Advanced imaging techniques developed for characterizing heavy-section pressure vessels offer an NDE alternative for hardness testing and other parts inspection.

PNNL applies its scientific capabilities and expertise to develop technology solutions for industry.

Pacific Northwest National Laboratory (PNNL), one of nine DOE multiprogram national laboratories, is dedicated to developing innovative solutions to federal and industrial problems in the areas of health, national security, high-performance computing, science, energy, advanced materials and manufacturing. Finding ways to do things better, faster and at lower cost has become the mantra for success in industry, and the lab is helping industries achieve their business goals through targeted research and development, especially in the area of materials characterization, a critically important aspect to operations in industries including chemical, aerospace, automotive and power generation.

Materials and component characterization is done in many instances using destructive testing, which is effective but time consuming, costly and awkward for feedback during manufacturing. Today, new, efficient, noninvasive materials characterization technologies are becoming the norm in industry, providing important cost reduction and quality benefits. State-of-the art nondestructive evaluation (NDE) and characterization technologies are providing industry with capabilities to advance their operations.

Advanced imaging capabilities being developed at PNNL are providing increased understanding of the internal quality of a material and its microstructural characteristics. Increased capabilities to confirm sound material quality, reveal and characterize anomalies and provide feedback during manufacturing enable more accurate predictions of how a part will perform in service. The benefit to industry is improved part reliability, improved efficiencies in planning proactive maintenance and safety improvements and increased efficiencies in conducting inspections and certification procedures.

PNNL's diverse research stems from a long history of imaging science development for the Nuclear Regulatory Commission (NRC) to characterize and detect anomalies in nuclear reactor pressure vessels. Researchers developed new technologies and applied standard imaging technologies in a unique manner to detect anomalies in reactor materials before there is material degradation and potential for a failure. Much of the experience in this area has led to advanced research and testing of high-alloy materials (such as stainless steels) and heat-treated parts, which could be applied in multiple industries including automotive, aviation and aerospace. Work on high-alloy materials has the potential to effectively and accurately evaluate specialty materials, such as those used in chemical and nuclear vessels and in corrosive environments, and work on hardness depth measurements has applications where the durability of heat-treated metal components is a concern.

Fig 1 View inside a vessel during synthetic aperture focusing technique for ultrasonic testing (SAFT-UT)

Imaging science for vessel analysis

In 1990, the NRC tapped PNNL to tackle a problem regarding fabrication flaws in U.S. light-water reactor pressure vessels (RPV), especially the problem of understanding the structural significance of flaws that were found. PNNL's methodology to estimate the density and size distribution of fabrication flaws in RPVs was to apply synthetic aperture focusing technology (SAFT) imaging, an ultrasonic imaging inspection system, in a unique, innovative manner to be able to differentiate between flaws and microstructural conditions. Researchers used SAFT on vessel material from cancelled nuclear plants and then verified the accuracy of detected flaws using destructive validation methods (Fig. 1). The combination of innovative imaging techniques and expertise in nondestructive evaluation methods proved to be a winning combination.

A key part of the process involved identifying key locations within the RPVs in which to take measurements to determine structural integrity, such as flaw location, dimensions, shape, orientation, and composition. Significant dynamic range and knowledge of the characteristics of the weld microstructure were needed to be able to distinguish fabrication flaws from existing microstructural conditions. The development of this type of imaging capability was essential to characterizing RPV flaws and is relevant today to industrial settings where nondestructive evaluation techniques are required.

The success of imaging the RPV product form, the usefulness of high-resolution techniques and the development of new SAFT reconstruction algorithms will continue to lead to improved imaging capabilities, allowing researchers to "see what they hear" during the material evaluation process. Results have shown that new algorithms can make SAFT calculations practical for large volumetric inspections, such as those done for interstate gas lines in the petroleum industry, for example. The work done with NRC also has led to more specialized studies of uniquely manufactured materials and components regardless of the industry application.

Fig 2 Two cross sections taken from the same arched section of a reactor-cooling loop illustrate the varied characteristics of coarse-grain materials. Both side A and side B were taken from the same piece of material, but a dramatic difference in texture and structural makeup exists at the locations just a few inches apart; Fig. 3 Varied microstructures result from the centrifugal casting process used to produce stainless steel piping for nuclear reactors; grain morphology changes from equiaxed at the inside diameter to columnar toward the outside diameter.

Evaluating coarse-grain materials

PNNL research with SAFT also encompasses the realm of new research and development with coarse-grain materials. The premise for this research was tied to the unique nature of statically or centrifugally cast materials. These types of materials have nonhomogeneous, coarse-grain microstructures (Fig. 2 and Fig. 3) with inherent structural anisotropy, which lead to significant difficulties for using traditional nondestructive examination techniques.

Ease of inspection was not a primary driver in the early development and use of cast stainless steel piping for commercial light-water reactors (LWR). Materials were developed to meet certain NRC specifications (for example, a 28 to 32 in., or 711 to 813 mm, diameter primary loop reactor-coolant piping section having 2 to 3 in., or 50 to 76 mm, thick walls) and did not take into account the need to regularly inspect and certify these types of reactor component materials.

Industrial applications often require materials that must be custom made in a variety of shapes and sizes. Similar to the LWR piping situation, this creates a significant challenge in being able to determine whether an inspection is showing a defect in a vital section of coolant piping, or is simply showing reflected acoustic energy from a grain boundary or geometrical reflector.

Conventional ultrasonic techniques are not effective for examining cast metals. However, that does not eliminate the need to inspect these materials to ensure both quality at the point of production and longevity in actual application. Current inspection procedures for cast materials continue to perform unsatisfactorily because of the coarse macrostructure of the material. Refraction and reflection of the sound beam occur at the grain boundaries, resulting in incorrectly reported defects or unexamined volumes of material. Existing ultrasonic examination techniques have been ineffective because of their inability to discriminate cracks from metallurgical reflectors. There is great potential in the use of low-frequency SAFT to provide high-resolution, high signal-to-noise ratio images of ultrasonic inspection, as well as in the design and development of state-of-the-art sensors (Fig. 4). PNNL is one of only a few domestic organizations that continue to work with coarse-grain materials using an ultrasonic approach-an approach that continues to show great promise.

Fig 4 High-bandwidth, low-frequency, dual-element ultrasonic search units currently used for coarse-grain material evaluation as part of PNNL's low-frequency SAFT data-acquisition system; Fig 5 Cross-section of hardened shaft

Hardness depth measurement

Thermal processing is a major part of manufacturing processes in a wide range of industries including automotive, power generation and aerospace to improve part properties such as wear resistance and fracture toughness. Metal surfaces, such as those on gears, cams and axels, wear in service when they rub against other hard surfaces, and surface hardening improves strength and resistance to wear and extends part life. Often, only specific areas need to be hardened. Surface hardening, such as case hardening, produces a hard surface to a certain depth, while the core remains softer. This is illustrated in a cross section of a part, such as a hardened shaft, which shows the surface, the hardened region, the transition zone, and the unhardened core (Fig. 5).

One of the testing methods used to determine whether a part has been properly heat treated is the hardness test, which can be destructive in nature if a part has to be sectioned to measure hardness or if it cannot tolerate any surface imperfections; i.e., the indentation from the hardness test. Hardness testing also can be time consuming with respect to testing in the lab and providing feedback of the results.

Fig 6 Color-coded computer images of two parts hardened to different depths to illustrate the differences observed by ultrasound

In an alternative approach to performing a physical hardness test, PNNL is advancing ultrasonic hardness depth measurement (UHDM) technology. UHDM is a fast, accurate, cost-effective nondestructive method to evaluate the hardness depth in metal parts. UHDM uses ultrasound to interrogate the microstructure of steel components, then translates the information into a quantified measurement that is objectively obtained. Figure 6 shows color-coded computer images of two parts hardened to different depths, which illustrates the differences observed by ultrasound. The two parts are cam lobes from an automotive manufacturer who provided parts having a factor-of-two difference in the hardness depth for a feasibility evaluation.

While images can help to visualize the information, reliable and quantitative measurements at localized spots are desirable for feedback in a manufacturing environment. The measurement number is what industry needs, and that is what a method like UHDM provides. This method also is highly applicable in a production environment because of its ease of use and because parts do not require pretreatment prior to examination. UHDM can be incorporated into an existing manufacturing production line, allowing quick change-out to other parts, a faster means of evaluating parts, and a potential reduction in evaluation time.

Frequent part change-outs that are common to agile manufacturing require quick feedback to bring a production line back on line after the change-out. Ensuring that parts are being manufactured correctly and eliminating unnecessary steps in the process increases the production rate, providing cost savings and a competitive advantage. These benefits can be achieved through applied research on methods like UHDM. PNNL already has licensed the technology for use in some industrial applications.

Fig. 7 Advanced imaging using ultrasonic hardness depth measurement (UHDM) technology allow researchers to differentiate between the inherent and desired characteristics of a component and actual flaws that should be addressed. Images of fasteners show threads in the form of staggered colored lines that run down the sides of the fasteners.

Other UHDM applications

Beyond application in a production environment, the microstructural analysis behind UHDM has led to work on the development of a portable ultrasonic scanner that potentially would help to eliminate the need to remove a fastener to conduct an inspection (also enabling preservation of a seal, if present). The method would allow in-service inspection of fasteners (Fig. 7 and Fig. 8) and allow predictive maintenance by periodically monitoring fasteners.

Stress corrosion cracking in a fastener is seen in the greenish clusters of the computer image (left); the destructively tested image of the same fastener (right) confirms the location of the weakened areas. Note that the dark line at the bottom of the second image is the result of the destructive test so the line does not appear in the previous computer image of the intact fastener.

Another potential application is in-situ inspection of highway-bridge hanger pins. Highway-bridge hanger pins are of particular interest when it comes to nondestructive testing to ensure the safety of motorists because of the dramatic increase in traffic volume coupled with an aging highway infrastructure. High-resolution images detect wear grooves and fatigue cracks at the shear plane of a hanger pin and allow inspectors to take proactive measures to address the problem.