Abstract
The crack growth rate in 9Cr-1Mo ferritic steel is measured at the upper expected service temperature 600 °C when the creep flow comes about. The samples of TDCB-type and mode of constant applied load are employed, the total test time ranging from ≈ 30 to ≈ 4500 h. In order to estimate a mechanical response to a prospective structural instability, two material variants differing in heat treatment are tested: normalized and tempered base metal and pre-aged material, which is the base metal subjected to long term static ageing before loading. Crack growth rates are presented in terms of load amplitude (parameter) C*, as it is usual for creeping cracked body. The load amplitude is obtained by means of rates of plastic displacements in line of applied load, provided that the stress exponents of creep power law for respective materials are known. From the point of view of crack growth resistance, a structure degradation as a consequence of some exposure to high temperatures is revealed. It is suggested to relate this effect to grain boundary precipitation. Using scanning electron microscopy of fracture surfaces, the fracture mechanism that controls the crack growth is estimated. This process seems to be initiated by grain boundary cavitation and completed by joining of grain boundary microcracks with a possible contribution of void coalescence.

Key words
creep crack growth, structure degradation, precipitation, fracture mechanisms, fracture surfaces

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