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Investigation of the effect of impact deformation in case-hardened steel materials
Abstract
Case carburized bearing cones were subjected to impact loadings using an experimental impact drop tester. The cones were manufactured from AISI-SAE 4320 steel by hot ring rolling, or machining from seamless tube. The microstructure consisted of retained austenite and tempered martensite. Some samples also contained fine dispersed carbides. The cones were impacted with energies in the range from 3 to 30 J. The acceleration of the impacter was measured in-situ. Post impact analysis included indentation surface profiling and cross section sampling for hardness, retained austenite and crack morphology. In general, results indicated that material response to impact loading was dominated by core thickness, microstructural constituents, and service history. Impacts on lubricated contact surfaces were more elastic, thereby limiting deformation. The critical or transition impact energy which caused case fracture was determined. The mode of case fracture depended on the retained austenite profile and load history: (1) Increased retained austenite levels beyond 40%, greatly reduced case fracture. (2) Increased load cycles moved the transition zone to lower impact energies. With increasing core thickness, the microstructure absorbed more energy. For high energy impacts which caused case fracture, the dependency on core thickness was enhanced. Resulting impact damage depended on the amount and condition of retained austenite. As the retained austenite content increased, more energy was absorbed through work hardening and transformation to martensite. This reduced the depth of the damaged microstructure, as well as the indentation size. Instantaneous case fracture was prevented and cracking of the case structure was largely reduced. During service life, the microstructure continually lost the capability to respond elastically to impact loading. As a result, the absorbed energy increased, damage appeared deeper below the surface and the transition zone beyond which case fracture occurred, moved to lower impact energies. The presence of carbides in the microstructure was beneficial only for very low impact loads, which limited deformation. As load increased, the carbides acted as stress concentration sites, which promoted crack initiation. The mode of deformation changed from displacement and transformation of retained austenite to a combination of displacement and case fracture, which absorbed minimum energy. As a result, the impact damage was far more severe, and extended deeper into the material.
Subject Area
Mechanical engineering|Materials science
Recommended Citation
Sommer, Joachim, "Investigation of the effect of impact deformation in case-hardened steel materials" (1999). ETD collection for University of Nebraska-Lincoln. AAI9929232.
https://digitalcommons.unl.edu/dissertations/AAI9929232