Properties and Workability of New Creep Strength Enhanced Steels as Known Grades 23, 24, 911 and 92

Abstract

The improvement of thermal efficiency by increasing the operating temperature and pressure of boilers has recently led to the development of new creep-resistant steels. For 9–12% Cr steels, grade T/P91, developed at the end of the seventies by CE (Combustion Engineering) and ORNL (Oak Ridge National Laboratories) [1] in the USA, marked the starting point for these developments. With its excellent mechanical characteristics at elevated temperatures and good workability, it was rapidly adopted worldwide for erecting new power generators. The tube design temperature is limited to around 610°C (1130°F) inside the combustion chamber. This limit depends on such factors as heat flow and corrosion. Steam lines in P91 are generally operated at temperatures below or slightly above 600°C (1112°F). More recent developments to produce new grades such as T/P911 and T/P92, have improved mechanical properties at high temperatures, in particular an increase in creep strength of 10 to 30% in 100,000 hrs at 600°C (1112°F). This makes it possible to reduce the wall thickness of the pipes and consequently improve their behavior to thermal fatigue. The new grades T/P23 and T/P24 are well suited for boiler components working at lower temperatures. While these grades were initially developed for manufacturing the water wall panels of Ultra Super Critical Boilers (USCB), they are also used for the superheaters and reheaters of conventional boilers and Heat Recovery Steam Generators (HRSG) in combined cycles. In addition to their excellent workability, they have the advantage to be used without post-weld heat treatment in case of thin wall tubes. Furthermore, owing to the good creep properties, they can be used to replace P22 and for some applications even P91, with the advantage of lower costs. This paper sets out the characteristics of these new grades and gives the main recommendations for processing them.