Generally, fatigue strength of steel correlates with hardness up to a certain point. However, the relationship of hardness and fatigue strength is more complicated for high strength materials, mainly because strength is sensitive to material defects. This paper presents a determination of defect size in carburized gear teeth and a strength evaluation method based on that size. A micro notch is artificially processed at the fillet of carburized gear tooth by means of a Focused Ion Beam, and a bending fatigue test is performed. Threshold notch depth the limit beyond which fatigue strength declines, is obtained by the test. Threshold notch depth is used as a measure to determine latent defect size in the surface layer. The of gears from which a nonmartensitic layer is removed by electro-polishing is shorter than the of gears with a nonmartensitic layer. The surface layer is observed by scanning electron microscope and electron probe micro analyzer to clarify latent defect characteristics. Many micro cracks, in addition to long cracks in the nonmartensitic layer, Cr oxides and Mn oxides near the surface, and Si segregated in grain boundary are observed. This demonstrates that they trigger fatigue crack initiation; and therefore defect size limits strength. Consequently, strength is expressed by an experimental formula of Vickers hardness and the square root of projected area of the defect obtained above.
Evaluation of Bending Strength of Carburized Gears Based on a Quantification of Defect Size in the Surface Layer
Contributed by the Power Transmission and Gearing Committee for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received December 2000. Associate Editor: R. F. Handschuh.
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Masuyama, T., Kato, M., Inoue, K., and Yamashita, T. (August 6, 2002). "Evaluation of Bending Strength of Carburized Gears Based on a Quantification of Defect Size in the Surface Layer ." ASME. J. Mech. Des. September 2002; 124(3): 533–538. https://doi.org/10.1115/1.1485091
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