Creep fatigue crack behavior of two power plant steels

Crack initiation and crack growth under creep fatigue conditions were experimentally determined on a bainitic turbine rotor steel (30CrMoNiV4-11) and a martensitic pipe steel (X10CrMoVNb9-1). Side grooved compact tension (CT) specimens with 25 and 50 mm thickness as well as double edge notch tensile (DENT) specimens with 15 and 60 mm thickness have been tested in order to observe possible influences of geometry and thus to check the transferability of the specimen test results to the behavior of components.The creep fatigue crack test results can be described with the usual fracture mechanics parameters. A modified two-criteria-method can be used to estimate the crack initiation under creep fatigue conditions. The creep fatigue crack growth can be calculated from the accumulation of fatigue crack growth which is described by the Forman-law and creep crack growth which is described by the C^*-parameter.     

Effect of intermittent unloading or reversed loading on high temperature crack growth behaviour of Grade 91 steel under constant tensile load

Abstract

As a part of the efforts for developing a reliable assessment procedure for crack growth in high temperature components, crack growth tests at various loading conditions were performed on Grade 91 steel. 1T compact tension specimens of 20 mm thickness were kept under constant tensile load at 600°C, but periodically unloaded or reversely loaded to compressive side to observe these effects on deformation behaviour as well as crack growth behaviour. It was found that periodical reversed loading accelerates crack growth due to re-acceleration of inelastic deformation during load holding, but its extent was not as large as predicted by creep J-integral in a conventional way. The predictions were improved by introducing an additional parameter to take account of creep damage recovery which was caused by the excursion to compressive load.     

Creep crack growth testing of P91 and P22 pipe bends

Laboratory component tests play an important role in the development of life assessment procedures for high temperature crack initiation and growth. Thus, the working programme of the project BE 1702 HIDA, which addressed the validation, expansion and harmonisation of existing procedures for high temperature defect assessment, included a comprehensive experimental programme with feature tests of components as its core. Because of their relevance for the high temperature industry, P91 and P22 were included in HIDA among five materials. This paper presents laboratory creep crack growth tests of P91 and P22 pipe bends, discusses the test experience and draws some conclusions for laboratory component tests in general. The components were prepared with spark-eroded notches at the outer surface. The test temperature was 625°C for P91 and 565°C for P22.     

Creep crack growth behaviour of Type 316H steels and proposed modifications to standard testing and analysis methods

Tests have been performed on Type 316H stainless-steel compact tension specimens from four ex-service components and creep crack growth rates from these tests have been characterised using C^*$C^{*}$. Several modifications to standard creep crack growth testing and analysis methods have been proposed, including an improved approach for determining whether widespread creep conditions have been developed in the specimens. The observed behaviour has then been compared with existing creep crack growth rate data for this material. A change in cracking mode from ductile to brittle intergranular fracture was observed with increasing test duration. In addition, creep crack growth rates for several of the longest-term tests lie above an extrapolation of existing data from shorter-term tests. Models based on ductility exhaustion have been used to derive new equations for predicting creep crack growth rates in Type 316H steel at temperatures of 525 and 550 °C.     

Fitness for service of welded components under creep and creep–fatigue loading

The welded materials are prone to damage particularly in heat-affected zones in base metal as well as in fusion line and weld metal. Therefore the treatment of welded components operating in the creep and creep–fatigue regime is of immediate industrial interest. It needs to be addressed by a methodology that will be developed and incorporated into design codes such as ASME, as well as in Fitness-for-Service assessment procedures. Assessment of welded components for Fitness-for-Service is addressed in fragments in various procedures. This is due to the complex microstructures of weldments that include base metal, heat-affected zone and weld metal. The R5 high temperature assessment procedure, produced by BE in the UK, formalized the methods for base metals loaded in static and creep–fatigue with sections describing the treatment of welds. Subsequently, procedures for assessing defects at high temperature, similar to those in R5, have been included in British Standards, the French A16 procedure and in API 579. Section 8 of the recent European thematic network FITNET Fitness-for Service procedure which is incorporated into R5, specifies methods for assessing defects in structures operating at high temperatures and subject to creep and creep–fatigue loading conditions. The reliability of the structural assessments following the codes depends strongly on availability of reliable data required as input data. Most of the data obtained to date on high temperature materials has been reported on creep crack growth (CCG), ignoring the creep crack initiation (CCI) where as creep–fatigue data is scarce. Recent European and international collaborative effort included EC projects CRETE and ECCC, and ESIS TC11, WG on High Temperature Testing of Welds concentrated on CCI as well as CCG testing and analysis of industrial specimens and assessment. This has led to development of a Code of Practice for High Temperature Testing of Weldments via Int. Institute of Welding (IIW) to be presented to ISO for standardization.     

Creep crack growth of high temperature weldments

The experimental programme of the EC supported project (SMT 2070) SOTA aimed at addressing a technical and industrial need to provide guidelines for creep crack growth (CCG) testing and data analysis of weldments. Mechanical and creep properties were determined on two pressure vessel steels of P22 (2.25Cr1Mo) and P91 (9Cr1MoVNb). The specimens were taken from pipe welds for weld metal (WM) tests, and simulated heat affected zone (HAZ) material for the HAZ tests.The CCG tests were carried out on cross-weld compact tension (CT) specimens machined out from weldment of pipes. The tests were done at 550 and 600°C on P22 and P91 materials, respectively. The CT specimens were notched using electrical discharge method, for a sharp starter crack. This method of initiating sharp starter crack was chosen to make sure that all partners will test specimens with starter crack location as specified in the work programme to study crack initiation and growth in WM and HAZ (both in the centre and type IV region). The CCG tests were carried out following the ASTM E1457-92 [ASTM E1457-92, Standard test method for measurement of creep crack growth rates in metals, ASTM, Philadelphia, PA 19103, USA]. The partners assessed their data and sent both assessed and their raw data to be further assessed centrally. All the data from partners were analysed and compared with those of partners' own assessed data.The present paper reports on the analyses of CCG data obtained in the programme including six laboratories from six European countries. The programme addresses the differences and difficulties in testing and the assessment of weldments, and provides guidelines for harmonisation of testing procedures for reliable data production for remanent life assessment of plants with welded components.     

Code of practice for high-temperature testing of weldments

The present paper reports on a code of practice (CoP) for high-temperature testing of weldments for industrially relevant specimens. Novel aspects of the CoP include advice for testing weldment zones using different specimen geometries. Those specimens differ from the standard compact tension C(T) specimen recommended in the only available creep crack growth (CCG) testing standard ASTM E1457. Recommendations for the required number of tests, techniques for testing, treatment of test records, reduction of test data and data analysis are presented. Associated specimen selection guidelines for industrial creep crack initiation (CCI) and growth testing are also described. Validation tests carried out on P22 and P91 weldments, and base metals of 316H steel and C-Mn steel using relevant specimen geometries are briefly described. The CoP contains recommended K and C* solutions, Y functions and η factors, which are used to determine values of the fracture parameters K and C* for the specimen geometries considered. Information from these new tests, together with a review of previous CCG tests on non-standard geometries, have been used in recommending the best method of analysis for CCI and CCG data for a range of creep brittle to creep ductile welded materials.     

Creep and damage analysis of a serviced tee intersection in a boiler header: Comparison between numerical and experimental results

In the last few years modelling of damage phenomena under creep condition has been developed in order to take into account the microstructural material evolution in life predictions of high temperature components. The new analytical methods based on “Continuum Damage mechanics” and experimental creep and creep-rupture data aim at describing both stress-strain and damage field in structures in order to predict crack initiation. These models are implemented in computer finite element programs and should be subjected to rigorous experimental verification for a practical use in power plant assessment activities.

In the present paper the numerical results obtained from some creep and damagement analyses of a header component (10 CrMo 910 steel) are shown and compared with the experimental ones. The creep analyses have been performed by the computer code ABAQUS and the damage evaluation has been carried out by means of proper in-house developed user's subroutine and post processor.     

Using a strain based failure assessment diagram for creep–brittle materials

Recently, there has been considerable interest in studying creep crack growth in creep–brittle materials. For example, the methodologies for assessing creep ductile materials, using fracture mechanics parameters like C^* and C_t, have been extended to include creep–brittle materials [1]. This paper begins by examining these recent developments and outlines the difficulties in adopting these developments. An alternative approach is then proposed. This new approach leads from recent work proposed by the authors [2], where a strain based failure assessment diagram (SB-FAD) is used. Experimental results from a series of tests on a simulated heat affected zone of a low alloy steel are explored. The application of the methodology for assessing the initiation and growth of a defect in a creep–brittle material is also demonstrated.     

Creep crack growth in service-exposed weld metal of 2.25Cr1Mo

Creep crack growth (CCG) has been studied for ex-service weld metal of 2.25Cr1Mo (P22). The testing was conducted at a temperature of 550°C and prior to testing, the material had been exposed to high temperature service for 110 000 h at 530°C. The results show a marked effect of the service exposure on the CCG properties of the material when compared to similar testing performed on a new material. The CCG rate was higher by a factor 3.1 in the service-exposed material, which should be compared to the model value of 3.2 based on the relations between the elongation values. The consumed deformation capacity was also estimated with the omega model for tertiary creep. In this case, an enhanced growth rate of 2.4 was obtained. Within a distance of about 10 mm in front of the propagating cracks, the number of creep cavities was significantly higher than in the surrounding material. The variation of the density of cavities as a function of distance from the crack tip was successfully modelled.     

A numerical creep analysis on the interaction of twin semi-elliptical cracks

The interaction and coalescence of multiple flaws will significantly influence the service life of components. In this paper, the interaction of two identical semi-elliptical cracks in a finite thickness plate subjected to the remote tension is investigated. The results indicated that interaction of multiple cracks is different between the time-dependent fracture characterized by C^*-integral and linear elastic fracture noted by SIF. The magnifying factors of creep fracture are obviously larger than that of the linear elastic fracture cases. Therefore, the current interaction and coalescence rule developed from linear elastic fracture analysis may lead to a non-conservative result when it is used in the assessment of creep crack. At the end, an empirical equation is developed based on the numerical results.     

Micro- and macro-creep damage formation for P92 under multiaxial stress related to circular notched specimen

Abstract

In this study, to clarify the behaviour of micro- and macro-creep damage progression for P92 under multiaxial stress field, interrupted creep tests, analysis of multiaxial stress and detailed the cross-sectional observations were conducted on a circular notched round bar specimen which produces the multiaxial stress field due to the plastic constraint. As a result, creep voids were initiated at the early stage and they were formed up to the final fracture. These phenomena were found to be detected using direct current potential drop (DCPD) method. These results concern the development of the measurement of creep crack initiation. The distribution of high void area fraction was in good agreement with that of high hydrostatic stress and high multiaxial stress. This result indicates that multiaxial stress affects the void formation. Furthermore, the micro-creep damage of each interrupted specimen was evaluated by using the electron backscatter diffraction (EBSD) method which can analyse crystallographic misorientation caused by creep strain. The results of EBSD analyses indicated that the value of grain reference orientation deviation (GROD) closely concerns the void initiation.     

Creep crack growth data and prediction for a P91 weld at 650 °C

Creep crack growth tests have been carried out on compact tension (CT) specimens machined from a P91 weldment. Four of these specimens were cut from the parent material side of the weld and another seven specimens were cut across the weld. For the cross-weld specimens, starter cracks were positioned into (or close to) the Type IV region. The creep tests were carried out under constant loads, at 650 °C. The results obtained showed that, the creep crack growth rates for parent material specimens are about ten times lower than those for the cross-weld specimens and that the scatter in the data is relatively high. In this respect, the accuracy of the crack tip location, in the cross-weld CT specimens, plays an important role. Finite Element (FE) analyses were carried out, on notched bar and CT models, using damage mechanics material behaviour models. These analyses were used to estimate the triaxial stress factor, α, for the parent material (PM), the weld metal (WM) and the heat affected zone (HAZ). FE analyses were then used to predict the creep crack growth in the CT specimens. Results from the FE analyses for both the PM and the cross-weld CT specimens were in good agreement with the corresponding experimental results. The effect of the potential drop versus crack length calibration on the calculated C* values was also investigated.     

The potential drop method for monitoring crack growth in real components subjected to combined fatigue and creep conditions: application of FE techniques for deriving calibration curves

The potential drop technique is a robust method to provide continuous in situ crack growth monitoring of real power-plant components. For a correct assessment of the crack depth, accurate calibration curves for the geometry at hand are required. The problem entails determining the electrical potential field in a body usually characterised by a complicated geometry as a function of the growing crack. Finite element analysis procedures are first applied to optimise the technique (i.e. to determine the best location for the PD leads) and secondly to provide theoretical calibration curves. The validity of this procedure has been assessed in laboratory component tests under both thermal fatigue and multiaxial creep loading. Post-test measurements of the crack depth underline the accuracy of the FE calibration technique.     

Difficulties in interpreting data from creep crack growth tests on type 316H weldments

Abstract

Experimental creep crack growth data are generally obtained by following standard methods such as ASTM E1457-07 and subsequently characterised using the C* parameter. These data are then used in assessment procedures, such as R5, together with reference stress estimates of C* in the component, to predict creep crack growth behaviour. Some modifications to the ASTM E1457 creep crack growth testing and analysis methods have already been proposed following a previous analysis of data from long term creep crack growth tests on type 316H parent material. This paper reports the results of creep crack growth tests on type 316H heat affected zone material at 550°C using compact tension (CT) specimens manufactured from non-stress relieved thick section butt welds. It is shown that interpretation of the data from these weldment tests is complicated by both the discontinuous nature of the cracking process and the presence of significant residual stresses in the CT specimens. Further modifications to creep crack growth testing and analysis methods are proposed to address difficulties arising from the discontinuous nature of the cracking process, and further work is identified to investigate the influence of the residual stresses present in the specimens on the observed crack growth behaviour.     

Effect of holding time in brazing cooling process on creep life of the solid oxide fuel cell with bonded compliant seal

In present paper, effect of holding time at 600 °C during the brazing cooling process on creep life of solid oxide fuel cell (SOFC) with bonded compliant seal (BCS) is investigated by the finite element method. The research indicates that creep crack initiation time in BCS structure increases significantly with the holding time increasing. Compared with that the traditional cooling method during the brazing process, the creep crack initiation time can be prolonged more than twice by the holding time of 150 h with the operating temperature of 600 °C, it increases from 14,949 h to 31,911 h. When the operating temperature is 800 °C, the creep crack initiation time of SOFC can hardly be affected if the holding time exceeds 10 h. Based on the creep damage analysis and considering the cost of the SOFC manufacturing process, it is recommended that the holding time should not be exceeded 300 h if the operating temperature is below 750 °C. And when the operating temperature is 800 °C, the recommended holding time should not be longer than 10 h. The research of the present paper can provide theoretical guidance for the long life manufacturing and reliability operation of SOFC.     

Finite element calculation of contour integral parameters for cracked P91 pipe weld

Abstract

This paper reports the results of elastic–creep finite element (FE) analyses of a P91 steel pipe weld with two external ‘type IV’ circumferential cracks, subjected to internal pressure and end (system) load using creep properties obtained at 650°C. Numerical contour integral calculations have been performed to obtain both transient and stationary high temperature fracture mechanics parameters. A mesh sensitivity analysis was performed in order to ensure the accuracy of FE analyses in the transient creep stage. The effects of load magnitude, the material mismatch near the crack surfaces and the crack depth on the stationary creep C* contour integrals have been investigated, and corresponding analytical correlations are presented.     

Time-dependent deformation and fracture of multi-material systems at high temperature

The present work reports several recent research activities on the time-dependent deformation and fracture performance of multi-material system and structures at elevated temperature. A micro region deformation measuring technique is developed to achieve the full creep strain fields of multi-material systems at high temperature in light of the long-distance microscope and digital speckle correlation method. The mean field approach based on the self-consistent method and the generalized self-consistent method are introduced to predict the time-dependent deformation of the dual-components material system under the creep condition. For the pressure vessels with functionally graded materials under both internal and external pressures, an asymptotic solution for creep stress and strain is derived based on the Taylor expansion series. For the time-dependent fracture issue of multi-material structures, a modified creep crack fracture parameter prediction method for C^∗ integral in the mismatched weldments and the particle-reinforced composite is proposed based on the generalized equivalent homogenous model. Finally, time-dependent failure assessment diagram (TDFAD) for multi-material system is derived for defect assessment of structures with various combinations of materials.     

Measurement of subcritical growth of defects in large components of nuclear power plants at elevated temperatures

An evaluation of fracture parameters of components to be integrated into high-power nuclear systems (1000 MW) is of a great importance because of extremely high safety requirements. Materials to be used for both pressure vessels and pipes including welds must comply with numerous requirements, fracture resistance being one of the most important of them. Properties of the high-performance steels to be used are experimentally verified on specimens. However, eventually, an important part of the experimental programme are fracture tests of full-scale models of real components.The paper deals with problems of measurement of initiation and subcritical growth of crack-like defects in large full-scale models of the connection of pipes to a reactor pressure vessel during fracture experiments at elevated temperatures. A modification of a direct current potential drop method (DCPD) was used to compensate influences of: (i) non-uniform cross-section of pipes, (ii) complicated shape and (iii) high temperatures. Methods how analytical calibration curves can be used in these specific complicated shapes are described. It was demonstrated that the high temperature is not a limiting factor for exact measurement of crack length if the compensation and computer controlled device is used. Results of a measurement on the component and an evaluation method of subcritical crack initiation and growth are presented.     

Creep crack growth in welded components — a numerical study and comparison with the R5 procedures

In the present study, creep crack growth (CCG) in a circumferentially welded low alloyed pipe is numerically investigated for a number of different combinations of weldment constituent material properties. A creep ductility based damage model describes the accumulation of creep damage ahead of the crack tip where a constraint parameter and the creep strain rate perpendicular to the crack plane are used as characterising parameters. It is assumed that a fully circumferential creep crack, located in the heat affected zone with a depth of one quarter of the pipe thickness, is growing at a constant rate from the outer surface towards the inside. The numerical results reveal that not only the properties of the zone containing the crack, but also the deformation properties of the surrounding material influence the CCG behaviour. This influence can be noted on the characterising parameters used for the CCG rate predictions as well as on the CCG rate itself. The mismatch influence on corresponding C^* values is, however, marginal. This indicates that determination of the CCG rate in weldments, based on the C^* value only, may result in uncertain estimates.The numerically investigated cases are also assessed by use of the R5 procedures for the sake of comparison. Considering the stress re-distribution, due to the mismatch effect, the CCG rate is determined for the different weldment configurations. The comparison shows that the assumption of plane strain or plane stress conditions in the R5 analysis is essential for the agreement of the results between R5 and the two-parameter approach. Assuming plane stress conditions at the crack tip results in a relatively good agreement for the axial stress dominated cases investigated. However, for the hoop stress dominated cases, the R5 procedures predict higher CCG rates by an order of magnitude.