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In the current experimental work, an effort has been made to explore the feasibility of fusion joints of 21-4-N nitronic steel employing microwave heating. These fusion joints were developed inside a domestic microwave applicator operating at 900 W. Microwave energy was used to fabricate the joints in hybrid heating mode by converting electromagnetic energy into heat at 2.45 GHz. Charcoal and SiC plates were used as susceptor and separator, respectively, and nickel powder was used as the interface material. The developed joints were characterized for their microstructural and mechanical properties. The microstructures indicate a complete fusion of nickel interfacing powder with the faying surfaces. XRD results show the formation of metallic nitrides and carbide phases (Cr2N, Fe3N, and Fe2C) and the FeNi phase at the weld zone. Furthermore, the observed average tensile strength of the fusion joints was approximately 61% of base metal. The reduction in the stress and elongation compared to the base metal were 38.67% and 12.68%, respectively. The average microhardness of the microwave joints was monitored as 407 ± 69.27 HV. The results indicate the feasibility of fusion joints of nitronic steel using microwave energy.

The objective of this research is to optimize cooling time for polyethylene fusion joints which will lead to higher output and cost savings to the industry. Conventionally, the butt fusion joints are prepared as per DVS standard with the following sequence: (i) preparation (alignment and facing of pipe & heating of plate) (ii) bead build up and heat soaking (iii) change over (iv) joining and cooling time. Among these stages cooling time is the most significant part as it governs the strength of the joint as well as the productivity. Higher is the cooling time lower is productivity. To the best of the authors’ knowledge, no study has been reported so far to address cooling time for the polyethylene fusion joints. Therefore, this work intends to study the cooling time and its effect on the joint integrity for these joints. Commonly used size of High Density Polyethylene (HDPE) pipes, OD 160 and OD 315 mm are used for the study. Sample joints are prepared as per the standard, with 100%, 80% and 60% cooling time. The samples are tested - tensile and bending tests. The comparison of the tensile properties and bending test results of the joints with different cooling time as well as with the parent material are analyzed to reach an optimum cooling time.

There is a need for ASME B&PV Code procedures and acceptance criteria for evaluation of flaws detected by inspection of high density polyethylene (HDPE) piping items in safety Class 3 systems. To support the development of flaw acceptance criteria for butt fusion joints in HDPE pipes, a series of coupon tests have been completed for specimens cut from butt fused HDPE pipes with surface or subsurface flaws placed in the joints prior to fusion process. Specimens containing known flaw sizes were tested under axial load at accelerated stresses and temperatures until failure; or until a prescribed number of test hours was reached. The failure time from the tests has been correlated to the net section stress and the stress intensity factor, and the results showed that the failure time can be better represented by the stress intensity factor. The test results were then used to fit the Brown and Lu formula that predicts the time to failure due to the slow crack growth of flaws as a function of stress intensity factor and temperature. With the developed Brown and Lu equation, the allowable stress intensity factors for a piping lifetime of 50 years at the maximum code allowable temperature of 60°C have been proposed for both surface and subsurface flaws in HDPE butt fusion joints. Examples of what might be corresponding allowable flaw sizes in the butt fusion joints of piping are also provided.

Polyethylene pipe has been used widely in gas transportation and nuclear safety-related cooling water applications due to its exceptional resistance to corrosion and erosion. Butt fusion joint is one of the main welding forms for polyethylene pipes. Ultrasonic technique is a typical nondestructive examination technique. To overcome the coupling problems when inspecting butt fusion structures, an inspection technique of ultrasonic phased array using water wedge is proposed to solve the coupling matching and to increase the ultrasound amplitudes. The influences on the imaging of the parameters (such as the angle and height of the water wedge), the array element specifications, and the aperture, were investigated via simulations. The parameter optimization was conducted to establish a suitable detection process. After that, the related probe and a simulated water wedge were designed based on the results and manufactured, in which the simulated wedge as a specific probe holder could adjust its own incident angle and array height. Meanwhile, a typical DN315 pipe of PE 100 was made with some typical artificial defects in it. Experiments were conducted, and the results showed that the proposed water-wedged ultrasonic phased array technique is suitable for butt fusion joint inspection.

The use of non-destructive examination (NDE) for assessing the quality of butt fusion joints in polyethylene (PE) pipes has been included in the draft Mandatory Appendix XXVI to Section III of the ASME Boiler and Pressure Vessel Code (Rules for construction of Class 3 buried polyethylene pressure piping). However, currently, there are no acceptance criteria for flaws in butt fusion joints in PE pipes. There is an ASME Task Group on flaw evaluation for PE pipe, which is developing a code case using linear elastic fracture mechanics (LEFM) to determine critical flaw sizes. However, the initial experimental crack growth data generated suggests that linear elastic fracture mechanics is not able to adequately describe slow crack growth in PE materials. In addition, this work is only considering planar lack of fusion flaws in the joint; it is not considering other critical flaw types that can occur in butt fusion joints, such as particulate contamination and cold fusion. TWI has developed procedures using mechanical testing to develop flaw acceptance criteria for butt fusion joints in PE pipes. This is based on inserting lack of fusion flaws of known size and particulate contamination flaws of known concentrations into butt fusion joints and determining the effect of these flaws on both the short-term and long-term integrity of the joints. An important aspect of this work is to determine which of the wide array of mechanical tests available for assessing the integrity of butt fusion joints in PE pipes are the most discriminating. This paper describes the procedures developed for inserting simulated flaws into butt fusion joints in PE pipes, the experimental work to compare the results from different standard short-term and long-term tests on flawed and unflawed joints and the procedures developed to determine flaw acceptance criteria. Results have shown that the most discriminating short-term test for butt fusion joints in PE pipes is a tensile test using a waisted specimen, such as those defined in EN 12814-2, EN 12814-7 and ISO 13953, and the most discriminating property is the energy to break the specimen. The most appropriate long-term test for butt fusion joints in PE pipes is the whole pipe tensile creep rupture test, as defined in EN 12814-3; this is the only long-term whole pipe test that consistently generates slow crack growth in the fused joint, even if it contains no flaws.

The desire to use high-density polyethylene (HDPE) piping in buried Class 3 service and cooling water systems in nuclear power plants is primarily motivated by the material’s high resistance to corrosion relative to that of steel alloys. The rules for construction of Class 3 HDPE pressure piping systems were originally published as an alternative to the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME BPVC) in ASME Code Case N-755 and were recently incorporated into the ASME BPVC Section III as Mandatory Appendix XXVI (2015 Edition). The requirements for HDPE examination are guided by criteria developed for metal pipe and are based on industry-led HDPE research and conservative calculations. Before HDPE piping will be generically approved for use in U.S. nuclear power plants, the U.S. Nuclear Regulatory Commission (NRC) must have independent verification of industry-led research used to develop ASME BPVC rules for HDPE piping. With regard to examination, the reliability of volumetric inspection techniques in detecting fusion joint fabrication flaws against Code requirements needs to be confirmed. As such, confirmatory research was performed at the Pacific Northwest National Laboratory (PNNL) from 2012 to 2015 to assess the ability of phased-array ultrasonic testing (PAUT) as a nondestructive evaluation (NDE) technique to detect planar flaws, represented by implanted stainless steel discs, within HDPE thermal butt-fusion joints. All HDPE material used in this study was commercially dedicated, 305 mm (12.0 in.) nominal diameter, dimension ratio (DR) 11, PE4710 pipe manufactured with Code-conforming resins, and fused by a qualified and experienced operator. Thermal butt-fusion joints were fabricated in accordance with or intentionally outside the standard fusing procedure specified in ASME BPVC. The implanted disc diameters ranged from 0.8–2.2 mm (0.03–0.09 in.) and the post-fabrication positions of the discs within the fusion joints were verified using normal- and angled-incidence X-ray radiography. Ultrasonic volumetric examinations were performed with the weld beads intact and the PA-UT probes operating in the standard transmit-receive longitudinal (TRL) configuration. The effects of probe aperture on the ability to detect the discs were evaluated using 128-, 64-, and 32-element PA-UT probe configurations. Results of the examinations for each of the three apertures used in this study will be discussed and compared based on disc detection using standard amplitude-based signal analysis that would typically be used with the ultrasonic volumetric examination methods found in ASME BPVC.

This paper presents a method for investigating the integrity of high density polyethylene (HDPE) butt fusion joints and mitered joints. The scanning technique utilizes an ultrasonic phased array pitch-catch scanning method that provides full volumetric coverage of the fusion zone for the detection of inclusions and surface-breaking defects. Empirical results supported by numerical modeling are presented. Additionally, the effects of temperature on focalization and the proper countermeasures are discussed.Copyright © 2015 by ASME

Defect classification is the basis of defect safety assessment because defects of different types can lead to failure in different forms. However, the identification of defect type has long been a critical issue in ultrasonic inspection. Wave acoustic was applied in this study to investigate the sound scattering of metal wires in polyethylene (PE), which provided theoretical support for ultrasonic feature extraction. A method of defect recognition for PE electro-fusion (EF) joints was proposed based on pattern recognition of ultrasonic inspection images. According to location, shape, signal intensity, and cluster conditions, typical defects of EF joints of PE pipes were distinguished and identified in phased array ultrasonic images. Furthermore, an automatic defect recognition software was designed based on the proposed approach; the software was improved and verified through defect inspection and identification experiments. Results showed that accuracy can reach 80% for joints with complex defects and 100% for those with single defects.Copyright © 2015 by ASME

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.

The butt-fusion process is an important method for connecting PE pipes, especially when the pipes are of large diameter and of thicker section. Since many different environmental factors can cause defects and flaws, the joints become the weak point that should be inspected before the pipes begin service. Especially cold welding caused by insufficient input of heat energy, which can not be detected by nondestructive test methods currently available, is one of the most dangerous defects in the butt-fusion joints of PE pipes. It has been discovered that there are three Eigen-lines in the butt-fusion joint of PE pipes by using the phased array ultrasonic focusing technique. Further research shows that the upper and lower Eigen-line indicates the solid-liquid interface and the middle Eigen-line indicates the initial butt interface. Based on a lot of experiments, it has been discovered that the distance between the upper and lower Eigen-line, is related with the input of heat energy during welding. So a new method called “Eigen-line method” is developed to assess cold welding by measuring the distance between the upper and lower Eigen-line. A set of experiments show that the method can detect cold welding in butt-fusion joints with the accuracy less than 6%, which can meet the testing requirement for welding joints in engineering.

Today, commercial nuclear power plants are installing High-Density Polyethylene (HDPE) in non-safety-related and safety-related applications. While this material has numerous advantages over the carbon steel pipes that historically have been used for the same applications, developing a way to accurately inspect for joint integrity in HDPE has become increasingly important to utilities and the U.S. Nuclear Regulatory Commission (USNRC). This paper will investigate the ability to quantify the levels of detection of flaws and detrimental conditions using ultrasonic phased array, in butt-fusion joints throughout the full spectrum of applicable HDPE pipe diameters and wall-thicknesses. Perhaps the most concerning joint condition is that of “Cold Fusion”. A cold-fused joint is created when molecules along the fusion line do not fully entangle or co-crystallize. Once the fusion process is complete, during visual examination, there is the appearance of a good quality joint. However, the joint does not have the strength needed, as the required co-crystallization along the pipe faces has not occurred. Performing a visual examination of the bead, as required by the current revision of ASME Code Case N-755, does not provide adequate guarantee of joint integrity. Therefore, volumetric examination is of special concern to the USNRC to safeguard against this type of detrimental condition. Factors addressed will include pipe diameter, wall-thickness, fusing temperature, interfacial pressure, dwell (open/close) time, and destructive verification of ultrasonic data.Copyright © 2010 by ASME

To establish a comprehensive classification method of the defects in fusion joints of polyethylene (PE) pipes, the geometrical characters of typical defects in electrofusion (EF) and butt fusion (BF) joints were researched by means of ultrasonic phased array inspection and naked-eye observation. Then the results were comparatively analyzed, and a novel classification method of the defects in PE pipe joints was proposed. Typical defects in EF joints were classified as lack of fusion, voids, wire dislocation, and cold welding. And those in BF joints were classified as crack, voids, inclusion on fusion interface, and process defects. Furthermore, characteristics of the classified defects, together with their main causes were briefly described.Copyright © 2010 by ASME

High-speed tensile and Charpy impact tests were conducted on fusion-joined specimens to clarify the impact properties of medium-density polyethylene pipes for gas distribution. Comparison of the impact energy of substrates with those of butt fusion joints reveals that the impact energy differs with resin grades, and that, in all cases, the impact energy of the fusion joints is lower than that of the substrates. The effects of fusion conditions and combination of different grades of resins on fusion strength are also discussed.

Remember the story of Cinderella? Cinderella was too poor and unworthy to ever become a princess. Similarly, when first introduced into the North American pipe market in the 1950s, PVC pipe was admonished by competitors and skeptics as having little chance to succeed. As a substitute for the well-established pipe mainstays of that era—iron, steel reinforced concrete, asbestos cement, and vitrified clay; PVC pipe was initially viewed as having insufficient strength and stiffness to be a viable contender. All of these early views had to be changed. This paper summarily chronicles how the questions and doubts surrounding PVC pipe’s performance capabilities were overcome, including the resolution of subsequently raised concerns. Through the consistent treatment of every issue with rational, technical, research based approaches; combined with an extensive in-service record of admirable performance; the use of PVC pipe has grown steadily through more than five decades. As a direct consequence, PVC pipes now account for the majority of all new water and sanitary sewer installations, exceeding the market shares of all the aforementioned established pipe materials combined. PVC pipe’s fairy godmother is proud, indeed!

This paper describes the application of the full-notch tensile fatigue (FNTF) test to fusion strength evaluation (butt, socket, saddle and electrofusion).When a full notch is made at the fusion interface in a joint, the fusion strength of the joint can be evaluated in the same manner as that of a substrate. While the conventional stress rupture test is not suitable for fusion interface strength measurement, our proposed method permits measurement of fusion strenth variation by means of the actual time to failure.Besides butt fusion strenth, the FNTF test method is applicable to the fusion strenth evaluation of various types of joints, determination of compatibility and selection of the optimum fusion conditions. The fusion strength correlates strongly with the melt flow rate of resin. The higher the melt flow rate is, the closer the fusion strength becomes to that of the substrate, providing a wider range of optimum fusion conditions. For butt fusion between two different brands of polyethylene, it is preferable to select resins having the same melt flow rate. The FNTF test method has already been specified in the JIS as one of the recommended evaluation methods for fusion joints.

Modern use of fusion joints instead of riveted or bolted connexions has produced a variety of new forms which offer numerous fields of inquiry covering a great range of problems. One set of problems relates to stress distribution directed more especially to the determination of the maximum stresses in fusion joints. This is almost entirely an experimental matter, since nearly all such joints have discontinuities imposed by practical necessities which render them insoluble by theoretical analysis, while little or no help can be gained from our existing knowledge and long experience of riveted and bolted joints on account of their essentially different nature. The present inquiry deals with problems of plane stress distribution in which models are used for measuring the stress at any point required by photo-elastic means, and in the first instance some typical butt welds of an unsymmetrical type are analysed experimentally. Other ways of connecting plates in line are then considered in which a joint is made by a lenticular section of weld metal on one side and a cover plate on the other, the ends of the latter being joined to the main plates by triangular sections of fusion metal. When the gap between the two main plates is sufficient the lenticular section of metal may be replaced by a pair of triangular fillets binding the inner ends of the plates to the cover strap, and this form is also dealt with. The effects of triangular fillets of lesser height than the butt straps and of corresponding fillets of curved quadrantal form are also examined with reference to a double butt strap joint. The paper concludes with a brief summary of the main results afforded by the experimental analysis.