The thermal activation energy for fatigue of Fe- 1 pct Cr- 0.5 pct Mo
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I.
INTRODUCTION
I T is well known that, as the temperature rises, rates of fatigue-crack propagation for a given waveform increase, m In this context, it has been frequently supposed that this increase in crack growth rate arises from the effect of temperature on some thermally activated rate-determining mechanism, t21 If this is the case, then examination of this activation energy might provide additional understanding of the fatigue process at elevated temperature. In addition, a knowledge of the magnitude of this activation energy, if it actually exists, would enable useful interpolations and extrapolations to be made from existing experimental data. Since the components of many industrial plants operating at elevated temperature are subject to variable temperatures, which may differ from those considered by the designer of the equipment, a reliable method for predicting fatiguecrack growth rates as a function of temperature could obviously be very useful. Surprisingly, there is very little information in the literature on the magnitudes of such activation energies for fatigue. Accordingly, the present work involved an examination of the data for a particular instance of elevated-temperature fatigue to show whether they did, in fact, support the existence of an activation energy. Secondly, this activation energy was measured. Thirdly, the mechanisms controlling fatigue-crack advance for the particular test conditions were considered and, finally, an attempt was made to explain the activation energies observed, in terms of the processes operating at the crack tip.
II.
EXPERIMENTAL PROCEDURE
The material tested was machined from a ferritic Fe1 pct Cr-0.5 pct Mo steam pipe that had previously experienced 125,000 hours of service in a thermal power station
M.B. CORTIE is Group Leader, Deformation and Fracture, Council for Mineral Technology, Republic of South Africa. G.G. GARRETT is Director, Materials Science and Technology, Council for Scientific and Industrial Research, Republic of South Africa. Manuscript submitted November 30, 1987.
METALLURGICALTRANSACTIONS A
at a nominal temperature of 520 ~ and steam pressure of 8.6 MPa. The microstructure of the material consisted of equiaxed ferrite and degenerate, spheroidized pearlite, with an average grain size of approximately 43/xm. All of the elevated-temperature crack growth rates reported in this paper were obtained by applying fatigue waveforms with a frequency of 1 Hz and an R-ratio of 0.1. The 1 Hz frequency was chosen because it is fast enough to permit a specimen to be tested in three to four days, but was considered slow enough to allow time-dependent effects such as creep and oxidation to take place. AK was used as the parameter against which fatigue crack growth was correlated. Although other, more complex, fracture mechanics parameters such as AJ and C* have also been used in the correlation of fatigue crack growth at elevated temperature, for example by Okazaki and Koizumi t31and Riedel and Wagner, N it has been widely observed that, for ferrous m
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