The prediction of creep damage in Type 347 weld metal: part II creep fatigue tests

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 part I of this paper the rupture strength and creep ductility data for a Type 347 weld metal were fitted to provide the material properties that are used to calculate creep damage. Part II of this paper examines whether the time fraction approach or the ductility exhaustion approach gives the better predictions of creep damage in creep–fatigue tests on the same Type 347 weld metal. In addition, a new creep damage model, which was developed by removing some of the simplifying assumptions that are made in the ductility exhaustion approach, was used. This new creep damage model is a function of the strain rate, stress and temperature and was derived from creep and constant strain rate test data using a reverse modelling technique (see part I of this paper). It is shown that the new creep damage model gives better predictions of creep damage in the creep–fatigue tests than the time fraction and the ductility exhaustion approaches.     

Mismatch effect of material creep strength on creep damage and failure probability of planar solid oxide fuel cell

The constraint effect with material parameters mismatch between every parts of planar solid oxide fuel cell (SOFC) plays an important role in the operation life. In this study, the mismatch effect of material creep strength coefficient on creep damage and failure probability of planar SOFC was investigated by finite element method. The results show that the maximum equivalent creep strain and failure probability of SOFC are located in the outer corner of sealant layer. With the increase of the creep strength coefficient of the sealant layer, the maximum creep damage, damage area and failure probability of the sealant layer all increase gradually. The creep strength coefficient of the sealant layer is suggested to be smaller than that of the frame material, which will improve service life of SOFC.     

Cr concentration dependence of overestimation of long term creep life in strength enhanced high Cr ferritic steels

Creep rupture data and microstructural degradation during aging of high Cr ferritic boiler steels with enhanced creep strength have been studied with special attention to prediction of long term creep rupture life. Tempered lath martensite structure in the high Cr ferritic steels remains unchanged during short term aging, whereas static recovery of the lath martensite structure proceeds when diffusion distance during aging becomes sufficiently long as is the case in long term creep. The static recovery brings about premature failure in long term creep and decreases in apparent activation energy for creep life. The decrease in activation energy is responsible for overestimation of rupture life reported in strength enhanced high Cr ferritic steels. The boundary from a short term region with high activation energy Q_H to a long term region with low activation energy Q_L moves towards longer time with decreasing Cr concentration. The difference in activation energy (Q_H − Q_L) primarily determines the extent of overestimation of rupture life predicted from short term data. In general, the extent of overestimation is less serious at 9%Cr as compared to 12%Cr.     

Modelling crack growth for creep and fatigue loading

Estimates of creep crack growth in engineering components under steady load conditions are usually based on the application of fracture mechanics concepts. In particular the creep parameter C* has become widely used together with creep crack growth data obtained from laboratory tests. There are now a number of practical methods to utilise experimental data. For high temperature components, which are subjected to cyclic (fatigue) as well as creep loading, the estimation of the fracture mechanics parameters becomes much more difficult, and consequently the extent to which the growth of pre-existing cracks grow by creep and fatigue is difficult to quantify. In this paper the response of Type 316L stainless steel is examined. This material progressively strain hardens under reversed cyclic loading, and the creep behaviour also changes. Using uniaxial fatigue and creep results, fracture parameter maps are developed to establish the appropriate regimes for creep-fatigue crack growth. Using the maps a model is developed which can predict the combined effect of fatigue and creep on crack growth. The implications of the model are discussed in relation to the limitations of obtaining results from laboratory tests at short times, and the assessment of practical engineering components.     

The prediction of creep damage in type 347 weld metal. Part I: the determination of material properties from creep and tensile tests

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 the case of the time fraction approach the rupture strength is used to calculate creep damage, whereas creep ductility is used in the ductility exhaustion approach. In part I of this paper the methods that are used to determine these material properties are applied to some creep and constant strain rate tests on a Type 347 weld metal. In addition, new developments to the ductility exhaustion approach are described which give improved predictions of creep damage at failure in these tests. These developments use reverse modelling to determine the most appropriate creep damage model as a function of strain rate, stress and temperature. Hence, the new approach is no longer a ductility exhaustion approach but is a true creep damage model.     

Evaluation of creep damage in heat affected zone of thick welded joint for Mod.9Cr–1Mo steel

Mod.9Cr–1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to the formation of Type IV cracking in heat affected zone (HAZ) at higher temperatures. The present paper aims to clarify the damage processes and mechanisms of the welded joint for Mod.9Cr–1Mo steel. Long-term creep tests of base metal, welded joint and simulated fine- grained HAZ were conducted at 550, 600 and 650 °C. Creep tests using thick plate welded joint specimen were interrupted at several time steps, and evolutions and distributions of creep damages were measured quantitatively using laser microscope. It is found that creep voids initiate at early stage of creep life (0.2 of life), the number of creep voids increases until 0.7 of life, and then voids coalesced into the macro crack at the later stage of life (0.8 of life). Creep damages concentrate mostly at a quarter depths of the plate thickness within the fine-grained HAZ of the present welded joint. The experimental creep damage distributions were compared with the computed results by using the FEM analysis. Both creep strain concentration and high stress triaxiality in fine-grained HAZ of welded joint are considered to accelerate the creep void formation and growth.     

Improved approach for predicting weld creep strength factors of ferritic steels

Abstract

The cross-weld (CW) creep strength of ferritic steels is typically lower than that for parent metal (PM), and in the past the ratio of CW to PM creep strength (weld strength factor – WSF) was assumed to be limited to ～80%. For newer Cr steels WSF can be significantly lower for a typical design life of 100 000 h or more. The possibility of low WSF is also accommodated in the current design codes such as EN 13445, but no suggested WSF values are given for guidance. Assuming a too high WSF for such welds obviously results in an unsafe (too long) predicted creep life. Unfortunately, as a further complication the WSF of the newer Cr steels can decrease when the operating temperatures are increased for improved efficiency of future power plants. It is hence important that reliable and sufficiently high values of WSF can be guaranteed. However, there is often much less extensive data on the creep strength of welds than on parent steel, and also the extrapolation to long term values of WSF can add more relative uncertainty than what is expected in extrapolating the long term creep strength of parent steel. Here an improved approach is proposed to predict WSF using the Wilshire creep model to obtain the relationship between the CW creep strength and the corresponding parent material (PM) strength. The Wilshire model directly provides the WSF value for each CW data point, when the expected normalised stress is based on the CW time to rupture at stress and temperature. The corresponding master curve parameters are those for PM, when the PM hot tensile strength is also known. The WSF data points for each CW test can then be fitted for temperature and stress dependence. This approach avoids fitting distortion in WSF, unlike the traditional assessment where a master curve is first obtained for the CW creep strength. As an example, WSF of welded P91 steel at 100 000 h is here predicted in the temperature range of 550–650°C.     

金沙江向家坝水电站坝基砂岩蠕变特性研究

选取了金沙江向家坝水电站坝基的典型砂岩试样,采用三轴压缩试验对砂岩蠕变特性进行研究,分析了流变失稳破坏时的特征及砂岩的轴向、侧向和体积应变的全过程蠕变曲线异同点,对砂岩的长期强度进行预测分析。试验结果表明,砂岩存在一个起始蠕变应力阈值,每级荷载下的蠕变曲线之前都存在一个瞬时应变且随着围压的增大和偏应力的增大幅度越来越小,轴向瞬时应变与偏应力具有很好的线性关系;侧向和体积变形则存在明显的蠕变三阶段,加速阶段要比轴向快且两者的蠕变曲线形状相似,在同一围压和同一级偏应力下侧向蠕变量比轴向及体积的大,其蠕变发展最快;砂岩的长期强度可用等时偏应力应变曲线簇来进行确定,采用体积偏应力应变曲线簇更适宜,在已有的流变模型中伯格斯模型能较好的反映砂岩蠕变特性。     

Study on creep-fatigue evaluation procedures for high-chromium steels—Part I: Test results and life prediction based on measured stress relaxation

Strong demand for improving thermal efficiency of power generation plants promoted the use of high-chromium steels, which have high creep strength and corrosion resistance. Aiming at cost reduction for future nuclear power plants, these materials are also regarded as candidates for structural materials, being favoured for lower thermal expansion rate compared with austenitic stainless steels. In structural design and life management of these plants, failure due to the combination of fatigue and creep damages has been considered as an important phenomenon to be evaluated, in addition to simple creep failure under sustained loading such as inner pressure. The author has been conducting a series of creep-fatigue tests for three types of high-chromium steels used in fossil power plants and the applicability of life prediction methods has been studied. It was found that the time fraction rule gives a relatively small amount of creep damage and overpredicts the failure life, whereas a simple ductility exhaustion method provides very large creep damage which leads to too conservative prediction of failure lives. A modified ductility exhaustion method developed on the re-definition of creep damage as a ductility consumer gave a moderate amount of creep damage and provided reasonable life predictability. Moreover, an empirical formula was derived which can represent the life reduction in compressive hold tests as a function of pure fatigue life and hold time.     

Remaining creep life estimation by strain assessment on plant

By utilising a generalised damage parameter first introduced in the Rabotnov-Kachanov equations for tertiary creep a model is proposed for remanent creep life prediction based on in situ strain assessment. It is shown that for relatively ductile materials the rupture life can be accurately related to a single strain or strain rate measurement without a knowledge of the rupture strain. Materials data are required only in the form of the minimum creep rate-rupture life product. Considerable evidence suggests that the latter is approximately constant for relatively low stresses in a range of materials so that the specific creep response of the component material is not required. It is also demonstrated that an assessment of strain at more than one stage of the life negates the need for materials data. Consideration is given to the effect of multiaxial stressing and the model is applied to the life prediction of low alloy ferritic steel tubes and pipes.     

一种预测TP347H钢长时持久强度的方法

基于多区LMP法,利用TP347H钢的高温（700℃、750℃）短时（≤5×10^3h）试验数据预测其长时持久强度（600～750℃,5×10^3～2×10^5h）．结果表明：在应力-时间图中根据口／dT。的特定比值对应力分区,用低应力-长时区LMP参数中的C值计算长时持久强度,其预测值与真实值吻合良好;与传统的单区LMP法相比,多区LMP法的应用不仅显著降低了持久强度的过估倾向而且大大缩短了试验时间,为这类钢的长时持久强度评估提供了准确而有效的方法．     

Method for determining remaining life

The ‘creep rate’ method described, which is based on short time creep tests up to 4000 h with accurate creep rate measurements, quantifies creep strain and remaining life for structural members subjected to creep stress. The evaluation is done using a time-temperature parameter accurately derived for the respective steel type which is used to determine the linear operation creep rate out of the linear creep rates of several creep tests. The creep strain and the remaining life up to a specified strain limit such as 2% (TRD 508) follow directly out of the linear creep rate determined for operation.With the method, which is comparatively accurate and conservative, seven pipes have been tested up to now. The shortest remaining life up to 2% strain was until 1999; the other pipes had far higher values. Intensive monitoring according to TRD 508 was only necessary in that shortest case because of 1% strain; in the other cases the usual level of monitoring was sufficient.     

A stochastic computation model for the creep damage of furnace tube

The service life of high temperature furnace tubes varies significantly in engineering practice. Predicting the service life of the tubes has thus long been a concern. Due to the difficulties in defining variability of creep data, previous research has mainly concentrated on deterministic creep damage models. In order to study the random nature of service life, a new stochastic creep damage model is proposed in this paper. A comparison with results calculated by use of the Monte Carlo method verifies the creep damage model. The randomness of the creep damage is demonstrated with a calculation on HK-40 furnace tubes which provides an effective means to assess the reliability of the furnace tubes.     

A numerical investigation of creep-fatigue life prediction utilizing hysteresis energy as a damage parameter

This paper explores the hypothesis that there exists an intrinsic material property, hysteresis damage energy at failure, which could be used as a creep-fatigue life prediction parameter. The connection between hysteresis energy and fatigue damage was introduced in the 1920’s by Inglis, but the use of hysteresis energy as a measure of damage was first presented by Morrow and Halford. Hysteresis energy shows promise in bridging the gaps associated with life prediction when the combination of both creep and fatigue scenarios are present. Numerical simulations which replicate experimental test configurations with 9Cr–1Mo steel were performed from which the hysteresis energy failure density (HEFD) could be calculated for each experiment. Taking the average of the HEFD values calculated for all of the experimental data as the parameter for failure (E_Intrinsic), creep-fatigue life predictions were made using a simplistic hysteresis energy based method as well as the time fraction/cycle fraction method endorsed by ASME Code and compared to experimental results. A good correlation with experimental results was obtained for life predictions using hysteresis energy density as a damage parameter. An investigation of the interaction between creep damage and fatigue damage based on the hysteresis energy method was also performed and compared with the damage interaction diagram utilized by the ASME and RCC-MR design codes. The hysteresis energy based method proved easy to implement and gave improved accuracy over the time fraction/cycle fraction method for low cycle creep-fatigue loading.     

On the development of creep damage constitutive equations: a modified hyperbolic sine law for minimum creep strain rate and stress and creep fracture criteria based on cavity area fraction along grain boundaries

This paper reports (1) the latest development and application of a modified hyperbolic sine law for minimum creep strain rate and stress for both low Cr and high Cr steels, and (2) the development of a creep fracture criterion based on cavity area fraction along grain boundaries for high Cr steel. This work is part of the fundamental development of creep damage constitutive equations which were identified through a critical literature review. In the former the application of the new law results in an improved fitting; in the latter, a new creep fracture criterion based on cavity area fraction along grain boundaries was derived and quantitatively calibrated using the latest detailed cavity nucleation and growth kinetics models for high Cr steel. Furthermore, this paper revealed the trend of nucleation rate coefficient with stress, and the trend of creep life time coefficient with stress, which provide reliable and universal prediction capabilities. This paper contributes to the specific knowledge on the minimum creep strain rate and stress function, the development of a scientifically sound and novel creep rupture criterion based on the cavity area fraction along grain boundaries for high Cr steel, and the provision of creep damage/life prediction tools.     

Modelling and experimental studies of alternative heat treatments in Steel 92 to optimise long term stress rupture properties

Abstract

The desire for power plant to give increased generating efficiency and decreased CO₂ emission has led to considerable effort over the last 10–15 years, to develop ferritic–martensitic steels which can be used for steam temperatures up to about 650°C. Examples are the addition of boron and increasing chromium content to 10–12 wt-%. However, high chromium levels have led to problems with long term precipitate stability. One approach which has not been widely explored, is the use of novel heat treatments to optimise the preservice microstructure to give the best long term creep rupture strength. Increased austenitising temperatures and lower tempering temperatures have been examined in Steel 92 (9Cr–0·5Mo–2W) and have produced significant improvements in creep rupture strength at temperatures up to 650°C compared with material given a conventional heat treatment. This has been achieved without any loss in ductility compared with conventional heat treatments. Test data for durations in excess of 40 000 h are presented. Modelling of microstructure evolution based on Monte Carlo simulations has shown important differences especially in the stability of grain boundary M₂₃C₆ and intragranular MX particles, between material with conventional and modified heat treatments. The model predictions are in good agreement with metallographic observations made on material before and after stress rupture testing. Continuum creep damage mechanics modelling based on the microstructural evolution has also been applied to predict creep life of Steel 92 and satisfactory agreement with creep rupture tests has been obtained.     

Analysis of cross-weld creep rupture data on the ½Cr½Mo¼V type IV zone

For the ferritic steel ½Cr½Mo¼V, the operating experience of medium and long term creep failures in power station welds has been of cavitation and cracking in the heat affected zone (HAZ) adjacent to the parent material, in what is referred to as the Type IV zone. This experience has led to the generation of uniaxial cross-weld creep rupture data on ½CrMoV weldments and to the development of life assessment procedures such as R5 Volume 7, which uses the rupture strength of the Type IV zone to calculate the life of power station components. Recently, the ECCC Working Group 3A has conducted a collation of the UK and German cross-weld creep rupture data on ½CrMoV weldments. Creep failure in ½CrMoV cross-weld specimens occurs in a variety of weldment zones; typically in the parent, the Type IV zone or the weld metal. Post test examination of the specimens has enabled those tests that failed in the Type IV zone to be identified and a creep rupture data assessment has been performed to derive a new model for the rupture strength of the Type IV zone.     

Robust methods of life assessment during creep

The paper presents a simplified, yet robust analysis procedure for structural problems occurring in the high temperature regime where the combined effects of permanent creep deformation and progressive material degradation must be taken into account. The method described here is based on the continuum damage concepts proposed by Kachanov and Rabotnov in the late fifties and early sixties, and on the reference stress notions that originated in the late sixties. These ingredients are combined with well-tried numerical techniques such as finite element procedures to form a basis for rational creep/damage analyses. The excessive computer capabilities required for this type of analysis, together with the scarcity and uncertainty of material data, have led to the need to de-couple the creep stress analysis from the material degradation. This approach gives the designer adequate results with accuracies commensurate with the inherent uncertainties in material data and service loadings, and within acceptable computing time limits. At the same time the robust approach gives the designer the ability to establish which effects are most important whereas the coupled analysis does not. The development of the method is described in detail in the paper and is illustrated with an example of a pressure vessel nozzle. Monte Carlo simulations methods for dealing with randomness of material and geometrical parameters are illustrated in other examples.     

Development of high-chromium steel for the sodium-cooled fast reactor in Japan and creep–fatigue assessment of the steel

This paper presents the provisional material specification and characteristics of the high-chromium (Cr) steel for the sodium-cooled fast reactor (SFR) in Japan and creep–fatigue assessment of the steel. Based on the mechanical test and metallurgical examination results, it was clarified that tungsten (W) should be diminished to achieve better ductility and toughness. Then the provisional specifications of the high-Cr steel for SFR components were proposed. Material characteristics, e.g. cyclic stress–strain relationship and creep strain curve, were also presented based on the material test results. Using these characteristics, a creep–fatigue strength assessment of the steel was performed. Conservative predictions were obtained and it was clarified that such conservatism resulted from over estimation of creep damage caused by too large initial stress at the beginning of stress relaxation. For precise assessment, it is essential to develop an appropriate model accounting for the effect of creep softening due to strain hold.