Effects of Deformation Behavior and Processing Temperature on the Fatigue Performance of Deep-Rolled Medium Carbon Bar S
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STRUCTURAL and power transfer components used in automotive and heavy equipment applications require materials that exhibit high fatigue resistance. Crankshafts, which are typically manufactured from steel or cast iron, require processing to improve the resistance to bending fatigue at fillets and other highly stressed locations. In comparison to cast iron, steel forgings offer greater strength and durability, improved noise suppression, and lower reciprocating mass, which enable high performance, fuel-efficient engines.[1] Common processes to improve the fatigue performance in crankshafts include shot peening, induction hardening, and deep rolling. Deep rolling has gained industrial interest because of its ability to provide substantial improvement in fatigue performance and offer cost advantages compared with shot peening.[2–4] Deep rolling is an axially symmetric deformation process for components with a circular cross section, in which a hardened profiled roller is pressed into a machined fillet inducing localized plastic deformation, while the workpiece is rotated.[2,3] The deformation M.D. RICHARDS, formerly with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401, is now Research Metallurgist at Evraz, Pueblo, CO 81004. Contact e-mail: [email protected] J.G. SPEER and D.K. MATLOCK, Professors, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines. M.E. BURNETT, Technologist, is with The Timken Co., Canton, OH 44706. Manuscript submitted July 15, 2011. Article published online September 22, 2012 270—VOLUME 44A, JANUARY 2013
burnishes the surface and introduces nonuniform plastic strain into the fillet cross section[2] which results in a strain hardened surface layer with substantial compressive residual stress. Fatigue performance is increased through an improvement in surface finish, work hardening of the surface material, and the introduction of compressive residual stress at the surface.[2,3] Parameters for the deep-rolling process are the applied force during deep rolling, the number of passes under load by the profiled roller on the workpiece (i.e., ‘‘over-rollings’’), rolling speed, lubrication, roller surface finish, and rolling profile.[2] Deformation induced during deep rolling is a result of the stress profile generated from contact between a roller and the workpiece and depends on the material properties and geometry of the two bodies. For a constant number of over-rollings, with an increase in applied rolling force, the fatigue performance increases to a maximum, beyond which a further increase in force degrades fatigue performance.[5–12] The change in fatigue performance indicates that the magnitude and distribution of the induced residual stress field can be modified by a change in the deep-rolling processing parameters. Development of compressive residual stress occurs in opposition of the plastic strain developed during deep rolling, and is considered to be the most influential factor which leads to improvement
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