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 delta 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 delta ferrite in the steel plates. The creep strength of the steel specimens was lower when the delta ferrite was present in the initial microstructure. The delta 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 delta ferrite disk. The residual delta ferrite dissolved completely after an additional normalizing heat treatment at 1050 degrees C for 30 min. The results obtained strongly indicate that the presence of delta ferrite is due to insufficient normalizing heat treatment time to remove segregation during solidification. The delta ferrite was also found to grow through a diffusional transformation during cooling from the normalizing heat treatment. (C) 2013 Elsevier B.V. All rights reserved.