Effect of Chemical Composition and Isothermal Treatment Duration on Nano-bainite Steels

Abstract

Effect of chemical composition and isothermal treatment duration on nano-bainite steels is investigated in this work. The steels that contain Co, V and Al elements with high C are designed and produced. The nano-bainite microstructures can be obtained by the isothermal treatment in the Co and Al co-alloyed sample. Its mechanical properties and wear resistance performance are tested. Its microstructures were detected using a FESEM. After the steel is austensitized at 950 ℃ for 30 minutes and is subsequently isothermally-treated at 260 ℃ for 2 hours, the nano-bainite structures can be obtained, whose microstructures also contain martenite and residual austenite in addition to nano-bainite. The isothermal time of 120 minutes is enough to obtain sufficient nano-bainite, which forms due to the addition of both Al and Co, which accelerate the austenite → bainite transformation, since Al and Co increase the free-energy difference between ferrite and austensite and the reaction rate for obtaining nano-bainites. The fracture toughness of the nano-bainite steel obtained by the isothermal transition is obviously better than the quenching + tempering steel for the Co and Al co-alloyed sample. The fracture morphology of the Charpy impact test shows that the fracture is ductile. Although nano-bainite steel has the poor wear-resistance performance the initial stage compared to quenching + tempering steel, it shows more and more wear resistance performance with increasing wear time.