Researches are underway worldwide to improve the performance of high chromium ferritic steels for high temperature applications up to 650℃. To investigate the effects of austenitizing temperature on the mechanical and creep strength of the 12% Cr-Mo-V-Nb steel, the austenitizing temperatures of 1000℃(2hr), 1050℃(2hr),1100℃(1hr) and 1150℃(1hr) were used. The creep test performed at 650℃ under constant load ranging from 100MPa to 150MPa.
The yield strength and ultimate tensile strength increased but the elongation and reduction of area decreased with increasing austenitizing temperature. Especially the Charpy absorbed energy drastically decreased and fracture mode changed from dimple to quasi-cleavage. These results can be explained by comparing the finess extent of martensite structure.
The creep rupture time increased with austenitizing temperature. The increase in austenitizing temperature resulted in not only an increase in grain size but also a decrease in the amount of undissolved carbides such as $M_{23}C_6$ and NbC. The dissolution of more carbide forming elements during the austenitizing treatments at higher temperature make more solid solution hardening and fine carbide precipitations which are benificial for creep rupture strength. However after long time tempering, from the results of tempering at 750℃, carbide coarsened more rapidly in the specimen austenitized at 1150℃ than that of austenitized at 1000℃. From the above results, the optimum austenitizing temperature is between 1050℃ and 1100℃.
