Pressurized creep tubes were used to evaluate the thermal creep behavior and the irradiation creep behavior of several variants of HT9 ferritic-martensitic steel in the temperature range from ∼490°C to 605°C. Examination of the thermal creep data as a function of time showed classic thermal behavior at all the test temperatures. A creep transient was followed by a period of steady state creep which, in some instances, was then followed by tertiary creep. The irradiation creep behavior also displayed a transient followed by steady state creep, but no tertiary creep was observed.

while the thermal creep behavior and the irradiation creep behavior were qualitatively similar, the magnitude of both the transient and the steady state creep rate were often different. At ∼490°C, the thermal creep specimens exhibited a greater transient strain, while at 600°C, the irradiation creep specimens exhibited a greater transient strain. For both the thermal creep specimens and the irradiation creep specimens, the transient strain was observed to increase with increasing applied stress. The steady state creep rates of both the thermal creep specimens and the irradiation creep specimens were observed to increase with increasing temperature. At lower stresses, the steady state creep rates of the thermal creep specimens and the irradiation creep specimens were often equal. At the higher test stresses, the steady state creep rate of the irradiation creep specimens was always higher.

At ∼490°C, the steady state thermal creep rate was observed to have a linear stress dependence up to the highest applied stress of 250 MPa. At 575°C, the stress exponent was between 1.5 and 2 for effective stresses less than 200 MPa. At 605°C, the stress exponent was ∼1 for effective stresses less than 70 MPa. For the irradiation creep specimens, a stress exponent between 1.5 and 2 was observed at ∼490°C and 550°C. At 600°C, the stress exponent was observed to be ∼1.