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A fatigue study was conducted to determine the performance of unnotched and notched carburized American Iron and Steel Institute (AISI) 8620H steel specimens and to evaluate the corresponding fatigue damage. Completely reversed, constant amplitude, load-controlled tests were conducted using cylindrical carburized steel specimens with stress concentrations of 1.00, 1.46, and 2.22. All tests were conducted at a frequency of 2 Hz under an ambient laboratory environment. Fatigue data plotted as stress amplitude versus mean fatigue life show that, at long life, both sets of notched specimens have similar performance. At shorter lives, the specimens with stress concentration of 1.46 exhibit superior performance compared with the specimens having a stress concentration of 2.22, as expected. The performance of smooth specimens is significantly better compared with the notched specimens at all lives.
Fractographic investigation conducted using scanning electron microscopy revealed the nature of initiation and the subsequent propagation of fatigue cracks in these specimens. At shorter lives, the crack initiation in smooth specimens was found to be surface-initiated at one or more sites on the surface, which then circumferentially converged and propagated through the carburized case into the core. The fracture surface of the case was smooth and flat, while in the core it was nonuniform and coarse. At higher lives, crack initiation in the smooth specimen originated at inclusions within the core. The subsurface crack initiation site showed convergence of radial lines toward a focal point, the site of a nonmetallic inclusion. Fractographs of notched specimens which failed at shorter lives show damage similar to that in smooth specimens. At longer lives, clear evidence of separation between the case and the core was found, with some evidence of subsurface initiation from nonmetallic silicon oxide inclusions having dimensions of 20 to 50 μm.
fatigue life, carburized steel, fatigue damage, fractography, nonmetallic inclusion, subsurface crack initiation
Principal research scientist, Battelle Memorial Institute, Columbus, OH