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.     

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.     

Analysis of cross-weld creep rupture data on the ½Cr½Mo¼V type IV zone

For the ferritic steel ½Cr½Mo¼V, the operating experience of medium and long term creep failures in power station welds has been of cavitation and cracking in the heat affected zone (HAZ) adjacent to the parent material, in what is referred to as the Type IV zone. This experience has led to the generation of uniaxial cross-weld creep rupture data on ½CrMoV weldments and to the development of life assessment procedures such as R5 Volume 7, which uses the rupture strength of the Type IV zone to calculate the life of power station components. Recently, the ECCC Working Group 3A has conducted a collation of the UK and German cross-weld creep rupture data on ½CrMoV weldments. Creep failure in ½CrMoV cross-weld specimens occurs in a variety of weldment zones; typically in the parent, the Type IV zone or the weld metal. Post test examination of the specimens has enabled those tests that failed in the Type IV zone to be identified and a creep rupture data assessment has been performed to derive a new model for the rupture strength of the Type IV zone.     

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.     

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.     

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.     

Fatigue crack initiation from defects at weld toes in steel

The mechanisms of fatigue initiation and microcrack growth at the toes of good quality manual and automatic T-plate welded joints in steel were investigated, using a compliance method to measure crack depth and closure levels, and three-stage ink staining, replica techniques and SEM microscopy to monitor crack growth. Minor undercuts, slag inclusions and surface spatter at the weld toes were detected as fatigue initiation sites in manual welds. Fatigue initiation in automatic welds was mainly associated with undercut-type defects. Details of their propagation and coalescence early in the fatigue process are documented. Delayed microcrack coalescence in the case of manual welds appears to be related to the more irregular toes of the manual welds, when compared to their automatic counterparts.     

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.     

A technique to produce automatic welds with enhanced fatigue crack propagation lives under transverse loading

In order to contribute to the development of an automatic weld process that introduces a controlled degree of irregularity to the weld toe, as a means to enhance the development and interaction of cracks and improve fatigue resistance, the double arc rotating technique (DART) was developed. Beadon-plate welds were fabricated using this method. Weld toes were fatigue tested by three point bending in the transverse and longitudinal directions. The fatigue propagation behaviour of cracks growing from the weld toes, and its relationship with defined geometric parameters, were modeled using a fracture mechanics model. Compared with standard straight welds, fatigue life improved up to 111% when loaded in the transversal direction, while decreased about 10% in the longitudinal direction. A fatigue life prediction method for welds with controlled toe irregularity, considering defined toe geometry parameters, is presented, and results are discussed.     

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.     

Weld repair of low alloy creep resistant steel castings without preheat and post-weld heat treatment

During recent years, a weld repair technique for low alloy CrMoV steel castings, using CrMoV weld metal, which dispenses with preheat and post-weld heat treatment, has been developed. This so-called ‘cold’ weld repair technique is economically attractive because of the potential reduction in down time that it offers. However, its adoption has been delayed because of fears of the risk of brittle fracture at start-up under the action of welding residual stresses and thermal stresses.The present report presents fracture toughness data for untempered CrMoV weld metal, deposited by CrMoV electrodes, so that the risk of fast fracture can be assessed. Additional data on creep ductility of the weld metal are presented and discussed with reference to the longer-term integrity of repairs.     

Enhanced methodologies used for the assessment of high-temperature header girth welds

Inspections performed during 2003 and 2004, have shown a significant increase in the number of occurrences of advanced cracking in superheater and reheat header girth welds. These cracks have been observed:

• at several plants within different utilities

• in both header body girth welds and header-to-tee girth welds

• in headers of different manufacturers and

• in headers with and without ligament or stub tube weld cracking.

In addition, there has also been an increase in the number of main steam and hot reheat piping girth weld failures that exhibit characteristics similar to the aforementioned header girth weld cracking. In a number of incidences cracking has been confirmed to have initiated mid-wall and in some cases propagated to through-wall failure. The use of linear phased array ultrasonic testing (UT) has been shown to be effective

• in determining girth weld profiles,

• in the detection and sizing of macro-cracking, and

• in locating macro-damage relative to the weld geometry.

Similarly, annular phased array UT has been shown to be effective in the detection of incipient creep cavity development before the onset of microcracking. In addition, long-range guided wave UT has been used on piping systems to determine the location and type (e.g., field or shop) welds without extensive scaffolding or insulation removal and with the anticipated advantage of being a screening technology for welds with extensive cracking.This paper will present Structural Integrity Associates (SI) experience with high-temperature header girth weld cracking and the enhanced non-destructive examination (NDE) techniques applied to the detection and quantification of damage.     

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.     

Application of a modified damage potential to predict ductile crack initiation in welded pipes

The mesh dependency of the cavity growth model due to Rice and Tracey has been overcome by integrating it over a process zone surrounding the crack tip. This integral represents a modified damage potential. The critical value of the integral for crack initiation in weld material has been determined from a welded CT specimen by comparing the computed J with the experimentally measured J-initiation value. This critical value is then employed in 8 and 12 in. welded pipes having different through-wall cracks at the centre to predict the crack initiation loads under four-point bending. Close agreement between the computed crack initiation loads and the experimentally measured values justifies the usefulness of the present modified damage potential.     

T23水冷壁管排鳍片焊缝裂纹的分析

SA213-T23钢因具有良好的高温蠕变强度和焊接工艺性能,而被电站锅炉广泛采用,但SA213-T23用于百万等级螺旋水冷壁,因焊接应力、热校正等因素造成管子与扁钢角焊缝中心出现裂纹.主要对该钢种应用在螺旋水冷壁鳍片管时,角焊缝产生结晶裂纹的形成原因及防止措施进行研究.     

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.     

A trigger of Type IV damage and a new heat treatment procedure to suppress it. Microstructural investigations of long-term ex-service Cr–Mo steel pipe elbows

Two seam welded Cr–Mo steel pipe elbows that had been used as main steam piping were investigated. Type IV damage was observed at heat-affected zones of seam welds. Microstructures were investigated by scanning electron microscopy and it was found that micro-cracks and polygonal hollows were distributed as if they were on the former austenite grain boundaries. Fine bainite grains surrounded by coarse carbides and creep voids were also distributed in the same manner.Dense precipitations of carbides (DPCs) were found on extraction replicas by transmission electron microscopy. Because of the similarities in shapes, sizes and distributions, DPCs were regarded as root causes of the hollows and the cracks. Heterogeneity of carbon content at former austenite grain boundaries was suggested to be the origin of DPCs. Based on the results of the investigation, a new heat treatment procedure to suppress Type IV damage was proposed.     

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.     

Crack initiation and crack growth assessment of a high pressure steam chest

Extensive cracking had occurred in a number of high pressure steam chests. An assessment was undertaken based on the R5 British Energy methodology to assess the components for both creep–fatigue damage initiation and crack growth analysis to determine fitness for purpose. The analysis determined that the remaining base rupture endurance life of the component was greater then 1 million hours, however, due to the start-up and shutdown ramp rates, creep–fatigue damage greater then unity has occurred leading to crack initiation in a number of locations. These cracks were confirmed during internal inspection of the steam chest. A subsequent crack growth analysis determined that the component could safely be returned to service for the expected future life of the station.