Creep Strength Enhanced Ferritic Steels Why They're Great and What Can Go Wrong

The steam operating conditions in both petrochemical plants and power generation facilities have increased though the years. Recent ethylene plant projects have been designed and built with steam pressures as high as 1700 psig/11.7 MPa and with a superheat temperature of 960°F/515°C. The power industry has long since moved past ethylene plant steam pressure and temperature levels to supercritical steam systems that operate at pressures in excess of 4400 psig/30.3 MPa and temperatures in excess of 1100°F/600°C. For these systems, creep strength enhanced ferritic steels (CSEFs) and austenitic steels are used at the higher-temperature locations. However, CSEF steels such as Grade 91 have also been increasingly selected as materials for steam piping in new construction projects for petrochemical plants, despite the lower temperatures and pressures where improved creep resistance is not necessarily required.

These steels are based on the conventional chrome-moly low alloy steels (e.g., 2¼Cr-1Mo and 9Cr-1Mo) but utilize small additions of carbo-nitride formers such as vanadium and niobium to provide enhanced creep strength. Due to the their normalized-and-tempered heat treatment condition, CSEF steels have considerably higher strength than conventional low alloy steels at all temperatures. This allows less material to be used (e.g., thinner walls, less weld metal, fewer supports, etc.), often making CSEF steels more cost-effective than low alloy steels even outside the creep range.

The power generation industry has routinely been using CSEFs since the 1980s, and a number of issues have been encountered in the transfer of these laboratory-developed steels to the real world of steam piping and headers. Unlike carbon and low alloy steels, the improved properties of Grade 91 and related CSEF steels rely on achieving and maintaining a specific martensitic microstructure. The martensitic microstructure is attained through very specific and controlled temperatures and cooling rates during the steel processing. Virtually any incident during manufacture, construction, or operation that disrupts this microstructure can compromise the material’s integrity and prevent it from achieving its designated properties.

Drawing on experience from the power generation industry, including work done with and by the Electric Power Research Institute (EPRI), this paper will present some of the history and background related to these steels and examples of current applications and issues.