This study presents creep characteristics of Mod. 9Cr-1Mo steel with various sized specimens and environment. Tensile creep tests were performed using three different sizes of specimen and three different types of testing environment. Creep specimens are a bulk specimen which has φ 6mm and 30mm in gage length, a miniature specimen which has φ 2mm and 10mm in gage length and a thin plate specimen which has 0.76mm in thickness, 1.5mm in width and 7.62mm in gage length. Three different types of testing environment are air, 99.99% Ar gas and vacuum. In the same environmental condition, there was no effect of specimen size on time to rupture. Time to rupture of Mod. 9Cr-1Mo steel in Ar gas was shorter than that in air and vacuum. Oxide thickness is not dominant factor in time to rupture. Fracture mode at specimen surface in Ar gas might be dominant factor in shorter time to rupture. Effect of specimen size and environment on creep strength of Mod. 9Cr-1Mo steel was evaluated on the basis of thinning effect.
