Effect of Machining Processes on the Fatigue Strength of Hardened AISI 4340 Steel

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.