Microstructural Evolution of Inverse Bainite in a Hypereutectoid Low-Alloy Steel

Abstract

Microstructural evolution of inverse bainite during isothermal bainite transformation of a hypereutectoid low-alloy steel at 773 K (500 °C) was investigated through a series of interrupted isothermal experiments using a quench dilatometer. Microstructural characterization revealed that the inverse bainitic transformation starts by the nucleation of cementite (Fe₃C) from parent austenite as a midrib in the bainitic microstructure. The inverse bainite becomes “degenerated” to typical upper bainite at prolonged transformation times. Crystallographic orientation relationships between the individual phases of inverse bainite microstructure were found to obey

$$ \begin{array}{*{20}l} { < 110 > _{\gamma } || < 1\overline{1} 0 > _{\theta } } \hfill & { < 111 > _{\alpha } || < 1\overline{1} 0 > _{\theta } } \hfill & { < 110 > _{\gamma } || < 111 > _{\alpha } } \hfill \\ {\{ 111\} _{\gamma } ||\{ \overline{2} 21\} _{\theta } } \hfill & {\{ 110\} _{\alpha } ||\{ \overline{2} 21\} _{\theta } \,} \hfill & {\,\{ 111\} _{\gamma } ||\{ 110\} _{\alpha } } \hfill \\ {\{ 111\} _{\gamma } ||\{ 211\} _{\theta } } \hfill & {\,\{ 110\} _{\alpha } ||\{ 211\} _{\theta } } \hfill & {} \hfill \\ \end{array} $$

Furthermore, the crystallographic orientation deviations between the individual phases of inverse bainite microstructure suggest that the secondary carbide nucleation occurs from the inverse bainitic ferrite. Thermodynamic driving force calculations provide an explanation for the observed nucleation sequence in inverse bainite. The degeneracy of inverse bainite microstructure to upper bainite at prolonged transformation times is likely due to the effects of cementite midrib dissolution at the early stage and secondary carbide coarsening at the later stage.