1995-02-01

Comparison of Single Gear Tooth and Cantilever Beam Bending Fatigue Testing of Carburized Steel 950212

The bending fatigue performance of gears, cantilever beam specimens, and notched-axial specimens were evaluated and compared. Specimens were machined from a modified SAE-4118 steel, gas-carburized, direct-quenched and tempered. Bending fatigue specimens were characterized by light metallography to determine microstructure and prior austenite grain size, x-ray analysis for residual stress and retained austenite measurements, and scanning electron microscopy to evaluate fatigue crack initiation, propagation and overload. The case and core microstructures, prior austenite grain sizes and case hardness profiles from the various types of specimens were similar. Endurance limits were determined to be about 950 MPa for both the cantilever beam and notched-axial fatigue specimens, and 1310 MPa for the single gear tooth specimens. The cantilever beam and single gear tooth specimens had surface compressive residual stress levels of 300 MPa and 720 MPa, respectively, a result which correlates with the higher endurance limit of the single gear tooth specimens. A detailed description of the five stages and zones of bending fatigue fractures in carburized steels is also presented.

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