction has been studied in order to show the influence of relative direction of loading to grain boundary on the creep behavior of the bicrystals. The numerical results of bicrystal models show that there are a high stress gradient and stress concentration near the void and grain boundary. The existing of the void has strong influence on creep durability life of the crystal. The stress distribution and creep strain characterization are dependent on the crystallographic orientations of the two crystals and the grain boundary direction as well as the existing of the void and loading directions. It is shown that the bicrystal model of the loading direction perpendicular to the grain boundary has the highest creep strain and creep damage, while that model of the of the loading direction parallel to the grain boundary has the minimum. This above conclusion is also same to the growth of void.     

Creep–fatigue life of Sn–8Zn–3Bi solder under multiaxial loading

This paper describes a creep–fatigue life of Sn–8Zn–3Bi solder under multiaxial loading. A push–pull and a reversed torsion tests were carried out using seven types of strain waveforms, which are a fast–fast, a fast–slow, a slow–fast and a slow–slow waveforms and three types trapezoidal strain waveforms with different strain holding times. The strain waveforms had a significant effect on creep–fatigue life and the shortest creep–fatigue life was found in the slow–fast strain waveform while the longest life in the slow–slow waveform in the push–pull and the reversed torsion tests. Creep–fatigue life in the reversed torsion test was approximately twice longer than that in the push–pull test at each strain waveform. Applicability of common used creep–fatigue damage models for life evaluation was discussed based on the obtained experimental results and only a grain boundary sliding model could evaluate the lives within a small scatter.     

A coupled creep damage evolution model and creep life evaluation

Due to the damage accumulation during creep deformation, creep failure after a certain service time is the most important failure mode for metal structures working at high temperatures. Considering the coupled damage evolution of geometric and material’s damage, a creep life evaluation method based on continuum damage mechanics has been proposed and examined. It is found that the geometric damage evolution model can be deduced theoretically from the creep constitutive equation, while the material’s damage evolution can be assumed in the same way as that for static fatigue problems. Through solving the coupled damage evolution models, creep lives under various stress levels and temperatures can be evaluated in a unified way, just by several material constants which can be determined by some creep tests only.     

热声激励下金属薄壁结构的随机疲劳寿命估算

金属薄壁结构在热声载荷作用下会发生复杂的大挠度非线性响应,结构内部快速变化的复杂应力严重降低了结构的疲劳寿命。在结构热声激振非线性响应分析基础上,采用雨流循环计数法对应力响应时间历程进行疲劳循环计数,通过Morrow TFS,SWT平均应力模型将疲劳循环进行零均值等效处理,结合Miner线性损伤累积理论,计算结构的热声疲劳寿命。以2024-T3型铝合金薄板为研究对象,计算得到了屈曲前后四种温度条件有限带宽高斯白噪声载荷作用下的非线性动态响应,并估算了疲劳寿命。分析结果表明,屈曲前结构的疲劳寿命随着温度升高下降,屈曲后结构持续跳变时的疲劳寿命持续下降直至最低,结构进入间歇跳变区域后疲劳寿命上升,结构热声疲劳寿命与非线性响应特征具有特定的对应关系。     

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.     

The effect of stress relaxation loading cycles on the creep behaviour of 2.25Cr-1Mo pressure vessel steel

This paper investigates the effects of repeated stress relaxation loadings and post stress relaxation creep to assess the stress relaxation-creep interaction and microstructural evolution of 2.25Cr-1Mo steel. Prior to creep testing, the microstructure of the material subjected to stress relaxation exhibited a structure which was non-conservative in predicting the remaining creep life of the material. The results obtained in the test program showed that the damage due to the effects of stress relaxation was crucial and had a significant effect on the creep life of this material. The study has also shown that the extent of metallurgical degradation, due to stress relaxation, may not be evident through microstructural assessment. Consequently, established life assessment procedures may not represent conservative estimates of remaining life because the microstructural indicators of damage, due to stress relaxation, are not evident.The effects of stress relaxation on the creep properties of ferritic pressure vessel steel are life limiting for critical high temperature power generation plant. In this study a comprehensive test program has been undertaken to assess the interaction of stress relaxation with creep and microstructural evolution in 2.25Cr-1Mo steel.     

Creep life assessment method for online monitoring of steam turbine rotors

The principal objective of work was to develop fast and accurate method for online creep life monitoring of steam turbine rotors. For this purpose, a characteristic strain model of creep was investigated and validated using test data of 2%CrMoV rotor steel. The model was then used to determine creep damage functions which are introduced to compute creep damage at a constant temperature based on the Robinson time fraction rule. Finite element (FE) creep calculations of a turbine rotor were performed using the same creep model to obtain reference damage results for validating the proposed method. Comparisons of the results obtained using both methods showed a very good accuracy of the online creep damage predictions.     

Fatigue behaviour and lifing of two single crystal superalloys

A model has been developed to predict the high temperature cyclic life of single crystal superalloys RR2000 and CMSX-4 under conditions of creep and fatigue. A combined creep–fatigue model is used, although it is found that failure always occurs by creep or fatigue separately, and that creep–fatigue interaction has a minor influence. Microstructural investigation of a series of interrupted high- and low-frequency tests are presented, these are combined with the results of a series of interrupted creep tests to identify the separate and interactive mechanisms of creep and fatigue. When creep damage is present the material behaves homogeneously. Under these conditions crack growth is initiation controlled, the mechanism of failure is surface or casting pore-initiated planar crack growth followed by shear on crystallographic planes. As the temperature is lowered or the cyclic frequency increased, the material behaves less homogeneously and shear bands are formed during cycling. Crack growth under these conditions is again initiation controlled and failure is by rapid crystallographic crack growth along shear bands. Such a failure is a distinct fatigue failure and occurs when little creep damage is present. Under certain cyclic conditions, mainly those where the crystallographic failure mechanism is dominant, the material shows an anomalous increase in fatigue resistance with temperature up to approximately 950 °C. This behaviour has been quantified by relating it to the effect of strain rate and temperature on the yield strength of the material.     

Effects of Stress Level and Stress State on Creep Ductility:Evaluation of Different Models

The last few decades have witnessed an increasing emphasis on the development of strain-based approach for predicting the creep life or damage of components operating at elevated temperatures. Creep ductility, as a key parameter in this approach, may vary with a number of factors including strain rate,state of stress, operating temperature, material microstructure, etc. The present paper, however, is focused on reviewing the state-of-the-art understanding of the effects of stress level and stress state on the creep ductility. Mechanisms involving the void growth and coalescence are presented to describe the role of stress level in the variation of uniaxial creep ductility. The prediction capacity of existing empirical ductility models is also assessed in light of uniaxial test data. On the other hand, a vast body of multiaxial creep test data, collected from open literature, is utilized to examine the influence of the state of stress on the creep ductility. Then, a variety of multiaxial ductility factor models are introduced and evaluated with the available experimental data. Finally, a brief discussion on the dependence of creep ductility on the stress triaxiality and Lode parameter, predicted by numerical methods, is provided.