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.     

Residual stresses in a CrMoV-2CrMo pipe weld: Part 2—The heat treated condition

The rate at which the three-dimensional residual stresses in a CrMoV-2CrMo main steam pipe weld decay during stress relieving has been determined by means of a finite element creep analysis. The analysis used a two-material model, weld metal and parent metal, and the creep laws ascribed to these materials were based upon appropriate uniaxial stress relaxation data. The manner in which multiaxial residual stresses decay during stress relief is revealed by examination of the stress-time history of relevant locations. The results of the analysis are shown to have significant implications upon stress relieving practice and upon the allowances made for residual stresses in the assessment of defects in such welds.     

The behaviour of ferritic weldments in thick section 12Cr12Mo14V 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 $\text{1}\text{2}\text{Cr}\text{1}\text{2}\text{Mo}\text{1}\text{4}\text{V}$ parent pipe using mild steel and low alloy 1CrMo, 2CrMo and $\text{1}\text{2}\text{CrMoV}$ 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.     

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.     

Analysis of creep in a welded ‘P91’ pressure vessel

The present work considers the uniaxial and multiaxial creep behaviour of the tempered martensite 9%Cr 1%Mo steel ‘P91’ and the creep behaviour of welds in a P91 pressure vessel. The microstructure of a base metal/weld metal transition in a thick section pipe was analysed by means of optical microscopy and hardness measurements. Special emphasis was given to three microstructural states: the base metal (BM), the weld metal (WM) and the intercritical heat affected zone material (IC-HAZ). A significant difference between these three microstructures was their subgrain size, which was measured in the transmission electron microscope and was found to be smallest for the weld metal and largest for the intercritical heat affected zone material. The uniaxial creep behaviour of the three material states was analysed and it was shown that the creep strength increased with decreasing subgrain size. The elastic modulus of P91 was measured and the uniaxial creep behaviour of the three material states was characterized and represented by (i) the Norton law and (ii) in terms of the Robinson model. A welded pressure vessel was creep tested and hoop and axial strains were measured for three welds in the vessel. A creep stress analysis of the welded pressure vessel was performed based on (i) Norton's law and (ii) the Robinson model concentrating on the accumulated hoop and axial strains in the welds. Measured and calculated axial and hoop strains were found to be in good agreement.     

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 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.     

Effect of the weld groove shape and pass number on residual stresses in butt-welded pipes

This study used finite element techniques to analyze the thermo-mechanical behaviour and residual stresses in butt-welded pipes. The residual stresses were also measured in some welds by using the Hole-Drilling method. The results of the finite element analysis were compared with experimentally measured data to evaluate the accuracy of the finite element modelling. Based on this study, a finite element modelling procedure with reasonable accuracy was developed. The developed FE modelling was used to study the effects of weld groove shape and weld pass number on welding residual stresses in butt-welded pipes. The hoop and axial residual stresses in pipe joints of 6 and 10 mm thickness of different groove shapes and pass number were studied. It is shown that these two parameters may have significant effects on magnitude and distribution of residual stresses in welded pipes.     

Residual stresses in a CrMoV-2CrMo pipe weld: Part 1—The as-welded condition

There is a complete lack of reliable data concerning the through thickness distribution of residual stress in welds, both for the as-welded and the stress relieved conditions. As a result, the assessment of defects in welds and current stress relieving practice are largely based on surface measurements of residual stress.To improve this situation a technique has been established which permits the complete distribution of residual stress to be determined. This technique has been applied to a ferritic main steam pipe weld, in the as-welded condition. In establishing this technique, which uses experimental measurements combined with finite element analyses, three different methods of solution were investigated involving line loads, band loads and triangular loads. Only the last was found to give acceptable accuracy.The results of the analysis show that, in general, the residual hoop stresses over the outer two-thirds of the weld are tensile, being compressive over the inner third. Axial stresses are tensile over the outer third and compressive over the inner two-thirds.The approximations and assumptions used in the analysis are discussed, together with the implications that the results have on defect assessment and stress relieving.     

Finite element simulation of welding residual stresses in martensitic steel pipes

Abstract

Finite element (FE) simulations of the welding of two high grade steel pipes are described. The first is a P91 steel pipe welded with a similar P91 weld consumable, and the second is a P92 steel pipe welded with dissimilar nickel–chromium based weld consumables. Both welds are multipass circumferential butt welds, having 73 weld beads in the P91 pipe and 36 beads in the P92 pipe. Since the pipes and welds are symmetric around their axes, the FE simulations are axisymmetric, allowing high FE mesh refinement and residual stress prediction accuracy. The FE simulations of the welding of the P91 and P92 pipes comprise thermal and sequentially coupled structural analyses. The thermal analyses model the heat evolution produced by the welding arc, determining the temperature history throughout the FE models. Structural analyses use the computed temperature history as input data to predict the residual stress fields throughout the models. Post-weld heat treatment (PWHT) of both pipes has also been numerically simulated by assuming that the FE models obey the Norton creep law during the hold time period at 760°C. The residual stresses presented here have all been validated by corresponding experimental measurements. Before PWHT, it has been found that, at certain locations in the weld region and heat affected zone (HAZ) in the pipes, tensile hoop and axial residual stresses approach the tensile strength of the material, presenting a high risk of failure. It has also been found that PWHT substantially reduces the magnitude of residual stresses by varying degrees depending on the material.     

Analysis of residual stresses at weld repairs

In contrast to initial fabrication welds, residual stresses associated with finite length weld repairs tend to exhibit some important invariant features, regardless of actual component configurations, materials, and to some degree, welding procedures. Such invariant features are associated with the severe restraint conditions present in typical repair welding situations. In this paper, residual stress results from several weld repair case studies, using both advanced computational modelling procedures and experimental measurement techniques, are presented and reviewed. From these results, it is evident that weld repairs typically increase the magnitude of transverse residual stresses along the repair compared with the initial weld and that the shorter the repair length the greater the increase in the transverse stress. Also, beyond the ends of the repair the transverse stress sharply falls into compression. For selected cases, predicted stresses are compared with detailed residual stress measurements and the adequacy of finite element simulation procedures is assessed. Welding procedure related parameters (pass lumping, heat input and inter-pass temperature) appear to be more important in analysing weld repairs than in initial fabrication welds. Also great care must be taken when employing simplified two-dimensional cross-section finite element models with applied restraint conditions to simulate the residual stress field at a specific point along the length of a repair.     

Using small punch test data to determine creep strain and strength reduction properties for heat affected zones

Abstract

The small punch (SP) test is a miniature technique that can provide information on creep performance of local features in welded components. However, the multiaxial stress and deformation history in SP test means that it can be a challenge to interpret the test results in a way that is comparable to uniaxial standard creep testing. This work aimed to compare SP test results from as new and service exposed P91 (9Cr–1Mo–V–Nb) base material (BM) and heat affected zones (HAZ) to uniaxial creep testing results from welded new and service exposed material. Two methods are proposed for predicting the uniaxial strain response for any zone of the weld: one alternative is to apply the SP rupture data and to accommodate the shape of the uniaxial base material creep curve to that of the zone of interest in the SP test. The other alternative is to use the SP deflection data and an appropriate translation function to the uniaxial creep curve. In both cases, the Wilshire creep equations have been used as the rupture model. The approach will also predict the stress reduction factors of welds and its constituent parts (including the subzones of HAZ). It is proposed that the approach is used to provide the local constitutive creep models for component assessment by finite element analysis (FEA).     

Effects of prior damage on the creep failure behaviour of similar and dissimilar welded CrMoV main steam pipes incorporating a partial repair

Creep damage finite element analyses, with the inclusion of “prior damage”, were performed for partially-repaired circumferential welds in a thick-walled, main steam, CrMoV pipe. The repair consists of aged parent material, weld metal and one HAZ region being partially excavated and replaced by new weld metal. The pipe welds were subjected to realistic internal pressure and uniform axial loading, the magnitude of the latter being up to that allowed by design codes. The material properties used are related to those of a CrMoV weldment at 640 °C. It is assumed that a full post-weld heat treatment has been carried out and that the effects of welding induced residual stresses reduce to negligible levels. The results obtained are used to examine the subsequent performance for “similar” and “dissimilar” welds with a range of “repair times” (defined as prior damage levels), magnitudes of axial (system) load, etc. From these results, the failure behaviour of this particular partial repair case was evaluated and discussed.     

The behavior of welded joint in steel pipe members under monotonic and cyclic loading

Most steel pipe members are joined by welding. The residual stress and weld metal in a welded joint have the influence on the behavior of steel pipes. Therefore, to accurately predict the behavior of steel pipes with a welded joint, the influence of welding residual stress and weld metal on the behavior of steel pipe must be investigated.In this paper, the residual stress of steel pipes with a welded joint was investigated by using a three-dimensional non-steady heat conduction analysis and a three-dimensional thermal elastic–plastic analysis. Based on the results of monotonic and cyclic loading tests, a hysteresis model for weld metal was formulated. The hysteresis model was proposed by the authors and applied to a three-dimensional finite elements analysis. To investigate the influence of a welded joint in steel pipes under monotonic and cyclic loading, three-dimensional finite elements analysis considering the proposed model and residual stress was carried out. The influence of a welded joint on the behavior of steel pipe members was investigated by comparing the analytical result both steel pipe with a welded joint and that without a welded joint.     

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.     

Experimental determination of stress corrosion crack rates and service lives in a buried ERW pipeline

Threshold stresses and crack growth rates for in-service stress corrosion cracking (SCC) of two electrical resistance weld (ERW) seam welded pipes from two 45-year-old oil pipelines were experimentally assessed. Seventeen high-pH SCC tests were carried out, in both base and ERW weld metals, at two temperatures (73 and 45 °C). Tapered specimens were used for base metal, and constant section specimens were developed for ERW tests, in which original surface conditions were preserved. It was found that susceptibility of the ERW seam welds is much higher than for base materials, so that the welds define the length of the pipe that is susceptible to SCC. Threshold pressure estimates for SCC initiation were defined from tests at elevated temperature, service temperature, and literature correlations. Fabrication residual stresses were also measured and taken into consideration. SCC threshold pressures for these lines are controlled by the ERW welds; the pipe tracts that are considered to be susceptible to SCC are those that undergo a service pressure of at least 2.4 MPa. For the case under study, this represents about 70% of the length of the pipeline.     

Analysis of residual stresses and distortions in T-joint fillet welds

T-joint fillet welds are extensively used in ship engineering and bridge structures. Localized heating from the welding process and subsequent rapid cooling induce tensile residual stress near the toe of the T-joint in fillet welds. Welding produces thermal stresses that cause structural distortions, which influence the buckling strength of the welded structures. This study describes the thermal elasto-plastic analysis using finite element techniques to analyse the thermomechanical behaviour and evaluate the residual stresses and angular distortions of the T-joint in fillet welds. Furthermore, this work employs the technique of element birth and death to simulate the weld filler variation with time in T-joint fillet welds. Also discussed are the effects of flange thickness, welding penetration depth, and restraint condition of welding on the residual stresses and distortions.     

The evaluation of weldment creep strength reduction factors by experimental and numerical simulations

The current design rules for welds are usually based upon the uniaxial creep rupture strength data. The effects of the stress multiaxility and the corresponding stress redistribution process of welded components are relatively ignored. As the present high-temperature testing techniques require large resources when testing welded components in full scale, the simulation of the effects will rely more on the numerical modelling. To evaluate the weldment joint efficiency this paper has proposed a general procedure in which the spatial distribution of constitutive parameters is determined by uniaxial testing while the creep process of components is simulated by numerical methods. Finite element methods are employed in the creep analysis of an AISI 316 butt-welded joint in pressurised tubes with a creep soft weld. To interpret the rupture behaviour of the tubes, different criteria are used to predict the rupture life. On the basis of the predicted structural rupture performance equations, the weldment creep reduction factors are evaluated for different design lifetimes. The reduction factors defined by the ASME code principle are found to be non-conservative in this case.     

Influence of weld misalignment and weld metal overmatch on the prediction of fracture in pipeline girth welds

$\text{J-}\text{integral}$ estimates using elastic-plastic finite element calculations, are shown to compare well with experimental results from single edge-notch bend (SENB) specimens made of X483 grade line pipe steel.The influence of weld misalignment and weld metal overmatch on fracture of cracked girth welds is predicted, also using elastic-plastic finite element calculations. Weld metal overmatch reduces plastic strain levels in the weld and appears beneficial in reducing $\text{J-}\text{integral}$ magnitudes. Predicted values of $\text{J-}\text{integral}$ are shown to increase with weld misalignment. However, $\text{J-}\text{integral}$ magnitudes near the critical value for crack growth initiation were only attained after considerable plastic straining at or near limit load. This result suggests that limit load calculations may be more appropriate for prediction of failure of cracked girth welds than conventional fracture techniques.