Predictions for fatigue crack growth life of cracked pipes and pipe welds using RMS SIF approach and experimental validation

The objective of the present study is to understand the fatigue crack growth behavior in austenitic stainless steel pipes and pipe welds by carrying out analysis/predictions and experiments. The Paris law has been used for the prediction of fatigue crack growth life. To carry out the analysis, Paris constants have been determined for pipe (base) and pipe weld materials by using Compact Tension (CT) specimens machined from the actual pipe/pipe weld. Analyses have been carried out to predict the fatigue crack growth life of the austenitic stainless steel pipes/pipes welds having part through cracks on the outer surface. In the analyses, Stress Intensity Factors (K) have been evaluated through two different schemes. The first scheme considers the ‘K’ evaluations at two points of the crack front i.e. maximum crack depth and crack tip at the outer surface. The second scheme accounts for the area averaged root mean square stress intensity factor (K_RMS) at deepest and surface points. Crack growth and the crack shape with loading cycles have been evaluated. In order to validate the analytical procedure/results, experiments have been carried out on full scale pipe and pipe welds with part through circumferential crack. Fatigue crack growth life evaluated using both schemes have been compared with experimental results. Use of stress intensity factor (K_RMS) evaluated using second scheme gives better fatigue crack growth life prediction compared to that of first scheme. Fatigue crack growth in pipe weld (Gas Tungsten Arc Welding) can be predicted well using Paris constants of base material but prediction is non-conservative for pipe weld (Shielded Metal Arc Welding). Further, predictions using fatigue crack growth rate curve of ASME produces conservative results for pipe and GTAW pipe welds and comparable results for SMAW pipe welds.     

Residual stress simulation in thin and thick-walled stainless steel pipe welds including pipe diameter effects

In this paper, residual stresses in welded components are discussed and a brief review of weld simulation is presented. The general methodology of the FE analysis methods used for welded sections of steel pipes is explained. FE analyses are performed for two axisymmetric butt welds in stainless steel pipes having a 4-pass or a 36-pass weld in a pipe with a wall thickness of 7.1 or 40.0 mm, respectively. In addition, more FE models with inside radius to wall thickness ratio ranging from 1 to 100 have been analysed to investigate the effect of pipe diameter on residual stresses. Residual axial and hoop stresses are plotted for the considered range of pipe diameters for the two simulated pipe wall thicknesses and the differences are discussed.     

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.     

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.     

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.     

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.     

Mechanical property variation within Inconel 82/182 dissimilar metal weld between low alloy steel and 316 stainless steel

In several locations of pressurized water reactors, dissimilar metal welds using Inconel welding wires are used to join the low alloy steel components to stainless-steel pipes. Because of the existence of different materials and chemistry variation within welds, mechanical properties, such as tensile and fracture properties, are expected to show spatial variation. For design and integrity assessment of the dissimilar welds, these variations should be evaluated. In this study, dissimilar metal welds composed of low alloy steel, Inconel 82/182 weld, and stainless steel were prepared by gas tungsten arc welding and shielded metal arc welding techniques. Microstructures were observed using optical and electron microscopes. Typical dendrite structures were observed in Inconel 82/182 welds. Tensile tests using standard and mini-sized specimens and micro-hardness tests were conducted to measure the variation in strength along the thickness of the weld as well as across the weld. In addition, fracture toughness specimens were taken at the bottom, middle, and top of the welds and tested to evaluate the spatial variation along the thickness. It was found that while the strength is about 50–70 MPa greater at the bottom of the weld than at the top of the weld, fracture toughness values at the top of the weld are about 70% greater than those at the bottom of the weld.     

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.     

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.     

Hydrogen environment embrittlement of orbital welded austenitic stainless steels at −50 °C

Different stainless steels were TIG orbital welded resulting in δ-ferrite contents up to 5% in the weld seam. Tensile specimens tested in He atmosphere did fracture at the fusion line/heat affected zone (FL/HAZ), which is the typical failure mode for welded structures. In contrast, all specimens (except the one made of 1.4301) tested in H₂ did not fracture in the FL/HAZ but in the base material. These results clearly show that for the tests performed here δ-ferrite contents up to 5% did not enhance susceptibility to HEE compared to the base material.     

Numerical simulation to study the effect of tack welds and root gap on welding deformations and residual stresses of a pipe-flange joint

This paper presents a three dimensional sequentially coupled non-linear transient thermo-mechanical analysis to investigate the effect of tack weld positions and root gap on welding distortions and residual stresses in a pipe-flange joint. Single-pass MIG welding for a single ‘V’ butt-weld joint geometry of a 100 mm diameter pipe with compatible weld-neck ANSI flange class # 300 of low carbon steel is simulated. Two tack welds at circumferentially opposite locations, with the crucial effect of the tack weld's orientation from the weld start position is the focus in this study. Four different angular positions of tack welds (0 and 180°, 45 and 225°, 90 and 270°, 135 and 315°) are analyzed. In addition, four cases for root gaps (0.8, 1.2, 1.6, 2.0 mm) are considered and computational results are compared. A basic FE model is also validated with experimental data for temperature distribution and deformations. From the results, the axial displacement and tilt of the flange face are found to be strongly dependent on the tack weld orientation and weakly dependent on the root gap.     

Influence of hydrogen content on the tensile properties and fracture of austenitic stainless steel welds

This study evaluates the effects of internal hydrogen on the tensile properties and fracture behaviours of a tungsten inert gas (TIG) weld and an autogenous electron beam (EB) weld of a 304L steel tube. Tensile specimens were thermally charged with hydrogen gas to achieve three different levels of hydrogen in these materials. Metallographic examination revealed that the TIG weld contained skeletal and lathy ferrite, whilst the EB weld displayed a fine dispersion of skeletal and vermicular ferrite. Average volume fractions of ferrite in these welds were 8% and 1%, respectively. The tensile data showed that hydrogen increased the yield and tensile strength, and caused a significant loss in ductility, particularly for the TIG weld. Fractographic analysis revealed that hydrogen induced a change in the fracture mode of the welds and promoted cracking at or near ferrite–austenite interfaces. The TIG weld was found to exhibit a higher susceptibility to hydrogen embrittlement than that of the EB and base metal.     

0Cr18Ni10Ti与（SA387Gr11c12＋堆焊层）异种钢的焊接工艺

针对产品加强管（0Ci118Ni10Ti）与筒体（SA387Gr11C12＋堆焊层）角焊缝异种钢焊接接头焊接问题，提出了先在筒体开孔厚度上焊接隔离层，然后再焊接角焊缝实践证明，采用这种焊接工艺进行焊接，可以获得优良的焊缝，能够保证设备的正常运行、     

Ductile fracture properties for assessing leak-before-break issues in ferritic weldments

A Leak-Before-Break (LBB) approach is being used by Ontario Hydro's Darlington nuclear generating station as a design alternative to pipe rupture restraint hardware on the large diameter piping of the primary heat transport system. The J-resistance curves of four different ferritic weldments, fabricated by either the submerged arc weld (SAW) or shielded metal arc weld (SMAW) process, were determined as part of this program.

Results indicated that the as-welded and post-weld heat treated (PWHT) welds were susceptible to varying degrees of static or dynamic strain aging at 200 and 250°C. Dynamic strain aging effects were most significant for as-welded welds, as evidenced by sudden load drops on the load-displacement curves and ductile crack jumping. The effect of loading displacement rate and PWHT on toughness was assessed and related to the weld's tensile properties and susceptibility to dynamic strain aging. Implications of strain aging to LBB assessments are discussed.     

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.     

The effect of residual stress on the creep deformation of welded pipe

The finite element method has been used to study the creep deformation of pipe butt welds in the presence of an initial residual stress distribution. The study has used values for pipe dimensions, temperature (575°C) and internal pressure (70 bar) which correspond to the conditions being used in a pressure vessel testing programme which is investigating transverse weld metal cracking in CrMoV reheat pipe welds. Two sets of steady state creep data for the weld metal have been used, one to represent a weld weaker than the parent and the other one that was stronger. A residual stress distribution, consistent with experimental data, has been generated as àn initial condition for the analysis. The results are presented, discussing the effects of residual stress on both the total and creep strain accumulations for the two weld properties. The steady state creep laws used in the analysis do not allow for damage accumulation, but the possibility of such an effect is discussed using the Kachanov model. The results of the finite element analysis are compared with the experimental creep strain data from the pressure vessel testing programme.     

Life prediction of repaired welds in a pressurised CrMoV pipe with incorporation of initial damage

Creep damage FE modelling was performed for fully and partially repaired, thick-walled, circumferential pipe weldments, in which initial damage was incorporated into the calculations to take account of the material degradation of the aged materials. The pipe welds were subjected to a realistic internal pressure and axial loading, the latter of which is allowed to vary within the range allowed by design codes. The material properties used are related to a CrMoV weldment at 640 °C. The initial damage distribution was numerically determined using an established procedure. A full post weld heat treatment is assumed to be carried out and the effects of welding induced residual stresses were neglected. The results obtained cover a number of initial damage levels, magnitudes of axial load, and repair excavation depth. On this basis, the sensitivities of the failure life of the repaired welds to these important factors can be evaluated. It was found that both the peak initial damage and the total life are very sensitive to the repair time, particularly when system load is high. The effect of the repair depth for depth: thickness ratios ≥0.5 is generally small for these loadings. There could be a significant benefit if the initial damage in the HAZ of the repair weld, which could be relatively high when the repair time is relatively large, could be reduced by repair welding or by post weld heat treatment.     

Weld overlay repairs from conception to long-term qualification

The weld overlay was initially developed in 1982 as an interim repair for intergranular stress corrosion cracking (IGSCC) flawed welds in boiling water reactor stainless steel piping. Since that time extensive work has been performed to qualify the weld overlay repair technique for longer-term service. Analytical studies of the weld overlay process, experimental programs on laboratory and field specimens, tests to demonstrate the beneficial effects of the weld overlay over its service life, and programs to develop nondestructive examination (NDE) techniques for use in inspecting weld overlays have all yielded positive results. These programs have been sponsored by the utility industry, either individually, through EPRI sponsorship or as part of the BWR Owners' Group (BWROG) effort, and by the US Nuclear Regulatory Commission (USNRC). Weld overlay repairs serve several design functions. These include: structurally reinforcing the flawed location to restore code margins to failure, providing an IGSCC-resistant barrier to crack growth, and imposing a favorable residual stress distribution in the inner portion of the flawed component to inhibit further flaw initiation and growth. The qualification programs confirm that these design functions are maintained with long-term service. A regulatory barrier to long-term operation with weld overlays was the need periodically to inspect overlays and thereby demonstrate continued integrity. Although IGSCC flaws have generally been detected by ultrasonic methods, the repair of these flaws by weld overlay made continued monitoring of the repaired flaw by conventional ultrasonic techniques difficult. Recent developments in ultrasonic examination equipment and techniques, including surface preparation criteria, have enhanced the inspectability of weld overlay repairs and the underlying base material. The original acceptance of weld overlay repairs by the USNRC was for limited service (one or two fuel cycles of operation). However, more recent regulatory guidance, including the recently issued NUREG-0313, Revision 2, recognizes the above technical advances, and provides criteria for acceptance of weld overlays as long-term repairs. Several US utilities have been successful to date in obtaining USNRC approval for extended operation with weld overlays. Some overlays at these plants are currently entering their fourth successful fuel cycle of operation.     

Material characterisation and selection for the international research project “PRIMAVERA”

The international research project “PRIMAVERA” aims to develop a physically validated prediction model for the transition temperature shift dependence (ΔT_K) of VVER-400 weld metals versus neutron fluence after reactor pressure vessel annealing. The reactor pressure vessel of VVER type has several ring welds between the forgings, characterised by large overall dimensions, structural heterogeneity and mechanical properties variations. The weld metal is a mild low-alloy steel with rather complex structure depending on chemical composition, manufacturing technology and heat treatment and the selection of the weld sample is crucial for the success of the project. A weld with high phosphorus content manufactured following standard VVER-440/230 weld technology, has been identified and selected for the project. The detailed characterisation of the selected weld in the un-irradiated condition is the first phase of the research programme and is presented in this paper. Significant phosphorus content variations through the weld could be identified; from 0.022 to 0.044 wt%. Only small variations in mechanical properties were identified; a maximum of 13 °C difference in ductile to brittle transition temperature, mainly related to the distance to the fusion boundary and the base metal. The study also showed that testing of 14 Charpy specimens is sufficient for a correct evaluation of transition temperature. A larger number of tests are required for base metal compared with the weld metal.