Journal of Ocean Engineering and Technology (한국해양공학회지)

Korean Society of Ocean Engineers (한국해양공학회)
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Variations of VHCF Characteristics by Microstructure of a Spring Steel to UNSM Treatment

  • Seung-Hoon Nahm (Korea Research Institute of Standards and Science) ;
  • Min-Soo Suh (Kyoto University) ;
  • Chang-Min Suh (School of Mechanical Engineering, Kyungpook National University)
  • Received : 2024.06.27
  • Accepted : 2024.08.19
  • Published : 2024.10.31
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Abstract

The bainitic structure resulting from the austempering of spring steel exhibits high strength and ductility. On the other hand, there appears to be no study on the effects of very high cycle fatigue (VHCF) and ultrasonic nanocrystal surface modification (UNSM) of this bainite structure. Therefore, this study compared the fatigue properties of VHCF using spring steel with bainitic and martensitic structures and bearing steel data. This study analyzed the characteristics of microstructure transformation associated with the heat treatment cycles and studied and evaluated the fatigue strength characteristics because of the UNSM in terms of fracture mechanics method and fracture surface analysis through electron backscatter diffraction, scanning electron microscopy fracture analysis, and energy-dispersive spectroscopy analysis. The fatigue limit of UNSM-treated spring steel was improved significantly by approximately 33% to 50% compared to the fatigue test results of the untreated material in the VHCF. In the long life range of bainized spring steel, fish-eye cracks appear in the form of the fine granular area, and fatigue cracks occur in the form of fish-eye cracks that occur in the bainite facet and matrix, resulting in a significant increase in fatigue strength and fatigue life.

Keywords

Acknowledgement

This study was conducted with support from the 2024 Korea Research Institute of Standards and Science's hydrogen station reliability evaluation technology development funds. (KRISS-2024-GP2024-0010).

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