Mechanism of Inverse Bainitic Transformation in High Carbon Steels

  • Author / Creator
    Kannan, Rangasayee
  • The formation of bainite is the most studied and the most debated phase transformation in steels, with research spanning almost a century. With two theories of transformation, the transformation mechanism of bainite is still a disputed subject among physical metallurgists. Upper bainite and lower bainite are the two common bainite morphologies in engineering steels, but the microstructure of bainite is complex with several other possible morphologies of bainite. Though the transformation of other forms of bainitic microstructure can be explained in terms of either of the theories of bainite transformation, the transformation of inverse bainite which is reported in eutectoid and hypereutectoid steels is still unclear. Fundamental understanding of bainitic microstructure is a key element in the understanding of the bainitic transformation from austenite. Little work has been made over the years to verify the existence of inverse bainite although its existence is of great importance for understanding of the eutectoid decomposition of austenite. Majority of the information presented in the literature was focused on presenting two dimensional micrographs to prove the existence of inverse bainite. However, the mechanism of the transformation and the microstructure evolution during the decomposition of austenite to inverse bainite still remain unknown. This thesis, describes the fundamental understanding of the inverse bainite transformation using both advanced characterization techniques and theoretical approaches to reveal its true nature.It was found that inverse bainite transformation proceeds by formation of cementite midrib from parent austenite through Negligible Partitioning Local Equilibrium (NPLE) kinetics as the first stage of transformation. During the second stage, inverse bainitic ferrite nucleation occurs in a Para-Equilibrium (PE) mode, and when the transformation time is increased, there is a transition in kinetics from PE to NPLE.Two different orientations of secondary carbides were observed to nucleate through PE kinetics within inverse bainitic ferrite during the third stage of the transformation. Firstly, the carbides near the cementite midrib/ferrite interface (V1 type) and the second type of carbides at the ferrite/martensite (prior austenite) interface oriented parallel to the growth direction of ferrite (V2 type). Through carbon concentration measurements within inverse bainitic ferrite, it was found that the inverse bainitic ferrite at the θm/α interface is supersaturated with carbon content much higher than the para-equilibrium carbon content. Carbon segregated θm/α interface acts as nucleation sites for the V1 type secondary carbides. The nucleation of secondary carbides in inverse bainite at the α/α′(γ) interface is a consequence of the relative competition between the incomplete transformation phenomenon of ferrite and the resumption offerrite transformation by the precipitation of carbides at the α/α′(γ) interface.When the transformation time is sufficiently long, inverse bainite microstructurebecomes degenerated to be conventional upper bainite, which is the fourth stage of the transformation. It was found that the cementite midrib dissolution starts because of the C diffusion flux between the cementite midrib and secondary cementite. The carbon diffusion flux is caused by the NPLE growth kinetics of cementite midrib, whereas secondary cementite growth in ferrite follows a PE kinetics. By the time the C diffusion flux is nullified, the secondary carbides are coarser than the cementite midrib, and by Gibb’s Thomson effect secondary carbides grow at the expense of cementite midrib. This results in transport of solutes from the cementite midrib to the secondary cementite. This process continues until the carbon concentration at the cementite midrib/ferrite interface reaches the matrix ferrite carbon concentration, and the location of the cementite midrib in the degenerated microstructure can be identified as a solute enriched region in ferrite.The experimentally observed transformation and microstructure evolution is further supported using suitable theoretical calculations. CALPHAD based thermodynamic driving force nucleation model and mathematical thermo-kinetic model using the principles of PE diffusion and Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetics has been shown to provide fair predictions regarding the nucleation conditions and the growth kinetics of the transformation.

  • Subjects / Keywords
  • Graduation date
    Spring 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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