An Investigation of bainitic transformation in low carbon alloy and high carbon railway steels

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2018
Öztürk, Zeynep
In this thesis study, bainitic transformation in high carbon railway steel and boron steel was investigated. Bainitic structure known as having high strength and high toughness at the same time. Mechanical properties of bainite were compared with the tempered martensite phase which is commonly used phase in industry. R260 and 32CrB4 steels were used in this study. Different heat treatments were applied to these steels for bainitic transformation. Austenitization was done at 850°C for R260 steel and at 880°C for 32CrB4 steel. It is followed by an isothermal holding at just above Ms temperatures. Studies show that, R260 steel do not complete bainitic transformation at 205°C (just above Ms) by isothermal holding for 7 days. 32CrB4 steel, on the other hand, can be transformed to fully bainite by holding 2 hours at 350°C, which is just above Ms. Two- step bainite transformation was applied in order to obtain secondary fine bainite sheaves. After holding 2 minutes at 350°C, sample was quenched to 300°C and hold 5.5 hours isothermally. Mechanical properties of two-step bainite transformation becomes higher relative to fully bainite phase. First step of two-step bainite transformation was investigated by holding 2 minutes at 350°C followed by water quenching. Additionally, tempering was applied at 450°C for 2 hours. Hardness testing, tensile testing and notched bar Charpy impact testing were applied to investigate the mechanical properties. 1410 MPa of tensile strength, 13.23% of tensile v strain and 50.5 Joules of impact energy were obtained in two-step bainite transformation.