Recent Status of ASME Code on Creep Strength Enhanced Ferritic Steels

Since the introduction to modified 9Cr-1Mo steel (Gr.91) in 1982, new grades of creep strength enhanced ferritic (CSEF) steels including the Gr.91 have been developed and practically used in the power plants worldwide in 1990s to date increasing the steam temperature to improve thermal efficiency for energy conservation and reduction of CO2 emission. The chemistry and heat treatment conditions for these CSEF steels are highly optimized and the elevated temperature properties such as creep strength are strongly influenced by the heat and strain cycles during the material manufacturing and fabrication. These steels have already been included into the international codes and standards, however a number of failure experiences with these steels throughout the manufacturing, fabrication and services have been reported. The American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel Code Committee has discussed on the material issues and taken the actions to improve or revise the specification of these steels based on the latest technical information and discussion results. This paper presents the recent status of ASME Code on CSEF steels.