Better welds can extend the lifetime of older components by decades and can save the industry billions of dollars. "A good weld extends plant life, enhances safety and reliability, and cuts down on operation and maintenance costs," said Vis Viswanathan, technical fellow and senior manager for materials application technology, Electric Power Research Institute (EPRI), Palo Alto, Calif. These benefits are especially important in nuclear plants, where a day of forced outage costs $300,000 to $750,000.

Through 2002, EPRI has spent $45 million to develop better welding technologies, according to RRAC manager Shane Findlan. The return on that investment would make any Wall Street analyst salivate: the industry has documented more than $2 billion in savings from the use of improved welding techniques.

Besides providing resources such as the RRAC, EPRI helps utilities help themselves through such methods as the development of repair guidelines.

Establishing Weld Repair Guidelines
Before 1996, few guidelines existed to identify the best weld repair technologies or to determine how long repairs were likely to extend a particular component's life. Original equipment manufacturers have little incentive to develop new repair technologies, and those that have done so guard their knowledge closely to maintain a competitive edge over other manufacturers and repair vendors and to recoup their technology development investment. Power companies, in contrast, are strongly motivated to repair rather than replace damaged equipment, both to minimize the length of forced outages and to extend component life. EPRI's efforts have resulted in a number of repair guidelines including a seven-volume report that presents guidelines for the weld repair of high-temperature and high-pressure parts

One of the first steps in helping utilities through the repair process was to survey the industry about its major concerns. The questions utilities wanted answers for included the following: What are my options if I find damage in a part? What caused the damage, and how do I avoid a recurrence? What's the best weld repair technique to use? How do I test the repair, inspect it, and service it in the future? What are the limitations of the repair? How do I select a repair vendor? Is the repaired part covered under warranty? What considerations must I address when planning an outage?

Today, "we're still going back to those questions to make sure we're staying on the right track with our answers," said David Gandy, the manager for Fossil Materials and Repair. The intent of the EPRI guidelines is not to provide utilities with the knowledge or tools for performing their own in-house repairs. Rather, it is to give them the precise information they need to make better decisions about their own equipment.

The RRAC gives utilities access to a centralized pool of experts, and the new guidelines can help utilities address problems even though the number of in-house experts shrinks. In addition, EPRI has developed weld technologies that do not require highly skilled welders. The guidelines describe state-of-the-art weld techniques for extending the life of components, explained Viswanathan. But by how much? Will the repaired part be as good as new, lasting another 40 years, or will it last only 5? Knowing the answer is important when deciding whether to replace the part and how often to inspect it. One thing the engineers have done is to examine damaged components to deduce what caused their failure. By studying weld failure experiences, they can characterize how certain types of repairs are likely to perform in the long run.

Typically, when the industry has used outdated repair methods instead of current practices, welds have failed in short order. Often, cracks have been found in the same places just a year later. Sometimes the repaired part has been operating outside its original design conditions. Sometimes welders have performed repairs without removing prior damage properly, and at other times, the root causes of failure, such as external stresses, have not been eliminated. Viswanathan described this formula for success: "If you eliminate the root cause of the problem and the extraneous out-of-design stresses and if you completely excavate the original damage and perform the right repairs, then you can get long life."

But nothing lasts forever. "After all, you're not repairing new components," Viswanathan said. "You may be repairing components that have been out there for 40 years and have generally aged. They've got creep damage. Microstructure changes have occurred. Eventually, you have to address the issue of repairability; you have to realize there's a point at which components are no longer repairable." Determining when that point has been reached is a big part of run-repair-replace decision making.

Following are some of the weld technologies that are being researched.

Temperbead Repair
Consider the innovation that has made the biggest impact on the industry: temperbead repair - Fig. 1. Traditionally, the codes that govern repairs - the National Board Inspection Code and the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code - have mandated postweld heat treatment (PWHT). This treatment softens, or tempers, the hardened material after a weld is performed and so relieves residual stresses. It also allows the diffusion of hydrogen, which is introduced into the metal during welding and can cause cracking. But PWHT is time-consuming and expensive, especially when the components involved are large or when many treatments are necessary. In nuclear plants, it can take up to 12 hours for a component to reach the desired temperature, 1 to 3 hours to perform the treatment, and another 8 to 12 hours for cooling. Sometimes PWHT may not even be possible because of the size or configuration of the flawed part. EPRI has spent a great deal of effort investigating alternatives to PWHT.

During 1995 and 1996, EPRI and many utilities cosponsored work to develop temperbead welding guidelines. They found the temperbead technique could produce welds that were tougher than conventional welds and could extend component life by at least 20 years. The temperbead repair guidelines, as well as a review of industry experience, the results of experimental studies on piping and casing steels, and a worldwide literature review, are included in the seven- volume report mentioned earlier.

Temperbead repairs save money. They saved Baltimore Gas and Electric $9 million and Yankee Atomic Electric $18 million. For the years 1996­2002, the Tennessee Valley Authority saved an estimated $18 million by performing casing repairs without PWHT, according to John Brooks, TVA project manager in the Technology Advancements Division. The Tennesee Valley Authority's investment in this effort, including R&D, training, and implementation, is estimated to be $1.2 million - a benefit-to-cost ratio of 15, which TVA considered impressive. "Without the teamwork and close coordination between TVA and the EPRI researchers, this could not have been achieved," Brooks said. Since the problem of cracked casings is industrywide, the total savings resulting from this temperbead repair application alone are expected to reach hundreds of millions of dollars.

Laser Beam Welding
The field of laser beam welding has also been advanced. In an industry survey, 9 out of 16 vendors would prefer to use laser welding over conventional welding for repairing combustion turbine blades. The reason is precision - Fig. 2. "With normal welding, heating and cooling occur over a large area," explained Viswanathan. "Laser welding is more like a surgical repair, affecting a localized, narrow weld width." Lasers also permit greater flexibility in alloy compositions, allowing welders to concoct complex mixtures of metal powders appropriate for specific applications. And, since workers have the option of defocusing the beam, lasers provide a means of in-situ heat treatment: after a highly focused beam melts the metal and the weld repair is performed, a defocused beam, whose energy is spread out over a larger area, can be used to heat-treat the surface.

Laser welding is a noncontact, line-of-sight process. Compared with conventional methods, automated laser welding is faster and requires less finishing and machining. In preliminary trials, laser welding has been successfully applied in the repair of Inco 738 Alloy combustion turbine blading to produce high-strength welds.

Automated Orbital FCAW System
In 1995, the power industry began looking for a way to improve the deposition rates of materials used to weld heavy-walled steam pipes found in both fossil and nuclear plants. Since conventional flux cored arc welding (FCAW) was ill-suited to weld pipes orbitally, EPRI and Magnatech partnered to develop a solution - an automated FCAW system that combines an orbital tracking mechanism for welding in odd positions and a power supply with real-time, fuzzy logic voltage and current control for improved arc stability. The system produces superior welds quickly, and deposition rates are three times higher than those achieved with conventional processes, Gandy said.

Repair of Rotating Components
Components that rotate - turbine rotors, disks, and blades - present a special challenge for weld repair since they are among the most critical and highly stressed components in nuclear and fossil power plants - Fig. 3. Moreover, they are expensive. Combustion turbine blades, for instance, can cost as much as $30,000 each, or $3 million per row.

Recently, EPRI reviewed current repair technologies for rotating components and developed comprehensive repair guidelines for turbine rotors, disks, and blades. The guidelines detail repair decision methodology, repair techniques, damage mechanisms, specifications, life assessment, and insurance considerations. The goal is to help utilities make well- informed, cost-effective repair decisions. The performance of previous repairs is also being documented. Case histories from utilities and repair vendors are being examined to see which repair techniques worked and which did not. Finally, EPRI is collecting information from utilities, industry experts, original equipment manufacturers, and repair vendors to develop a detailed methodology for making run-repair-replace decisions about rotating components.

Technology Transfer
Innovations in welding repair offer the power industry practical ways to improve safety and profits. At least a dozen EPRI-developed repair technologies, applying to areas as diverse as underwater welding, repair of rotating components, and novel alloys, have been patented and licensed. Research has also been helpful to regulatory bodies in updating codes for the industry; at least nine new repair codes address topics ranging from laser welding of steam generator tubing to weld overlays in service-water piping. These technical advances will mean little, however, unless they are transferred, and effectively sharing knowledge is an ongoing challenge.

To facilitate the transfer of EPRI- developed weld repair technologies, the 20 to 25 utilities participating in the RRAC meet twice a year to discuss their progress in applying these new methods. The group also promotes the acceptance of innovative welding technologies to facilitate the updating of codes. Members attend an international conference on weld repair advances every two years, and they receive in-house training at the RRAC.

EPRI also conducts major international collaborations with other welding research organizations in order to identify optimal repair techniques and procedures. Key topics include the development of technologies for important power plant components, such as stationary components in fossil plants and rotating components in both fossil and nuclear plants, and technologies that eliminate the need for PWHT. Various utilities from around the world, including Electricité de France, ENEL of Italy, Ontario Power Generation, and Taiwan Electric Power, join with U.S. utilities in cofunding these R&D efforts.

Why is international collaboration so important? Industry standards and guidelines resulting from such global teamwork are more likely to gain widespread acceptance and application.