Materials Laboratory Failure Analysis
Failure can be defined as any change in a component that prevents satisfactory performance of its intended function. Understanding why a component failed is important for several reasons. First, a determination of the failure mode is an essential component of any root cause analysis. Second, understanding a component failure provides essential information for preventing additional or similar failures. Perhaps most importantly, an erroneous or incomplete analysis can be worse than no analysis at all, since this can prompt inappropriate responses that do not address the basic cause of failure or increase the potential for additional failures.
The degrees to which a failure analysis can be applied range from simply determining the failure mode to performing a full root cause analysis of the failure event. For each situation that occurs, the approach required should be evaluated to determine the level of metallurgical evaluation or engineering effort that should be expended. Identifying the mode of a failure can range from relying on a thorough visual examination by a competent failure analyst to a full laboratory analysis consisting of some or all of the following tasks:
- Visual Examination and Photo-Documentation
- A preliminary diagnosis of the mechanism can often be made based on visual examination of the macroscopic damage features; this will determine the number and location of specimens to be removed for destructive analysis.
- Photo-documentation will record distinctive features of the damage prior to sample cutting and can preserve information on locations of specimens removed for destructive analysis.
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Cracking identified by WFMT A ring section showing external wallloss as well as swelling Microhardness traverse through fractured weld
Non-Destructive Examination (NDE), Where Appropriate Before the component is sectioned, it might be examined by various non-destructive techniques (e.g., dye penetrant inspection or phased array ultrasonics) to help identify areas for sectioning or assess the effectiveness of an NDE technique to locate similar damage in other components.
- Chemical Composition Analysis A chemical analysis is performed to determine if the component material is within specification, and if any particular additional elements (unspecified or trace) are present that could adversely affect material performance.
- Dimensional Measurements Dimensional measurements include the major dimensions such as outer and inner diameters and component thickness. They can be used to identify the location and magnitude
of material wastage or wall loss. For tubular components, dimensional measurements can also be used to identify the degree of service-induced swelling, which is a measure of accumulated creep damage. - Hardness Evaluation and/or Mechanical Properties Testing Hardness tests can easily be performed on metallurgical sections and provide an indication of the metallurgical condition of the material. Hardness values can also be used to estimate tensile strength. Other mechanical property testing, including elevated temperature properties (creep strength) and fracture toughness, can be performed as part of more detailed investigations.
- Metallography Metallographic evaluation allows for assessment of component microstructure and microstructural degradation (spheroidization, graphitization or transformation). It also provides information on damage type, extent, and morphology (cracking/fracture path, rupture features, corrosion, pitting, and cavitation). The appearance and thickness of oxides/scales/deposits can also be assessed.
- Fractography If the sample includes a fracture surface, this surface can be examined to evaluate the fracture characteristics. The morphology of the fracture surface provides insight into the mode of failure (transgranular/intergranular, ductile/brittle, etc.) and may also indicate the presence of precipitates, cavities or foreign species that may have caused or contributed to the failure. Fracture surfaces are commonly examined with a stereomicroscope, digital Keyence microscope, or with a scanning electron microscope, which provides both depth of field as well as high magnification views of the damage.
- Characterization of Oxide/Deposit (EDS/XRD) Often surface oxides, deposits, or corrosion products play a significant role in the damage mechanism either by directly causing wall loss or attack of the metal, or by acting as a secondary contributor to a failure (e.g. internal oxide scale “insulating” a steam touched tube and causing an increase in the tube metal temperature). Characterization of these oxides or deposits includes identifying the elements or compounds present, mapping the elemental constituents to show how they are dispersed throughout the layers, and examining the morphology at low and high magnifications.
of
Elemental maps of an oxide from a supercritical boiler waterwall tube showing an inner chromium rich spinel layer and an outer iron oxide layer.
- Characterization of Crack Deposits and Corrosion Products/Deposits Elemental analysis of crack deposits or corrosion products can help identify the damage mechanism. In the case where contaminants cause or contribute to failure, identification of such contaminants can bolster findings and support recommendations for avoiding future failures.
Chloride stress corrosion cracking in an austenitic stainless steel and EDS analysis results from crack deposits showing a very high chlorine peak
For any situation involving material property characterization, Structural Integrity has an experienced group of materials specialists and a full-service metallurgical testing laboratory that can help.
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SI Solutions Expands Structural Engineering Expertise and Adds Controls Engineering Division with the Acquisition of SC Solutions, Inc.
CHARLOTTE, NC – SI Solutions is pleased to announce the purchase of SC Solutions, Inc., a respected provider of innovative engineering solutions for the advanced process controls and structural engineering industries. Founded in 1987, SC Solutions has served its clients in Silicon Valley and beyond, expanding to Sacramento, CA, Portland, OR, and Atlanta, GA.
SI Solutions, a privately held company, is a leading provider of mission-critical engineering, construction, testing, and maintenance services to the energy, process, and infrastructure markets. SI Solutions has over 500 employees and ten offices serving customers across the U.S. and internationally. SI’s capabilities span multiple technical disciplines, with a focus on specialized engineering, instrumentation, electrical design and construction, and advanced nondestructive examination.
SC Solutions, Inc. (SC), based in Sunnyvale, CA, specializes in structural engineering and advanced process controls. SC’s structural division has over 35 years of experience with numerical analysis of complex infrastructure assets, including those subjected to extreme loads and events such as earthquakes, dynamic impact, thermal shock, construction transients, and soil-structure-fluid interaction effects. SC’s controls engineering team has extensive experience in control design, modeling of physical systems, real-time software, signal processing, optimization, system identification, and fault diagnostics for the semiconductor, advanced materials manufacturing, energy, infrastructure, and defense industries.
Mark W. Marano, President and CEO of SI Solutions, stated: “Adding SC Solutions to our existing platform will bolster our Critical Infrastructure division, enhancing our ability to serve clients’ needs through new technical capabilities and expanded engineering offerings. SC Solutions’ highly respected process controls division will add a new market segment and a third reputable brand under the SI Solutions umbrella, diversifying our service offerings worldwide.”
Greg Loy, Chairman and co-founder of SC Solutions noted, “I’m proud of our team for the business we’ve built over the years; this is the next step in the company’s evolution. The additional investment from new ownership will allow us to further develop the products and services our clients have come to rely upon.”
Chris Martin, Managing Director of Jumana Capital and Chairman of SI Solutions, remarked, “The addition of SC Solutions to our family of brands grows the strength and capabilities of SI Solutions to meet the growing need for asset management, regulatory compliance, maintenance and upgrades within nuclear power, power services, process industries, pipeline integrity, critical structures, and controls engineering sectors in the United States and abroad. We are excited about the future for SI Solutions as we continue to build toward an extraordinary future.”
www.sisolutions.com
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News and Views, Volume 54 | Materials Lab Featured Damage Mechanism
STRAIN-INDUCED PRECIPITATION HARDENING (SIPH) IN AUSTENITIC STAINLESS BOILER TUBES
By: Wendy Weiss
Structural Integrity’s Metallurgical Laboratory offers comprehensive metallurgical laboratory services to support client material issues.
Strain-Induced Precipitation Hardening, also known as SIPH, is a commonly misinterpreted boiler tube failure mechanism that occurs when austenitic stainless steel tubing is cold or warm worked during fabrication and then is installed with either improper or no solution annealing heat treatment. While the basic mechanism and the root cause are understood, the complex interaction between heat chemistry, quantity of cold or warm work, and subsequent thermal history makes it very difficult to predict under precisely what circumstances damage due to SIPH will result in failure of a boiler tube.
Longitudinally oriented crack at the extrados of a bend in a stainless steel superheater tube.
Mechanism
SIPH occurs when a heat of austenitic stainless steel containing certain precipitate-forming elements (e.g., niobium, titanium, vanadium, etc.) either intentionally or as residuals is cold or warm worked during subsequent material processing. The cold or warm working creates excess defects in the sub-structure of the material, which serve as preferred sites for precipitation of temper-resistant carbides or carbo-nitrides. Precipitation occurs when the material is heated to a sufficiently high temperature that is well below the solution annealing temperature. This can occur rapidly during a poorly executed heat treatment if the material does not reach the proper solution annealing temperature, or it can occur more slowly at typical operating temperatures for superheater or reheater tubing in utility-type boilers.
Once formed, precipitates anchor to the defects, resulting in a substantial increase in the elevated temperature or creep strength of the interior of the grains. At the same time, there is a narrow zone of material immediately adjoining the grain boundaries that remains largely precipitate-free due to the diffusional characteristics of the grain boundary itself. Ultimately, the interior portion of the individual grains becomes very strong at elevated temperatures while the material immediately adjoining the grain boundaries becomes comparatively creep weak. In addition, any surface-active elements that may be present in the material, such as arsenic, tin, antimony, etc., will tend to concentrate at the grain boundaries, further reducing their strength.
Regardless of when the precipitation occurs, once the interior of the grains has been strengthened, the grain boundary regions are weakened. Any strain imposed on the material in response to an applied or residual stress is forced to concentrate in the grain boundary region, which substantially magnifies its effect. For example, suppose the bulk strain experienced by a cold-worked stainless superheater tube segment is very small – a fraction of a percent – and the material has undergone the SIPH reaction. The strengthened grain interiors will undergo no strain. Conversely, within the much smaller volume of the comparatively weak grain boundaries, the accumulated strain will be orders of magnitude higher than the bulk level.
Typical Locations
- Bends
- Offsets
- Swages
- Welded attachments
Features
- Can initiate at midwall
- Early-stage damage consists largely of grain boundary cavities and microfissures
- Intergranular cracking
Root Causes
The single root cause of SIPH is the failure to properly solution anneal susceptible heats of austenitic stainless steel tubing that has been either cold or warm worked during fabrication.
An overall, cross-sectional view of the intergranular crack (image A) with higher magnification views of some of the secondary grain boundary microfissures and voids (image C). Slip bands (parallel lines in image B) indicate local deformation.
For any situation involving material property characterization, Structural Integrity has an experienced group of materials specialists and a full-service metallurgical testing laboratory that can help.
Visit Our Metallurgical Laboratory Services Page
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News & Views, Volume 53 | Serviceability Assessment of an L-Grade Stainless Steel Pipe Fitting
By: Terry Totemeier
A client recently ordered a Type 316 stainless steel pipe coupling fitting for use in a high-pressure, high-temperature steam line operating at 1005°F. The fitting that was received was so-called dual grade Type 316/316L stainless steel. Given the limitations on using “L” grades of stainless steel at high temperatures, the client requested that SI perform a serviceability assessment for the fitting to determine if it could be safely used until the next scheduled outage when a replacement non-L grade fitting would be available.
BACKGROUND
The fitting ordered was a ½” nominal diameter (NPS ½), 6000# (Class 6000) full coupling socket-welding fitting in accordance with the ASME B16.11 specification, material ASME SA-182 forging, Type 316 stainless steel (designated as F316 in SA-182). The fitting supplied was dual grade F316/316L material with a carbon content of 0.023% per the material test certificate. The designation of this material as “dual grade” means that it meets the requirements of both F316 and F316L material grades. This is possible because the chemical composition requirements of these two grades overlap, with the primary difference between them being carbon content. For F316 the carbon content is specified to be 0.08% maximum (no minimum), while for F316L the carbon content is specified to be 0.030% maximum. Therefore, material with carbon content less than 0.030% will meet the requirements for both grades. It is worth noting that the carbon content of “H” grade of 316 stainless steel (F316H per SA-182) is specified to be 0.04-0.10%. The H grade is intended for use at high temperatures.
The received fitting was installed in a main steam valve pressure equalizing line with a steam temperature/pressure of 2750 psia/1015°F at design conditions and 2520 psia/1005°F at operating conditions. The fitting was welded to Grade P11 pipe on one side and Grade P22 pipe on the other side. The applicable code was stated to be ASME BPVC Section I.
With a reported carbon content of less than 0.04%, the fitting is technically not permitted for use in ASME Section I construction above a temperature of 1000°F. Per the ASME Boiler and Pressure Vessel Code (BPVC) Section II, Part D, Table 1A, the allowable stresses for SA-182, F316 material are valid at or above 1000°F only when the carbon content is greater than 0.04% (Note G12). Per the same table, SA-182, F316L material is only permitted for use in Section I construction up to 850°F. The reason for this temperature limitation is that the long-term creep-rupture strength of Type 316 stainless steel with lower carbon content is reduced compared to material with higher carbon content because fewer carbides form during service to strengthen the grain boundaries. There are no other adverse impacts of the lower carbon content, e.g., on fatigue strength or oxidation resistance.
The short-term serviceability of the fitting with low carbon content was assessed by comparing bounding pressure stresses in the fitting with the reported creep-rupture strength for Type 316L material. Per the ASME B16.11 specification, Class 6000 socket-welding fittings are compatible with NPS Schedule 160 pipe, meaning that pressure stresses in the fitting will be less than those in Sch 160 pipe with minimum wall thickness according to ASME B36.10 (pipe dimension specification), in other words, the fitting will be at least as strong as the pipe.
ASSESSMENT
The dimensions of NPS ½, Schedule 160 pipe per the ASME B36.10 pipe specification are 0.84” outer diameter (OD), 0.165” minimum wall thickness (MWT). For an operating steam pressure of 2,520 psi, the reference hoop stress per the equation in ASME BPVC Section I, Appendix A-317 is 5.05 ksi. Per the general design guidance in ASME B16.11 (Section 2.1.1) the pressure stresses in the fitting must be less than this.
Figure 1. Schematic diagram for a socket-welding coupling fitting. Per ASME B16.11, an NPS ½, Class 6000 fitting has relevant dimensions B = 0.875” maximum, C = 0.204” minimum, and D = 0.434” minimum.
Since the fitting in question is cylindrical, comparative hoop stresses can also be calculated from dimensions given in ASME B16.11, although these may not be exact due to the varied wall thickness in the fitting. According to Table I-1 of ASME B16.11, the central body of the fitting is 1.283” OD and 0.395” MWT (Figure 1). The reference hoop stress calculated using the A-317 equation at 2,520 psi stream pressure and these dimensions is 2.63 ksi, considerably less than 5.05 ksi. In the female socket ends of the fitting, the OD is also 1.283”, but the minimum wall thickness is 0.204”, leading to a calculated reference hoop pressure stress of 6.58 ksi. Note that the actual stresses in the socket ends will be much less than this because the pipe will be inserted and welded into the socket, taking up the pressure loading, but the calculated stress can be taken as a bounding value.
Creep-rupture strengths for Type 316L stainless steel have been reported in ASTM Data Series DS 5S2 publication, “An Evaluation of the Yield, Tensile, Creep, and Rupture Strengths of Wrought 304, 316, 321, and 347 Stainless Steels at Elevated Temperatures” (ASTM, 1969). According to Table 7 in this report, the average 10,000 hour creep-rupture strengths for Type 316L at 1000°F and 1050°F are 34.5 and 25 ksi, respectively. Minimum creep-rupture strengths are typically taken as 80% of the average strength, so the inferred minimum strengths at 1000°F and 1050°F are 27.6 and 20 ksi, respectively.
The reported 10,000 hour creep-rupture strengths in the temperature range of interest are more than twice the calculated bounding pressure stresses in the fitting, so it was judged that there is very little risk of failure of the fitting by creep-rupture in the next 10,000 hours of service.
This result is unsurprising since the 1005°F is barely into the creep range for Type 316 regardless of carbon content. The carbon content effects become more pronounced at higher temperatures (approximately 1100°F and above).
CONCLUSION
Based on the above assessment, it was SI’s opinion that the Type 316L fitting with carbon content less than 0.03% was suitable for a limited period of service (less than 10,000 hours) until it can be replaced. Given that the fitting is reportedly welded to low-alloy steel pipe on either side, SI also recommended that a Grade 22 (2.25Cr-1Mo) low-alloy steel fitting be considered as a replacement, which would eliminate dissimilar metal welds (DMWs) between the fitting and pipes. DMWs are prone to premature failure due to thermal fatigue, weld fusion line cracking, and decarburization of the ferritic material. This voluntary recommendation made by SI, was not part of the original scope of work, but may have been just as critical a finding as it shed light upon a failure risk previously unknown by the client.
SIA Receives SOC 2, Type II Certifications for our AIMS Software Platform.
At Structural Integrity Associates (SIA), we take cybersecurity very seriously for ourselves and our clients, who expect us to protect the sensitive information shared with us with the same rigor as their internal policies.
Recognizing this dedication, we are excited to announce that SI received the Service Organization Control (SOC) 2, Type II certification for our Asset Information Management System (AIMS) platform. In short, this means that all products built on AIMS (see sidebar) fully comply with the latest cybersecurity standards.
SOC 2 is an auditing framework created by the American Institute of Certified Public Accountants (AICPA). It reviews an organization’s processes and procedures regarding client security, confidentiality, privacy, availability, and processing integrity. The comprehensive review includes extensive auditing over a prolonged period. To be certified, there must be detailed planning, constant monitoring, and continual improvements to uphold the challenging requirements of the AICPA. The SOC 2, Type II certification is a significant milestone that demonstrates to our clients that we operate in a secure environment.
AIMS is an industry-agnostic asset management platform used to build applications that store, visualize, and analyze information for industrial assets. It is a low-code application platform that can be configured to manage any industrial asset. Its flexible structure, easy integration with time series data, and advanced analytics make it ideal for managing critical assets and building Digital Twins for industrial resources.
“The AIMS Digital Solutions platform is integral to Structural Integrity’s mission to be the most trusted provider of best-in-value, innovative, fully integrated asset lifecycle solutions. Digital products paired with our expertise in Engineering, Inspections, and Analytics help us provide holistic asset management solutions to our clients. Receipt of the SOC 2, Type II certification assures users of the integrity of the software of the AIMS applications.”
Anup Aggarwal – Director of Digital Transformation
Structural Integrity is proud to continue supporting our clients by providing these applications for critical asset management and ensuring their cyber security.
AIMS Platform
- PlantTrack™
- MapPro™
- SIIQ™ (Online Monitoring)
- SI Pipe Evaluation (SIPE)
- Material Verification Intelligence (MVI)
- Maximum Allowable Operating Pressure (MAOP)
- Assessment Planning
- TRU Compliance
- ICON
Dan Patten Appointed Director, Strategic Business Development
Structural Integrity Associates, Inc. (SIA) is pleased to announce the addition of Daniel (Dan) Patten as Director, Strategic Business Development. Mr. Patten comes to SIA with a diverse background in the power generation industry. Most recently, Mr. Patten served as the Fleet Engineering Manager for Energy Harbor, where he held responsibility for Fleet Programs Engineering (Buried Pipe, Flow Accelerated Corrosion, Inservice Inspection, Non-Destructive Examination, etc.), Probabilistic Risk Assessment, Life Cycle Management (for critical components), Procurement, and more. Mr. Patten has also held multiple leadership roles at PSEG, supporting critical nuclear and fossil fleet initiatives. Mr. Patten’s expertise extended into serving on multiple EPRI committees, including acting as Subgroup Chairman from 1997 through 2023 and, most recently, with the Welding and Repair Technology Committee (WRTC).
Regarding the arrival of Mr. Patten to Structural Integrity Associates, Mike Battaglia, SIA’s Vice President, Nuclear and Chief Nuclear Officer, stated, “Dan’s breadth of experience and demonstrated leadership in the nuclear industry is widely recognized. In addition, he will bring the perspective of the nuclear utility clients to better position our products and services. We are excited and fortunate to welcome Dan into the fold at Structural Integrity Associates.”
Mr. Patten has a Master of Science degree in Materials Engineering and a Master of Science in Engineering Management.
News & Views, Volume 53 | Encoded Phased Array Ultrasonic Examination Services for Cast Austenitic Stainless Steel (CASS) Piping Welds
IN PRESSURIZED WATER REACTOR (PWR) COOLANT SYSTEMS
By: John Hayden and Jason Van Velsor
The CASS piping welds present in many PWR plants provide numerous and complicated challenges to their effective ultrasonic examinations. To this point, a viable ultrasonic examination solution for the inspection of these piping components, as required by ASME Code Section IX, had previously not been available. By leveraging our technical expertise in materials, technology development, and advanced NDE deployment, Structural Integrity Associates, Inc (SI) has developed a new system that will provide a meaningful solution for the examination of CASS piping components. The result of this program will be the first commercial offering for the volumetric examination of CASS components in the nuclear industry.
BACKGROUND INFORMATION
ASME Section XI Class 1 RCS piping system welds fabricated using CASS materials pose serious and well-understood challenges to their effective ultrasonic examination. For decades, utilities and regulators have struggled with the administrative and financial burdens of Relief Requests, which were, and still are, based on the inability to perform meaningful volumetric examinations of welds in CASS components.
Many years of futility and frustration may have fostered the belief that technology allowing effective and meaningful examination of CASS materials would never be achievable. This is no longer the case.
The failure mechanism for CASS material occurs through the loss of fracture toughness due to thermal aging embrittlement. The susceptibility of CASS material to thermal aging embrittlement is strongly affected by several factors, primary of which are system operating time and temperature, the casting method used during component manufacture, and molybdenum and ferrite content. In addition to the existing ASME Section XI requirements for the examination of welds in CASS materials, the susceptibility to thermal aging embrittlement drives the requirement for additional examinations (including ultrasonic examinations) as directed by several NRC-published NUREGs required for plant license renewal. The existence of a viable, effective examination capability for CASS materials plays a very important part in both currently required Inservice Inspections (ISI) and plant license renewal.
Figure 1. An example of the widely-varying microstructure of a centrifugally cast piping segment. False-color imaging is used to aid visualizing grain variations. (Image from NUREG/CR-6933 PNNL-16292)
CASS MATERIAL PROPERTIES AND EFFECT ON ULTRASONIC EXAMINATION
Metallurgical studies have revealed that the microstructure of CASS piping can vary drastically in the radial (through-wall) direction, as well as around the circumference and along the length of any given piping segment. Large and small equiaxed, columnar and mixed (combinations of equiaxed and columnar grains), and banding (layers of substantially different grain structures) are commonly observed in CASS piping materials. None of these conditions favor the performance of effective ultrasonic examinations.
Figure 2. PWR RCS Major Components
The very large and widely varying types (equiaxed, columnar, and randomly mixed), sizes and orientations of the anisotropic grains in CASS material are very problematic. Anisotropic is defined as an object or substance having a physical property that has a different value when measured in different directions. Such physical properties strongly affect the propagation of ultrasound in CASS material by causing severe attenuation (loss of energy through beam scattering and absorption), beam redirection, and unpredictable changes in ultrasonic wave velocity. These factors are responsible for the inability of ultrasonic examination to completely and reliably interrogate the Code-required volume (inner 1/3 Tnom) of welds in CASS piping material. Interestingly, CASS materials less than 1.6” Tnom (Pressurizer Surge Piping) can be effectively examined, while CASS materials over 2.00” (Main RCS Coolant Loop Piping) are less effectively examined. Consequently, an ASME Section XI, Appendix VIII qualification program for CASS piping components has not been established and remains in the course of preparation. Nonetheless, ASME Section XI requirements to conduct inservice examinations of RCS piping welds fabricated from CASS components remain fully in force.
ASME CODE ACTIONS AFFECTING CASS PIPING EXAMINATIONS
ASME Section XI Code Case N-824, “Ultrasonic Examination of Cast Austenitic Piping Welds From the Outside Surface,” was approved by ASME in October 2012 and by the NRC in October 2019. This Code Case provides the first approved direction for the ultrasonic examination of welds joining CASS piping components. The ASME B&PV Code, Section XI, 2015 Edition, incorporates Code Case N 824 into Mandatory Appendix III in the form of Mandatory Supplement 2. To date, these two ASME Section XI Code documents remain the sole sources approved by ASME and NRC that provide specific direction for the examination of CASS RCS piping system welds and, therefore, form the foundation of SI’s approach for the development of our CASS ultrasonic examination solution.
SI’S CASS PROGRAM DESCRIPTION
SI is developing the industry’s most well-conceived and capable ultrasonic system for the examination of welds in CASS piping components. To accomplish this objective, SI has drawn upon our internal knowledge and experience, supplemented by a careful study of numerous authoritative bodies of knowledge relating to the examination of CASS components. The development of the SI examination system has been guided by both SI’s industry-leading 17 years of experience conducting phased array examinations in nuclear power plants and the knowledge acquired through the careful study of the topical information contained within industry-recognized publications. These published results of extensive industry research provided both guidance for the selection of phased array system components and CASS-specific material insights that strengthen the technical content of our Appendix III-based procedure.
Figure 3. RCS Coolant Pump and Crossover Piping
CASS PROGRAM ELEMENTS
SI believes that the procedure, equipment and personnel featured in this program will be equivalent or superior to those that will form the industry-consensus approach for CASS ultrasonic examinations needed to successfully achieve Appendix VIII, (future) Supplement 9, “Qualification Requirements for Cast Austenitic Piping Welds.”
Ultrasonic Procedure – SI has crafted an ultrasonic examination procedure framework that is fully compliant with ASME Section XI, Mandatory Appendix III, Supplement 2, along with referenced Section XI Appendices as modified by the applicable regulatory documents.
Ultrasonic Equipment – SI has acquired and assembled the ultrasonic system components required by Code Case N-824 and Appendix III, Supplement 2, which includes the following:
- Ultrasonic instrumentation capable of functioning over the entire expected range of examination frequencies. The standard examination frequency range extends from low-frequency, 500 KHz operation for RCS main loop piping welds through 1.0 MHz for pressurizer surge piping.
SI has designed and acquired additional phased array transducers that meet the physical requirements of frequency, wave mode, and aperture size and are capable of generating the prescribed examination angles with the required focal properties. SI has designed and fabricated an assortment of wedge assemblies that will be mated with our phased array probes to provide effective sound field coupling to the CASS components being examined. SI’s wedge designs consider the CASS pipe outside diameter and thickness dimensions and employ natural wedge-to-material refraction to assure optimal energy transmission and sound field focusing.
SI also possesses several data encoding options that are necessary to acquire ultrasonic data over the expected range of component access and surface conditions. The encoding options will include:
- Fully-automated scanning system, capable of driving the relatively large and heavy 500KHz phased array probes
- The SI-developed Latitude manually-driven encoding system, which has been deployed during PDI-qualified dissimilar metal DM weld examinations in nuclear power plants
Figure 4. Steam Generator Details
Examination Personnel – SI’s ultrasonic examination personnel are thoroughly trained and experienced in all elements of encoded ultrasonic data acquisition and analysis in nuclear plants. SI’s examiners have a minimum of 10 years of experience and hold multiple PDI qualifications in manual and encoded techniques. SI recognizes the challenges that exist with the examination of CASS piping welds and has developed a comprehensive program of specialized, mandatory training for personnel involved with CASS examinations. This training includes descriptions of coarse grain structures, their effect on the ultrasonic beam, and the expected ultrasonic response characteristics of metallurgical and flaw reflectors, as well as the evaluation of CASS component surface conditions.
ULTRASONIC TECHNQUE VALIDATION
Although not required by the ASME Code, SI has arranged for access to CASS piping system specimens from reputable sources to validate the efficiency of our data acquisition process and the performance of our ultrasonic examination techniques. The specimens represent various pipe sizes and wall thicknesses and contain flaws of known location and size to permit the validation and optimization of SI’s data acquisition and analysis processes. SI will thoroughly analyze, document, and publish the results of our system performance during the examination of the subject CASS specimens.
Figure 5. Pressurizer and Surge Line Details
CASS PIPING SYSTEM APPLICATIONS
Typical CASS Piping Weld Locations in PWR Reactor Coolant Systems
The following graphic illustrates the location and extent of CASS materials in the RCS of many PWR plants.
RCS Main Loop Piping Welds: This portion of the RCS contains large diameter butt welds that join centrifugally cast stainless steel (CCSS) piping segments to statically cast stainless steel (SCSS) elbows and reactor coolant pump (RCP) casings. RCS main loop piping includes the following subassemblies:
- Hot leg piping from the Reactor Vessel Outlet to the SG Inlet
- Cross-over piping from the SG Outlet to the RCP Inlet
- Cold leg piping from the RCP Outlet to the RPV Inlet
Steam Generator Inlet / Outlet Nozzle DM Welds: These terminal end DM butt welds are present in PWR plants, both with and without safe ends between the SCSS elbows and the ferritic steel nozzle forgings.
Pressurizer Surge Piping Welds: This portion of the RCS contains a series of butt welds fabricated using CCSS piping segments to SCSS elbows between the Pressurizer Surge nozzle end and the Hot Leg Surge nozzle.
SUMMARY
The CASS piping welds present in many PWR plants provide numerous and complicated challenges to their effective ultrasonic examinations. SI’s new CASS ultrasonic examination system will provide a new and meaningful solution.
PROJECT TIMELINE
SI is working to complete the development, integration and capability demonstrations of the CASS ultrasonic examination system described in this document for limited (emergent) fall 2023 and scheduled deployments beginning in spring 2024.
News & Views, Volume 53 | Phased Array Ultrasonic Testing (PAUT) Monitoring with Ultrasonic Thick-Film Arrays
Traditional nondestructive examination (NDE) activities are planned based on hours of service, number of load cycles, time elapsed since previous inspections, or after the emergence of clear and obvious damage in a component. While engineering judgment and risk analysis can, and should, be used to prioritize inspections, these prioritizations are not based on the actual physical condition of the component or material it is constructed from but on precursory conditions that may or may not lead to eventual damage. Alternatively, continuous monitoring approaches can facilitate advanced planning and the optimization of Operations and Maintenance (O&M) spending by enabling the prioritization of inspections based on a component’s actual current condition. Furthermore, continuous monitoring enables earlier detection, which allows the extension of the component’s remaining useful life through modified operation.
SI’s recent advances with thick-film are breakthrough technologies for long-term monitoring and imaging of crack growth in critical components.
Figure 1. Installed thick-film UT sensors for thickness monitoring of elbows.
Given the trend of fewer on-site resources and tighter O&M budgets, the energy industry has a strong motivation to progress toward condition-based inspection and maintenance. To facilitate this evolution in asset management strategy, new monitoring sensor technologies are needed, ones that provide meaningful monitoring data directly correlated to the condition of the material or asset. To support this need Structural Integrity has developed a novel thick-film ultrasonic sensor solution. Initially developed for basic applications, such as thickness monitoring, SI’s recent advances with this technology make long-term monitoring and imaging of crack growth in critical components possible.
BACKGROUND
Ultrasonic thick-films are comprised of a piezoelectric ceramic coating that is deposited on the surface of the component that will be monitored. A conductive layer is then placed over the ceramic layer, and the ceramic layer deforms when an electric potential is applied across the film. When a sinusoidal excitation pulse in the ultrasonic frequency range is applied across the film, the vibration of the film is transferred into the test component as an ultrasonic stress wave.
Structural Integrity initially developed our thick-film ultrasonic sensors for real-time thickness monitoring and has demonstrated the performance and longevity of this technology through laboratory testing and installation in industrial power plant environments, as seen in the photograph in Figure 1, where the sensors have been installed on multiple high-temperature piping components that are susceptible to wall thinning from erosion. In this application, the sensors are fabricated directly on the pipe’s external surface, covered with a protective coating, and then covered with the original piping insulation. Following installation, data can either be collected and transferred automatically using an installed data acquisition instrument, or a connection panel can be installed that permits users to acquire data periodically using a traditional off-the-shelf ultrasonic instrument. Example ultrasonic datasets are shown in Figure 2.
Figure 2. Ultrasonic datasets from an installed thick-film UT sensor at two different points in time.
TECHNOLOGY ADVANCEMENTS
Recently, SI has demonstrated the ability to create thick-film sensors with complex element arrays that can be individually controlled to steer and focus the sound field, as with traditional phased-array ultrasonic testing (PAUT). Moreover, data from individual array elements can be acquired and post-processed using full-matrix capture (FMC) techniques. FMC is a data acquisition technique where all elements in the array are used to both transmit and receive ultrasonic waves. The result is a large data matrix that can be used for further processing with various post-processing techniques. Compared to more traditional active focusing, FMC is well-suited for a fixed transducer array, as scanning speed is not a concern. Another advantage is that the electronics needed for data acquisition can be simplified – requiring only a single pulsing channel.
A thick-film Linear-Phased Array (LPA) installed on a standard calibration block is shown in Figure 3. The two images shown on the right were generated using the Total Focusing Method (TFM) post-processing algorithm, with the image on the far right having an adjusted color scale to highlight the imaging of the notches toward the bottom of the calibration block. TFM is an amplitude-based image reconstruction algorithm where the A-scans from the FMC dataset are used to synthetically focus on every point in a defined region of interest.
Figure 3. FMC TFM results from a thick-film linear phased array installed on a calibration block.
Using other information from the FMC dataset, such as the phase of the waveforms, has proven to be beneficial in certain cases. At each focal point in the region of interest, a large phase coherence among all the waveforms can be indicative of a focused reflector. This can then be applied to the TFM image at each focal point as a weighting factor (also known as the Phase Coherence Factor (PCF)) to improve the signal-to-noise ratio.
Figures 4 and 5 illustrate the results of applying the phase coherence imaging technique to the FMC datasets collected with thick film transducer arrays. The sample is a section of high-energy piping approximately 1.7 inches thick with cracking at various positions along a girth weld. The sample has a counterbore with ID-initiated cracks up to approximately 0.5 inches in length coming from the taper of the counterbore. The thick film transducer arrays were located at different positions along the weld.
SUMMARY
The energy industry is moving away from traditional scheduled-based planning for inspection and maintenance activities and toward “smart plant” concepts that rely more heavily on data correlated to actual component conditions. To accomplish this, there is a need for new and novel monitoring technologies that are both unobtrusive and able to withstand the harsh conditions of industrial facilities. Collecting robust and meaningful monitoring data will be critical in ensuring that safety and asset reliability are maintained and even improved. Structural Integrity’s thick-film UT technology has been developed to achieve this goal and continues to evolve for higher-temperature components and more advanced applications. We are ready to support a variety of in-field applications, contact one of SI’s experts if you have questions or a potential application that could benefit from installed thick-film UT sensors.
Figure 4. Phase coherence imaging result from a thick film transducer array on a cracked weld sample.
Figure 5. Phase coherence imaging result from a thick film transducer array on a cracked weld sample.
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