Heat-to-heat variation of long-term creep strength property observed f
or 12Cr steel was investigated from viewpoints of creep deformation be
haviour and stability of microstructure. Heat-to-heat variations of cr
eep deformation behaviour and creep rupture strength properties were d
escribed by the differences in the parameter Lu in alpha modified thet
a projection method which indicates the acceleration in creep rate at
the tertiary stage. It was shown that the magnitude of alpha and its c
hange with creep depend on the morphology and stability of initial mic
rostructure, and correlate with microstructural changes accompanied by
recovery and recrystallization under tertiary creep. It is concluded
therefore that the creep strength and rupture life can be controlled b
y the microstructure and its stability of 12Cr steels and expressed on
ly by the rate constant alpha.