The formation and dissolution of residual δ ferrite in ASME Grade 91 steel plates

This paper investigated the relationship between creep strength and the existence of residual δ ferrite in the initial microstructure in three heats of Grade 91 steel plates and examined the origin, the transformation process and the dissolution of the δ ferrite in the steel plates. The creep strength of the steel specimens was lower when the δ ferrite was present in the initial microstructure. The δ ferrite showed a disk shape and a bumpy interface, and had MX carbonitride particle arrays inside and coarse M23C6 carbide particles on the bumpy interface. Chromium and molybdenum were found to segregate in/around the δ ferrite disk. The residual δ ferrite dissolved completely after an additional normalizing heat treatment at 1050 °C for 30 min. The results obtained strongly indicate that the presence of δ ferrite is due to insufficient normalizing heat treatment time to remove segregation during solidification. The δ ferrite was also found to grow through a diffusional transformation during cooling from the normalizing heat treatment.