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

Abstract

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 longterm 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 M 23 (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.

Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

“…Although Co addition has been reported as effective for creep strengthening in ferritic heat-resistant steels, 14,[22][23][24][25][26][27][28] its effectiveness depends on the composition of the steel or the temperature and time of the creep test. [29][30][31][32][33][34] With respect to chemical composition, Co addition was effective for creep strengthening in heat-resistant steels with Cr contents of 9 or 15 mass%, 14,[22][23][24][25][26][27][28] but in these cases, the samples had a single-phase martensite or ferrite structure. In contrast, several other studies concluded that the addition of Co was not effective for creep strengthening, but these investigations were conducted on heat-resistant steels with 10.5-12 mass% Cr contents that contained residual δ ferrite in part of the martensitic microstructure.…”