Experimental Investigation of Ductile Tearing Properties for API X70 and X80 Pipeline Steels

This work presents an investigation of the ductile tearing properties for API 5L X70 and X80 pipeline steels using experimentally measured crack growth resistance curves (J-R curves). Testing of the pipeline steels employed compact tension (C(T)) fracture specimens to determine the J-R curves based upon the unloading compliance method using a single specimen technique in accordance with the ASTM E1820 standard procedure. Conventional tensile tests and Charpy V-Notch tests were also performed to determine the mechanical and impact properties for the tested materials. Severe splitting running parallel with the crack propagation path with varied lengths was observed in all tested fracture specimens, particularly for the API X80 material. The occurrence of splits makes the determination of JIc and resistance curves more difficult, as delamination of interfaces positioned normal to the crack front decreases the effective thickness of the test piece, inducing plane stress conditions deep inside the specimen. This experimental characterization provides additional toughness and mechanical data against which the general behavior of X70 and X80 class pipeline steel can be compared.

Impact of PWSCC and Current Leak Detection on Leak-Before-Break Acceptance

This paper assesses the effect of using primary water stress corrosion cracking (PWSCC) crack morphology parameters (roughness, number of turns, and actual flow path/pipe thickness) in determining the difference in the leakage crack length, and how the difference in the leaking crack lengths changes typical margins from past LBB submittals and published reports. Several past LBB submittal cases were selected; in addition, cases from generic LBB reports published by EPRI were also selected. The results of the analyses showed that the past submittals by nuclear steam system supply (NSSS) companies frequently used the surface roughness comparable to an air-fatigue crack with no turns and the actual flow path equal to the thickness of the pipe. This condition would give the shortest possible leakage flaw length. The roughness, number of turns, and actual flow path to thickness ratio for PWSCC cracks were determined from photomicrographs of service-removed cracks. When using the PWSCC crack morphology parameters that corresponded to the crack growing parallel to the long direction of the dendritic grains (V.C. Summer and Ringhals cases), then the leakage flaw length increased 69 percent over the air-fatigue crack length at the same leak rate. Using the same critical crack length as was used in the initial LBB submittals and the published documents, the margins on the crack length changed from 1.77 to 6.0 for the initial submittals (which we also reproduced) to 0.88 to 2.74 from our calculations for a PWSCC crack. If the crack grew in the buttered region of the bimetallic weld, then based on metallographic sections from service-removed flaws, there would be a more tortuous flow path. For this crack condition, in all but one case, the margins on the normal operating versus N+SSE crack lengths were below the safety factor of two required for LBB approval. The average margin decreased from 3.39 for the air-fatigue crack to 1.55 for the PWSCC crack growing transverse to the long direction of the dendritic grains. This was about an additional 20 percent decrease in the margin from the case of having the PWSCC grow parallel to the long direction of the dendritic grains. These results show that LBB is difficult to satisfy for PWSCC susceptible pipe using the current SRP 3.6.3 LBB approach. This LBB assessment did not consider the possible development of a long circumferential surface crack, which would be more detrimental to LBB behavior. Such cracking behavior would violate the LBB screening criterion.

SMILE: A European R&D Program for the Inclusion of Warm Pre-Stress in RPV Assessment

The Reactor Pressure Vessel (RPV) is an essential component liable to limit the lifetime duration of PWR plants. The assessment of defects in RPV subjected to PTS transients made at an European level generally do not take necessary into account the beneficial effect of load history (warm pre-stress WPS). A 3-year Research & Development program — SMILE — has been started in January 2002 as part of the Fifth Framework of the European Atomic Energy Community (EURATOM). The SMILE project (“Structural Margin Improvements in aged-embrittled RPV with Load history Effects”) is one of a “cluster” of Fifth Framework projects in the area of Plant Life Management. It aims to give sufficient elements to demonstrate, to model and to validate the beneficial WPS effect in a RPV integrity assessment. Finally, this project aims to harmonize the different approaches in the European Codes and Standards regarding the inclusion of the WPS effect in a RPV structural integrity assessment. Within the framework of the project, an important experimental work has been conducted including WPS type experiments on CT specimens and also a PTS type transient experiment on a large component. The experimental results on CT specimens confirm the beneficial effect of warm pre-stress, with an effective significant increase of the material resistance regarding the risk of brittle failure. The WPS type experiment on the cylinder has been successfully conducted, with a final brittle failure during the reloading. The present paper describes the aims and objectives of the SMILE project, the main experimental results, and the corresponding analyses based on engineering methods, finite element elastic and elastic-plastic computations, and local approach to fracture.

Reactor Pressure Vessel Modification Approaches

Stress corrosion cracking of Alloy 600 has lead to the modification and replacement of many nuclear power plant components. Among these components are the Bottom Mounted Nozzles (BMN) of the Reactor Pressure Vessel (RPV). Modifications of these components have been performed on an emergent basis. Since that time, Framatome ANP has developed state-of-the-art modification methods for the repair of BMNs using the Electrical Power Research Institute (EPRI) managed Materials Reliability Program (MRP) attributes for an ideal repair as a basis for evaluation of modification concepts. These attributes were used to evaluate the optimal modification concepts and develop processes and tooling to support future modification activity. This paper details the BMN configurations, modification evaluation criteria, several modification concepts, and the development of the tooling to support the optimal modification scenarios.

Predicting Speed of Crack Running in Functionally Graded Material

A theoretical and computational methodology for the analysis of the functionally graded material (FGM) is introduced, and its application is made to the problem of a dynamically propagating crack running transversely in the FGM, where the intensity of the estimated crack-tip severity is managed to keep in valance with the graded material toughness in the FGM during the propagation. To detect the crack-tip severity, an integral fracture parameter, T*, is used. The crack is propagated so that the value of T* is equated to the prescribed varying critical values of T* for the graded material. Emphasis is placed on the use of a fuzzy inference technique in order to control the crack speed, which is deduced from a few T* values immediately preceding the current crack position. As to describing the constitutive law for the FGM, micro-spherical particles of arbitrary size in mesoscale are considered to be randomly dispersed in the matrix medium. By assuming that the volume fraction of the inclusion is continuously varied from 0 to 100 percent in the material, the grading is modeled. For modeling the constitutive law for the FGM composite media of thermo-elastoplasticity, a closed form SCC-LRM constitutive model describing the nonlinear material mechanics of the particle-dispersed medium is used. The model is based on the self-consistent scheme and uses Eshelby’s equivalent inclusion method. Unprecedented analytical results of predicting the crack speed of a crack running transversely in the FGM plate are obtained. In some cases of material grading, apparent crack arresting is observed as the crack runs into the metal rich area of the FGM.

Creep Crack Initiation in a Weld Steel: Effects of Residual Stress

In this paper, the effect of residual stress on the initiation of a crack at high temperature in a Type 347 austenitic steel weld is examined using the finite element method. Both two and three dimensional analyses have been carried out. Residual stresses have been introduced by prior mechanical deformation, using a previously developed notched compact tension specimen. It has been found that for the 347 weld material, peak stresses in the vicinity of the notch are approximately three times the yield strength at room temperature and the level of stress triaxiality (ratio between hydrostatic and equivalent stress) is approximately 1 (considerably higher than that for a uniaxial test). The finite element analysis includes the effects of stress redistribution and damage accumulation under creep conditions. For the case examined the analysis predicts that crack initiation will occur under conditions of stress relaxation if the uniaxial creep ductility of the material is less than 2.5%. Furthermore, the predicted life of the component under constant load (creep conditions) is significantly reduced due to the presence of the residual stress field.

Advanced Post Weld Treatment of a Repair Weld in an Austenitic Header

The repair or replacement of nuclear power components outwith code is a very cumbersome, costly and time consuming exercise which is responsible for long plant shut down time and corresponding loss of electricity production. The aim of the European Union sponsored project ENPOWER was to produce advanced repair techniques and procedures for application in such circumstances to improve the integrity and life of the component and eliminate the need for expensive and sometimes difficult to execute global Post Weld Heat Treatment (PWHT). This paper describes the development and validation of an Alternative Post Weld Treatment (APWT) for a repair weld in an austenitic header component consisting of a 217mm outside diameter by 202mm thick tube plate welded to a 64mm thick cylindrical header. The development of the method using finite element (FE) analysis and the predicted residual stresses before and after application of the APWT are given. Application of the method to a full size mock up of the header is described. Comparison is made between the FE predictions and Deep Hole Drilling through thickness residual stress measurements. This leads to the conclusion that the APWT method is a suitable alternative to a standard PWHT for the control of residual stress in repair welds.

Residual Stress Analysis of Tube Attachment Weld in Pressure Vessel Forging: Comparison of FE Predictions and Measurements

Residual stresses were analysed in a partial penetration weld attaching a tube inside a thick pressure vessel forging, both made of SA508 steel. 2D finite element (FE) analyses methods were used to simulate this multi-pass manual TIG weld. The weld preps are buttered and the forging subsequently heat-treated prior to making the closure weld. Buttering of the forging J-prep and subsequent PWHT creep stress relaxation were modelled. Generally the buttering was found to have minimal influence on the final stress state, although some difference in local peak stress and stress gradients were calculated. Representative test blocks were manufactured, with and without buttered weld preps. Each test block contained two tube penetrations and attachment welds, in order to examine interaction effects. Welding details were captured and peak temperatures recorded by thermocouples were reasonably consistent with the FE model predictions. Surface stresses were measured both in the as-welded condition and after machining, using the hole drilling strain gauge method. Good agreement with FE results was achieved in surface stress levels in the vessel forging, buttering and tube wall. However the 2D model overestimates weld hoop stresses. Large yield magnitude tensile stresses in the vicinity of the joint are balanced by lower compressive stresses in the surrounding PV forging. Interaction effects between the stress fields produced by adjacent tube welds are negligible.

Fatigue Crack Growth Properties of 16MnR and 316L Steels at High Temperature

Fatigue failures often take place in high temperature pressure vessels and equipment because of fluctuation of pressure and temperature. Fatigue crack growth properties of materials at high temperatures are very important for safety assessment of high temperature equipment. A series of fatigue crack growth tests were carried out, and fatigue crack growth rates were determined at 25∼500°C for typical steels 316L and 16MnR. The laws of fatigue crack growth of two materials at different temperatures and the effect of temperature on fatigue crack growth rates were studied. The results show that the crack growth rates increase with temperature for 316L steel. Both the exponent n and constant C for Paris law change with temperature. The fatigue cracks of 16MnR propagate at 150 °C and 300 °C more slowly than at room temperature and 425 °C. The fatigue crack growth rate at 425 °C is the highest for temperature range of 25–425 °C.

Integrity Assessment of Cracked Piping Components: Experimental, Analytical and Numerical Studies

Integrity assessment of piping components is very essential for safe and reliable operation of power plants. Over the last several decades, considerable work has been done throughout the world to develop a methodology for integrity assessment of pipes and elbows, appropriate for the material involved. However, there is scope of further development/improvement of issues, particularly for pipe bends, that are important for accurate integrity assessment of piping. Considering this aspect, a comprehensive Component Integrity Test Program was initiated in 1998 at Bhabha Atomic Research Centre (BARC), India. In this program, both theoretical and experimental investigations were undertaken to address various issues related to the integrity assessment of pipes and elbows. Under the experimental investigations, fracture mechanics tests have been conducted on pipes and elbows of 200–400 mm nominal bore (NB) diameter with various crack configurations and sizes under different loading conditions. Tests on small tensile and three point bend specimens, machined from the tested pipes, have also been done to evaluate the actual stress-strain and fracture resistance properties of pipe/elbow material. The load-deflection curve and crack initiation loads predicted by non-linear finite element analysis matched well with the experimental results. The theoretical collapse moments of throughwall circumferentially cracked elbows, predicted by the recently developed equations, are found to be closer to the test data compared to the other existing equations. The role of stress triaxialities ahead of crack tip is also shown in the transferability of J-Resistance curve from specimen to the component.