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ASME Conference Presenter Attendance Policy and Archival Proceedings

2014;():V06BT00A001. doi:10.1115/PVP2014-NS6B.
FREE TO VIEW

This online compilation of papers from the ASME 2014 Pressure Vessels and Piping Conference (PVP2014) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Materials and Technologies for Nuclear Power Plants

2014;():V06BT06A001. doi:10.1115/PVP2014-28013.

This paper presents an overview of the work undertaken by Rolls-Royce to justify the use of a Hot Isostatically Pressed (HIP) Reactor Circulating Pump (RCP) bowl manufactured in 316L stainless steel for a Pressurised Water Reactor (PWR) application. It presents the work from a design justification/manufacturing quality assurance perspective, rather than from a pure metallurgical perspective.

Although the HIP process is not new, it was new in its application to Rolls-Royce designed nuclear reactor plant. As a consequence, Rolls-Royce has implemented an evolving, staged approach, starting with HIP bonding of solid valve seats into small bore valve pressure boundaries. This was followed by powder HIP consolidation of leak-limited, thin-walled toroids, and has culminated in the powder HIP consolidation of thick-walled components such as RCP bowls. The paper provides an overview of each of these stages, the method of manufacture for the RCP bowl, and the approach taken with respect to justification.

In previous Rolls-Royce applications of HIP to PWR plant components, the component section thickness has been fairly consistent. For the RCP bowl application, the section thickness varies quite considerably. To assess any variation in properties as a result of section thickness variation, a stepped wedge technology demonstrator was first manufactured and non-destructively and destructively examined to assess whether material properties remain within specification. The paper presents material property results for ‘Near Surface’ and ‘Buried’ samples taken from section thicknesses covering 50mm to 300mm. Yield, Ultimate Tensile Strength (UTS), Charpy ‘V’ notch impact strength and Strauss test results were found to be within specification for all section thicknesses.

An analysis of the data shows no statistical difference in material properties across the different section thicknesses for both near surface and buried properties, except for Charpy ‘V’ notch impact strength, for which, above 50mm, mean ‘Buried’ strength is significantly higher than mean strength for ‘Near Surface’.

It is observed that the mean ‘Near Surface’ yield and tensile strength appears to increase with section thickness; however, there are insufficient data to demonstrate whether or not the increase is statistically significant.

There appears to be a reduction in mean ‘Buried’ yield strength when the section thickness is at 300mm, compared to thinner sections, but again, the current data are insufficient to determine whether or not the reduction is statistically significant. It could be postulated that there is a point at which the section thickness starts to influence the cooling rate, and as a consequence grain growth may be more prominent at the ‘Buried’ position. This could result in yield and tensile material properties being detrimentally affected. However, this is not supported by an analysis of Charpy ‘V’ notch impact strength results, for which a similar reduction would be expected; the highest ‘Buried’ Charpy results occur in the thickest section. Further work is ongoing to understand this observation.

Non-destructive examination results of a prototype RCP bowl are presented. This shows no defects identified from dye penetrant surface examination and ultrasonic testing for a rejection level set at a 3mm flat bottomed hole.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A002. doi:10.1115/PVP2014-28040.

Ensuring mechanical properties of carbon and low alloy steels after deformation is of major concern since the building process of heavy (i.e. thick-walled) pressure vessels may be directly impacted. Indeed, thick plates encounter forming and welding operations that may modify as-delivered properties. From both technical and economical points of view, cold forming is usually preferred. This technique is nowadays widespread and new rolling equipments display sufficient power to handle plates up to at least 250mm thick.

Current limitations are now mainly related to maximum admissible strain in materials and regulation rules resulting from construction codes. The ASME Boilers and Pressure Vessels Construction Code on the American side and the EN 13445 Unfired Pressure Vessels Construction Code on the European side, both allow the use of as-strained material up to maximum 5% plastic (i.e. permanent) strain without any subsequent heat treatment operation.

Above 5% plastic deformation, on one hand the European code requires a full quality treatment (meaning high temperature austenitization treatment, then cooling in air (normalizing – N) or in accelerated conditions (quenching – Q or accelerated cooling – NAC), followed by a Tempering treatment T) and on the other hand the ASME code only requires Tempering that can even be carried out using the mandatory Post Weld Heat Treatment (PWHT) needed by welded zones.

However, it is of high importance to note that thick vessels are always submitted to a final PWHT to insure sufficient toughness in welded zones. This final PWHT is performed whatever the deformation obtained during plate rolling. In practice, there are no thick vessels made out of plates in as-strained conditions.

Avoiding a full quality treatment as demanded per EN 13445 rules is of major interest for fabricators as it allows to decrease the delivery time, the risk of appearance of problematic issues (uncontrolled deformations of the vessel during high temperature treatments…) and significantly reduces the overall fabrication costs.

This paper focuses on the effect of strain on conventional mechanical properties for steel grades widely used for the fabrication of heavy pressure equipments (i.e. tensile properties, hardness, Charpy V toughness) for different strain levels. In particular, it points out and discusses PWHT effects on properties of various pre-strained materials, showing that there is no need for full quality heat treatment.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A003. doi:10.1115/PVP2014-28091.

During Refueling Outage 18 (RFO18, April 2011) Pilgrim Nuclear Power Station (PNPS) identified crack-like indications on the Steam Separator Lifting Lugs. A multi-disciplinary engineering effort was undertaken to determine the cause of the cracking and prepare the technical justification for long term operation of the lifting lugs. This approach focused on addressing the potential for future degradation due to the existing indications, and the resulting effects on the hardware and its function. A materials evaluation concluded that intergranular stress corrosion cracking (IGSCC), most likely associated with cold work present in the as-fabricated steam separator, was the cause of the indications found on the lifting lugs. To support long term operation of the lifting lugs at PNPS, a structural evaluation was completed using ANSI N14.6 and NUREG-0612 criteria with a conservative bounding configuration. Crack growth rates, based on BWRVIP-76 guidance of 5E-5 in/hr for length and 2.2E-5 in/hour for depth, were used in the analyses. The evaluation concluded that PNPS could continue with long term operation of the Steam Separator. Consistent with standard practice, a general heavy loads examination was performed in 2013 (RFO19), confirming no discernable changes. The general examination will be repeated in 2015 (RFO20), and an examination of the lifting lugs is planned for the 2017 Refueling Outage (RFO21) to confirm that the indication behavior is consistent with the evaluation results.

Topics: Steam
Commentary by Dr. Valentin Fuster
2014;():V06BT06A004. doi:10.1115/PVP2014-28237.

Low cycle fatigue tests were conducted for carbon steel, STS410, low alloy steel, SFVQ1A, and austenitic stainless steel, SUS316NG, which were used for nuclear power plants, in order to investigate the mechanism of fatigue damage when the plants were subjected to huge seismic loads. In these tests, the surface behavior of fatigue crack initiation and growth was observed in detail using cellulose acetate replicas, while the interior behavior was detected in terms of fracture surface morphology developed by multiple two-step strain amplitude variations with periodical surface removals. Fatigue crack growth rates were evaluated by elasto-plastic fracture mechanics approach. For SFVQ1A and SUS316NG, the fracture mechanics approach is available in order to predict the crack growth life from the metallurgical crack initiation size to the final crack length of the specimens. For STS410, numerous small cracks initiated, grew and coalesced each other on the specimen surface under low cycle fatigue regime.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A005. doi:10.1115/PVP2014-28424.

In the nuclear industry surface mechanical treatments are used in order to improve the surface integrity of the component, which increases their lifetime regarding corrosion and fatigue damages. A good understanding of these processes and their consequences is required to ensure the efficiency and perpetuity of such mitigation treatment.

This study focuses on the ultrasonic shot peening process. It consists in shooting at high speed small steel beads on the part to be treated by using a high frequency vibration device. Parameters such as the number and the size of beads, the input frequency and the dimensions of the chamber can induce large ranges of impact velocity and coverage. In order to help manufacturers to control the treatment applied on their components, a numerical model has been developed. It accounts for the shocks of the beads against the walls of the chamber, the peening head and between beads, describing their motions accurately.

In this paper, we will introduce the numerical model developed to simulate the motions of beads in the peening chamber. Special attention will be taken to the determination of the restitution rates related to the different materials. Results of the model will be shown for different process parameter (e.g. the number of beads), and a thorough analysis of their effects on the workpiece will be presented, including a comparison with some experimental results.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A006. doi:10.1115/PVP2014-28443.

In this work, we outline the development of an automated, high-throughput robotic system designed for the structural characterization of radioactive samples at the X-ray Powder Diffraction beamline of the National Synchrotron Light Source-II (NSLS II).

Commentary by Dr. Valentin Fuster
2014;():V06BT06A007. doi:10.1115/PVP2014-28504.

The paper will cover the general approach of welding requirements in RCC-M 2012Ad2014[1], from a technical and quality point of view.

It will be clarified how EN standards has been taken in account, mainly due to developments pushed by Pressure Equipment Directive (97/23/EC)[2] in Europe. It will be explained how nuclear activities for welding need to restrain the use of standards developed firstly for the needs of non nuclear industries. New ISO/ EN standards related to welding (ISO 9606-1, ISO 15614-1, ISO 3834) will be considered, and how they may lead to international cohabitation, compatibilities or harmonization, between nuclear codes.

Topics: Welding
Commentary by Dr. Valentin Fuster
2014;():V06BT06A008. doi:10.1115/PVP2014-28919.

The standard master curve approach has the major limitation, which is only applicable to homogeneous datasets. In nature, steels are macroscopically inhomogeneous and thus the fracture toughness has larger scatters than expected by a conventional master curve approach. RPV steel has different fracture toughness with varying distance from the inner surface of the wall. Regarding this, a clear tendency was reported in that the toughness extracted near the surface had to be higher than in the center region due to the higher quenching rate at the surface (deterministic material inhomogeneity). On the other hand, the T0 value itself behaves like a random parameter when the datasets have a large scatter due to the datasets consisting of several different materials such as welding region (random inhomogeneity). In the present paper, four regions, the surface, 1/8T, 1/4T and 1/2T, were considered for fracture toughness specimens of KSNP (Korean Standard Nuclear Plant) SA508 Gr. 3 steel to provide deterministic material inhomogeneity and random inhomogeneity effect. Specimens were extracted from these four regions and fracture toughness tests were performed at various temperatures in the transition region. Several concepts were provided for the master curve of inhomogeneous materials such as a bimodal and random inhomogeneous master curve scheme, and among them, the bimodal master curve analyses were reviewed and compared with a conventional master curve approach to find the random inhomogeneity. The bimodal master curve considering inhomogeneous materials provides better description of scatter in fracture toughness data than conventional master curve analysis, but it is unclear to provide evidence that the bimodal analysis lines follow the data more closely than the conventional master curve analysis.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Materials for Hydrogen Service

2014;():V06BT06A009. doi:10.1115/PVP2014-28181.

Understanding the micromechanisms of hydrogen-assisted fracture in multiphase metals is of great scientific and engineering importance. By using a combination of scanning electron microscopy (SEM), scanning tunneling microscopy (STM), atomic force microscopy (AFM) and magnetic force microscopy (MFM), the micromorphology of fracture surface and microcrack formation in hydrogen-precharged super duplex stainless steel 2507 are characterized from microscale to nanoscale. The results reveal that the fracture surfaces consist of quasi-brittle facets with riverlike patterns at the microscale, which exhibit rough irregular patterns or remarkable quasi-periodic corrugation patterns at the nanoscale that can be correlated with highly localized plastic deformation. The microcracks preferentially initiate and propagate in ferrite phase and are stopped or deflected by the boundaries of the austenite phase. The hydrogen-assisted cracking mechanisms in super duplex stainless steel are discussed according to the experimental results and hydrogen-enhanced localized plasticity theory.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A010. doi:10.1115/PVP2014-28236.

Hydrogen gas container is one of the critical components for fuel cell electric vehicle (FCEV), which is expected for CO2-free personal transportation. In order to choose an appropriate material for its metal boss and liner, crack growth resistance should be evaluated for various aspects such as fatigue crack growth (FCG) and stress corrosion cracking (SCC) in salt water or humid air environments for the purpose of commercial vehicle use. In the present study, FCG tests were carried out for A6061 and A6066 alloys in laboratory air and in 3.5% NaCl solution for compact (CT) and single edge notched (SEN) specimens. Some SEN specimens were cut from machined hydrogen container made of A6066 at the neck and the shoulder locations. SCC tests were carried out for A6061, A6066 and A6351 (fine and coarse grains) alloys in 3.5% NaCl solution and in humid air for CT specimen.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A011. doi:10.1115/PVP2014-28280.

In order to reveal the effect of indentation load on Vickers hardness of austenitic stainless steel after hydrogen charging, the Vickers hardness measurements have been conducted with three different indentation load of 0.49, 1.96 and 9.80 N on the surface of type 316L austenitic stainless steel after hydrogen charging. Relationship between plastic deformation behavior during indentation process and hydrogen absorption behavior was revealed. In the Vickers hardness test, Vickers hardness keeps same value though the indentation load varies. Needless to say, the value did not depend on magnitude of the indentation load before hydrogen charging in the present study. However, the Vickers hardness increased along with hydrogen charging time and, interestingly, the increase in the Vickers hardness due to the presence of hydrogen depends on magnitude of the indentation load. In the load of 0.49 N and 9.80 N, the Vickers hardness has a maximum value of 3.04 and 2.04 GPa which is 1.58 and 1.15 times larger than value of 1.73 and 1.70 GPa before hydrogen charging, respectively. The hydrogen-induced hardening behavior observed by the Vickers hardness tests employing different indentation load would be evaluated by the relationship between the plastic deformation depth and the hydrogen absorption depth.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A012. doi:10.1115/PVP2014-28291.

The current international standards and codes dedicated to the design of pressure vessels do not properly ensure fitness for service of such vessel used for gaseous hydrogen storage and subjected to hydrogen enhanced fatigue. Yet, hydrogen can reduce the fatigue life in two ways: by decreasing the crack initiation period and by increasing the fatigue crack growth rate. The European project MATHRYCE aims are to propose an easy to implement vessel design methodology based on lab-scale tests and taking into account hydrogen enhanced fatigue.

The study is focused on a low alloy Cr-Mo steel, exhibiting a tempered bainitic and martensitic microstructure, and classically used to store hydrogen gas up to 45 MPa. Due to hydrogen diffusion at room temperature in such steel, tests have to be performed under hydrogen pressure to avoid outgassing.

In the present work, experimental procedures have been developed to study both crack initiation and crack growth. The specimens and tests instrumentation have been specifically designed to quantitatively measure in-situ these two stages under high hydrogen pressure. We developed and tested crack gages located close to a small drilled notch. This notch simulates the presence of steel nonmetallic inclusions and other microstructural features that can affect fatigue crack initiation and propagation. The experimental results addressing the effects of the testing conditions, such as stress ratio, frequency and hydrogen pressure will be compared to the local strain and stress fields obtained by Finite Element Method and correlated to the possible hydrogen enhanced fatigue mechanisms involved.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A013. doi:10.1115/PVP2014-28364.

Due to the increasing demands for light oil in newly industrializing countries and depletion of conventional oil resources, upgrading of heavy oil and coal-to-liquid processes have been a focus in recent years. The efficiency of these processes depends on temperature and pressure conditions, where a higher temperature, around 500°C, is likely to be used. However, 2¼Cr-1Mo-V steels which have been widely used for heavy-wall pressure vessels for many years cannot be applied to a high temperature process around 500°C since the design temperature of this material is limited to 482°C by ASME Code Section VIII, Division 2 [1].

On the other hand, 9Cr-1Mo-V steels (Grade 91), which has an excellent performance at high temperature in mechanical properties and hydrogen resistance, has been used for tubing and piping materials in power industries and it can be a candidate material for the high temperature processes. However it has not been used for pressure vessels in refining industries.

In order to manufacture heavy-wall pressure vessels using 9Cr-1Mo-V steels, essential techniques including manufacture of large forged shell rings, thick wall welding and overlay welding have been developed.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A014. doi:10.1115/PVP2014-28381.

In this study, the effect of gaseous hydrogen on the fatigue crack growth behavior in a precipitation-hardened martensitic stainless steel is investigated. It is known that the degradation in fatigue crack growth behavior derives from a complex interaction between the fatigue damage and the amount of hydrogen enriching the crack tip, which is dependent on the hydrogen pressure, loading frequency, and stress intensity factor amplitude. Therefore, fatigue crack growth tests were performed in a range of 0.09 to 40 MPa under gaseous hydrogen at a frequency of 20 and 0.2 Hz. The fatigue data as well as fracture morphologies obtained so far indicate a sharp increase in crack growth rates in a narrow range of stress intensity factor amplitudes. Also, it is shown that by decreasing the loading frequency to 0.2 Hz at a given pressure of hydrogen the transition occurs at lower values of stress intensity factor amplitudes accompanied by a change in fracture mode. Scanning electron microscope (SEM) observations of the fracture surfaces are used to support the explanations proposed to account for the observed phenomena.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A015. doi:10.1115/PVP2014-28511.

Slow strain rate tests using smooth specimens of two types of steels, low alloy steel JIS-SCM435 and carbon steel JIS-SM490B, were carried out in nitrogen gas and hydrogen gas under a pressure of 115 MPa at three different temperatures: 233 K, room temperature and 393 K. In nitrogen gas, these steels exhibited the so-called cup-and-cone fracture at every temperature. On the other hand, in hydrogen gas, in both steels a number of cracks initiated on the specimen surface and coalesced with each other at every temperature, which led to a marked reduction in ductility. Nonetheless, even in hydrogen gas, JIS-SCM435 exhibited a certain reduction of area after the stress-displacement curve reached the tensile strength (TS), whereas JIS-SM490B exhibited little, if any, necking in hydrogen gas. In addition, tension-compression fatigue testing at room temperature revealed that in both steels there was no noticeable difference between the fatigue strengths in air and 115MPa hydrogen gas, especially in a relatively long life regime. Considering that there was little or no hydrogen-induced degradation in either TS or fatigue strength in JIS-SCM435, it is suggested that JIS-SCM435 is eligible for fatigue limit design on the basis of a safety factor (i.e. TS divided by the allowable design stress) for mechanical components used in hydrogen gas up to 115 MPa.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A016. doi:10.1115/PVP2014-28578.

9Cr-1Mo-V steel with excellent high temperature strength is one of candidate materials for advanced pressure vessels in oil-refining plants, whose process temperature is expected to be around 500°C. Although 9Cr-1Mo-V steel has been applied as boiler tube material in power generation for a few decades, it was reported that embrittlement occurred after long-term aging around 600°C which is accelerated condition for pressure vessel operation. Since pressure vessels are more sensitive in stress-concentration around crack tip than boiler tube because of its large wall thickness, fracture toughness is an important property of concern when 9Cr-1Mo-V steel is applied to pressure vessels. In this research, 9Cr-1Mo-V steel with tempered-martensitic microstructure was aged up to max. 10000 hr at 500, 550 and 600°C, and fracture toughness was evaluated after the aging by Charpy impact test.

The influence of heat treatment conditions such as austenitizing, tempering and PWHT were also investigated, because the heat treatment conditions used in pressure vessels are different from those of boiler tube. In case of samples treated under the conditions for pressure vessels, Charpy impact values at 0°C were sufficient around 200J before aging, and decreased after aging depending on its conditions, and longer time and higher temperature led to more severe degradation. When the aging time at 550°C and 600°C was converted to the equivalent aging time at 500°C by Larson-Miller-equation, the impact value was estimated to keep over 50J after several decades at the operating temperature for pressure vessels. In contrast to the conditions for pressure vessels, the heat treatment conditions used in boiler tube made initial impact value decreased significantly, because tempering and PWHT were shorter than those of pressure vessels. Therefore, the samples heat treated under boiler tube conditions showed lower impact values around 50J in the earlier stage of aging. Considering all obtained results, it was suggested that the serious degradation of fracture toughness in 9Cr-1Mo-V after long term aging would not occur in actual service time for pressure vessels.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A017. doi:10.1115/PVP2014-28581.

In this paper, the tensile fatigue tests using small compact specimen were conducted by the corrosion and hydrogen embrittlement fatigue testing method which authors have proposed. Based on these results, the quantitative separated estimations of the sensitivity of Corrosion, Hydrogen Embrittlement (HE) and Facet-like Fracture Hydrogen Induced Dislocation Pile-up (F-HIDP) were made by using the acceleration factor, D*, that is the ratio of fatigue crack growth rate to that under atmospheric condition.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A018. doi:10.1115/PVP2014-28584.

Hydrogen penetrates into the metal and causes Hydrogen embrittlement due to the increase in hydrogen concentration. This is caused by the local stress fields such as residual stress field at the site of welding or local stress field around a crack tip. It accompanied with incubation time of several hours since the components were exposed to hydrogen atmospheric condition. This incubation time is time lag of hydrogen diffusion and concentration at the site where the hydrogen embrittlement occurs. Therefore, clarification of the hydrogen diffusion behavior is important to prevent from fracture of hydrogen embrittlement.

In this paper, the numerical analyses of hydrogen diffusion around weld part including HAZ (Heat Affected Zone) under residual stress coupled with that of heat transfer during the cooling process before and after weld were conducted and the behaviors of hydrogen concentration were analyzed. On the basis of these analyses, the method of heat treatment to prevent from hydrogen concentration at the weld part was investigated. Results obtained by these analyses showed that pre weld heat treatment is effective in the prevention of hydrogen concentration and combined pre weld heat treatment with post weld heat treatment was found to be the most effective treatment.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A019. doi:10.1115/PVP2014-28604.

Pressure cycle tests were performed on two types of Cr-Mo steel pressure vessels with inner diameters of 306 mm and 210 mm and notches machined on their inside under hydrogen-gas pressures, varied between 0.6 and 45 MPa at room temperature. One of the Cr-Mo steels had a fine microstructure with tensile strength of 828 MPa, while the other had a coarse microstructure with tensile strength of 947 MPa. Fatigue-crack growth (FCG) and fracture-toughness tests of the Cr-Mo steels were also carried out in gaseous hydrogen. The Cr-Mo steels showed accelerated FCG rates in gaseous hydrogen compared to ambient air with an upper bound corresponding to an approximately 30-times higher FCG rate. Furthermore, in gaseous hydrogen, the fracture toughness of the Cr-Mo steel with coarse microstructure was significantly smaller than that of the steel with fine microstructure. Four pressure vessels were tested; then, all of the pressure vessels failed by leak-before-break (LBB). Based on the fracture-mechanics approach, the LBB failure of one pressure vessel could not be estimated by using the fracture toughness in gaseous hydrogen. The fatigue lives could be estimated by using the upper bound of the accelerated FCG rates in gaseous hydrogen.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A020. doi:10.1115/PVP2014-28640.

Slow strain rate tests (SSRTs) were performed with two types of high-strength austenitic stainless steels, Types AH and BX, as well as with two types of conventional austenitic stainless steels, Types 304 and 316L. The tests used the following combinations of specimen types and test atmospheres: (i) non-charged specimens tested in air, (ii) hydrogen-charged specimens tested in air (tests for internal hydrogen), and (iii) non-charged specimens tested in hydrogen gas at pressures of 78 ∼ 115 MPa (tests for external hydrogen). Type 304 exhibited a marked reduction of ductility in the tests for both internal hydrogen and external hydrogen, whereas Types AH, BX and 316L exhibited little or no degradation. In addition, fatigue crack growth (FCG) tests for the four types of steels were also carried out in air and hydrogen gas at pressures of 100 ∼ 115 MPa. In Type 304, FCG in hydrogen gas was more than 10 times as fast as that in air, whereas the acceleration rate remained within 1.5 ∼ 3 times in Types AH, BX and 316L. It was presumed that, in Types AH and BX, a small amount of additive elements, e.g. nitrogen and niobium, increased the strength as well as the stability of the austenitic phase, which thereby led to the excellent resistance against hydrogen.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A021. doi:10.1115/PVP2014-28677.

This paper addresses the material selection and safety validation of hydrogen steel tanks for stationary applications. Tensile, fatigue and crack growth behavior of HSLA steels in high pressure gaseous hydrogen are briefly reviewed and material qualification procedure using fatigue analysis was proposed.

Topics: Steel , Safety , Hydrogen
Commentary by Dr. Valentin Fuster
2014;():V06BT06A022. doi:10.1115/PVP2014-28720.

Hydrogen substantially reduces fracture properties such as threshold stress intensity factor KIH and tearing resistance dJ/da in conventional Cr-Mo steels. In order to enable the life assessment of a reactor with a hydrogen-induced crack using a failure assessment diagram (FAD), an experimental database of hydrogen-assisted subcritical crack growth rates da/dt is requisite. However, there are very few studies concerning the effects of hydrogen- and temper-embrittlement on da/dt at ambient temperatures in 2.25Cr-1Mo steels with high and low impurity levels. In this paper, vacuum melted lab heats of 2.25Cr-1Mo steel were supplied with compositional controls. Some specimens were embrittled by step cooling heat treatment (SCHT). Subcritical crack growth rate at a constant load was obtained by means of the potential drop method for 2.25Cr-1Mo steel with initial internal hydrogen (3.2 mass ppm).

Commentary by Dr. Valentin Fuster
2014;():V06BT06A023. doi:10.1115/PVP2014-28811.

Type 316/316L austenitic stainless steels are considered the benchmark for resistance to hydrogen embrittlement in gaseous hydrogen environments. Type 316/316L alloys are used extensively in handling systems for gaseous hydrogen, which has created engineering basis for its use. This material class, however, is relatively expensive compared to other structural metals including other austenitic stainless steels, thus the hydrogen fuel cell community seeks lower-cost alternatives. Nickel content is an important driver of cost and hydrogen-embrittlement resistance; the cost of austenitic stainless steels is largely determined by nickel content, while high nickel content generally improves resistance to hydrogen embrittlement. These circumstances create the perception that less-expensive grades of austenitic stainless steels are not appropriate for hydrogen service. While other grades of austenitic stainless steels are generally more susceptible to hydrogen embrittlement, in many cases the hydrogen-affected properties are superior to the properties of materials that are considered acceptable, such as aluminum alloys and A-286 austenitic stainless steel. In this paper, the properties of a variety of austenitic stainless steels are compared with the aim of promoting the consideration of a wider range of austenitic stainless steels to reduce cost and reduce weight of high-pressure components for hydrogen service.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A024. doi:10.1115/PVP2014-28815.

Recently, the measurement of threshold stress intensity factors for various low alloy ferritic steels in high-pressure hydrogen gas of 103 MPa was performed, and it was revealed that the subcritical cracking threshold under rising displacement was lower than the subcritical cracking threshold for crack arrest under constant displacement. These experimental results demonstrate the importance of the testing method for evaluating the fracture properties in high-pressure hydrogen gas. We measured the subcritical cracking threshold under rising displacement for ASME SA-372 Grade J ferritic steels in high-pressure hydrogen gas at pressure up to 115MPa. In contrast to other reported procedures where the applied displacement was increased continuously, in this study crack length was determined using an unloading elastic compliance method. The values of the subcritical cracking threshold measured by the unloading elastic compliance method are consistent with previous measurements in which the applied displacement continuously increased. These results suggest the possibility that subcritical cracking thresholds do not depend on the applied displacement path, i.e., periodic unloading vs. continuously rising displacement.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A025. doi:10.1115/PVP2014-28858.

Aluminum alloys offer significant advantages for hydrogen service such as low weight, improved uniformity of properties relative to forged austenitic stainless steels, and immunity to embrittlement in the presence of dry hydrogen. For these reasons aluminum alloys are now being considered for high-pressure hydrogen isotope pressure vessel applications where forged stainless steels have been the standard materials of construction for decades. In particular, alloy AA2219 is being evaluated due to its excellent weldability, microstructural stability, and good mechanical and fracture toughness properties. Prototype AA2219 pressure vessels have been fabricated and tested, including electron beam weld development, weld hardness and tensile testing prior to and after post-weld heat treatment, and burst testing. The design, manufacture, and testing of AA2219 pressure vessels will be discussed, including an ongoing long-term shelf storage program where pressure vessels are loaded with gaseous hydrogen at pressure of 103 MPa (85% of the burst pressure for these vessels).

Commentary by Dr. Valentin Fuster
2014;():V06BT06A026. doi:10.1115/PVP2014-28938.

The National Institute of Standards and Technology has been testing pipeline steels for about 3 years to determine the fatigue crack growth rate in pressurized hydrogen gas; the project was sponsored by the Department of Transportation, and was conducted in close collaboration with ASME B31.12 Committee on Hydrogen Piping and Pipelines. Four steels were selected, two X52 and two X70 alloys. Other variables included hydrogen gas pressures of 5.5 MPa and 34 MPa, a load ratio, R, of 0.5, and cyclic loading frequencies of 1 Hz, 0.1 Hz, and a few tests at 0.01 Hz. Of particular interest to ASME and DOT was whether the X70 materials would exhibit higher fatigue crack growth rates than the X52 materials. API steels are designated based on yield strength and monotonic tensile tests have historically shown that loss of ductility correlates with increase in yield strength. The X70 materials performed on par with the X52 materials in fatigue. The test matrix, the overall set of data, implications for the future, and lessons learned during the 3-year extensive test program will be discussed.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A027. doi:10.1115/PVP2014-28943.

A primary barrier to the widespread use of gaseous hydrogen as an energy carrier is the creation of a hydrogen-specific transportation network. Research performed at the National Institute of Standards and Technology, in conjunction with the U.S. Department of transportation and ASME committee B31.12 (Hydrogen Piping and Pipelines), has resulted in a phenomenological model to predict fatigue crack growth of API pipeline steels cyclically loaded in high-pressure gaseous hydrogen. The full model predicts hydrogen-assisted (HA) fatigue crack growth (FCG) as a function of applied load and hydrogen pressure. Implementation of the model into an engineering format is crucial for the realization of safe, cost-effective pipelines for the nation’s hydrogen infrastructure. Working closely with ASME B31.12, two simplified iterations of the model have been created for an engineering-based code implementation. The engineering-based iterations are detailed here and the benefits of both are discussed. A case study is then presented detailing the use of both versions. The work is concluded with a discussion of the potential impact that model implementation would have upon future hydrogen pipeline installations.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A028. doi:10.1115/PVP2014-28964.

This study presents prediction on initiation of hydrogen-induced delayed cracking (HIDC) in hydrogen pre-charged high-strength steel notched bars under a constant load based on hydrogen influenced cohesive zone modeling (CZM). The prediction is implemented by using a three-step sequential coupling finite element procedure including elastic-plastic stress analysis, stress-assisted hydrogen diffusion analysis and cohesive stress analysis with cohesive elements embedded along the potential crack path. Hydrogen influenced linear traction separation law is applied to the cohesive elements. The predicted initiation time of HIDC gives a good agreement with the experimental fracture time reported in a literature. The prediction reproduces the experimental trend that the critical hydrogen concentration for crack initiation is independent of the initial hydrogen concentration, while decreases with increasing load or stress concentration factor of the notch. CZM has a potential to predict HIDC of high-strength steel.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Mechanistic Modelling of Materials

2014;():V06BT06A029. doi:10.1115/PVP2014-28039.

This paper intends to present the development of a Charpy V-notched (CVN) mastercurve to fit and/or extrapolate the transition curve of ferritic steels (including martensitic, bainitic and ferritic materials).

The purpose of a mastercurve is to define a general mathematical model able to represent the overall behaviour:

• of a material for different test conditions,

• or for a set of materials for given tests conditions.

The role of a mastercurve is then to allow extrapolating or interpolating values from other data on the basis of a robust model, thereby permitting to avoid execution of new tests. It is in general based on practical general observations and research for a common behaviour through a large database of experimental results.

This kind of approach is largely used to fit fracture mechanics data and can also be used in the case of Charpy toughness. Some changes have to be performed in order to take into account specificities of Charpy (such as dependence of transition slope with strength of materials) as well as some verifications linked to statistical distribution of failure probability.

This work is based on the assessment of a very large database collected for years at Industeel’s Research Center (tensile results, Charpy transition curves). This database represents 286 Charpy V-Notched transition curves of ferritic steels. It covers a wide range of materials and applications. Yield strengths of studied materials are ranging from 290 to 1180MPa while ultimate tensile strengths are in the range of 480 to 1690MPa.

The target of this mastercurve is to predict the correct shape of the CVN transition curve with a reasonable safety margin on the basis of a limited amount of data (tensile properties at room temperature at least and when available, few Charpy data).

A brief description of the database used within this study is given in the first part of this contribution as well as a description of the concept and underlying models. Finally, a validation of the work will be given as well as an illustration of its performance in a practical industrial case.

Topics: Steel
Commentary by Dr. Valentin Fuster
2014;():V06BT06A030. doi:10.1115/PVP2014-28062.

This work describes a micromechanics methodology based upon a local failure criterion incorporating the strong effects of plastic strain on cleavage fracture coupled with statistics of microcracks. A central objective is to gain some understanding on the role of plastic strain on cleavage fracture by means of a probabilistic fracture parameter and how it contributes to the cleavage failure probability. A parameter analysis is conducted to assess the general effects of plastic strain on fracture toughness correlations for conventional SE(B) specimens with varying crack size over specimen width ratios. Another objetive is to evaluate the effectiveness of the modified Weibull stress (σ̃w) model to correct effects of constraint loss in PCVN specimens which serve to determine the indexing temperature, T0, based on the Master Curve methodology. Fracture toughness testing conducted on an A285 Grade C pressure vessel steel provides the cleavage fracture resistance (Jc) data needed to estimate T0. Very detailed non-linear finite element analyses for 3-D models of plane-sided SE(B) and PCVN specimens provide the evolution of near-tip stress field with increased macroscopic load (in terms of the J-integral) to define the relationship between σ̃w and J. For the tested material, the Weibull stress methodology yields estimates for the reference temperature, T0, from small fracture specimens which are in good agreement with the corresponding estimates derived from testing of much larger crack configurations.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A031. doi:10.1115/PVP2014-28238.

As an effective method for performance analysis of in-service component, small punch test (SPT) can be used in various situations to analyze material characterization of structural component. In order to improve stress distribution and fatigue strength of materials, ultrasonic impact treatment (UIT) has been widely researched in recent years. However, the nanocrystalline layer is so thin that can not be analyzed by the conventional tests. In this paper, the mechanical properties of AISI304 which were improved by ultrasonic impact treatment were evaluated by small punch test. Meanwhile, as two important factors, the temperature and grain size were considered. The influence of impact time on surface treatment of AISI304 was analyzed. It can be found that the grain size decreases with the increase of impact time. Based on the variation of Load-Displacement (L-D) curves obtained by small punch test, the strengths of treated specimens were obtained. Finally, the mechanical property evolutions with different surface grain sizes were discussed at elevated temperature.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A032. doi:10.1115/PVP2014-28333.

This research develops an engineering approach which permits the treatment of Charpy specimen absorbed energy data in the lower transition of Charpy specimen fracture behaviour. The procedure has been shown to be applicable to a ferritic steel study material. The calculation method comprises several steps to correct the input Charpy data to the equivalent material fracture toughness of a ferritic steel under consideration. The engineering procedure develops existing methods for constraint and notch correction to data [Sherry et al, EFM 2005] [Horn and Sherry, IJPVP 2012].

Micromechanical modeling of cleavage fracture behaviour has been applied in conjunction with sequential experimental testing. This work addresses the important geometric differences between a single edge notch bend, SEN(B), fracture toughness specimen and the standard Charpy V-notch specimen. The engineering approach is demonstrated using a suitable study ferritic steel material and by undertaking an experimental laboratory testing programme comprising standard fracture toughness specimens and non-standard U-notch and V-notch Charpy sized specimens with a range of notch geometries.

It has been found that constraint and notch assessment methodologies premised upon micro-mechanical modeling of cleavage fracture offer an accurate probabilistic description of fracture behaviour in these specimen geometries. Refinement of a notch angle correction is necessary within the procedure. These findings permit the extension of the approach to develop a material specific guidance to practitioners undertaking structural integrity assessments. The final extension of the research to Charpy impact data requires the measurement of ferritic steel material flow behaviour under dynamic conditions and represents further research.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A033. doi:10.1115/PVP2014-28425.

This paper concerns the numerical simulation of elastic and elastic-plastic crack growth in welded components. Three-dimensional, spline-based, automatic crack re-meshing algorithms have been developed at TWI to simulate crack propagation using the commercial finite element analysis software ABAQUS. These methods allow for fatigue crack growth simulations employing the Paris law, mean stress effects and more advanced elastic crack growth laws, and incorporate nodal release techniques or iterative stationary crack methods coupled with experimentally measured tearing resistance curves for elastic-plastic crack growth. The flexibility, stability and accuracy of these numerical methods are demonstrated through several examples. The application of the crack growth simulations to full-life engineering critical assessments (ECA) of offshore structures is also described and demonstrated.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A034. doi:10.1115/PVP2014-28543.

A theoretical, multi-scale model to predict the fracture toughness of ferritic steels in the ductile-to-brittle fracture mode transition temperature region has been implemented into the DISlocation-based FRACture (DISFRAC) computer code to permit fracture safety assessments of ferritic structures. The theoretical basis of this model provides a means of predicting fracture behavior outside of the ranges of data currently used in deriving empirically-based models and should provide a means of improving the understanding of fracture behavior in the fracture mode transition region. Testing has been conducted to verify the model behavior as coded into DISFRAC_V2 and to validate the code predictions against trends observed in mechanical test data. Sensitivity studies were also conducted to identify the models and parameters having the greatest effect on the predicted results. The DISFRAC model was found to be most sensitive to the definition of a “specimen size” relative to the number of particles considered. Specimen size had a large effect, both on the temperature-dependence observed across the range of predicted fracture toughness as well as on the range of KJc values simulated at each test temperature. This sensitivity of KJc to specimen size needs more study to ensure definition of an appropriate specimen size based strongly on the particles most relevant to the fracture process.

Topics: Testing
Commentary by Dr. Valentin Fuster
2014;():V06BT06A035. doi:10.1115/PVP2014-28656.

This paper analyses and compares a range of Notch Failure Assessment Diagram (NFAD) methods for assessing the fracture resistance of structures and components that contain defects with non-sharp tips. As micromechanistic failure criteria for predicting fracture from notch tips have developed, several forms of NFADs have been proposed over the last 20 years with notable developments having been made in the last 10 years. This paper quantifies the differences between four different types of NFAD approach and uses test results from test specimens containing notches of varying acuities to evaluate each approach. The results highlight significant differences in fracture predictions between the different NFAD approaches due to differences in the definition of the NFAD axes, the failure loci, the assumed failure mechanism and the corresponding micromechanistic failure criteria employed by each method.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A036. doi:10.1115/PVP2014-28904.

This paper provides simulation of ductile crack growth in full-scale cracked pipe tests using an element-size dependent damage model. This method is based on the stress-modified fracture strain damage model. The stress-modified fracture strain model is determined to be incremental damage in terms of stress triaxiality and fracture strain for dimple fracture from tensile test result with FE analyses technique. To validate the proposed method, this research analyses STPT 410 cracked pipes test at 300°C taken from CRIEPI (Central Research Institute of Electric Power Industry). In order to calibrate the stress-modified fractures strain model, tensile tests and fracture toughness tests were compared with simulated results using element-size dependent damage model. Tensile specimen and fracture toughness specimen were extracted from STPT 410 steel pipe. The calibrated damage model predicts ductile crack growth in 5 type circumferential cracked pipes bending test. And these results were compared with the experimental results. The results show that the proposed method can simulate ductile crack growth in full-scale cracked pipe tests.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Pipeline Integrity

2014;():V06BT06A037. doi:10.1115/PVP2014-28339.

In this paper, single-edge cracked plate (SECP) specimens are analyzed using three-dimensional finite element analysis under mode I loading conditions. The stress intensity factor K, T11 and T33 stresses along the crack front of the SECP specimens with the crack surface subjected to uniform, linear, parabolic or cubic stress distributions are calculated. The relative crack depth, a/W, is varied by 0.2, 0.4, 0.6 or 0.8. And the relative thickness, t/W, is chosen by 0.1, 0.2, 0.5, 1.0, 2.0 or 4.0, respectively. For engineering applications, empirical equations of normalized stress intensity factor K, T11 and T33 stress at the mid-plane are also obtained. By superposition, the results enable the calculations of these fracture mechanics parameters under the loading conditions of tension, bending and nonlinear stress distributions.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A038. doi:10.1115/PVP2014-28415.

Single-edge notched tension (SE(T) or SENT) specimens has been increasingly proposed as a low-constraint toughness test to measure toughness of line pipe materials, as the crack tip constraint approximates a circumferential surface flaw in a pipe under loading. The clamped SE(T) single-specimen procedures recently developed by Shen and Tyson [1, 2] and Tang et al. [3] have in common the use of a clamped single-specimen of similar geometry and rely on unloading compliance technique for crack size estimation. In the former case, a single clip gauge is attached to the integral knife edge and the crack-tip opening displacement (CTOD) is estimated by means of a J-integral-to-CTOD conversion, similar to the procedure of ASTM E1820. The latter uses a pair of clip gauges mounted to an attachable raised set of knife edges to estimate CTOD at the original crack tip position by a triangulation rule. Consolidating these two sets of clip gauges in a specimen makes direct comparisons of two SE(T) methods on identical test conditions: material, specimen geometry, equipment, test temperature and operator [4]. In this study, SE(T) testing employing these two SE(T) methods on a single specimen was conducted on BxB shallow-cracked (a/W∼0.35) specimens of two x70 pipeline girth welds. This paper discusses the details of two SE(T) methods and techniques on the same specimen.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A039. doi:10.1115/PVP2014-28983.

The present paper evaluates the traditional strength design criteria and recently developed plastic flow criteria used in the structural design and integrity assessment for pressure vessels. This includes (1) a brief review of the traditional strength criteria used in ASME Boiler and Pressure Vessel (B&PV) Code, (2) a discussion of the shortcoming of existing strength criteria when used to predict the burst pressure of pressure vessels, (3) an analysis of challenges, technical gaps and basic needs to improve the traditional strength design criteria, (4) a comparison of strength theory and flow theory for ductile pressure vessels, (5) an evaluation of available flow criteria and their shortcoming in prediction of failure pressure of pressure vessels, (6) an introduction of newly developed multi-axial flow criterion and its application to pressure vessels, and (7) a demonstration of experimental validations of the new flow criterion when used to predict the burst pressure of pressure vessels. On this basis, several recommendations are made for further study to improve the existing strength design and integrity assessment methods of pressure vessels.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Plastic Pipe

2014;():V06BT06A040. doi:10.1115/PVP2014-28008.

Polyethylene (PE) has now been widely used to make plastic pipe for gas transportation. Because of its excellent ductility, processes for repair and maintenance of PE pipe allow squeeze-flattening the pipe to reduce the gas flow. Our previous study has shown that stretch of PE, even at a low strain level, can cause damage of the material, and significance of the damage depends on the strain rate. This paper presents results from a follow-up study, to investigate the possibility of quantifying the influence of damage on the mechanical properties. Coupon specimens used in this study have the same geometry as that used previously. Each specimen was tested twice. The 1st test was to introduce the damage, by stretching the specimen at a crosshead speed of 1 mm/min; the 2nd test to characterize the influence of the damage on the mechanical properties, by stretching the specimen at 1 μm/min. The two crosshead speeds were chosen because the former (1 mm/min) is known to introduce much more damage than the latter (1 μm/min) at a same strain level. Therefore, change in mechanical properties observed from the 2nd test should mainly come from the damage generated in the 1st test. To avoid influence of viscous recovery from the 1st test on the results from the 2nd test, the two tests were conducted more than one month apart. Test results show that even by stretching the specimen to a strain level below the yield point in the 1st test (i.e. with the strain less than 0.1), damage introduced to the specimen can cause a detectable decrease in the mechanical properties, such as the tangent modulus at the strain 0.01 and the stress response at the strain 0.1, from the 2nd test. The results also show that the rate of decrease of the above values with the increase of strain becomes significant when the strain level introduced in the 1st test is above the yield point. By stretching the specimen to a strain level about 0.5 in the 1st test, though yet to cause apparent necking or stress whitening, the tangent modulus and the stress response in the 2nd test are decreased by about one-third of the values for the virgin specimen. This amount of change is significant and should not be ignored for long-term applications such as for gas transportation.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A041. doi:10.1115/PVP2014-28431.

The integrity of high density polyethylene (HDPE) piping and fusion joints are a topic of interest to the nuclear industry, regulators, ASME code, and the plastics pipe industry. The ASME Code Case N-755-1 has been approved and addresses the use of HDPE in safety related applications. Over the last few years some of the concerns identified with the parent HDPE pipe material and the fusion joints have been addressed while others are still being resolved. One such unresolved concern is the effect of the fusion process on the integrity of the joint, specifically, the introduction of flaws during the fusion process. The potential impact of flaws in the fusion joint on the service life of the HDPE piping is being evaluated.

The current study calculates stress intensity factors (SIF) for circumferential flaws and uses them to evaluate the potential structural integrity of HDPE fusion joints in pipes. The recent API 579-1/ASME FFS-1 standard provides SIF (KI) solutions to various semi-elliptical and full-circumferential (360°) surface cracks/flaws on the outer surface (OD) and the inner surface (ID). The API 579-1/ASME FFS-1 standard SIF tables and finite element analysis (FEA) of selected cases were used to develop simplified SIF relations for full-circumferential surface flaws that can be used for plastic pipes with diameters ranging from 101.6 mm (4 inch) through 914.4 mm (36 inch) and dimensional ratios (DRs) from 7 through 13. Further, the SIF of embedded flaws akin to lack-of-fusion regions was evaluated. The results from this study serve as precursors to understanding and advancing experimental methods to address important issues related to the critical tolerable flaw size in the butt-fusion joint material and were utilized to select the specimen tests and hydrostatic pipe tests used to evaluate various joining processes. Further, they will help with understanding the essential variables that control the long-term component integrity and structural performance of HDPE pipe joints in ASME Class 3 nuclear piping.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2014;():V06BT06A042. doi:10.1115/PVP2014-28609.

Polyethylene (PE) pipe has many advantages such as good flexibility, corrosion resistance and long service life. It has been introduced into nuclear power plants for transportation of cooling water both in U.S. and Europe. Recently, one Chinese nuclear power plant in Zhejiang Province also introduced four polyethylene pipelines in essential cooling water system with operating pressure of 0.6MPa and operating temperature of no more than 60°C. The PE pipes used in this nuclear power plant are DN762 SDR9 (30in OD, 3.3in wall), which are much larger and thicker than traditional natural gas PE pipe. As the pipe wall is so thick that the ultrasonic phased array instrument used in inspection of PE pipe with diameter less than 400mm has been improved. Results of field inspection in the Sanmen nuclear plant are reported, and the presented ultrasonic inspection technique proves to be effective for high density polyethylene (HDPE) pipe of large size in nuclear power plant.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A043. doi:10.1115/PVP2014-28682.

Reinforced Thermoplastic Pipe (RTP) is generally connected by butt fusion joint or electro-fusion joint with intensified sleeves. Contrast to regular electro-fusion joints for PE (polyethylene) pipe, those for RTP are of larger length and thickness. Based on the ultrasonic inspection research of electro-fusion joint of PE pipe, the authors designed a special phased array probe for the electro-fusion sleeve of RTP considering its complicated structure and large size. A special C-scan technique was proposed including a longitudinal scan carried out with electronic method and circumferential scan carried out with mechanical method. Meanwhile, special software was designed to ensure the synchronization and precision control between longitudinal electronic scan and circumferential mechanical scan, and the consistency control between ultrasonic information and position information. Based on the results of detection experiments of RTP electro-fusion joint with typical defects, it was proven that this technique has the capability to detect typical defects in RTP electro-fusion joint, and high testing sensitivity was obtained.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A044. doi:10.1115/PVP2014-28718.

High density polyethylene (HDPE) is commonly used in pipe fabrication for water and natural gas systems, due to its versatility, low cost and lightweight. A piping system is subject to service conditions such as impact and cyclic loads as a consequence of internal pressure or external pressure fluctuations, and the existence of discontinuities in the material. These conditions cause material damage, cracking and weakening, and have to be considered in the piping design. The Boundary Element Method (BEM) is a numerical method based on integral equations that consider only the contour of the solid (meaning an easier meshing). Crack modeling is one of the most important applications for the BEM, since it allows an accurate stress analysis around the crack tip. In this work, a computational study based on the BEM in two dimensions whose aim is to determine the stress intensity factors (SIFs) in order to evaluate the mechanical resistance to fracture of HDPE PE100 pipes and its comparison with the results obtained by previous experimental tests, is developed. Numerical simulations of specimens subject to three point bending loads (SENB specimens) using the characteristics of the linear elastic fracture mechanic (LEFM), are developed. As a first attempt, the numerical models of different SENB geometries are validated comparing the numerical solution versus the results given by a reference solution from literature. The results show that the BEM under the LEFM approach is valid for loads within the linear range of HDPE since LEFM gives an upper bound of the fracture load of HDPE specimens; however, an Elastic-Plastic fracture analysis could be required for loads in the plastic range of the material.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Probabilistic Assessment of Failure

2014;():V06BT06A045. doi:10.1115/PVP2014-28114.

Low cycle fatigue (LCF) induces damage accumulation in structural components used in various applications. LCF typically describes conditions for which plastic strains are larger than elastic strains. In order to certify and qualify a structural component, manufactured from a given material, that requires high reliability for operation and safety, fundamental material properties should be experimentally investigated and validated. The traditional strain–life approach serves as the underlying experimental method for most LCF investigations. Building upon that background, the purpose of this paper is to investigate the statistical variability and appropriately model that variability for life in LCF. Specifically, the variability associated with the median behavior in a strain–life graph for data is examined. The ensuing analyses are based on data for a cold-rolled, low carbon, extra deep drawing steel; ASTM A969 which is appropriate for applications where extremely severe drawing or forming is envisioned. It is frequently used in the automotive industry for components such as inner door components and side body components. For substantiation of the proposed modeling techniques, data for 9Cr-1Mo steel is also investigated. Such steel is frequently used in the construction of power plants and other structures that experience operating temperatures in excess of 500°C. The commonly used universal slopes approach for fatigue life modeling for which the strain–life computation employs the standard Coffin–Manson relationship is compared to a statistical methodology using a distribution function frequently used in structural reliability. The proposed distribution function for characterizing the fatigue life is a generalized Weibull distribution function that empirically incorporates load history and damage accumulation.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A046. doi:10.1115/PVP2014-28150.

In the framework of the R6 development programme on leak before break NRG developped a probabilistic fracture-mechanics model for analyzing circumferential through-walled cracked pipe made of Type 304 stainless steel, subjected to bending loads. An elastic-plastic analysis has been carried out using ANSYS for estimating J-integral as a function of moment load. The validity of the J-integral based on the ANSYS calculations was evaluated by comparison with LBB.ENG2 method and Rahman’s calculations.

Probabilistic fracture analysis has been carried out using ANSYS Probabilistic Design System (PDS) module to find out the failure probability of a pipe as a function of applied moment. These results have been compared with LBB.ENG2 probabilistic calculations, which has been developed using MATLAB.

To achieve high efficiency, accuracy and robustness to design structural component with a low probability of failure, Advanced sampling methods (ADIS) have been used for probabilistic calculations. These ADIS results have been compared with Monte Carlo probabilistic results.

The probabilistic method is subsequently extended to a Leak Before Break case (LBB). It is demonstrated that the probability of failure reduces when more probabilistic data for input parameters is added instead of using deterministic safety factors.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A047. doi:10.1115/PVP2014-28877.

How to quantitatively measure the uncertainty of engineering failure data is an important but still unsolved task in probabilistic risk analysis. This paper aims to fill the gap first by specifying the requirements for a robust uncertainty measure to meet the criteria. Complexity and uncertainty measurements in computational complexity, classical statistical mechanics and information theory are also reviewed for possible inspiration. In this paper, a new groundbreaking parameter, which is related to reliability or survival function, is selected to characterize the uncertainty of engineering failure data with given probabilistic distributions. The uncertainty formulae based on the Shannon entropy and the new uncertainty parameter for various distribution functions are also provided. Finally, several examples are given to demonstrate the applicability of the new uncertainty measure in durability and reliability analyses.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A048. doi:10.1115/PVP2014-28880.

Multiple-modal statistical distributions, which are caused by multiple failure mechanisms and modes, have been observed in several materials for several types of physical entities such as micro-crack size and cycles to failure under fatigue loading. However, the underlying damage mechanisms related to these entities are still not well understood, and the associated modeling is still lacking. In this paper, the possible underlying driving forces of the multiple-modal distribution functions are investigated based on the concept of crack or damage evolution.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A049. doi:10.1115/PVP2014-29118.

The safety integrity level (SIL) of equipment used in safety instrumented functions is determined by the average probability of failure on demand (PFDavg) computed at the time of periodic inspection and maintenance, i.e., the time of proof testing. The computation of PFDavg is generally based solely on predictions or estimates of the assumed constant failure rate of the equipment. However, PFDavg is also affected by maintenance actions (or lack thereof) taken by the end user. This paper shows how maintenance actions can affect the PFDavg of spring operated pressure relief valves (SOPRV) and how these maintenance actions may be accounted for in the computation of the PFDavg metric. The method provides a means for quantifying the effects of changes in maintenance practices and shows how these changes impact plant safety.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Welding Residual Stress and Distortion Simulation and Measurement

2014;():V06BT06A050. doi:10.1115/PVP2014-28030.

Weld residual stress (WRS) is known to be an important driver for stress corrosion cracking. Therefore, in probabilistic Leak-Before-Break calculations for safety-related nuclear piping systems, it is necessary to offer best-estimate WRS inputs along with an informed characterization of the associated uncertainty. This paper discusses the finite element (FE) analysis work performed to develop the WRS inputs for probabilistic models used for dissimilar metal (DM) welds at pressurized water reactors (PWRs). Three different weld geometry cases were considered representing typical nuclear piping system DM welds. In accounting for modeling uncertainty, the FE results from three independent analysts for the various weld geometry cases were compared and evaluated. This paper describes the modeling strategies used to develop the best-estimate inputs and the uncertainty distributions, including approaches to eliminate unnecessary sources of uncertainty, such as inconsistent post processing.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A051. doi:10.1115/PVP2014-28042.

Welding residual stresses around a cylindrically symmetric weld between two steel components, a necked steel forging and a uniform thickness plate, were modeled using a finite-element modeling approach. The modeling included a preheat period, a 22-pass weld and a limited cooling rate after welding, all with a sequential thermal and mechanical analysis approach. The configuration, with a J-groove bevel at the outside of the forging flange, allowed the flange to be joined to a thick steel plate. Welding residual stresses were large and tensile in the weld area. A region of surface residual tensile stress on the flange extended to the neck at more than 5 weld widths from the weld joint in a region that was not heated more than 100°C. Modifications of the welding to reduce the driving forces for residual stress changed the distribution near the weld, but did not greatly affect the remote stresses. A change of fixturing on the faying surface did significantly reduce the remote tensile stresses by more than 30%.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A052. doi:10.1115/PVP2014-28084.

Welding is often listed as a production operation that companies would like to reduce overall cost and improve productivity; however, most companies merely implement cost reduction programs focused on lowering welding consumable costs. Though significant and important, these associated material costs typically represent only a small percentage to the total cost, i.e., 10 to 20% (welding consumables 8 to 15% and power and equipment 2 to 5%) of the overall welding cost in a typical U.S. welding operation. To further reduce welding costs, companies need to look further. Since labor and overhead, which relates directly to productivity, represents approximately 80 to 85% of the overall cost of any given welding operation they also offer the greatest opportunities for significant cost reduction. Simply changing from Shielded Metal-Arc Welding (SMAW) to Flux-Cored Arc Welding (FCAW) can reduce labor cost and increase productivity. Due to the increased deposition efficiency and operating factor of FCAW the weld deposition rate increases thus translating into increased productivity. The increase in productivity, in turn, reduces labor cost by reducing the man-hours required for the completion of any given weld. An added benefit gained by using FCAW is that it also significantly reduces the skill level required by the welder to produce welds of equal quality. When all of these benefits are combined FCAW yields significant cost savings opportunities by reducing labor and simultaneously improving productivity.

Topics: Welding , Arc welding
Commentary by Dr. Valentin Fuster
2014;():V06BT06A053. doi:10.1115/PVP2014-28151.

In Pressurized Water Reactors, most of heavy components and pipes have a large thickness and their manufacturing processes often require multi-pass welding. Despite the stiffness of these components, the distortion issue may be important for operational requirements (e.g. misalignment) or controllability reasons (Non Destructive Examinations have to be achievable, therefore ovalization should be limited). These requirements may be difficult to achieve by simply adjusting welding processes. Indeed because of the complexity of mechanisms involved during a welding operation and the high number of influencing parameters, this process is still essentially based on the experience of the welder. Furthermore the experimental estimation of the stress and distortion level in the component remains a difficult task that is subject to errors even if techniques are currently improved to become more accurate. These are the reasons why AREVA has put a large effort to improve welding numerical simulations, in order to have a better understanding of the involved physical phenomena and also to predict the residual state through the structure. Computational welding mechanics is used to qualify the manufacturing processes in the very early phase of the welded component design. Within the framework of a R&D program whose main objective was to improve tools for the numerical simulation of welding regarding industrial needs, AREVA has decided to validate new methodologies based on 3D computation by comparison with measurements. For this validation task the chosen industrial demonstrator was a Control Rod Drive Mechanism (CRDM) Nozzle with a J-groove attachment weld to the vessel head. For such an application, operations of post-joining straightening have to be limited, if not prohibited, because of their cost or the impossibility to use them in front of a steel giant. The control of distortion during welding operations is a key issue for which simulation can be of great help. Regarding distortion issues, both accurate metal deposit sequence modeling and respect of the real welding parameters are mandatory, especially for multi-pass operation on such a complex geometry.

The aim of this paper is to present the simulation of the distortion of a peripheral adapter J-groove attachment weld mock-up. This new full 3D simulation improves the result of the previous one based on lumped pass deposits. It is the result of a fruitful collaboration between AREVA and ESI-Group.

Topics: Simulation , Vessels
Commentary by Dr. Valentin Fuster
2014;():V06BT06A054. doi:10.1115/PVP2014-28163.

The current paper presents a finite element analysis of an eight-pass groove weld in a 316L austenitic stainless steel plate. A dedicated welding heat source modelling tool was employed to produce volumetric body power density data for each weld pass, thus simulating weld-induced thermal loads. Thermocouple measurements and cross-weld macrographs taken from a weld specimen were used for heat source calibration. A mechanical finite element analysis was then conducted, using the calibrated thermal loads and a Lemaitre-Chaboche mixed work-hardening model. The predicted post-weld residual stresses were validated using contour method measurements: good agreement between measured and simulated residual stress fields was observed. A sensitivity analysis was also conducted to identify the boundary conditions that best represent a tack-welded I-beam support, which was present on the specimen back-face during the welding.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A055. doi:10.1115/PVP2014-28185.

Residual stress caused by welding processes affects characteristics of strength and fracture of equipment and piping in power plants. Numerical thermal elastic-plastic analysis is a powerful tool to evaluate weld residual stress in actual plants. However, the conventional three-dimensional precise analysis for a welding process such as multi-pass welding, machining and thermal treatment requires enormous computation time though it can provide accurate results. In this paper, the finite element analysis code based on the iterative substructure method that was developed to carry out thermal elastic-plastic analysis efficiently, with both high computational speed and accuracy, was proposed to simulate the welding process of plant equipment and piping. Furthermore, optimization of the proposed analysis code was examined and the computational efficiency and accuracy were also evaluated.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A056. doi:10.1115/PVP2014-28209.

Weld residual stresses can significantly impact the performance of structural components. Tensile residual stresses are of particular concern due to their ability to accelerate failure. For example, the presence of tensile residual stress can cause initiation and accelerate growth of primary water stress corrosion cracking (PWSCC). The contour method is a residual stress measurement technique capable of generating two dimensional maps of residual stress, which is particularly useful when applied to welds since they typically contain spatially varying residual stress distributions. The two-dimensional capability of the contour method enables detailed visualization of complex weld residual stress fields. This data can be used to identify locations and magnitude of tensile residual stress hot-spots. This paper provides a summary of the contour method and presents detailed results of contour method measurements made on a mock-up from the NRC/EPRI weld residual stress (WRS) program [1].

Commentary by Dr. Valentin Fuster
2014;():V06BT06A057. doi:10.1115/PVP2014-28217.

This paper aims to provide a detailed assessment of some of the existing residual stress profiles prescribed in widely used fitness-for-service assessment codes and standards, such as BS 7910 Appendix Q, by taking advantage of some comprehensive residual studies that become available recently. After presenting a case study on which residual stress measurements are available for validating finite element based residual stress solution procedure, residual stress profiles stipulated in BS 7910 for girth welds are evaluated in the context of a series of parametric finite element results and a shell theory based full-field residual stress estimation scheme. As a result, a number of areas for improvement in residual stress profile development are identified, including some specific considerations on how to attain some of these improvements.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A058. doi:10.1115/PVP2014-28223.

Typically, the distortion from welding is mitigated with the use of fixtures, clamps, tack welds and so on. Also the welding current and traveling speed are normally set constant during welding along a weld-path. The authors have developed and implemented an advanced control method that adaptively changes welding current and traveling speed depending on the state of deformation, in order to mitigate the final distortion without the use of additional hardware such as fixtures, clamps, and/or tack welds. It predicts the distortion before actual happening and adjusts parameters to counteract the deformation during welding. The present work implements this advanced method by applying an optimized, varying welding current and traveling speed on an edge-welded bar of Aluminum 5052-H32. A comparison is made between the final welding distortion with the new method, versus the regular method at constant welding current and traveling speed. A virtual predictive model was established to simulate and control the adaptive change of welding current and traveling speed, the optimized profile of the process parameters were performed by a robot, and the transient distortion was measured by state-of-the-art 3D photogrammetry cameras in real–time.

Topics: Welding
Commentary by Dr. Valentin Fuster
2014;():V06BT06A059. doi:10.1115/PVP2014-28247.

The application of procedures such as R6 or BS7910 for the structural assessment of defects in pressurised components containing residual stresses requires knowledge of the through-wall residual stress profile. Currently there is much interest in improving the residual stress profiles that are provided in the procedures. In this paper we present an improved analysis of residual stresses of the pipe girth welds by applying the developed heuristic method to one set of extended residual stress measurement data. The through-thickness residual stress is decomposed into three stress components: membrane, bending and self-equilibrating. The heuristic method was applied to the three components separately, so that the residual stress profile was a combination of the three stress components. This form provides not only a clear physical basis for the residual stress profile, but is also convenient for defect assessment where only the membrane and bending stress components are important.

Topics: Stress , Pipes
Commentary by Dr. Valentin Fuster
2014;():V06BT06A060. doi:10.1115/PVP2014-28328.

This paper describes a sequence of residual stress measurements made to determine a two-dimensional map of biaxial residual stress in a nozzle mockup having two welds, one a dissimilar metal (DM) weld and the other a stainless steel (SS) weld. The mockup is cylindrical, designed to represent a pressurizer surge nozzle of a nuclear pressurized water reactor (PWR), and was fabricated for Phase 2a of the NRC/EPRI welding residual stress round robin. The mockup has a nickel alloy DM weld joining a SS safe end to a low-alloy steel cylinder and stiffening ring, as well as a SS weld joining the safe end to a section of pipe. The biaxial mapping experiments follow the approach described earlier, in PVP2012-78885 and PVP2013-97246, and comprise a series of experimental steps and a computation to determine a two-dimensional map of biaxial (axial and hoop) residual stress near the SS and DM welds. Specifically, the biaxial stresses are a combination of a contour measurement of hoop stress in the cylinder, slitting measurements of axial stress in thin slices removed from the cylinder wall, and a computation that determines the axial stress induced by measured hoop stress. At the DM weld, hoop stress is tensile near the OD (240 MPa) and compressive at the ID (−320 MPa), and axial stress is tensile near the OD (370 MPa) and compressive near the mid-thickness (−230 MPa) and ID (−250 MPa). At the SS weld, hoop stress is tensile near the OD (330 MPa) and compressive near the ID (−210 MPa), and axial stress is tensile at the OD (220 MPa) and compressive near mid-thickness (−225 MPa) and ID (−30 MPa). The measured stresses are found to be consistent with earlier work in similar configurations.

Topics: Metals , Stress , Nozzles
Commentary by Dr. Valentin Fuster
2014;():V06BT06A061. doi:10.1115/PVP2014-28488.

In this paper, a 3-D thermo-mechanical finite element model (FEM) is developed to simulate the process of friction stir seal welding (FSSealW) of tube-tubesheet joint, using a commercial finite element (FE) package considering temperature dependent material properties. The model is used for the prediction of temperature and stress distributions, as well as the prediction of the residual stresses in the seal welded joint, including the expanded tube and surrounding ligaments. Validation of the model is achieved using experimental temperature measurements. The FEM results are found to be in good agreement with experimental ones. Temperatures of the joint material away from the processed zone are below the annealing temperature. The calculated residual stresses are found to be compressive and help to enhance the contact stress in the tube-tubesheet joint.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A062. doi:10.1115/PVP2014-28565.

A circular disc containing a partial ring weld has been devised to permit high levels of residual stress to be created in a relatively small specimen. The purpose of this research is to investigate the residual stress within the weld whilst developing a residual stress measurement method called the over-coring deep hole drilling (oDHD) method. The welding simulation, incremental deep hole drilling (iDHD) simulation and measurement and neutron diffraction were previously studied and reported in [1]. In this paper, the welding simulation results were mapped into a 3D model that included the necessary mesh and boundary conditions to simulate the process of residual measurement using the oDHD method. An experimental programme of residual stress measurement using the oDHD method was then conducted on a welded circular disc. The results from the oDHD simulation and measurement matched well with previous iDHD simulations on the original stress field in the ring weld, which also matched earlier neutron diffraction results.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A063. doi:10.1115/PVP2014-28571.

Friction stir welding (FSW) is a relatively new solid state metallurgical joining technique. It flourishes on the simple principle of utilising frictional heat by the stirring motion of a non-consumable rotating tool to create the seam. Feasibility of FSW aided by a newly designed probeless tool was investigated for fabricating copper-tungsten mechanical composite. The most effective parameter combination was determined by conducting a parametric study of the probeless tool aided FSW copper. Strength of the mechanical composite fabricated at this condition was evaluated through punch shear testing. Punch shear testing established that the friction stir welded interface of the copper-tungsten composite was 87% as strong as the base metal (i.e. copper). Advantages of the designed technique have been summarised.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A064. doi:10.1115/PVP2014-28737.

When manufacturing cylindrical or conical structures from metal plates, residual stresses originate from both welding and bending. The sequence in which these fabrication steps are carried out is essential as it can radically change the final distribution of residual stresses. To study this effect, detailed welding and bending simulations have been performed on both axial and circumferential X-welds of a 316L stainless steel cylindrical vessel. Bending after welding is shown to reduce residual stresses markedly more than bending before welding and the benefit on critical crack size is illustrated by a Fracture Mechanics analysis.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A065. doi:10.1115/PVP2014-28775.

Pressurized Water Reactor components are welded by Gas Tungsten Arc Welding (GTAW). To achieve good corrosion resistance and mechanical properties, Ni base alloy 690 is used to manufacture these components. The understanding of physical phenomena involved during welding and the prediction of induced residual stresses are crucial to guarantee high quality of these components. Welding induces drastic changes in the microstructure of the molten zone and heat-affected zone of metallic alloys especially for multi-pass welding. These changes may deteriorate the mechanical properties of the assembly. In order to reproduce the complex thermo-mechanical loading occurring within the heat affect zone, experiments on a thermo-mechanical simulator Gleeble 3500 have been carried out. In order to characterize the base alloy, isothermal tensile tests have been performed at various strain rates and temperatures (from 25 to 1100°C). A constitutive law has been proposed to predict the mechanical properties under different strain rates and temperatures. Tensile tests have also been performed after several thermal cycles to understand the effect of welding on mechanical properties of Ni alloy 690. In parallel, grain size evolution and carbide precipitation have been characterized and correlated to measured mechanical properties.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A066. doi:10.1115/PVP2014-28776.

Two-dimensional (2D) and three-dimensional (3D) weld-induced residual stress finite element analyses have been performed for 2-inch Schedule 80 Type-304 stainless steel pipe sections joined by a multi-layer segmented-bead pipe weld. The analyses investigate the similarities and differences between the two modeling approaches in terms of residual stresses and axial shrinkage induced by the pipe weld. The 2D analyses are of axisymmetric behavior and evaluate two different pipe end constraints, namely fixed-fixed and fixed-free, while the 3D analysis approximates the non-axisymmetric segmented welding expected in production, with fixed-free pipe end constraints.

Based on the results presented, the following conclusions can be drawn. The welding temperature contour results between the 2D and 3D analyses are very similar. Only the 3D analysis is capable of simulating the non-axisymmetric behavior of the segmented welding technique. The 2D analyses yield similar hoop residual stresses to the 3D analysis, and closely capture the maximum and minimum ID surface hoop residual stresses from the 3D analysis. The primary difference in ID surface residual stresses between the 2D fixed-fixed and 2D fixed-free constraints cases is the higher tensile axial stresses in the pipe outside of the weld region. The 2D analyses under-predict the maximum axial residual stress compared to the 3D analysis. The 2D ID surface residual stress results tend to bound the averaged 3D results. 2D axisymmetric modeling tends to significantly under-predict weld shrinkage. Axial weld shrinkage from 3D modeling is of the same magnitude as values measured in the laboratory on a prototypic mockup.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A067. doi:10.1115/PVP2014-28778.

Weld residual stress (WRS) distributions are an important input into fracture mechanics evaluations necessary to determine the residual lives of dissimilar metal welds (DMWs). Since the DMW geometry and the presence or absence, size, and location of weld repairs is nozzle specific, finite element WRS analysis is often used to predict through-wall weld residual stress distributions. It is important to note that despite small differences in plant specific geometry or weld location specific weld repair geometry there are substantial similarities between the configurations that have been evaluated in the numerous weld specific finite element WRS analyses documented in the literature. Important insight can be gained from parametric studies of simplified geometries in order to understand the significance of different parameters on the resulting WRS distributions. The results of such studies can allow engineers to focus resources on refining accuracy of critical inputs and to support simplified model development suitable for incorporation into design and fitness for service codes. This paper documents the results of various studies performed to validate the ability to use a simplified pipe-to-pipe model for simulating relative effects on through-wall WRS distributions of pipe and weld repair geometry, investigate the effect of pipe mean radius to wall thickness ratio, weld repair depth (ID and OD), and weld repair sequence. Fifteen cases are analyzed. The dimensions selected for each case span a range of wall thickness, Rm/t and depth of repair values representative of typical Boiling Water Reactor (BWR) nozzle DMWs. The results are used as input into a simplified WRS model presented in a separate paper [17].

Commentary by Dr. Valentin Fuster
2014;():V06BT06A068. doi:10.1115/PVP2014-28808.

In assessment of stress corrosion cracking behavior of susceptible welded materials, the contribution of weld residual stress is a key input for stress intensity factor calculations, which in turn are used to determine anticipated crack growth and to plan for inspection or repair. Without accurate weld residual stress information, it is challenging to develop an optimal plan for plant management. Weld residual stress simulations, based on non-linear finite element computations, provide a means to estimate residual stresses in components. However, there is no established, consensus approach for weld residual stress model validation, which could be used to judge model quality, specifically with respect to the influence of residual stress output on plant management decisions. A consensus model validation approach would benefit a broad range of stakeholders in pressure vessel technology.

The paper provides technical detail of example approaches for weld residual stress model validation, and applies these approaches to a set of weld residual stress model outputs that were developed in the context of an industry round robin. The set of outputs is from Phase 2a of the international round robin organized cooperatively by the U.S. Nuclear Regulatory Commission and the Electric Power Research Institute. Example validation approaches include comparisons of output from one model with output from other models, as well as comparisons of model output with data from residual stress measurements. The figures of merit used for comparisons range from simple (e.g., evaluation of mechanical section forces) to complex (e.g., comparison of predicted crack growth behavior). Applying a range of validation approaches provides information for use within the technical community, to support development of a consensus approach for weld residual stress model validation.

Topics: Simulation , Stress
Commentary by Dr. Valentin Fuster
2014;():V06BT06A069. doi:10.1115/PVP2014-28809.

This paper describes the results of weld model analysis and deep hole-drilling measurements undertaken to evaluate residual stress distributions in austenitic and ferritic steel thick section electron beam welds. The work was undertaken in support of a Rolls-Royce and TWI development programme in the UK, for a Reduced Pressure Electron Beam (RPEB, 0.1 to 1mbar) welding process using a mobile local vacuum seal for the manufacture of thick section pressure vessel and pipe welds for nuclear power plant applications. Measurements were undertaken on representative mock-ups including a 160mm thick SA508-3 forging circumferential seam weld, in both the as-welded and furnace post weld heat treated condition. A 316L stainless steel plate butt weld and a 304L pipe girth weld of 80mm and 36mm thickness respectively were also analysed.

There is now considered to be sufficient understanding of the residual stress fields generated by thick Electron Beam (EB) welds, to propose through thickness ‘upper bound’ R6 Level 2 stress profiles for use in defect tolerance assessments. The intention is to incorporate residual stress profiles of this type into the R6 structural integrity assessment procedure, following peer review. This would represent a significant step forward in demonstrating technology readiness for plant applications. It is also anticipated that an ASME Code Case will be drafted and proposed for the RPEB welding process.

EB welding is a relatively low heat input process, compared with a multi-pass arc weld, such that the fusion zone and heat affected zone are narrow. The centre of an EB weld is the last region to solidify and cool-down, so typically there is a high degree of restraint to weld metal contraction, thereby generating a highly tri-axial yield magnitude tensile stress state at the mid-thickness location. The stress components acting in the longitudinal welding direction and through-thickness orientation tend to be large in the centre of EB welds of high aspect ratio (depth / width). By contrast, lower stress levels are produced on the surfaces acting transverse to the weld plane compared to conventional multi-pass metal arc welds. The transverse stress component is most likely to be required for the assessment of any postulated EB welding defects. The residual stress field decays rapidly with distance from the EB joint into the adjacent parent metal. Symmetrical stress distributions are typically generated in a 1-pass EB plate weld and stress fields are characteristically sinusoidal of wavelength between 1 and 4 times the section thickness.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A070. doi:10.1115/PVP2014-28828.

Two simplified models are developed and benchmarked for predicting through-wall axial and hoop weld residual stress (WRS) distributions in single V groove dissimilar metal welds (DMWs) joining cylindrical components such as piping or nozzles. The models can be used to predict WRS distributions for different pipe mean radius to wall thickness ratios (Rm/t) without an inside surface repair and WRS distributions at a single Rm/t for various inside surface weld repair depth to pipe thickness ratios (x/t). The models are developed by approximating the through-wall WRS distribution using a finite Fourier series where the coefficient of each term in the Fourier series is determined using a linear equation in which the Rm/t or x/t is the independent parameter. The model for the unrepaired condition has been benchmarked against two plant specific finite element WRS analyses of BWR nozzle to safe end welds as well as experimental and FEA WRS data from the PWR pressurizer safety/relief nozzle to safe end weld documented in MRP-317. The weld repair model has been benchmarked against the pressurizer surge nozzle experimental data presented in MRP-316. The models have been used to perform numerous plant specific DMW residual life calculations and can save significant time and money when performing weld specific fracture mechanics analyses.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A071. doi:10.1115/PVP2014-28864.

Measuring plastic strains is very useful method for validating finite element model of weld residual stress, which is very important for understanding welding process and facilitating other engineering applications. In this work, the distribution of plastic strains in a multi-pass dissimilar metal weld comprised of Nickel Alloy 82 and austenitic stainless steel 304L is evaluated quantitatively through micro-hardness mapping. An experiment procedure was developed to separate the contribution to hardness from the plastic strain (work hardening) that forms the chemistry variation in the dissimilar metal weld. It is found that high equivalent plastic strains are predominately accumulated in the buttering layer, the root pass, and the heat affected zone, which experience multiple welding thermal cycles. The final cap passes, experiencing only one or two welding thermal cycles, exhibit less plastic strain accumulation. Moreover, the experimental residual plastic strains are compared with those predicted using an existing weld thermo-mechanical model with two different strain hardening rules.

Topics: Metals , Microhardness
Commentary by Dr. Valentin Fuster
2014;():V06BT06A072. doi:10.1115/PVP2014-28869.

Primary water stress corrosion cracking (PWSCC) is a major materials challenge for dissimilar metal welds (DMW) in pressurized water reactors. The reliability of structure integrity assessment of DMW is strongly dependent on the reliable determination of the weld residual stress (WRS) field, which is one of the primary driving forces for PWSCC. Recent studies have shown that WRS prediction using today’s DMW WRS models strongly depends upon the choice of strain-hardening constitutive model. The commonly used strain hardening models (isotropic, kinematic, and mixed) are all time-independent ones that are inadequate to accounting for the time-dependent (viscous) plastic deformation at the elevated temperatures during welding. Recently, a dynamics strain hardening constitutive model has been proposed and the application of such a model has resulted in improved WRS prediction when compared to the WRS measurement results by contour method and deep-hole drilling method. In this study, the dynamic strain hardening behavior, under uniaxial tensile loading conditions, of several stainless steels and nickel alloys (SS304, Alloy 600, Alloy 82 and Alloy 52) commonly used in pressure vessel nozzle DMW are experimentally determined and compared. The extent of softening due to different duration of high-temperature exposure is studied and its influence on final residual stresses is discussed. An empirical correlation combining both the time and temperature effects on dynamic strain hardening is proposed for weld residual stress modeling.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A073. doi:10.1115/PVP2014-28965.

Internal components in nuclear reactor pressure vessels are joined to the ferritic vessel by use of dissimilar metal welds which commonly include nickel base weld material Alloy 182. It has turned out that Alloy 182 sometimes is susceptible to stress corrosion cracking (SCC) for the operating environment in reactors. Tensile residual stress has a large influence on SCC and it is important to carefully characterize the residual stresses generated at manufacturing. The manufacturing of these welds includes welding Alloy 182 to the ferritic steel to form a buttering, post-weld heat treatment (PWHT) of the buttering, and finally attachment welding between the internal component and the buttering.

An experimental program was designed for measurement and numerical analysis for validation of residual stresses in a nickel base Alloy 182 weld between the core shroud support leg and reactor pressure vessel. Two full-scale mock-ups were manufactured according to the original procedures for the buttering to the ferritic steel and the final attachment weld to the core shroud support. The mock-up was also carefully designed to produce correct boundary conditions for the support leg. Measurements were performed by the deep-hole drilling technique (DHD/iDHD). The residual stress fields from welding and heat treatments were predicted by detailed numerical modelling. Comparison between the numerical results and the measurement results shows very good agreement and validates the predicted residual stresses. It was concluded that the PWHT of the vessel only partly relieve weld residual stresses in the nickel base buttering.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A074. doi:10.1115/PVP2014-28972.

The development of residual stresses in the Heat Affected Zone (HAZ) during welding of a ferritic steel can be critical to weld structural integrity. The Prior Austenite Grain Size (PAGS), the thermo-mechanical properties of the phases that develop during phase transformation, and the transformation strains are some of the key parameters that can alter residual stress development during welding. Understanding the trend in variation of these parameters is crucial for Finite Element (FE) modelling of residual stress development in weld. In this study, the effect of PAGS on the phase transformation in SA508 grade 4 was determined. For this purpose, samples were heated up to 900, 1050, 1250, and 1350°C and held for various time intervals to produce different austenite grain sizes. The measured austenite grain sizes were then used to fit parameters in an exponential equation implemented in an FE User MATerial subroutine (UMAT) for the modelling of welds. With performing various free dilatometry experiments, it is shown that the only phase that austenite transforms to upon cooling is martensite. In addition, the mechanical properties of as-received material, austenite, and martensite as a function of temperature were measured. Also, various uni-axial loads were applied during cooling cycles, and before the onset of phase transformations, to measure the evolution of transformation strain to generate an empirical formulation for numerical modelling.

Topics: Steel
Commentary by Dr. Valentin Fuster
2014;():V06BT06A075. doi:10.1115/PVP2014-29005.

A number of girth-welded pipe mock-ups have been manufactured and investigated during the STYLE project, using a wide range of measurement techniques accompanied by extensive finite element simulation campaigns. This paper gives an overview of the work carried out and presents preliminary conclusions on the performance of finite element weld residual stress simulation techniques in the different mock-up designs.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A076. doi:10.1115/PVP2014-29015.

The New Nuclear Manufacturing (NNUMAN) programme was established in the UK in late 2012, to develop new manufacturing technologies for nuclear components. One of the themes of this programme is research to develop candidate advanced joining methods using arc and high-energy welding processes, for components manufactured using nuclear grade reactor pressure vessel steel SA508 Grade 3 Class 1. The key outcomes of this study are the comparison of residual stresses and mechanical properties of welded mock-ups as a function of different welding processes, together with the development and validation of numerical models to predict residual stresses and mechanical properties. This paper gives an overview of the NNUMAN welding programme, coupled with design of experiments to date. The ultimate objective of this research is aimed at determining the effect of the selection of welding process, on the performance of nuclear components with operational lifetimes of 60 years or greater.

Commentary by Dr. Valentin Fuster
2014;():V06BT06A077. doi:10.1115/PVP2014-29092.

Residual stresses within stainless steel pipe welds may impact both component inspections and in-service cracking. Various investigators have examined piping residual stresses in large diameter piping using both finite element modeling and experimental techniques, but limited information is available for small diameter piping. This investigation uses both experimental methods and analytical modeling to evaluate the impact of welding on the residual stresses along the inner diameter (ID) of two small diameter pipe specimens.

Results of the investigation showed that tensile axial residual stresses were observed in the heat affected zone (HAZ) along the ID of the thin-wall pipe specimen with distinct regions of tensile and compressive axial stress which correlate well with the location where the last weld segments of the final weld pass were deposited. Higher stresses were also observed in the HAZ on the side where the final weld pass was deposited. By contrast, testing of the thick-wall pipe specimen showed significantly lower levels of tensile stresses along the pipe ID with the higher stress regions being biased toward the pipe outer diameter (OD).

Commentary by Dr. Valentin Fuster
2014;():V06BT06A078. doi:10.1115/PVP2014-29100.

The level and distribution of residual stresses in welds arises from the complex thermo-mechanical history of heat flow and thermal expansion at very high temperatures. It is not possible to make assessments of these with the methods that are used to determine service stresses. Simulation techniques have been developed over many years making it increasingly possible to predict residual stresses. These models need accurate materials data including, where applicable, the effect of phase transformations. In nuclear reactor pressure vessel welds, it is necessary to consider welding as a metallurgical problem as well as a thermo-mechanical one and FE simulations of these require a wide range of material data in order to create suitable input parameters.

It is crucial that models of ferritic steel welds simulate the effects of phase transformations because the different phases have different thermal expansion coefficients. Partly due to differences in thermal expansion coefficient attributed to the different phases, but more significantly because of the associated transformation strain and transformation plasticity. Further to this, predicting the distribution of the phase fractions enables structural simulations to account for the distribution of mechanical properties throughout a weld. In this work, a simplified approach to producing an empirical model to simulate phase transformations in SA-508 Gr3 pressure vessel steel is presented. A commercial finite element package is used to implement the model which calculates the volume fraction of bainite, martensite and austenite and the thermal strains that evolve over the thermal excursions. The results of these FE simulations are compared to experimental data.

Commentary by Dr. Valentin Fuster

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