The effect of three surface finishes of roughnesses 4 to 5, 20 to 25, and 70 to 80 microinches rms on the fatigue properties were deter-mined for low-carbon N-155 alloy of grain size A.S.T.M. 1 at temperatures of 80°, 1000°, 1350 , and 1500° F. The fatigue properties for the vari-ous finishes differed appreciably at room temperature;however, after short periods at 1000° F and for all periods investigated at temperatures above 1000° F, specimens having different finishes had the same fatigue strength. It was concluded that the primary cause of the difference in room-temperature strength was due to compressive stresses set up in the surface and that at elevated temperatures these compressive stresses were relieved by annealing. Apparently, roughness alone did not signifi¬cantly affect the fatigue strength of the material investigated.
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