The effect of hardness on the residual stress in the machined surface of AISI 4340 steel was studied. Chips produced during the machining also were examined and the surface structure was investigated using optical microscope and scanning transmission microscope. Reflection electron diffraction study revealed the existence of austenite in the white layer of the machined surface together with untempered martensite. The residual stress near the machined surface or hardened steel is a compressive stress, and it changes to tensile stress as the hardness decreases. Chip segmentation was observed when steel with hardness of over Rc 50 was machined.
Residual stress remaining in machined parts can be detrimental. Previous experimental evidence shows that hardness has a significant effect on its formation. Yet, no satisfactory explanation is available for the causes of such a phenomenon. This work seeks to understand the mechanism of residual stress formation and explain the effect of hardness on it. The analysis is based on the existence of several measurable factors that influence the stress field in the work-material during the cutting process. The sensitivity of these factors to hardness allows establishment of relationships between the hardness and the material loading cycle. The results of the analysis indicate that the residual stress pattern is correlated most strongly to the orientation of the primary deformation zone in metal cutting. This correlation provides a good explanation for the role of the material hardness on the residual stress formation.
The effect of two finishing processes, namely, cutting and grinding, on the fatigue strength of hardened AISI 4340 steel was investigated. Three sets of flat tensile specimens were prepared by first machining into the general shape of the fatigue specimen standard, then they were hardened to HRC 54. The final grinding was carefully performed on one set of specimens. Two sets of specimens were fly cut to obtain a surface finish comparable to the ground surface. The residual stress distribution, surface structure, and surface profiles were determined. Fatigue testing was accomplished on these specimens in tension under load control. All the residual stress patterns were compressive, but the residual stress created by fly cutting reached a much deeper layer than that created by grinding. Fly cutting also produced a surface with a higher fatigue strength than the grinding did.
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