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.     

Predicting weld creep strength reduction for 9% Cr steels

In design standards and in post-service life assessment, the cross-weld (CW) creep strength of ferritic steels is nearly universally assumed to be 80% of the corresponding value for the parent material (PH). However, CW data assessment of some 9% Cr steels such as E911 and P91 suggests that this would not hold at least at the high temperature end of the testing range. The resulting weld creep strength factor (WSF) is then attaining values well below 0.8 when extrapolated to typical design life of 100 000 h or more. Under such conditions the conventional value of 0.8 would result in non-conservative (too long) predicted life for structures subjected to CW loading in the creep regime.To accommodate the CW strength data for realistic values of WSF requires appropriate correction based on actual data. For this purpose, an alternative assessment approach, rigidity parameter correction (RPC), is proposed. This approach can be used to predict CW rupture strength from the PM master curves, with any PM rupture model optimized to correspond to the welded materials data.     

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.     

Assessment of creep behaviour of a narrow gap weld

Finite element creep analyses have been performed using Norton's secondary creep law and continuum damage equations for a thick-walled narrow gap pipe weld. The creep stress and failure life were obtained using combinations of material properties which allowed different strengths for the parent material, heat-affected zone and weld metal for 1/2Cr1/2Mo1/4V: 2 1/4Cr1Mo welds. The stationary-state failure prediction was performed based on a steady-state rupture stress. The failure times obtained from continuum damage modelling were used for assessing the results and the accuracy of the steady-state approach. The results show that the creep stress and failure life of the narrow gap weldments of both materials data options were similar to those of the V-shaped weldments over a range of narrow gap weld widths from 8 to 12 mm. The conservatism of the steady-state analysis method is illustrated, from the life estimates. The data also allow estimation of a possible effect of the presence of a weld in a plain pressurised pipe.     

Creep behaviour and lifetime of large welds in X 20 CrMOV 12 1—results based on simulation and inspection

Weld creep performance in steam lines of X 20 CrMoV 12 1 (X20) has been studied by simulation and plant inspection. By finite-element simulations, the influences of (i) the heat affected zone creep rate, (ii) the material constant α, related to the multiaxial rupture criterion, and (iii) axial system stresses on creep life, have been studied. The results show that an increase of each factor (i)-(iii) reduces the creep life, especially when two of them act at the same time. The inspection study covers over two decades of replica inspection of steam line welds in Danish power plants with service times up to 190.000 h. Analysis of the results shows that large butt joints and T-butt joints in X20 perform exceptionally well. They are unlikely to develop detectable creep damage within service times up to 200.000 h. One of the reasons for this good performance of the steel is its high time safety margin.     

Creep fatigue crack development in dissimilar metal welded joints between steels and nickel based alloy

Abstract

Creep and strain controlled cyclic/hold creep fatigue tests have been performed at temperatures in the range of 550–575°C on specimens extracted from dissimilar metal welded (DMW) joints between two classes of steel and a nickel based alloy. The details and results of the tests are described. While crack development in the cyclic/hold creep fatigue test specimens tends to be creep dominated, the microstructural paths followed in the steels in the vicinity of their heat affected zones are not identical to those observed in creep rupture testpieces taken from the same DMW joint. In pure creep tests, cracking may occur adjacent to the fusion line and/or in the fine grain heat affected zone (FGHAZ), with rupture location being dependent on temperature stress and microstructural condition. In contrast, creep dominated creep fatigue cracking typically occurs in the intercritical heat affected zone/FGHAZ or the overtempered parent material on the steel side of such weldments, depending on the composition of the joint.     

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.     

Continuum damage mechanics analyses of type IV creep failure in ferritic steel crossweld specimens

A major high temperature failure mechanism for weldments in ferritic steel steam pipework is circumferential creep cracking within the region of the heat affected zone, adjacent to the parent material, that experiences the lowest temperatures during the welding process. This is commonly known as type IV cracking. In recent years a number of experimental studies have investigated the occurrence of type IV failure in laboratory test pieces, however, there have been few attempts at theoretical modelling of type IV failure to assist in the formulation of design and assessment procedures. This report discusses the use of the creep continuum damage mechanics method for the analysis of the deformation and failure of weldments that are known to fail within the type IV region.The creep behaviour of each of the material regions of a weldment is described with a set of physically based constitutive equations, which incorporate a number of state variables. The finite element creep continuum damage mechanics method is used, with the physically based constitutive equations, to analyse the deformation and failure of the welded testpieces. The computations are shown to be in good agreement with the experimental results. The implications of the analyses are discussed with reference to the assessment of weldments that are susceptible to type IV failure.     

Numerical investigation on the creep damage induced by void growth in heat affected zone of weldments

In structural welded joints after long-term service at high temperature, fracture occurs mainly in the fine-grained heat affected zone (HAZ). Recently, the nucleation and growth of creep voids in the fine-grained HAZ of weldments, recognized as Type IV fracture, have become an important problem for low alloy ferritic heat resisting steels. In this paper, a new constitutive model was introduced to analyze the creep damage development in HAZ induced by void growth. This model is based on the equations of continuum damage mechanics (CDM) and combines a micromechanism-based method to account for the void growth process, which is different from the previous studies of creep damage. By coding a user-defined material subroutine (UMAT) in the FEA software ABAQUS, the proposed model was used to investigate the creep damage development in HAZ of a multi-material cross-weld specimen and a medium bore welded branched pipe where four different material properties: base material, coarse-grained HAZ, fine-grained HAZ, and weld material, were taken into account.     

Effect of specimen size and shape on creep rupture behavior of creep strength enhanced ferritic steel welds

The creep strength enhanced ferritic (CSEF) steels such as Grades 91, 92, 122, 911, 23 and 24 have become very important key materials for high efficiency fossil-fired power plants for last decades, however the long-term creep rupture strength and strength reduction in welds due to Type IV failure of these steels are serious problem to be urgently resolved. In order to use CSEF steel welds safely setting new weld strength reduction factors have internationally discussed. For instance ASME Boiler and Pressure Vessel Code Committee recently developed a new creep strength reduction factors for CSEF steels intercritically post-weld heat treated to be 0.5 based upon the creep rupture data obtained for standard creep specimens. However it is needed to make further consideration on the specimen size/shape effect on the creep strength of the welds to determine more appropriate weld strength reduction factors and joint influence factors. Present report provides comprehensive creep rupture test results of the specimens with various size and shape, and full size components dealing with creep rupture location/behavior and specimen size/shape effect on the creep rupture strength of CSEF steel welds.     

Microstructure evolution in HAZ and suppression of Type IV fracture in advanced ferritic power plant steels

The effect of boron and nitrogen on the microstructure evolution in heat affected zone (HAZ) of 9Cr steel during simulated heating and on the Type IV fracture in welded joints has been investigated at 650 °C. Gr.92 exhibits a significant decrease in time to rupture after thermal cycle to a peak temperature near A_C3, while the creep life of Gr.92N, subjected to only normalizing but no tempering, and 9Cr-boron steel is substantially the same as that of the base metals. In Gr.92 after A_C3 thermal cycle, very few precipitates are formed along PAGBs in the fine-grained microstructure. In the P92N and 9Cr-boron steel after A_C3 heat cycle, on the other hand, not only PAGBs but also lath and block boundaries are covered by M₂₃C₆ carbides in the coarse-grained microstructure. It is concluded that the degradation in creep life in Gr.92 after the A_C3 thermal cycle is not caused by grain refinement but that the reduction of boundary and sub-boundary hardening is the most important. Soluble boron is essential for the change in α/γ transformation behavior during heating and also for the suppression of Type IV fracture in welded joints. Newly alloy-designed 9Cr steel with 160 ppm boron and 85 ppm nitrogen exhibits much higher creep rupture strength of base metal than P92 and also no Type IV fracture in welded joints at 650 °C.     

The behaviour of ferritic weldments in thick section pipe at elevated temperature

A major collaborative research programme is being carried out within the CEGB to examine the correlation between data, produced from a range of test methods, which are currently used in the design of welded steam pipes. In the part of the programme reported here, the elastic and creep deformation occurring in low alloy ferritic steel pipe-to-pipe weldments has been studied in pressure vessel experiments conducted at 565°C and 455 bar internal steam pressure. The welds were made in parent pipe using mild steel and low alloy 1CrMo, 2CrMo and weld metals. All the weldments were post-weld heat treated for 3 h at 700°C prior to testing. In addition, the weldments, represented as parent material, heat affected zone and weld metal, have been analysed to determine stresses and strains using a finite element three-material model.The main features of the macro- and micro-structures of the four weldments are briefly described. Results are then presented for the elastic and creep deformations observed in both the hoop and axial directions in the weldments. The experimental creep strain data are then used as a basis for calculating the stationary state stresses present on the surface of the weldments. The surface stationary state stress distribution and corresponding steady state strain rates, determined using the finite element model, are then presented.The pressure vessel experimental results and the data from the finite element analysis are discussed in terms of the hoop and axial deformation in the weldments. An assessment is then made of the correlation between the results from the experimental and analytical approaches. Finally, the practical implications of the present results are considered with respect to the design of operating plant.     

Creep crack growth behavior in the HAZ of weldments of W containing high Cr steel

The creep and creep crack growth properties of W strengthened 11Cr–0.4Mo–2W steel welded joints have been investigated at 923 K. The joints were prepared using gas tungsten arc (GTA) welding and electron beam (EB) welding. Most of the joint specimens were ruptured in their heat affected zone (HAZ), and inevitably resulted in shorter creep lives than those of the base metals. The investigation of creep properties of simulated HAZ specimens showed that fine grains produced by heating around A_c3 were obviously responsible for the degradation of creep strength in welded joints. The creep lives of smooth specimens for EBW joints were about twice longer than those for GTAW joints, however brittle type IV fracture occurred even in the EBW joints with narrower HAZ width for long-term creep test. The FEM analysis used creep data from simulated HAZ specimens and so the experimental results for creep properties of welded joints could be explained. The creep crack growth properties in the HAZ of weldments were investigated using CT specimens. In the pre-cracked CT specimens, the crack initiation time was affected by mechanical constraint, whereas the difference of the crack growth rate between welded joints and base metal was negligible for the present high-strengthened steel.     

Creep degradation in welds of Mod.9Cr-1Mo steel

Creep life assessment technology has not yet been developed for Mod.9Cr-1Mo steel welded joints due to the unique degradation and failure mechanisms in the heat affected zone. Nevertheless, there is strong demand from power plant operators for the development of nondestructive damage detection and life assessment technology for weldments of this steel. In this study creep rupture testing using a large-size welded joint was conducted, and creep and damage detection tests were carried out to elucidate the degradation mechanism. Also the microstructure and hardness changes in the heat affected zones of the welds were investigated to clarify the degradation mechanisms of such welds in comparison with base metal during creep. In general, the changes in hardness distribution along the welds were difficult to correlate with the life consumption, although degradation in welds due to creep was successfully detected. Accordingly, a new approach to degradation evaluation and creep life assessment by hardness measurement method for Mod.9Cr-1Mo steel welds is proposed.     

Creep behaviour of 12Cr-Mo-V steel weldments

This paper describes an investigation into the creep behaviour of weldments in 12Cr-Mo-V steel, with particular emphasis on the effect of welding conditions and type of filler metals. At 600°C, the creep lives of all the weldments fall below the DIN 17175 scatterband and are not influenced by a variation in welding regime, number of postweld heat treatments and type of filler metal. This is attributed to the premature creep damage in the ‘type IV’ zone, the outer part of the fine grained zone, which possesses the poorest creep resistance.     

Some issues in life assessment of longitudinal seam welds based on creep tests with cross-weld specimens

In order to reduce production costs, it is of great interest to use longitudinal seam welds when manufacturing large diameter pipes. The cost reduction can be as high as 30%. However, severe inservice accidents for this type of pipes working in the creep regime have occurred mainly due to mismatch in weldment creep properties.

In many cases, creep tests of cross-weld specimens, taken from the seam weld, are used to predict the behaviour of the seam weld, assuming that the creep behaviour of specimen and weldment is equivalent. Experiences of this procedure indicate that further knowledge is required before translation between specimen and component can be made.

In the present paper, both full scale seam welded pipes and cross-weld specimens are studied with the damage mechanics concept using finite element, FE, technique. The same mechanical model of multiple material zones is used for the two components. Both the influence of differences in creep properties between the weldment constituents and the size effect of the cross-weld specimen, are studied.

It is found that the cross-weld test results can not directly be translated to the full scale component. Factors such as the creep properties and the relative geometry of the weldment constituents and the size of the cross-weld specimen have to be considered when performing creep life assessment.     

Comparison of cross-weld creep testing and service welds

Creep testing of cross-weld specimens is often used to predict weld performance. The global stress state in a cross-weld creep test is substantially different to that occurring in service welds. Using finite element analysis (FEA), the results of cross-weld creep testing were compared with a number of service weld situations. This work was compared to previous published data. Modelling suggested that the results of cross-weld creep testing are not indicative of expected weld performance in service welds.     

Creep crack growth in welds: a damage mechanics approach to predicting initiation and growth of circumferential cracks

The results of damage mechanics finite element analyses have been used to estimate the initiation and growth of type IV cracks in a series of internally pressurised circumferential pipe welds, in main steam pipelines made of 1/2CrMoV steel. The material properties used, for the various zones of new, service-aged and repaired welds, were produced from creep test data at 640°C. Damage distributions and accumulation with time within the HAZ are presented, from which the crack initiation times and positions for these welds, under a closed-end condition, and with additional axial (system) loading, were identified. By investigating the propagation of damage through the wall thickness, the remaining lives of the various weld types were estimated. The method provides a means for predicting the initiation and growth of type IV cracks in these CrMoV weldments, and for estimating the length of time a weld can safely be left in service, after damage, or type IV cracking, is identified during inspection.     

Using the results of creep crack incubation tests on CrMoV steel for predicting long time creep rupture properties

The results of a series of creep crack incubation (CCI) tests have been used to examine the effectiveness of the LICON methodology for predicting long duration uniaxial rupture strength of a CrMoV steel at 550° C. The study has revealed that effective long time predictions can be made, but only with the availability of additional information including: an awareness of the short and long time rupture mechanisms and the associated multi-axial rupture criteria obeyed by the material, the results of uniaxial and multi-axial creep tests, and a knowledge of the steady state creep stress conditions existing in the testpieces (structures) forming part of the evaluation.     

Creep constitutive equations for parent,Type IV,R-HAZ,CG-HAZ and weld material in the range 565–640 °C for Cr–Mo–V weldments

Creep deformation and rupture data over the temperature range 565–640 °C for the parent, Type IV, refined-heat affected zone, coarse grained-heat affected zone and weld materials, associated with a 0.5Cr–0.5Mo–0.25V ferritic steel parent material welded with a 2.25Cr–1Mo steel, has been collected and used to determine a constitutive equation parameter set. The constitutive equation parameter set has been generated for use in a future investigation to analyse a welded medium-bore branched vessel tested at 590 °C; and, also to permit extrapolation and interpolation over the temperature range 565–640 °C.