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The comparison of bainitic transformations and mechanical properties of 60simn5 and 55cr3 steels

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2019
Bakır, Simg
In this study, lower bainitic and martensitic transformations in 55Cr3 and 60SiMn5 heavy duty spring steels were studied. 55Cr3 and 60SiMn5 steels are used in the production of springs that can withstand medium and high tensile and compression loads, especially as leaf springs, helical springs and ring springs in vehicles. A high elastic deformation behavior is expected in spring steels. Therefore, it is important that the spring steel has high yield strength. The transformation mechanisms and mechanical properties of lower bainite were compared with the tempered martensite of 55Cr3. The samples of both steels were austenitized at 860°C for 30 minutes. It was followed by isothermal treatment of 55Cr3 samples at just above Ms temperature, which is 270°C, for 80 minutes in order to obtain fully lower bainitic microstructure. Additionally, incomplete bainitic transformations at either 270°C for 8 minutes or at 270°C for 3 hours were also studied. Moreover, martensitic samples of 55Cr3 were tempered in the range 220°C-550°C. Another set of samples were also treated under Ms in order the obtain mixture of lower bainite and tempered martensite. Although the tempered martensitic samples yielded good impact toughness values, their tensile properties were lower than the lower bainitic samples of 55Cr3. It was seen from the results that the lower bainitic sample of 55Cr3 (270°C, 80min) had 583HV hardness,1645MPa yield strength, 1978MPa ultimate tensile strength, 9.7% elongation and 12,7J impact toughness values. Moreover, the carbide-free lower bainite of 60SiMn5 was seen to have 600HV, 1781MPa yield strength, 2107MPa ultimate tensile strength, 5,68% elongation and 9J impact toughness values.