Abstract
The influence of holding time during tempering on the long-term creep rupture strength of mod.9Cr-1Mo steel was investigated in this study, so as to elucidate proper heat treatment for boiler applications. Tempering was conducted at 770°C for 0.5h, 1h, 3h, 10h and 100h for the test materials, after re-normalization at 1050°C for 1h in all cases. Creep rupture tests were conducted at 600°C, and ruptured specimens were investigated to better understand the microstructural changes, including changes in the number density of precipitates, in order to observe and discuss their creep strength. All creep rupture test results for materials tempered within 10h exceeded the average creep strength of T91. Shorter tempering times such as 0.5h and 1h were clearly correlated with longer time to rupture at 600°C under 80MPa to 100MPa stress conditions. Reduction of area in creep-ruptured specimens decreased principally with lowered creep stress. Materials tempered for 0.5h and 100h showed the lowest reduction of area at 90MPa and 100MPa respectively, and their reduction of area recovered at lower than those stress levels. These stresses, showing minimum reduction of area, met inflection stress in the creep rupture strength curve.
