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Investigation of fatigue behavior of a high strength lower bainitic structure in 60SIMN5 steel via a modified testing method

Uluğ, Emirhan
A steel with high silicon content can yield carbide-free bainite after an isothermal bainitic transformation. In this thesis study, fatigue behavior of a high strength carbide-free lower bainitic structure, which was produced from 60SiMn5 steel, was investigated. The fatigue tests of the same steel with tempered martensitic microstructure were also carried out for comparison purposes. Before fatigue testing, the mechanical properties of the specimens were also determined, which include hardness, yield strength, tensile strength and modulus of elasticity. The fatigue tests were performed on the fatigue specimens, which have rectangular cross sections, under single amplitude loading (stress ratio, R = 0) using bending fatigue mechanism. In accordance with this purpose, a new fatigue test machine and a new type fatigue specimen were designed. The design of the fatigue specimen was based on ASTM B 593 – 96 Standard, and the design of Krouse-type fatigue specimen in the standard was revised via numerical analyses according to the mechanical properties of the specimens to be tested. S-N curves and fractographic investigations of the fatigue specimens were used while comparing fatigue behaviors of the structures. It has been observed that there is no clear difference between the fatigue lives and endurance limits of lower bainitic and tempered martensitic fatigue specimens as most commonly vi observed for high strength bainitic and martensitic structures in literature. It has been demonstrated that bending fatigue application, in ASTM B 593 – 96 Standard, is also applicable to high strength steels by revising Krouse-type specimen specified in the standard.