Effect of manufacturing process parameters on long-term microstructural evolution and accumulation of creep damage in Grade 91 material

The grade 91 creep strength-enhanced ferritic (CSEF) steel is nowadays used extensively in the power generation industry for the manufacturing of various high temperature, high pressure tubing and piping. Its elevated temperature strength is ensured by a microstructure consisting of tempered martensite and a homogeneous dispersion of secondary phases, namely carbides and carbo-nitrides. This microstructure is the result of the manufacturing process, including steelmaking, hot forming, and final heat treatment; therefore, reliable manufacturing process control is required to ensure the desired in-service properties and thus reliable operation.

The effect of manufacturing parameters on the microstructure and short-term creep-rupture properties of T91 tubes was previously investigated by Gligor et al. (EPRI 2019). More recently, an update of the creep testing program was presented, including test results in excess of 30,000 h, and correlating rupture strength and steady-state creep rate to the manufacturing conditions and the original product’s microstructure (Locatelli et al., ECCC 2023).

Following completion of most long-duration tests, this work presents the metallurgical characterization of ruptured test specimens. The manufacturing parameters are correlated not only to sheer creep strength, but also to ductility and evolution of the microstructure after exposure.