Abstract
The construction of highly efficient Ultra Supercritical (USC) boiler systems to operate with steam temperatures up to 760°C (1400°F) and with steam pressures up to 34.5 MPa (5000 psi) will require the use of advanced high temperature, high strength materials. As part of a 5-year project to qualify advanced boiler materials for USC power plants, a number of austenitic materials have been selected for further development and use in USC boiler systems, including alloys 230, 740, CCA 617, HR6W, and Super 304H. In one task of this project, boiler fabrication guidelines appropriate for the use of these alloys were investigated. Because it is recognized that cold formed and mechanically strained austenitic materials can degrade in material creep strength, a study to investigate the limits of strain and temperature exposure for the USC alloys was undertaken. An objective of this work was to determine for each USC alloy a relationship between the level of cold strain and the conditions of time and temperature that will cause recrystallization and significant microstructural change. The ultimate goal of this work was to determine limits of strain, due to cold forming, that can be tolerated before heat treatment is required, similar to those limits provided for the austenitic materials (e.g., 300-series stainless steels, alloy 800H) in Table PG-19 in Section I of the ASME Boiler and Pressure Vessel Code. This paper will describe the technical approach for 1) preparing specimens having discrete cold strains ranging from about 1 to 40 percent, 2) exposing these strained specimens for selected times at various elevated temperatures, 3) identifying the onset of recrystallization in the microstructures of the exposed specimens, and 4) establishing a useful engineering method to predict conditions for the onset of recrystallization in the USC alloys using the experimental results.
