Effect of Cobalt and Boron on Long-term Creep Rupture Strength of 12Cr Cast Steels

Microstructure of creep ruptured specimens is investigated in order to understand the effect of Co and B on long term creep rupture strength of 12Cr cast steels. The specimen with 2.5% Co contains no delta-ferrite and has better tensile and impact properties than Co free specimens including about 1 vol% delta-ferrite. In short-term creep rupture region, Co containing steel has shown better strength than Co free steel, while in long term region such as over 7000 h at 923K, Co free steel has shown better strength. The addition of Co promoted recovery of microstructure. The difference between 20 ppm B and 50 ppm B steels in short-term creep rupture property was not observed, while in long-term region, 50 ppm B steel has a tendency to show better strength. From ATE method, it is considered that some of B have been included in carbide such as M23(CB)6, and retarded coarsening of carbide. According to calculation of thermodynamic equilibrium, solid solution content of W and Mo was reduced by Co addition. Therefore the thermodynamic stabilization of microstructure associated with the solid solution strengthening of W and Mo has decreased and the formation of Laves phase has been enhanced. It has been concluded that improvement of stabilization and enhancement of formation of Laves phase by 2.5% Co addition is one of the reasons of the deterioration of long-term creep rupture strength.