Improvement in Creep Rupture Ductility of Creep Strength Enhanced Ferritic Steel by New Heat Treatment Process with Intermediate Tempering

Creep rupture ductility of creep strength enhanced ferritic (CSEF) steels has been investigated. Creep rupture ductility of CSEF steels decreases in the long-term corresponding to decrease in stress below 50% of 0.2% offset yield stress at the temperatures which is referred as a half yield. Boundary condition of half yield is regarded to be an elastic limit of the steels. Creep deformation mechanism of the steels is considered to be different in high- and low-stress regimes divided by a half yield. Accuracy of creep rupture life prediction is improved by region splitting analysis in which creep rupture life is analyzed independently for high- and low-stress regimes, in consideration of half yield. The higher strength steel tends to indicate more significant decrease in creep rupture ductility in the long-term. High ductility in the high-stress regime above half yield should be provided by easy plastic deformation, and it has been speculated that a remarkable drop in ductility in the low-stress regime is derived from a concentration of creep deformation into a tiny recovered region formed at the vicinity of grain boundary. Preferential recovery at the vicinity of grain boundary is caused mainly by lowered creep strength due to plastic deformation during martensitic phase transformation. New heat treatment process with intermediate tempering on partially transformed dual phase microstructure obtained by cooling from normalizing temperature to the temperature between Ms and Mf, retains the potential to improve creep rupture ductility without loss of creep rupture life. Improvement in creep rupture ductility of Grade P92 steel has been obtained by a modified heat treatment process with an intermediate tempering which reduces residual stress introduced by martensitic phase transformation.