In recent years, damage has been found early in the life of creep strength enhanced ferritic (CSEF) steels, including Grade 91, used in the boiler tubes, piping and headers of fossil-fuel-fired and combined-cycle power plants. This discovery raised concerns among users due to the potential for tube failures, and the implications for plant personnel safety and equipment reliability. Additionally, utilities currently have few tools at their disposal to assess CSEF components in-service and to accurately predict the life of these steels.
The Electric Power Research Institute initiated a broad-based collaborative project to address the critical issues associated with the use of CSEF steel. The project, which brings together experts from around the world in a 4-year collaborative effort, provides information and tools to assist utilities with material procurement, shop fabrication, field erection, evaluation of the in-service behavior of the base metal and weldments, life prediction and maintenance optimization. More than 35 companies have joined the project to date, including
Testing of high-temperature strain gauges at Tampa Electric In-service, high-temperature strain gauges directly monitor strain accumulation at critical sites on a component during plant operation. By monitoring the rate of strain accumulation in material in an area of high stress on a component, the gauges allow plant personnel to more accurately assess the material’s time-to-rupture. This assessment would enable plants to ensure both timely replacement of deficient material, where the operating conditions are particularly rigorous, and to avoid unnecessary replacement of deficient material, where the operating conditions are not severe. In the TECO project, the cheeks (sides) of two elbows, made of Grade 91 steel, in the hot reheat piping system were identified as having relatively low hardness values. The two elbows were instrumented with high-temperature capacitive strain gauges and thermocouples, and the sensors were monitored for a period of three months. A straight section of pipe also was instrumented as a baseline. Additionally, plant operating conditions data — such as temperature, pressure and generator output — were collected from the existing plant instrumentation. The project found that creep strain rates could be successfully measured, resulting in proof-of-concept. However, under the operating conditions for the monitored locations at TECO Bayside, there was not a discernable difference in creep strain rates between relatively hard and soft Grade 91 material. |
CSEF steel
Grade 91 steel, one of several CSEF steels, was initially developed in the 1970s for use in nuclear reactors. The alloy has a number of enhanced mechanical properties, including high creep-rupture strength and fracture toughness. This higher strength makes Grade 91 attractive for fossil plant applications because of the potential for higher operating temperatures — up to 1,100°F (593°C) — and lower wall thicknesses. This means Grade 91 is suitable for use as a retrofit material in conventional subcritical power plants, and as a building material for advanced supercritical plants.
Grade 91 steel was first used in fossil power plants in the early 1980s, and wide-scale use began in the early 1990s. In retrofit applications, Grade 91 steel can be used to replace conventional ferritic steels in sections of the boiler that have a high rate of creep failures. Sections include superheater and reheater tubes, headers and piping systems. In new plant applications, Grade 91 steel has been used extensively in combined-cycle plants in main steam and reheat piping, as well as in boiler and steam generator components.
Other CSEF steels developed subsequent to Grade 91 include Grades 92, 122, 23 and 24.
Deficiencies and irregularities of CSEF steels
Recent in-service experience showed that a small but significant amount of Grade 91 steel in U.S. power plants has substantially degraded properties due to irregularities during fabrication and/or installation. The primary areas of concern are early cracking and “soft,” or degraded microstructure, which can lead to component failures (see Figure 1). Inspections at a number of new and operating plants, both standard fossil-fuel-fired and heat recovery steam generators (HRSGs), uncovered evidence in multiple components of “soft” material with an undesirable condition of microstructure.
Degraded properties include rupture properties approaching those of lower-temperature materials, such as Grade 9 or Grade 22 steel, low hardness, reduction in tensile/yield strength and mixed ferritic/martensitic microstructures. The deficient material has involved both base metal and weldments.
Adding to these challenges, utilities lack concise specifications for procuring or processing CSEF steel, field inspection techniques to identify degraded materials and clear methods for predicting component remaining life. In most cases, too, there is no adequate documentation to determine how the deficient condition developed, so utilities cannot accurately gauge the extent of the deficiencies without resorting to costly inspection.
Specification, ordering and processing
Because the issues with CSEF steel begin even before the material arrives at the plant, the first focus of the EPRI project involves developing information to assist utilities with the “front-end” issues: specifying and ordering the optimum CSEF steel for a specific plant, and monitoring how the steel is processed and which quality controls are employed during processing.
