Creep behavior of seam-welded reheat steam pipes in thermal fossil power plant: feedback analysis and life assessment methodology

A methodology is developed to evaluate a residual life of the oldest seam-welded reheat pipes. The first step consists of a design concept based using a weld parameter depending on welding process, in order to classify pipes by their creep life fraction. This part is in progress. The second step consists of a metallurgical approach to optimize life assessment of the most damaged pipes, by estimating a residual life before crack initiation in the intercritical zone of a pipe tempered weld. Creep tests on a simulated intercritical microstructure were subjected to different creep life fractions. Creep damage was identified as the creep cavity density evaluated by image analysis. Tests results have shown that creep damage appears in the intercritical microstructure in the very end of life (last 16 ± 2 % of creep life). These results warrant a residual life of at least 24000 hours for a 150000 hours aged pipe under operating conditions, if no creep damage was found. Extra examinations have been performed on the oldest seam-welded (MMA process) reheat pipe of EDF thermal fossil power plants. Application of this metallographic approach have enabled one to estimate a residual life of at least 9000 hours before the creep crack initiation stage, in good agreement with creep tests performed on extracted welds. Applicability of this new approach seems to be very interesting for in service seam-welded reheat pipes, but it requires the previously identification of the most creep damaged section. In that way, NDT must be improved to be able to detect creep damage in the bulk.     

Creep stress concentrations in mis-matched weldments—starting points for early creep damage and creep cracking

The material creep properties of weldments, i.e. the minimum creep strain rate, the creep rupture strength and the creep ductility, do in general differ from those of the parent material. It is often assumed that if the weld material creep strength is higher than that of the parent, the life of the weldment will also be higher. This is not always true. In fact, when subjected to a prescribed displacement, e.g. a circumferential weldment in a pipe subjected to internal pressure, a creep hard weld develops a stress concentration which may result in a reduced life expectancy of the weldment system. Similarly, the properties of the heat affected zone often differ from those of the parent and weld material properties which may be one reason for early Type IV cracking. In addition, the weld preparation geometry, the width of the HAZ and other geometric parameters influence the stress distribution. The present paper summarizes some of the effects of material property variations in weldments and their impact on the creep life expectancy. It is shown that the stress concentrations resulting from material property mis-matching may be high enough to produce premature creep damage in the weldment, or will cause creep cracking starting from initial defects.     

Practical use of defect assessment procedures for industrial component integrity assessment

Abstract

The practical use of defect assessment procedures for industrial component integrity assessment is described through two practical examples in this paper. In the first example, the procedure is used to perform low temperature fitness-for-service (FFS) analysis of a longitudinal seam welded vessel manufactured from a duplex stainless steel. The impact energy obtained from the Charpy impact test performed on the weld of the vessel was found lower than the value of the minimum impact energy criteria given by the British Standard BS 5500. Charpy impact energy and fracture toughness correlation and failure assessment diagram (FAD) methodology were used in this FFS analysis to determine the fracture resistance of the vessel. For the assumed defect size used in the assessment, the weld was found to meet the fracture resistance criteria and therefore would still be fit-for-purpose. The FAD analysis was, however, repeated using the J-values obtained from the CTOD test to gain better confidence as the Charpy impact test does not provide direct measurement of the fracture toughness. The FAD was later used to determine the critical surface crack length which was then represented as a function of crack depth. The Charpy impact energy correlation method was found to be more conservative than the method of evaluation using the CTOD J-values. The lack of side wall fusion (LOF) which is a typical defect in this type of vessel would usually be influenced by the size/diameter of the electrode wire used in the weld and the number of runs. In this case, the fracture resistance of the 15mm thick vessel with the longitudinal seam weld should be adequate if less than 5mm diameter of electrode wire is used.

The second example illustrates defect assessment of a high temperature plant component. A defect was found in the high pressure final superheater header. A defect assessment incorporating FAD on the header showed that the defect was non-critical. This led to the need to perform creep and fatigue crack growth calculations and remaining life assessment in order to determine the mitigation plan for the engineers. The deterministic approach, which mainly considers the worst case scenario, suggested that the remaining life of the header was approximately 4.5 years. Probabilistic analysis showed that the component could still be fit for service up to 6 years. This will allow the engineers to mitigate a more efficient plan with a decision to either repair or replace the header and when. The use of probabilistic lifing methodologies and algorithms could consequently bring considerable financial benefits to the plant owners/operators e.g. by avoiding premature component repair or replacement. Nevertheless, it would be in the management’s interest to avoid a forced outage. The recommendation would be that the component could still be fit to use until their next minor outage (in 3 years). From then, the remedy option would be to grind down to a depth of suspected crack, excavate a small area/surrounding which might be affected, followed by correct and regular monitoring, or alternatively replace the component.     

HIDA activity on P91 steel

The 9%-12% Cr-steels are strategic materials for new power plant and for component substitution for plant life extension. One of these steels, P 91 was included in the project BE-1702 (HIDA) to provide crack initiation and growth data for the improvement and validation of procedures for high-temperature defect assessment. The paper presents an outline of the testing programme and the initial results for P 91. In addition to uniaxial and static/cyclic creep crack growth tests on standard fracture mechanics geometries, feature tests are also included in the experimental programme. These consist of internally pressurised pipe welds, pipe bends and 4-point bend pipes, and C-shaped specimens. The majority of these tests are still ongoing. The static and cyclic loading conditions are being employed to consider the range of creep/fatigue interaction in this alloy. All tests are being conducted at 625°C.     

High temperature crack initiation and defect assessment of P22 steel weldments using time dependent failure assessment method

High temperature deformation and crack resistance of low alloy ferritic grade P22 steel weldments applied in power plants are reported. The creep crack initiation (CCI) and creep crack growth (CCG) data were determined using compact type (C(T)) and C-Shape (CS(T)) fracture mechanics specimens at 550 °C. The deformation and crack growth behaviour of similar weldment zones and significance of CCI and CCG in defect assessment of components were addressed. The weldments with industrially relevant properties were produced in butt welded pipe joint from which test specimens are sampled. The studied material covers a spectrum of microstructures and ductility over the weldment zones to give representative for a welded component. The emphasis is placed on the measurement and particularly analysis of crack initiation for failure assessment in P22 steel weldments. The particular importance of construction of isochronous curves for time dependent failure assessment diagram (TDFAD) method is reported. It is aimed to contribute to establishing guidelines for acceptable methodologies for testing, analysis and assessment of welded components using TDFAD for high temperature service.     

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.     

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.     

Analysis of creep lifetime of a ASME Grade 91 welded pipe

A multi-material local approach to creep damage was applied to a ferritic-martensitic ASME Grade 91 steel welded joint at 625 °C. Focus was made on the detailed analysis of the most sensitive area of the weld i.e. the intercritical heat affected zone. Prediction of creep failure of the weld well agrees with experimental results. The model was then applied to the case of a seam-welded pipe exhibiting a roof defect, creep tested at 580 °C, showing consistent results with a more classical engineering assessment.     

Some key effects on the failure assessment of weld repairs in CrMoV pipelines using continuum damage modelling

Some typical results obtained from finite element (FE) creep and continuum damage mechanics analyses, for assessing weld repair performance, are presented. The work outlines some developments in failure analyses of repaired welds in pressurised, thick-walled, main steam circumferential pipe weldments made of CrMoV steels. Methods involved in determining the material properties for constitutive equations are briefly described. Results presented cover a range of analyses, taking account of the effects of repair profiles/dimensions, geometry change during creep, end (system) loading, reheating effects in the weld metal of partial repair welds and initial damage level, etc., on the failure life of the repaired welds. The potential uses and limitations of the damage analysis for weld repair performance assessment are discussed.     

Use of R5 in plant defect assessment

The R5 defect assessment procedures are widely used to assess components in Nuclear Electric's Advanced Gas-Cooled Reactor plant. In this paper, the use of R5 is illustrated by calculating the creep- fatigue crack growth in a specific component. The resulting crack size is compared with the limiting crack size calculated using R6. The implications of considering ductile tearing in the latter calculation, the possible interaction of tearing with creep crack growth, and the use of leak before break arguments are discussed.     

Comparison of creep crack growth rate in heat affected zone of welded joint for 9%Cr ferritic heat resistant steel based on C, dδ/dt, K and Q parameters

Creep crack growth behavior is very sensitive to the materials’ micro-structures such as the heat affected zone of a weld joint. This is a main issue to be clarified for 9%Cr ferritic heat resistant steel for their application in structural components. In this paper, high temperature creep crack growth tests were conducted on CT specimens with cracks in the heat affected zone of weld joints of W added 9%Cr ferritic heat resistant steel, ASME grade P92. The creep crack growth behavior in the heat affected zone of welded joint was investigated using the Q^∗ concept following which the algorithm of predicting the life of creep crack growth has been proposed. Furthermore, three-dimensional elastic-plastic creep FEM analyses were conducted and the effect of stress multiaxiality of welded joint on creep crack growth rate was discussed as compared with that of base metal.     

管道焊缝裂纹扩展的流固磁耦合表征

本工作利用虚拟裂纹闭合技术(Virtual crack closure technique,VCCT)提出一种单裂纹和多裂纹扩展通用的流固磁耦合方法,研究了流体压力的动态施加、流体-管道焊缝结构-磁场耦合作用下管道焊缝裂纹的扩展问题。该流固磁耦合方法每递增一次流体压力载荷,则完成一次裂纹增量扩展,更新裂纹几何形状,并重构网格,循环进行裂纹扩展计算和磁场分析,实现铁磁性管道焊缝裂纹扩展的流固磁耦合。以管道焊缝不同环向位置分布的单裂纹、多裂纹等六种工况为数值模拟算例,根据扩展结果中描述裂纹扩展的裂纹张开距离、裂纹扩展长度、磁感应强度水平分量峰值、磁感应强度垂直分量峰值等特征值,衡量裂纹所在位置的危险等级,判断管道焊缝损伤部位与损伤程度,识别多裂纹与单裂纹。该方法的实现可为在役管道焊缝裂纹的漏磁检测与评价、管道再制造修复等提供理论基础。     

Multiaxial fatigue life assessment of welded structures

Welded structures, such as welded pressure vessel components subjected to multiaxial cyclic loading, are particularly susceptible to fatigue damage. In this paper, a new path-length-based effective stress range is proposed to assess the fatigue life of weld joints under multiaxial fatigue loading. The path-length measure, a function of both normal and shear components on a critical crack plane, has a solid root in classic fracture mechanics and its application is validated by correlating nominal fatigue data including pure-bending, pure-torsion, in-phase, and out-of-phase loading. Path-Dependent Maximum Range (PDMR), a unique general-purpose fatigue life assessment package for multiaxial variable-amplitude loading, is introduced in this paper. Finally, the application of PDMR to multiaxial fatigue life assessment of complex loading cases is also discussed.     

Creep crack growth in joints of a thermoplastic

An investigation of creep crack growth in butt heat fusion joints in a high density polyethylene (HDPE) is performed to quantify their life expectancy. Three point bend specimens containing a centrally located notched joint are used in creep crack growth tests at ambient and elevated temperatures. A quasi-nonlinear viscoelastic fracture mechanics model is used to deduce the crack growth histories from the measured load-point displacement histories. The initiation time for crack growth and the rate of crack growth are correlated with the stress intensity factor for combinations of initial crack lengths, applied loads and test temperatures. The elevated temperature data are shifted bidirectionally, utilizing shift functions derived from stress relaxation tests, to develop master curves for the initiation time and rate of crack growth. These master curves are used to predict the life of a girth joint containing an initial circumferential surface crack extending through 10 percent of the thickness of a pressurized pipe.     

钢桥面板纵肋与横隔板焊接细节表面缺陷及疲劳效应研究

初始焊接缺陷是影响结构件疲劳性能的关键因素之一。在断裂力学评估框架下引入特征化初始焊接缺陷,结合相互作用积分法与复合断裂准则解决由表面缺陷所导致的复合型疲劳裂纹扩展问题,在此基础上编写裂纹扩展模拟程序,建立表面焊接缺陷效应评价方法,通过分析揭示了不同形态和尺度的初始焊接缺陷对于钢桥面板纵肋与横隔板构造细节裂纹扩展关键性度量指标和疲劳寿命预测的影响。结果表明:所建立的方法可有效用于评估焊趾部位表面焊接缺陷对于疲劳性能的效应;面状缺陷对于裂纹扩展度量指标和疲劳寿命预测结果的影响更为显著,其初始缺陷深度和形态均是影响疲劳性能的关键因素,体积型缺陷对于疲劳寿命的影响主要由深度方向的缺陷尺寸决定;焊接缺陷的形态和几何参数取值应根据工程实际和规范建议值共同确定,直接简化为面状缺陷会低估结构件的疲劳寿命;考虑焊接缺陷不确定性的可靠度评估方法尚需进一步研究。     

Fatigue crack growth in a fillet welded joint

Existing theories for the growth of cracks at weld toes have proved difficult to verify because of a lack of experimental proof at short crack depths and slow growth rates. Arbitrary initial defect sizes have been employed in life calculations coupled with approximate two-dimensional stress analyses. In this study, the fatigue performance of a stress relieved fillet weld is determined by both theory and experiment. Crack growth results for shallow (less than 1 mm depth) elliptical cracks at weld toes are used to test an elastic expression for stress intensity using a correction factor from a three-dimensional stress analysis. No evidence of higher than expected growth rates, observed by others for very short cracks and cracks in notch plastic zones, is apparent. Integration of a growth law that includes the threshold stress intensity factor provides fatigue life predictions for various stress ratios and from experimentally measured defect depths. Needle peening the weld toe improves the fatigue life by retarding crack growth up to 1 mm below the weld toe.     

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.

The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.     

Characterization of incubation time on creep crack growth for weldments of P92

Most structures are mainly fabricated by welding which are likely to be regions of crack initiation and propagation. In such weldment, it is known that the multi-axial stress fields appear due to the plastic constraint induced by the differences in material micro-structure between the weld metal, heat affected zone (HAZ) and base metal. In the present study, the experiments of creep crack growth tests and the structural mechanical analyses of weldment were conducted to understand the effects of stress multi-axiality of weldment on the characteristics of creep crack growth, creep deformation and creep ductility. Additionally, to characterize the time of creep crack initiation up to the start of a brittle type creep crack growth, the distribution of stress multi-axiality (TF) through the base metal, fine-grain HAZ, coarse-grain HAZ to weld metal were investigated.     

Ultimate fracture capacity of pressurised pipes with defects - Comparisons of large scale testing and numerical simulations

In this paper results from large scale 4-point bending tests of pipe-segments are compared with numerical analyses using LINK_pipe. The experiments were carried out as a part of the joint industry project Fracture Control - Offshore Pipelines. The comparisons between large scale testing of pipelines and numerical analyses also address the effect of biaxial loading on the strain capacity. The defect is positioned on the tension side of the pipe when applying the load. A parametric study on changing the nominal wall thickness of the pipe is carried out. Due to variation in the yield stress, a parametric study to see the effect of this variation was also performed. The results demonstrate that ductile crack growth and biaxial loading are important elements in fracture assessment procedures for pipelines.