The comparison of microstructure and mechanical properties of a low carbon bainitic forging steel after isothermal transformation and direct cooling

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2022-4-27
Balcılar, Muhammed Mustafa
Bainitic forging steels have emerged in recent decades as an alternative to Quench and Temper steels due to their low cost and energy requirements and optimum combination of strength, ductility, and toughness. Although effect of continuous cooling on bainitic forging steels have been studied, the effect of isothermal transformation on microstructure and mechanical properties of microalloyed bainitic forging steels is an open field. A low carbon microalloyed bainitic forging steel is examined in this study, with a composition of 0.2C-1.3Si-1.5Mn-1.45(Cr+Mo), microalloyed with V-Ti-Nb. The steel is isothermally transformed at various temperatures and continuously cooled and compared in terms of microstructural features by optical microscopy and scanning electron microscopy, phase analysis by XRD measurements, and mechanical properties. Effect of isothermal holding time is also studied at each isothermal transformation temperature, varying from 15 minutes to 30 hours. Isothermal transformation just above Ms gives the maximum amount of bainite with the smallest lath thickness, which is proven by microstructural characterization. Retained austenite measurements demonstrated that in all samples, similar amounts of austenite remain in the system. Hardness measurements show that bainitic samples are similar in terms of hardness, but the specimen transformed at the lowest temperature gave the hardest microstructure ranging from 391-420 HV. The bainitic microstructure hardness values for other specimens range from 378HV to 421HV. The isothermal transformation just above the Ms temperature has given the most ductile and toughest steel, with an elongation of 17% and a UTS of 1264 MPa. The results show that isothermally transformed specimens have superior mechanical properties and isothermal transformation heat treatment routes should be designed to obtain materials with the best performance.

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Citation Formats
M. M. Balcılar, “The comparison of microstructure and mechanical properties of a low carbon bainitic forging steel after isothermal transformation and direct cooling,” M.S. - Master of Science, Middle East Technical University, 2022.