The effect of Si and Mo on the isothermal transformation treatment and tempering of a bainitic medium C steel

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2021-12

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Kafadar, Gülten

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Bainitic steels have become popular in recent years that generally include medium-high C(~0.5-1%), high Mn(~1-2%) and high Si (~1-2%) because of their high mechanical properties. However, their transformation time is very long (7-14 days). In this study, one of the main goal is to obtain bainitic steels which have shorter bainitic transformation times. This may be achieved by changing Mo and Si ratios in the alloys. In this thesis study, isothermal transformation treatment and tempering of a bainitic medium C steel are investigated. This research work aims to study the effect of 2wt%Mo on bainitic transformation kinetics and bainite morphology in 0.6wt%C-1.2wt%Si-1.0wt%Mn (60SiMn5) steel. Different heat treatments were applied for bainitic transformation. In the first set of specimens, austenitization was done at 950°C. It is followed by an isothermal holding at just above MS and higher temperatures. Second set of specimens were austenitized at 950°C, rapidly cooled in salt bath and isothermally treated between 200°C–300°C for different time intervals. Another set of specimens were rapidly cooled in oil after an austenitization treatment and then tempered in the temperature range of 200°C–550°C. Hardness testing and tensile testing were applied to investigate the mechanical properties. In 60SiMn5 specimens, the amount of retained austenite increases with increasing the amount of bainite but no retained austenite is observed in bainitic 2wt% Mo addition of 60SiMn5 specimens. 2wt% Mo addition of 60SiMn5 specimens shows the secondary hardening effect peak upon tempering at 500°C. The examination of the secondary hardening would also contribute to the behavior of Mo in tempered bainitic steels.

Subject Keywords

Bainite Transformation, Microstructure, Secondary Hardening, Bainite, Tempering

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https://hdl.handle.net/11511/94951

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Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

G. Kafadar, “The effect of Si and Mo on the isothermal transformation treatment and tempering of a bainitic medium C steel,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.